nmap - Online in the Cloud

PROGRAM:

NAME

nmap - Network exploration tool and security / port scanner

SYNOPSIS

nmap [Scan Type...] [Options] {target specification}

DESCRIPTION

Nmap (“Network Mapper”) is an open source tool for network exploration and security
auditing. It was designed to rapidly scan large networks, although it works fine against
single hosts. Nmap uses raw IP packets in novel ways to determine what hosts are available
on the network, what services (application name and version) those hosts are offering,
what operating systems (and OS versions) they are running, what type of packet
filters/firewalls are in use, and dozens of other characteristics. While Nmap is commonly
used for security audits, many systems and network administrators find it useful for
routine tasks such as network inventory, managing service upgrade schedules, and
monitoring host or service uptime.

The output from Nmap is a list of scanned targets, with supplemental information on each
depending on the options used. Key among that information is the “interesting ports
table”.. That table lists the port number and protocol, service name, and state. The
state is either open, filtered, closed, or unfiltered. Open. means that an application
on the target machine is listening for connections/packets on that port. Filtered. means
that a firewall, filter, or other network obstacle is blocking the port so that Nmap
cannot tell whether it is open or closed. Closed. ports have no application listening on
them, though they could open up at any time. Ports are classified as unfiltered. when
they are responsive to Nmap's probes, but Nmap cannot determine whether they are open or
closed. Nmap reports the state combinations open|filtered. and closed|filtered. when it
cannot determine which of the two states describe a port. The port table may also include
software version details when version detection has been requested. When an IP protocol
scan is requested (-sO), Nmap provides information on supported IP protocols rather than
listening ports.

In addition to the interesting ports table, Nmap can provide further information on
targets, including reverse DNS names, operating system guesses, device types, and MAC
addresses.

A typical Nmap scan is shown in Example 1. The only Nmap arguments used in this example
are -A, to enable OS and version detection, script scanning, and traceroute; -T4 for
faster execution; and then the hostname.

Example 1. A representative Nmap scan

# nmap -A -T4 scanme.nmap.org

Nmap scan report for scanme.nmap.org (74.207.244.221)
Host is up (0.029s latency).
rDNS record for 74.207.244.221: li86-221.members.linode.com
Not shown: 995 closed ports
PORT STATE SERVICE VERSION
22/tcp open ssh OpenSSH 5.3p1 Debian 3ubuntu7 (protocol 2.0)
| ssh-hostkey: 1024 8d:60:f1:7c:ca:b7:3d:0a:d6:67:54:9d:69:d9:b9:dd (DSA)
|_2048 79:f8:09:ac:d4:e2:32:42:10:49:d3:bd:20:82:85:ec (RSA)
80/tcp open http Apache httpd 2.2.14 ((Ubuntu))
|_http-title: Go ahead and ScanMe!
646/tcp filtered ldp
1720/tcp filtered H.323/Q.931
9929/tcp open nping-echo Nping echo
Device type: general purpose
Running: Linux 2.6.X
OS CPE: cpe:/o:linux:linux_kernel:2.6.39
OS details: Linux 2.6.39
Network Distance: 11 hops
Service Info: OS: Linux; CPE: cpe:/o:linux:kernel

TRACEROUTE (using port 53/tcp)
HOP RTT ADDRESS
[Cut first 10 hops for brevity]
11 17.65 ms li86-221.members.linode.com (74.207.244.221)

Nmap done: 1 IP address (1 host up) scanned in 14.40 seconds

The newest version of Nmap can be obtained from https://nmap.org. The newest version of
this man page is available at https://nmap.org/book/man.html. It is also included as a
chapter of Nmap Network Scanning: The Official Nmap Project Guide to Network Discovery and
Security Scanning (see https://nmap.org/book/).

OPTIONS SUMMARY

This options summary is printed when Nmap is run with no arguments, and the latest version
is always available at https://svn.nmap.org/nmap/docs/nmap.usage.txt. It helps people
remember the most common options, but is no substitute for the in-depth documentation in
the rest of this manual. Some obscure options aren't even included here.

Nmap 7.01 ( https://nmap.org )
Usage: nmap [Scan Type(s)] [Options] {target specification}
TARGET SPECIFICATION:
Can pass hostnames, IP addresses, networks, etc.
Ex: scanme.nmap.org, microsoft.com/24, 192.168.0.1; 10.0.0-255.1-254
-iL <inputfilename>: Input from list of hosts/networks
-iR <num hosts>: Choose random targets
--exclude <host1[,host2][,host3],...>: Exclude hosts/networks
--excludefile <exclude_file>: Exclude list from file
HOST DISCOVERY:
-sL: List Scan - simply list targets to scan
-sn: Ping Scan - disable port scan
-Pn: Treat all hosts as online -- skip host discovery
-PS/PA/PU/PY[portlist]: TCP SYN/ACK, UDP or SCTP discovery to given ports
-PE/PP/PM: ICMP echo, timestamp, and netmask request discovery probes
-PO[protocol list]: IP Protocol Ping
-n/-R: Never do DNS resolution/Always resolve [default: sometimes]
--dns-servers <serv1[,serv2],...>: Specify custom DNS servers
--system-dns: Use OS's DNS resolver
--traceroute: Trace hop path to each host
SCAN TECHNIQUES:
-sS/sT/sA/sW/sM: TCP SYN/Connect()/ACK/Window/Maimon scans
-sU: UDP Scan
-sN/sF/sX: TCP Null, FIN, and Xmas scans
--scanflags <flags>: Customize TCP scan flags
-sI <zombie host[:probeport]>: Idle scan
-sY/sZ: SCTP INIT/COOKIE-ECHO scans
-sO: IP protocol scan
-b <FTP relay host>: FTP bounce scan
PORT SPECIFICATION AND SCAN ORDER:
-p <port ranges>: Only scan specified ports
Ex: -p22; -p1-65535; -p U:53,111,137,T:21-25,80,139,8080,S:9
--exclude-ports <port ranges>: Exclude the specified ports from scanning
-F: Fast mode - Scan fewer ports than the default scan
-r: Scan ports consecutively - don't randomize
--top-ports <number>: Scan <number> most common ports
--port-ratio <ratio>: Scan ports more common than <ratio>
SERVICE/VERSION DETECTION:
-sV: Probe open ports to determine service/version info
--version-intensity <level>: Set from 0 (light) to 9 (try all probes)
--version-light: Limit to most likely probes (intensity 2)
--version-all: Try every single probe (intensity 9)
--version-trace: Show detailed version scan activity (for debugging)
SCRIPT SCAN:
-sC: equivalent to --script=default
--script=<Lua scripts>: <Lua scripts> is a comma separated list of
directories, script-files or script-categories
--script-args=<n1=v1,[n2=v2,...]>: provide arguments to scripts
--script-args-file=filename: provide NSE script args in a file
--script-trace: Show all data sent and received
--script-updatedb: Update the script database.
--script-help=<Lua scripts>: Show help about scripts.
<Lua scripts> is a comma-separated list of script-files or
script-categories.
OS DETECTION:
-O: Enable OS detection
--osscan-limit: Limit OS detection to promising targets
--osscan-guess: Guess OS more aggressively
TIMING AND PERFORMANCE:
Options which take <time> are in seconds, or append 'ms' (milliseconds),
's' (seconds), 'm' (minutes), or 'h' (hours) to the value (e.g. 30m).
-T<0-5>: Set timing template (higher is faster)
--min-hostgroup/max-hostgroup <size>: Parallel host scan group sizes
--min-parallelism/max-parallelism <numprobes>: Probe parallelization
--min-rtt-timeout/max-rtt-timeout/initial-rtt-timeout <time>: Specifies
probe round trip time.
--max-retries <tries>: Caps number of port scan probe retransmissions.
--host-timeout <time>: Give up on target after this long
--scan-delay/--max-scan-delay <time>: Adjust delay between probes
--min-rate <number>: Send packets no slower than <number> per second
--max-rate <number>: Send packets no faster than <number> per second
FIREWALL/IDS EVASION AND SPOOFING:
-f; --mtu <val>: fragment packets (optionally w/given MTU)
-D <decoy1,decoy2[,ME],...>: Cloak a scan with decoys
-S <IP_Address>: Spoof source address
-e <iface>: Use specified interface
-g/--source-port <portnum>: Use given port number
--proxies <url1,[url2],...>: Relay connections through HTTP/SOCKS4 proxies
--data <hex string>: Append a custom payload to sent packets
--data-string <string>: Append a custom ASCII string to sent packets
--data-length <num>: Append random data to sent packets
--ip-options <options>: Send packets with specified ip options
--ttl <val>: Set IP time-to-live field
--spoof-mac <mac address/prefix/vendor name>: Spoof your MAC address
--badsum: Send packets with a bogus TCP/UDP/SCTP checksum
OUTPUT:
-oN/-oX/-oS/-oG <file>: Output scan in normal, XML, s|<rIpt kIddi3,
and Grepable format, respectively, to the given filename.
-oA <basename>: Output in the three major formats at once
-v: Increase verbosity level (use -vv or more for greater effect)
-d: Increase debugging level (use -dd or more for greater effect)
--reason: Display the reason a port is in a particular state
--open: Only show open (or possibly open) ports
--packet-trace: Show all packets sent and received
--iflist: Print host interfaces and routes (for debugging)
--append-output: Append to rather than clobber specified output files
--resume <filename>: Resume an aborted scan
--stylesheet <path/URL>: XSL stylesheet to transform XML output to HTML
--webxml: Reference stylesheet from Nmap.Org for more portable XML
--no-stylesheet: Prevent associating of XSL stylesheet w/XML output
MISC:
-6: Enable IPv6 scanning
-A: Enable OS detection, version detection, script scanning, and traceroute
--datadir <dirname>: Specify custom Nmap data file location
--send-eth/--send-ip: Send using raw ethernet frames or IP packets
--privileged: Assume that the user is fully privileged
--unprivileged: Assume the user lacks raw socket privileges
-V: Print version number
-h: Print this help summary page.
EXAMPLES:
nmap -v -A scanme.nmap.org
nmap -v -sn 192.168.0.0/16 10.0.0.0/8
nmap -v -iR 10000 -Pn -p 80
SEE THE MAN PAGE (https://nmap.org/book/man.html) FOR MORE OPTIONS AND EXAMPLES

TARGET SPECIFICATION

Everything on the Nmap command-line that isn't an option (or option argument) is treated
as a target host specification. The simplest case is to specify a target IP address or
hostname for scanning.

Sometimes you wish to scan a whole network of adjacent hosts. For this, Nmap supports
CIDR-style. addressing. You can append /numbits to an IPv4 address or hostname and Nmap
will scan every IP address for which the first numbits are the same as for the reference
IP or hostname given. For example, 192.168.10.0/24 would scan the 256 hosts between
192.168.10.0 (binary: 11000000 10101000 00001010 00000000) and 192.168.10.255 (binary:
11000000 10101000 00001010 11111111), inclusive. 192.168.10.40/24 would scan exactly the
same targets. Given that the host scanme.nmap.org. is at the IP address 64.13.134.52, the
specification scanme.nmap.org/16 would scan the 65,536 IP addresses between 64.13.0.0 and
64.13.255.255. The smallest allowed value is /0, which targets the whole Internet. The
largest value is /32, which scans just the named host or IP address because all address
bits are fixed.

CIDR notation is short but not always flexible enough. For example, you might want to scan
192.168.0.0/16 but skip any IPs ending with .0 or .255 because they may be used as subnet
network and broadcast addresses. Nmap supports this through octet range addressing. Rather
than specify a normal IP address, you can specify a comma-separated list of numbers or
ranges for each octet. For example, 192.168.0-255.1-254 will skip all addresses in the
range that end in .0 or .255, and 192.168.3-5,7.1 will scan the four addresses
192.168.3.1, 192.168.4.1, 192.168.5.1, and 192.168.7.1. Either side of a range may be
omitted; the default values are 0 on the left and 255 on the right. Using - by itself is
the same as 0-255, but remember to use 0- in the first octet so the target specification
doesn't look like a command-line option. Ranges need not be limited to the final octets:
the specifier 0-255.0-255.13.37 will perform an Internet-wide scan for all IP addresses
ending in 13.37. This sort of broad sampling can be useful for Internet surveys and
research.

IPv6 addresses can only be specified by their fully qualified IPv6 address or hostname.
CIDR and octet ranges aren't yet supported for IPv6.

IPv6 addresses with non-global scope need to have a zone ID suffix. On Unix systems, this
is a percent sign followed by an interface name; a complete address might be
fe80::a8bb:ccff:fedd:eeff%eth0. On Windows, use an interface index number in place of an
interface name: fe80::a8bb:ccff:fedd:eeff%1. You can see a list of interface indexes by
running the command netsh.exe interface ipv6 show interface.

Nmap accepts multiple host specifications on the command line, and they don't need to be
the same type. The command nmap scanme.nmap.org 192.168.0.0/8 10.0.0,1,3-7.- does what you
would expect.

While targets are usually specified on the command lines, the following options are also
available to control target selection:

-iL inputfilename (Input from list) .
Reads target specifications from inputfilename. Passing a huge list of hosts is often
awkward on the command line, yet it is a common desire. For example, your DHCP server
might export a list of 10,000 current leases that you wish to scan. Or maybe you want
to scan all IP addresses except for those to locate hosts using unauthorized static IP
addresses. Simply generate the list of hosts to scan and pass that filename to Nmap as
an argument to the -iL option. Entries can be in any of the formats accepted by Nmap
on the command line (IP address, hostname, CIDR, IPv6, or octet ranges). Each entry
must be separated by one or more spaces, tabs, or newlines. You can specify a hyphen
(-) as the filename if you want Nmap to read hosts from standard input rather than an
actual file.

The input file may contain comments that start with # and extend to the end of the
line.

-iR num hosts (Choose random targets) .
For Internet-wide surveys and other research, you may want to choose targets at
random. The num hosts argument tells Nmap how many IPs to generate. Undesirable IPs
such as those in certain private, multicast, or unallocated address ranges are
automatically skipped. The argument 0 can be specified for a never-ending scan. Keep
in mind that some network administrators bristle at unauthorized scans of their
networks and may complain. Use this option at your own risk! If you find yourself
really bored one rainy afternoon, try the command nmap -Pn -sS -p 80 -iR 0 --open. to
locate random web servers for browsing.

--exclude host1[,host2[,...]] (Exclude hosts/networks) .
Specifies a comma-separated list of targets to be excluded from the scan even if they
are part of the overall network range you specify. The list you pass in uses normal
Nmap syntax, so it can include hostnames, CIDR netblocks, octet ranges, etc. This can
be useful when the network you wish to scan includes untouchable mission-critical
servers, systems that are known to react adversely to port scans, or subnets
administered by other people.

--excludefile exclude_file (Exclude list from file) .
This offers the same functionality as the --exclude option, except that the excluded
targets are provided in a newline-, space-, or tab-delimited exclude_file rather than
on the command line.

The exclude file may contain comments that start with # and extend to the end of the
line.

HOST DISCOVERY

One of the very first steps in any network reconnaissance mission is to reduce a
(sometimes huge) set of IP ranges into a list of active or interesting hosts. Scanning
every port of every single IP address is slow and usually unnecessary. Of course what
makes a host interesting depends greatly on the scan purposes. Network administrators may
only be interested in hosts running a certain service, while security auditors may care
about every single device with an IP address. An administrator may be comfortable using
just an ICMP ping to locate hosts on his internal network, while an external penetration
tester may use a diverse set of dozens of probes in an attempt to evade firewall
restrictions.

Because host discovery needs are so diverse, Nmap offers a wide variety of options for
customizing the techniques used. Host discovery is sometimes called ping scan, but it goes
well beyond the simple ICMP echo request packets associated with the ubiquitous ping tool.
Users can skip the ping step entirely with a list scan (-sL) or by disabling ping (-Pn),
or engage the network with arbitrary combinations of multi-port TCP SYN/ACK, UDP, SCTP
INIT and ICMP probes. The goal of these probes is to solicit responses which demonstrate
that an IP address is actually active (is being used by a host or network device). On many
networks, only a small percentage of IP addresses are active at any given time. This is
particularly common with private address space such as 10.0.0.0/8. That network has 16
million IPs, but I have seen it used by companies with less than a thousand machines. Host
discovery can find those machines in a sparsely allocated sea of IP addresses.

If no host discovery options are given, Nmap sends an ICMP echo request, a TCP SYN packet
to port 443, a TCP ACK packet to port 80, and an ICMP timestamp request. (For IPv6, the
ICMP timestamp request is omitted because it is not part of ICMPv6.) These defaults are
equivalent to the -PE -PS443 -PA80 -PP options. The exceptions to this are the ARP (for
IPv4) and Neighbor Discovery. (for IPv6) scans which are used for any targets on a local
ethernet network. For unprivileged Unix shell users, the default probes are a SYN packet
to ports 80 and 443 using the connect system call.. This host discovery is often
sufficient when scanning local networks, but a more comprehensive set of discovery probes
is recommended for security auditing.

The -P* options (which select ping types) can be combined. You can increase your odds of
penetrating strict firewalls by sending many probe types using different TCP ports/flags
and ICMP codes. Also note that ARP/Neighbor Discovery (-PR). is done by default against
targets on a local ethernet network even if you specify other -P* options, because it is
almost always faster and more effective.

By default, Nmap does host discovery and then performs a port scan against each host it
determines is online. This is true even if you specify non-default host discovery types
such as UDP probes (-PU). Read about the -sn option to learn how to perform only host
discovery, or use -Pn to skip host discovery and port scan all target hosts. The following
options control host discovery:

-sL (List Scan) .
The list scan is a degenerate form of host discovery that simply lists each host of
the network(s) specified, without sending any packets to the target hosts. By default,
Nmap still does reverse-DNS resolution on the hosts to learn their names. It is often
surprising how much useful information simple hostnames give out. For example, fw.chi
is the name of one company's Chicago firewall. Nmap also reports the total number of
IP addresses at the end. The list scan is a good sanity check to ensure that you have
proper IP addresses for your targets. If the hosts sport domain names you do not
recognize, it is worth investigating further to prevent scanning the wrong company's
network.

Since the idea is to simply print a list of target hosts, options for higher level
functionality such as port scanning, OS detection, or ping scanning cannot be combined
with this. If you wish to disable ping scanning while still performing such higher
level functionality, read up on the -Pn (skip ping) option.

-sn (No port scan) .
This option tells Nmap not to do a port scan after host discovery, and only print out
the available hosts that responded to the host discovery probes. This is often known
as a “ping scan”, but you can also request that traceroute and NSE host scripts be
run. This is by default one step more intrusive than the list scan, and can often be
used for the same purposes. It allows light reconnaissance of a target network without
attracting much attention. Knowing how many hosts are up is more valuable to attackers
than the list provided by list scan of every single IP and host name.

Systems administrators often find this option valuable as well. It can easily be used
to count available machines on a network or monitor server availability. This is often
called a ping sweep, and is more reliable than pinging the broadcast address because
many hosts do not reply to broadcast queries.

The default host discovery done with -sn consists of an ICMP echo request, TCP SYN to
port 443, TCP ACK to port 80, and an ICMP timestamp request by default. When executed
by an unprivileged user, only SYN packets are sent (using a connect call) to ports 80
and 443 on the target. When a privileged user tries to scan targets on a local
ethernet network, ARP requests are used unless --send-ip was specified. The -sn option
can be combined with any of the discovery probe types (the -P* options, excluding -Pn)
for greater flexibility. If any of those probe type and port number options are used,
the default probes are overridden. When strict firewalls are in place between the
source host running Nmap and the target network, using those advanced techniques is
recommended. Otherwise hosts could be missed when the firewall drops probes or their
responses.

In previous releases of Nmap, -sn was known as -sP..

-Pn (No ping) .
This option skips the Nmap discovery stage altogether. Normally, Nmap uses this stage
to determine active machines for heavier scanning. By default, Nmap only performs
heavy probing such as port scans, version detection, or OS detection against hosts
that are found to be up. Disabling host discovery with -Pn causes Nmap to attempt the
requested scanning functions against every target IP address specified. So if a class
B target address space (/16) is specified on the command line, all 65,536 IP addresses
are scanned. Proper host discovery is skipped as with the list scan, but instead of
stopping and printing the target list, Nmap continues to perform requested functions
as if each target IP is active. To skip ping scan and port scan, while still allowing
NSE to run, use the two options -Pn -sn together.

For machines on a local ethernet network, ARP scanning will still be performed (unless
--disable-arp-ping or --send-ip is specified) because Nmap needs MAC addresses to
further scan target hosts. In previous versions of Nmap, -Pn was -P0. and -PN..

-PS port list (TCP SYN Ping) .
This option sends an empty TCP packet with the SYN flag set. The default destination
port is 80 (configurable at compile time by changing DEFAULT_TCP_PROBE_PORT_SPEC. in
nmap.h).. Alternate ports can be specified as a parameter. The syntax is the same as
for the -p except that port type specifiers like T: are not allowed. Examples are
-PS22 and -PS22-25,80,113,1050,35000. Note that there can be no space between -PS and
the port list. If multiple probes are specified they will be sent in parallel.

The SYN flag suggests to the remote system that you are attempting to establish a
connection. Normally the destination port will be closed, and a RST (reset) packet
sent back. If the port happens to be open, the target will take the second step of a
TCP three-way-handshake. by responding with a SYN/ACK TCP packet. The machine running
Nmap then tears down the nascent connection by responding with a RST rather than
sending an ACK packet which would complete the three-way-handshake and establish a
full connection. The RST packet is sent by the kernel of the machine running Nmap in
response to the unexpected SYN/ACK, not by Nmap itself.

Nmap does not care whether the port is open or closed. Either the RST or SYN/ACK
response discussed previously tell Nmap that the host is available and responsive.

On Unix boxes, only the privileged user root. is generally able to send and receive
raw TCP packets.. For unprivileged users, a workaround is automatically employed.
whereby the connect system call is initiated against each target port. This has the
effect of sending a SYN packet to the target host, in an attempt to establish a
connection. If connect returns with a quick success or an ECONNREFUSED failure, the
underlying TCP stack must have received a SYN/ACK or RST and the host is marked
available. If the connection attempt is left hanging until a timeout is reached, the
host is marked as down.

-PA port list (TCP ACK Ping) .
The TCP ACK ping is quite similar to the just-discussed SYN ping. The difference, as
you could likely guess, is that the TCP ACK flag is set instead of the SYN flag. Such
an ACK packet purports to be acknowledging data over an established TCP connection,
but no such connection exists. So remote hosts should always respond with a RST
packet, disclosing their existence in the process.

The -PA option uses the same default port as the SYN probe (80) and can also take a
list of destination ports in the same format. If an unprivileged user tries this, the
connect workaround discussed previously is used. This workaround is imperfect because
connect is actually sending a SYN packet rather than an ACK.

The reason for offering both SYN and ACK ping probes is to maximize the chances of
bypassing firewalls. Many administrators configure routers and other simple firewalls
to block incoming SYN packets except for those destined for public services like the
company web site or mail server. This prevents other incoming connections to the
organization, while allowing users to make unobstructed outgoing connections to the
Internet. This non-stateful approach takes up few resources on the firewall/router and
is widely supported by hardware and software filters. The Linux Netfilter/iptables.
firewall software offers the --syn convenience option to implement this stateless
approach. When stateless firewall rules such as this are in place, SYN ping probes
(-PS) are likely to be blocked when sent to closed target ports. In such cases, the
ACK probe shines as it cuts right through these rules.

Another common type of firewall uses stateful rules that drop unexpected packets. This
feature was initially found mostly on high-end firewalls, though it has become much
more common over the years. The Linux Netfilter/iptables system supports this through
the --state option, which categorizes packets based on connection state. A SYN probe
is more likely to work against such a system, as unexpected ACK packets are generally
recognized as bogus and dropped. A solution to this quandary is to send both SYN and
ACK probes by specifying -PS and -PA.

-PU port list (UDP Ping) .
Another host discovery option is the UDP ping, which sends a UDP packet to the given
ports. For most ports, the packet will be empty, though some use a protocol-specific
payload that is more likely to elicit a response. The payload database is described
at https://nmap.org/book/nmap-payloads.html.. --data, --data-string, and
--data-length options.

The port list takes the same format as with the previously discussed -PS and -PA
options. If no ports are specified, the default is 40125.. This default can be
configured at compile-time by changing DEFAULT_UDP_PROBE_PORT_SPEC. in nmap.h.. A
highly uncommon port is used by default because sending to open ports is often
undesirable for this particular scan type.

Upon hitting a closed port on the target machine, the UDP probe should elicit an ICMP
port unreachable packet in return. This signifies to Nmap that the machine is up and
available. Many other types of ICMP errors, such as host/network unreachables or TTL
exceeded are indicative of a down or unreachable host. A lack of response is also
interpreted this way. If an open port is reached, most services simply ignore the
empty packet and fail to return any response. This is why the default probe port is
40125, which is highly unlikely to be in use. A few services, such as the Character
Generator (chargen) protocol, will respond to an empty UDP packet, and thus disclose
to Nmap that the machine is available.

The primary advantage of this scan type is that it bypasses firewalls and filters that
only screen TCP. For example, I once owned a Linksys BEFW11S4 wireless broadband
router. The external interface of this device filtered all TCP ports by default, but
UDP probes would still elicit port unreachable messages and thus give away the device.

-PY port list (SCTP INIT Ping) .
This option sends an SCTP packet containing a minimal INIT chunk. The default
destination port is 80 (configurable at compile time by changing
DEFAULT_SCTP_PROBE_PORT_SPEC. in nmap.h). Alternate ports can be specified as a
parameter. The syntax is the same as for the -p except that port type specifiers like
S: are not allowed. Examples are -PY22 and -PY22,80,179,5060. Note that there can be
no space between -PY and the port list. If multiple probes are specified they will be
sent in parallel.

The INIT chunk suggests to the remote system that you are attempting to establish an
association. Normally the destination port will be closed, and an ABORT chunk will be
sent back. If the port happens to be open, the target will take the second step of an
SCTP four-way-handshake. by responding with an INIT-ACK chunk. If the machine running
Nmap has a functional SCTP stack, then it tears down the nascent association by
responding with an ABORT chunk rather than sending a COOKIE-ECHO chunk which would be
the next step in the four-way-handshake. The ABORT packet is sent by the kernel of the
machine running Nmap in response to the unexpected INIT-ACK, not by Nmap itself.

Nmap does not care whether the port is open or closed. Either the ABORT or INIT-ACK
response discussed previously tell Nmap that the host is available and responsive.

On Unix boxes, only the privileged user root. is generally able to send and receive
raw SCTP packets.. Using SCTP INIT Pings is currently not possible for unprivileged
users..

-PE; -PP; -PM (ICMP Ping Types) .
In addition to the unusual TCP, UDP and SCTP host discovery types discussed
previously, Nmap can send the standard packets sent by the ubiquitous ping program.
Nmap sends an ICMP type 8 (echo request) packet to the target IP addresses, expecting
a type 0 (echo reply) in return from available hosts.. Unfortunately for network
explorers, many hosts and firewalls now block these packets, rather than responding as
required by RFC 1122[2].. For this reason, ICMP-only scans are rarely reliable enough
against unknown targets over the Internet. But for system administrators monitoring an
internal network, they can be a practical and efficient approach. Use the -PE option
to enable this echo request behavior.

While echo request is the standard ICMP ping query, Nmap does not stop there. The ICMP
standards (RFC 792[3]. and RFC 950[4]. ) also specify timestamp request, information
request, and address mask request packets as codes 13, 15, and 17, respectively. While
the ostensible purpose for these queries is to learn information such as address masks
and current times, they can easily be used for host discovery. A system that replies
is up and available. Nmap does not currently implement information request packets, as
they are not widely supported. RFC 1122 insists that “a host SHOULD NOT implement
these messages”. Timestamp and address mask queries can be sent with the -PP and -PM
options, respectively. A timestamp reply (ICMP code 14) or address mask reply (code
18) discloses that the host is available. These two queries can be valuable when
administrators specifically block echo request packets while forgetting that other
ICMP queries can be used for the same purpose.

-PO protocol list (IP Protocol Ping) .
One of the newer host discovery options is the IP protocol ping, which sends IP
packets with the specified protocol number set in their IP header. The protocol list
takes the same format as do port lists in the previously discussed TCP, UDP and SCTP
host discovery options. If no protocols are specified, the default is to send multiple
IP packets for ICMP (protocol 1), IGMP (protocol 2), and IP-in-IP (protocol 4). The
default protocols can be configured at compile-time by changing
DEFAULT_PROTO_PROBE_PORT_SPEC. in nmap.h. Note that for the ICMP, IGMP, TCP (protocol
6), UDP (protocol 17) and SCTP (protocol 132), the packets are sent with the proper
protocol headers. while other protocols are sent with no additional data beyond the
IP header (unless any of --data, --data-string, or --data-length options are
specified).

This host discovery method looks for either responses using the same protocol as a
probe, or ICMP protocol unreachable messages which signify that the given protocol
isn't supported on the destination host. Either type of response signifies that the
target host is alive.

-PR (ARP Ping) .
One of the most common Nmap usage scenarios is to scan an ethernet LAN. On most LANs,
especially those using private address ranges specified by RFC 1918[5], the vast
majority of IP addresses are unused at any given time. When Nmap tries to send a raw
IP packet such as an ICMP echo request, the operating system must determine the
destination hardware (ARP) address corresponding to the target IP so that it can
properly address the ethernet frame. This is often slow and problematic, since
operating systems weren't written with the expectation that they would need to do
millions of ARP requests against unavailable hosts in a short time period.

ARP scan puts Nmap and its optimized algorithms in charge of ARP requests. And if it
gets a response back, Nmap doesn't even need to worry about the IP-based ping packets
since it already knows the host is up. This makes ARP scan much faster and more
reliable than IP-based scans. So it is done by default when scanning ethernet hosts
that Nmap detects are on a local ethernet network. Even if different ping types (such
as -PE or -PS) are specified, Nmap uses ARP instead for any of the targets which are
on the same LAN. If you absolutely don't want to do an ARP scan, specify
--disable-arp-ping.

For IPv6 (-6 option), -PR uses ICMPv6 Neighbor Discovery instead of ARP. Neighbor
Discovery, defined in RFC 4861, can be seen as the IPv6 equivalent of ARP.

--disable-arp-ping (No ARP or ND Ping) .
Nmap normally does ARP or IPv6 Neighbor Discovery (ND) discovery of locally connected
ethernet hosts, even if other host discovery options such as -Pn or -PE are used. To
disable this implicit behavior, use the --disable-arp-ping option.

The default behavior is normally faster, but this option is useful on networks using
proxy ARP, in which a router speculatively replies to all ARP requests, making every
target appear to be up according to ARP scan.

--traceroute (Trace path to host) .
Traceroutes are performed post-scan using information from the scan results to
determine the port and protocol most likely to reach the target. It works with all
scan types except connect scans (-sT) and idle scans (-sI). All traces use Nmap's
dynamic timing model and are performed in parallel.

Traceroute works by sending packets with a low TTL (time-to-live) in an attempt to
elicit ICMP Time Exceeded messages from intermediate hops between the scanner and the
target host. Standard traceroute implementations start with a TTL of 1 and increment
the TTL until the destination host is reached. Nmap's traceroute starts with a high
TTL and then decrements the TTL until it reaches zero. Doing it backwards lets Nmap
employ clever caching algorithms to speed up traces over multiple hosts. On average
Nmap sends 5–10 fewer packets per host, depending on network conditions. If a single
subnet is being scanned (i.e. 192.168.0.0/24) Nmap may only have to send two packets
to most hosts.

-n (No DNS resolution) .
Tells Nmap to never do reverse DNS resolution on the active IP addresses it finds.
Since DNS can be slow even with Nmap's built-in parallel stub resolver, this option
can slash scanning times.

-R (DNS resolution for all targets) .
Tells Nmap to always do reverse DNS resolution on the target IP addresses. Normally
reverse DNS is only performed against responsive (online) hosts.

--system-dns (Use system DNS resolver) .
By default, Nmap resolves IP addresses by sending queries directly to the name servers
configured on your host and then listening for responses. Many requests (often dozens)
are performed in parallel to improve performance. Specify this option to use your
system resolver instead (one IP at a time via the getnameinfo call). This is slower
and rarely useful unless you find a bug in the Nmap parallel resolver (please let us
know if you do). The system resolver is always used for IPv6 scans.

--dns-servers server1[,server2[,...]] (Servers to use for reverse DNS queries) .
By default, Nmap determines your DNS servers (for rDNS resolution) from your
resolv.conf file (Unix) or the Registry (Win32). Alternatively, you may use this
option to specify alternate servers. This option is not honored if you are using
--system-dns or an IPv6 scan. Using multiple DNS servers is often faster, especially
if you choose authoritative servers for your target IP space. This option can also
improve stealth, as your requests can be bounced off just about any recursive DNS
server on the Internet.

This option also comes in handy when scanning private networks. Sometimes only a few
name servers provide proper rDNS information, and you may not even know where they
are. You can scan the network for port 53 (perhaps with version detection), then try
Nmap list scans (-sL) specifying each name server one at a time with --dns-servers
until you find one which works.

PORT SCANNING BASICS

While Nmap has grown in functionality over the years, it began as an efficient port
scanner, and that remains its core function. The simple command nmap target scans 1,000
TCP ports on the host target. While many port scanners have traditionally lumped all ports
into the open or closed states, Nmap is much more granular. It divides ports into six
states: open, closed, filtered, unfiltered, open|filtered, or closed|filtered.

These states are not intrinsic properties of the port itself, but describe how Nmap sees
them. For example, an Nmap scan from the same network as the target may show port 135/tcp
as open, while a scan at the same time with the same options from across the Internet
might show that port as filtered.

The six port states recognized by Nmap

An application is actively accepting TCP connections, UDP datagrams or SCTP
associations on this port. Finding these is often the primary goal of port scanning.
Security-minded people know that each open port is an avenue for attack. Attackers and
pen-testers want to exploit the open ports, while administrators try to close or
protect them with firewalls without thwarting legitimate users. Open ports are also
interesting for non-security scans because they show services available for use on the
network.

A closed port is accessible (it receives and responds to Nmap probe packets), but
there is no application listening on it. They can be helpful in showing that a host is
up on an IP address (host discovery, or ping scanning), and as part of OS detection.
Because closed ports are reachable, it may be worth scanning later in case some open
up. Administrators may want to consider blocking such ports with a firewall. Then they
would appear in the filtered state, discussed next.

Nmap cannot determine whether the port is open because packet filtering prevents its
probes from reaching the port. The filtering could be from a dedicated firewall
device, router rules, or host-based firewall software. These ports frustrate attackers
because they provide so little information. Sometimes they respond with ICMP error
messages such as type 3 code 13 (destination unreachable: communication
administratively prohibited), but filters that simply drop probes without responding
are far more common. This forces Nmap to retry several times just in case the probe
was dropped due to network congestion rather than filtering. This slows down the scan
dramatically.

The unfiltered state means that a port is accessible, but Nmap is unable to determine
whether it is open or closed. Only the ACK scan, which is used to map firewall
rulesets, classifies ports into this state. Scanning unfiltered ports with other scan
types such as Window scan, SYN scan, or FIN scan, may help resolve whether the port is
open.

Nmap places ports in this state when it is unable to determine whether a port is open
or filtered. This occurs for scan types in which open ports give no response. The lack
of response could also mean that a packet filter dropped the probe or any response it
elicited. So Nmap does not know for sure whether the port is open or being filtered.
The UDP, IP protocol, FIN, NULL, and Xmas scans classify ports this way.

This state is used when Nmap is unable to determine whether a port is closed or
filtered. It is only used for the IP ID idle scan.

PORT SCANNING TECHNIQUES

As a novice performing automotive repair, I can struggle for hours trying to fit my
rudimentary tools (hammer, duct tape, wrench, etc.) to the task at hand. When I fail
miserably and tow my jalopy to a real mechanic, he invariably fishes around in a huge tool
chest until pulling out the perfect gizmo which makes the job seem effortless. The art of
port scanning is similar. Experts understand the dozens of scan techniques and choose the
appropriate one (or combination) for a given task. Inexperienced users and script
kiddies,. on the other hand, try to solve every problem with the default SYN scan. Since
Nmap is free, the only barrier to port scanning mastery is knowledge. That certainly beats
the automotive world, where it may take great skill to determine that you need a strut
spring compressor, then you still have to pay thousands of dollars for it.

Most of the scan types are only available to privileged users.. This is because they send
and receive raw packets,. which requires root access on Unix systems. Using an
administrator account on Windows is recommended, though Nmap sometimes works for
unprivileged users on that platform when WinPcap has already been loaded into the OS.
Requiring root privileges was a serious limitation when Nmap was released in 1997, as many
users only had access to shared shell accounts. Now, the world is different. Computers are
cheaper, far more people have always-on direct Internet access, and desktop Unix systems
(including Linux and Mac OS X) are prevalent. A Windows version of Nmap is now available,
allowing it to run on even more desktops. For all these reasons, users have less need to
run Nmap from limited shared shell accounts. This is fortunate, as the privileged options
make Nmap far more powerful and flexible.

While Nmap attempts to produce accurate results, keep in mind that all of its insights are
based on packets returned by the target machines (or firewalls in front of them). Such
hosts may be untrustworthy and send responses intended to confuse or mislead Nmap. Much
more common are non-RFC-compliant hosts that do not respond as they should to Nmap probes.
FIN, NULL, and Xmas scans are particularly susceptible to this problem. Such issues are
specific to certain scan types and so are discussed in the individual scan type entries.

This section documents the dozen or so port scan techniques supported by Nmap. Only one
method may be used at a time, except that UDP scan (-sU) and any one of the SCTP scan
types (-sY, -sZ) may be combined with any one of the TCP scan types. As a memory aid, port
scan type options are of the form -sC, where C is a prominent character in the scan name,
usually the first. The one exception to this is the deprecated FTP bounce scan (-b). By
default, Nmap performs a SYN Scan, though it substitutes a connect scan if the user does
not have proper privileges to send raw packets (requires root access on Unix). Of the
scans listed in this section, unprivileged users can only execute connect and FTP bounce
scans.

-sS (TCP SYN scan) .
SYN scan is the default and most popular scan option for good reasons. It can be
performed quickly, scanning thousands of ports per second on a fast network not
hampered by restrictive firewalls. It is also relatively unobtrusive and stealthy
since it never completes TCP connections. SYN scan works against any compliant TCP
stack rather than depending on idiosyncrasies of specific platforms as Nmap's
FIN/NULL/Xmas, Maimon and idle scans do. It also allows clear, reliable
differentiation between the open, closed, and filtered states.

This technique is often referred to as half-open scanning, because you don't open a
full TCP connection. You send a SYN packet, as if you are going to open a real
connection and then wait for a response. A SYN/ACK indicates the port is listening
(open), while a RST (reset) is indicative of a non-listener. If no response is
received after several retransmissions, the port is marked as filtered. The port is
also marked filtered if an ICMP unreachable error (type 3, code 0, 1, 2, 3, 9, 10, or
13) is received. The port is also considered open if a SYN packet (without the ACK
flag) is received in response. This can be due to an extremely rare TCP feature known
as a simultaneous open or split handshake connection (see
https://nmap.org/misc/split-handshake.pdf).

-sT (TCP connect scan) .
TCP connect scan is the default TCP scan type when SYN scan is not an option. This is
the case when a user does not have raw packet privileges. Instead of writing raw
packets as most other scan types do, Nmap asks the underlying operating system to
establish a connection with the target machine and port by issuing the connect system
call. This is the same high-level system call that web browsers, P2P clients, and most
other network-enabled applications use to establish a connection. It is part of a
programming interface known as the Berkeley Sockets API. Rather than read raw packet
responses off the wire, Nmap uses this API to obtain status information on each
connection attempt.

When SYN scan is available, it is usually a better choice. Nmap has less control over
the high level connect call than with raw packets, making it less efficient. The
system call completes connections to open target ports rather than performing the
half-open reset that SYN scan does. Not only does this take longer and require more
packets to obtain the same information, but target machines are more likely to log the
connection. A decent IDS will catch either, but most machines have no such alarm
system. Many services on your average Unix system will add a note to syslog, and
sometimes a cryptic error message, when Nmap connects and then closes the connection
without sending data. Truly pathetic services crash when this happens, though that is
uncommon. An administrator who sees a bunch of connection attempts in her logs from a
single system should know that she has been connect scanned.

-sU (UDP scans) .
While most popular services on the Internet run over the TCP protocol, UDP[6] services
are widely deployed. DNS, SNMP, and DHCP (registered ports 53, 161/162, and 67/68) are
three of the most common. Because UDP scanning is generally slower and more difficult
than TCP, some security auditors ignore these ports. This is a mistake, as exploitable
UDP services are quite common and attackers certainly don't ignore the whole protocol.
Fortunately, Nmap can help inventory UDP ports.

UDP scan is activated with the -sU option. It can be combined with a TCP scan type
such as SYN scan (-sS) to check both protocols during the same run.

UDP scan works by sending a UDP packet to every targeted port. For some common ports
such as 53 and 161, a protocol-specific payload is sent to increase response rate, but
for most ports the packet is empty unless the --data, --data-string, or --data-length
options are specified. If an ICMP port unreachable error (type 3, code 3) is returned,
the port is closed. Other ICMP unreachable errors (type 3, codes 0, 1, 2, 9, 10, or
13) mark the port as filtered. Occasionally, a service will respond with a UDP packet,
proving that it is open. If no response is received after retransmissions, the port is
classified as open|filtered. This means that the port could be open, or perhaps packet
filters are blocking the communication. Version detection (-sV) can be used to help
differentiate the truly open ports from the filtered ones.

A big challenge with UDP scanning is doing it quickly. Open and filtered ports rarely
send any response, leaving Nmap to time out and then conduct retransmissions just in
case the probe or response were lost. Closed ports are often an even bigger problem.
They usually send back an ICMP port unreachable error. But unlike the RST packets sent
by closed TCP ports in response to a SYN or connect scan, many hosts rate limit. ICMP
port unreachable messages by default. Linux and Solaris are particularly strict about
this. For example, the Linux 2.4.20 kernel limits destination unreachable messages to
one per second (in net/ipv4/icmp.c).

Nmap detects rate limiting and slows down accordingly to avoid flooding the network
with useless packets that the target machine will drop. Unfortunately, a Linux-style
limit of one packet per second makes a 65,536-port scan take more than 18 hours. Ideas
for speeding your UDP scans up include scanning more hosts in parallel, doing a quick
scan of just the popular ports first, scanning from behind the firewall, and using
--host-timeout to skip slow hosts.

-sY (SCTP INIT scan) .
SCTP[7] is a relatively new alternative to the TCP and UDP protocols, combining most
characteristics of TCP and UDP, and also adding new features like multi-homing and
multi-streaming. It is mostly being used for SS7/SIGTRAN related services but has the
potential to be used for other applications as well. SCTP INIT scan is the SCTP
equivalent of a TCP SYN scan. It can be performed quickly, scanning thousands of ports
per second on a fast network not hampered by restrictive firewalls. Like SYN scan,
INIT scan is relatively unobtrusive and stealthy, since it never completes SCTP
associations. It also allows clear, reliable differentiation between the open, closed,
and filtered states.

This technique is often referred to as half-open scanning, because you don't open a
full SCTP association. You send an INIT chunk, as if you are going to open a real
association and then wait for a response. An INIT-ACK chunk indicates the port is
listening (open), while an ABORT chunk is indicative of a non-listener. If no response
is received after several retransmissions, the port is marked as filtered. The port is
also marked filtered if an ICMP unreachable error (type 3, code 0, 1, 2, 3, 9, 10, or
13) is received.

-sN; -sF; -sX (TCP NULL, FIN, and Xmas scans) .
These three scan types (even more are possible with the --scanflags option described
in the next section) exploit a subtle loophole in the TCP RFC[8] to differentiate
between open and closed ports. Page 65 of RFC 793 says that “if the [destination] port
state is CLOSED .... an incoming segment not containing a RST causes a RST to be sent
in response.” Then the next page discusses packets sent to open ports without the
SYN, RST, or ACK bits set, stating that: “you are unlikely to get here, but if you do,
drop the segment, and return.”

When scanning systems compliant with this RFC text, any packet not containing SYN,
RST, or ACK bits will result in a returned RST if the port is closed and no response
at all if the port is open. As long as none of those three bits are included, any
combination of the other three (FIN, PSH, and URG) are OK. Nmap exploits this with
three scan types:

Null scan (-sN)
Does not set any bits (TCP flag header is 0)

FIN scan (-sF)
Sets just the TCP FIN bit.

Xmas scan (-sX)
Sets the FIN, PSH, and URG flags, lighting the packet up like a Christmas tree.

These three scan types are exactly the same in behavior except for the TCP flags set
in probe packets. If a RST packet is received, the port is considered closed, while no
response means it is open|filtered. The port is marked filtered if an ICMP unreachable
error (type 3, code 0, 1, 2, 3, 9, 10, or 13) is received.

The key advantage to these scan types is that they can sneak through certain
non-stateful firewalls and packet filtering routers. Another advantage is that these
scan types are a little more stealthy than even a SYN scan. Don't count on this
though—most modern IDS products can be configured to detect them. The big downside is
that not all systems follow RFC 793 to the letter. A number of systems send RST
responses to the probes regardless of whether the port is open or not. This causes all
of the ports to be labeled closed. Major operating systems that do this are Microsoft
Windows, many Cisco devices, BSDI, and IBM OS/400. This scan does work against most
Unix-based systems though. Another downside of these scans is that they can't
distinguish open ports from certain filtered ones, leaving you with the response
open|filtered.

-sA (TCP ACK scan) .
This scan is different than the others discussed so far in that it never determines
open (or even open|filtered) ports. It is used to map out firewall rulesets,
determining whether they are stateful or not and which ports are filtered.

The ACK scan probe packet has only the ACK flag set (unless you use --scanflags). When
scanning unfiltered systems, open and closed ports will both return a RST packet. Nmap
then labels them as unfiltered, meaning that they are reachable by the ACK packet, but
whether they are open or closed is undetermined. Ports that don't respond, or send
certain ICMP error messages back (type 3, code 0, 1, 2, 3, 9, 10, or 13), are labeled
filtered.

-sW (TCP Window scan) .
Window scan is exactly the same as ACK scan except that it exploits an implementation
detail of certain systems to differentiate open ports from closed ones, rather than
always printing unfiltered when a RST is returned. It does this by examining the TCP
Window field of the RST packets returned. On some systems, open ports use a positive
window size (even for RST packets) while closed ones have a zero window. So instead of
always listing a port as unfiltered when it receives a RST back, Window scan lists the
port as open or closed if the TCP Window value in that reset is positive or zero,
respectively.

This scan relies on an implementation detail of a minority of systems out on the
Internet, so you can't always trust it. Systems that don't support it will usually
return all ports closed. Of course, it is possible that the machine really has no open
ports. If most scanned ports are closed but a few common port numbers (such as 22, 25,
53) are filtered, the system is most likely susceptible. Occasionally, systems will
even show the exact opposite behavior. If your scan shows 1,000 open ports and three
closed or filtered ports, then those three may very well be the truly open ones.

-sM (TCP Maimon scan) .
The Maimon scan is named after its discoverer, Uriel Maimon.. He described the
technique in Phrack Magazine issue #49 (November 1996).. Nmap, which included this
technique, was released two issues later. This technique is exactly the same as NULL,
FIN, and Xmas scans, except that the probe is FIN/ACK. According to RFC 793[8] (TCP),
a RST packet should be generated in response to such a probe whether the port is open
or closed. However, Uriel noticed that many BSD-derived systems simply drop the packet
if the port is open.

--scanflags (Custom TCP scan) .
Truly advanced Nmap users need not limit themselves to the canned scan types offered.
The --scanflags option allows you to design your own scan by specifying arbitrary TCP
flags.. Let your creative juices flow, while evading intrusion detection systems.
whose vendors simply paged through the Nmap man page adding specific rules!

The --scanflags argument can be a numerical flag value such as 9 (PSH and FIN), but
using symbolic names is easier. Just mash together any combination of URG, ACK, PSH,
RST, SYN, and FIN. For example, --scanflags URGACKPSHRSTSYNFIN sets everything, though
it's not very useful for scanning. The order these are specified in is irrelevant.

In addition to specifying the desired flags, you can specify a TCP scan type (such as
-sA or -sF). That base type tells Nmap how to interpret responses. For example, a SYN
scan considers no-response to indicate a filtered port, while a FIN scan treats the
same as open|filtered. Nmap will behave the same way it does for the base scan type,
except that it will use the TCP flags you specify instead. If you don't specify a base
type, SYN scan is used.

-sZ (SCTP COOKIE ECHO scan) .
SCTP COOKIE ECHO scan is a more advanced SCTP scan. It takes advantage of the fact
that SCTP implementations should silently drop packets containing COOKIE ECHO chunks
on open ports, but send an ABORT if the port is closed. The advantage of this scan
type is that it is not as obvious a port scan than an INIT scan. Also, there may be
non-stateful firewall rulesets blocking INIT chunks, but not COOKIE ECHO chunks. Don't
be fooled into thinking that this will make a port scan invisible; a good IDS will be
able to detect SCTP COOKIE ECHO scans too. The downside is that SCTP COOKIE ECHO scans
cannot differentiate between open and filtered ports, leaving you with the state
open|filtered in both cases.

-sI zombie host[:probeport] (idle scan) .
This advanced scan method allows for a truly blind TCP port scan of the target
(meaning no packets are sent to the target from your real IP address). Instead, a
unique side-channel attack exploits predictable IP fragmentation ID sequence
generation on the zombie host to glean information about the open ports on the target.
IDS systems will display the scan as coming from the zombie machine you specify (which
must be up and meet certain criteria). This fascinating scan type is too complex to
fully describe in this reference guide, so I wrote and posted an informal paper with
full details at https://nmap.org/book/idlescan.html.

Besides being extraordinarily stealthy (due to its blind nature), this scan type
permits mapping out IP-based trust relationships between machines. The port listing
shows open ports from the perspective of the zombie host. So you can try scanning a
target using various zombies that you think might be trusted. (via router/packet
filter rules).

You can add a colon followed by a port number to the zombie host if you wish to probe
a particular port on the zombie for IP ID changes. Otherwise Nmap will use the port it
uses by default for TCP pings (80).

-sO (IP protocol scan) .
IP protocol scan allows you to determine which IP protocols (TCP, ICMP, IGMP, etc.)
are supported by target machines. This isn't technically a port scan, since it cycles
through IP protocol numbers rather than TCP or UDP port numbers. Yet it still uses the
-p option to select scanned protocol numbers, reports its results within the normal
port table format, and even uses the same underlying scan engine as the true port
scanning methods. So it is close enough to a port scan that it belongs here.

Besides being useful in its own right, protocol scan demonstrates the power of
open-source software. While the fundamental idea is pretty simple, I had not thought
to add it nor received any requests for such functionality. Then in the summer of
2000, Gerhard Rieger. conceived the idea, wrote an excellent patch implementing it,
and sent it to the announce mailing list. (then called nmap-hackers).. I
incorporated that patch into the Nmap tree and released a new version the next day.
Few pieces of commercial software have users enthusiastic enough to design and
contribute their own improvements!

Protocol scan works in a similar fashion to UDP scan. Instead of iterating through the
port number field of a UDP packet, it sends IP packet headers and iterates through the
eight-bit IP protocol field. The headers are usually empty, containing no data and not
even the proper header for the claimed protocol. The exceptions are TCP, UDP, ICMP,
SCTP, and IGMP. A proper protocol header for those is included since some systems
won't send them otherwise and because Nmap already has functions to create them.
Instead of watching for ICMP port unreachable messages, protocol scan is on the
lookout for ICMP protocol unreachable messages. If Nmap receives any response in any
protocol from the target host, Nmap marks that protocol as open. An ICMP protocol
unreachable error (type 3, code 2) causes the protocol to be marked as closed while
port unreachable (type 3, code 3) marks the protocol open. Other ICMP unreachable
errors (type 3, code 0, 1, 9, 10, or 13) cause the protocol to be marked filtered
(though they prove that ICMP is open at the same time). If no response is received
after retransmissions, the protocol is marked open|filtered

-b FTP relay host (FTP bounce scan) .
An interesting feature of the FTP protocol (RFC 959[9]) is support for so-called proxy
FTP connections. This allows a user to connect to one FTP server, then ask that files
be sent to a third-party server. Such a feature is ripe for abuse on many levels, so
most servers have ceased supporting it. One of the abuses this feature allows is
causing the FTP server to port scan other hosts. Simply ask the FTP server to send a
file to each interesting port of a target host in turn. The error message will
describe whether the port is open or not. This is a good way to bypass firewalls
because organizational FTP servers are often placed where they have more access to
other internal hosts than any old Internet host would. Nmap supports FTP bounce scan
with the -b option. It takes an argument of the form username:password@server:port.
Server is the name or IP address of a vulnerable FTP server. As with a normal URL, you
may omit username:password, in which case anonymous login credentials (user: anonymous
password:-wwwuser@) are used. The port number (and preceding colon) may be omitted as
well, in which case the default FTP port (21) on server is used.

This vulnerability was widespread in 1997 when Nmap was released, but has largely been
fixed. Vulnerable servers are still around, so it is worth trying when all else fails.
If bypassing a firewall is your goal, scan the target network for port 21 (or even for
any FTP services if you scan all ports with version detection) and use the ftp-bounce.
NSE script. Nmap will tell you whether the host is vulnerable or not. If you are just
trying to cover your tracks, you don't need to (and, in fact, shouldn't) limit
yourself to hosts on the target network. Before you go scanning random Internet
addresses for vulnerable FTP servers, consider that sysadmins may not appreciate you
abusing their servers in this way.

PORT SPECIFICATION AND SCAN ORDER

In addition to all of the scan methods discussed previously, Nmap offers options for
specifying which ports are scanned and whether the scan order is randomized or sequential.
By default, Nmap scans the most common 1,000 ports for each protocol.

-p port ranges (Only scan specified ports) .
This option specifies which ports you want to scan and overrides the default.
Individual port numbers are OK, as are ranges separated by a hyphen (e.g. 1-1023).
The beginning and/or end values of a range may be omitted, causing Nmap to use 1 and
65535, respectively. So you can specify -p- to scan ports from 1 through 65535.
Scanning port zero. is allowed if you specify it explicitly. For IP protocol scanning
(-sO), this option specifies the protocol numbers you wish to scan for (0–255).

When scanning a combination of protocols (e.g. TCP and UDP), you can specify a
particular protocol by preceding the port numbers by T: for TCP, U: for UDP, S: for
SCTP, or P: for IP Protocol. The qualifier lasts until you specify another qualifier.
For example, the argument -p U:53,111,137,T:21-25,80,139,8080 would scan UDP ports 53,
111,and 137, as well as the listed TCP ports. Note that to scan both UDP and TCP, you
have to specify -sU and at least one TCP scan type (such as -sS, -sF, or -sT). If no
protocol qualifier is given, the port numbers are added to all protocol lists. Ports
can also be specified by name according to what the port is referred to in the
nmap-services. You can even use the wildcards * and ? with the names. For example, to
scan FTP and all ports whose names begin with “http”, use -p ftp,http*. Be careful
about shell expansions and quote the argument to -p if unsure.

Ranges of ports can be surrounded by square brackets to indicate ports inside that
range that appear in nmap-services. For example, the following will scan all ports in
nmap-services equal to or below 1024: -p [-1024]. Be careful with shell expansions and
quote the argument to -p if unsure.

--exclude-ports port ranges (Exclude the specified ports from scanning) .
This option specifies which ports you do want Nmap to exclude from scanning. The port
ranges are specified similar to -p. For IP protocol scanning (-sO), this option
specifies the protocol numbers you wish to exclude (0–255).

When ports are asked to be excluded, they are excluded from all types of scans (i.e.
they will not be scanned under any circumstances). This also includes the discovery
phase.

-F (Fast (limited port) scan) .
Specifies that you wish to scan fewer ports than the default. Normally Nmap scans the
most common 1,000 ports for each scanned protocol. With -F, this is reduced to 100.

Nmap needs an nmap-services file with frequency information in order to know which
ports are the most common. If port frequency information isn't available, perhaps
because of the use of a custom nmap-services file, Nmap scans all named ports plus
ports 1-1024. In that case, -F means to scan only ports that are named in the services
file.

-r (Don't randomize ports) .
By default, Nmap randomizes the scanned port order (except that certain commonly
accessible ports are moved near the beginning for efficiency reasons). This
randomization is normally desirable, but you can specify -r for sequential (sorted
from lowest to highest) port scanning instead.

--port-ratio ratio<decimal number between 0 and 1>
Scans all ports in nmap-services file with a ratio greater than the one given. ratio
must be between 0.0 and 1.1.

--top-ports n
Scans the n highest-ratio ports found in nmap-services file after excluding all ports
specified by --exclude-ports. n must be 1 or greater.

SERVICE AND VERSION DETECTION

Point Nmap at a remote machine and it might tell you that ports 25/tcp, 80/tcp, and 53/udp
are open. Using its nmap-services. database of about 2,200 well-known services,. Nmap
would report that those ports probably correspond to a mail server (SMTP), web server
(HTTP), and name server (DNS) respectively. This lookup is usually accurate—the vast
majority of daemons listening on TCP port 25 are, in fact, mail servers. However, you
should not bet your security on this! People can and do run services on strange ports..

Even if Nmap is right, and the hypothetical server above is running SMTP, HTTP, and DNS
servers, that is not a lot of information. When doing vulnerability assessments (or even
simple network inventories) of your companies or clients, you really want to know which
mail and DNS servers and versions are running. Having an accurate version number helps
dramatically in determining which exploits a server is vulnerable to. Version detection
helps you obtain this information.

After TCP and/or UDP ports are discovered using one of the other scan methods, version
detection interrogates those ports to determine more about what is actually running. The
nmap-service-probes. database contains probes for querying various services and match
expressions to recognize and parse responses. Nmap tries to determine the service protocol
(e.g. FTP, SSH, Telnet, HTTP), the application name (e.g. ISC BIND, Apache httpd, Solaris
telnetd), the version number, hostname, device type (e.g. printer, router), the OS family
(e.g. Windows, Linux). When possible, Nmap also gets the Common Platform Enumeration
(CPE). representation of this information. Sometimes miscellaneous details like whether
an X server is open to connections, the SSH protocol version, or the KaZaA user name, are
available. Of course, most services don't provide all of this information. If Nmap was
compiled with OpenSSL support, it will connect to SSL servers to deduce the service
listening behind that encryption layer.. Some UDP ports are left in the open|filtered
state after a UDP port scan is unable to determine whether the port is open or filtered.
Version detection will try to elicit a response from these ports (just as it does with
open ports), and change the state to open if it succeeds. open|filtered TCP ports are
treated the same way. Note that the Nmap -A option enables version detection among other
things. A paper documenting the workings, usage, and customization of version detection
is available at https://nmap.org/book/vscan.html.

When RPC services are discovered, the Nmap RPC grinder. is automatically used to
determine the RPC program and version numbers. It takes all the TCP/UDP ports detected as
RPC and floods them with SunRPC program NULL commands in an attempt to determine whether
they are RPC ports, and if so, what program and version number they serve up. Thus you can
effectively obtain the same info as rpcinfo -p even if the target's portmapper is behind a
firewall (or protected by TCP wrappers). Decoys do not currently work with RPC scan..

When Nmap receives responses from a service but cannot match them to its database, it
prints out a special fingerprint and a URL for you to submit if to if you know for sure
what is running on the port. Please take a couple minutes to make the submission so that
your find can benefit everyone. Thanks to these submissions, Nmap has about 6,500 pattern
matches for more than 650 protocols such as SMTP, FTP, HTTP, etc..

Version detection is enabled and controlled with the following options:

-sV (Version detection) .
Enables version detection, as discussed above. Alternatively, you can use -A, which
enables version detection among other things.

-sR. is an alias for -sV. Prior to March 2011, it was used to active the RPC grinder
separately from version detection, but now these options are always combined.

--allports (Don't exclude any ports from version detection) .
By default, Nmap version detection skips TCP port 9100 because some printers simply
print anything sent to that port, leading to dozens of pages of HTTP GET requests,
binary SSL session requests, etc. This behavior can be changed by modifying or
removing the Exclude directive in nmap-service-probes, or you can specify --allports
to scan all ports regardless of any Exclude directive.

--version-intensity intensity (Set version scan intensity) .
When performing a version scan (-sV), Nmap sends a series of probes, each of which is
assigned a rarity value between one and nine. The lower-numbered probes are effective
against a wide variety of common services, while the higher-numbered ones are rarely
useful. The intensity level specifies which probes should be applied. The higher the
number, the more likely it is the service will be correctly identified. However, high
intensity scans take longer. The intensity must be between 0 and 9.. The default is
7.. When a probe is registered to the target port via the nmap-service-probesports
directive, that probe is tried regardless of intensity level. This ensures that the
DNS probes will always be attempted against any open port 53, the SSL probe will be
done against 443, etc.

--version-light (Enable light mode) .
This is a convenience alias for --version-intensity 2. This light mode makes version
scanning much faster, but it is slightly less likely to identify services.

--version-all (Try every single probe) .
An alias for --version-intensity 9, ensuring that every single probe is attempted
against each port.

--version-trace (Trace version scan activity) .
This causes Nmap to print out extensive debugging info about what version scanning is
doing. It is a subset of what you get with --packet-trace.

OS DETECTION

One of Nmap's best-known features is remote OS detection using TCP/IP stack
fingerprinting. Nmap sends a series of TCP and UDP packets to the remote host and examines
practically every bit in the responses. After performing dozens of tests such as TCP ISN
sampling, TCP options support and ordering, IP ID sampling, and the initial window size
check, Nmap compares the results to its nmap-os-db. database of more than 2,600 known OS
fingerprints and prints out the OS details if there is a match. Each fingerprint includes
a freeform textual description of the OS, and a classification which provides the vendor
name (e.g. Sun), underlying OS (e.g. Solaris), OS generation (e.g. 10), and device type
(general purpose, router, switch, game console, etc). Most fingerprints also have a Common
Platform Enumeration (CPE). representation, like cpe:/o:linux:linux_kernel:2.6.

If Nmap is unable to guess the OS of a machine, and conditions are good (e.g. at least one
open port and one closed port were found), Nmap will provide a URL you can use to submit
the fingerprint if you know (for sure) the OS running on the machine. By doing this you
contribute to the pool of operating systems known to Nmap and thus it will be more
accurate for everyone.

OS detection enables some other tests which make use of information that is gathered
during the process anyway. One of these is TCP Sequence Predictability Classification.
This measures approximately how hard it is to establish a forged TCP connection against
the remote host. It is useful for exploiting source-IP based trust relationships (rlogin,
firewall filters, etc) or for hiding the source of an attack. This sort of spoofing is
rarely performed any more, but many machines are still vulnerable to it. The actual
difficulty number is based on statistical sampling and may fluctuate. It is generally
better to use the English classification such as “worthy challenge” or “trivial joke”.
This is only reported in normal output in verbose (-v) mode. When verbose mode is enabled
along with -O, IP ID sequence generation is also reported. Most machines are in the
“incremental” class, which means that they increment the ID field in the IP header for
each packet they send. This makes them vulnerable to several advanced information
gathering and spoofing attacks.

Another bit of extra information enabled by OS detection is a guess at a target's uptime.
This uses the TCP timestamp option (RFC 1323[10]) to guess when a machine was last
rebooted. The guess can be inaccurate due to the timestamp counter not being initialized
to zero or the counter overflowing and wrapping around, so it is printed only in verbose
mode.

A paper documenting the workings, usage, and customization of OS detection is available at
https://nmap.org/book/osdetect.html.

OS detection is enabled and controlled with the following options:

-O (Enable OS detection) .
Enables OS detection, as discussed above. Alternatively, you can use -A to enable OS
detection along with other things.

--osscan-limit (Limit OS detection to promising targets) .
OS detection is far more effective if at least one open and one closed TCP port are
found. Set this option and Nmap will not even try OS detection against hosts that do
not meet this criteria. This can save substantial time, particularly on -Pn scans
against many hosts. It only matters when OS detection is requested with -O or -A.

--osscan-guess; --fuzzy (Guess OS detection results) .
When Nmap is unable to detect a perfect OS match, it sometimes offers up near-matches
as possibilities. The match has to be very close for Nmap to do this by default.
Either of these (equivalent) options make Nmap guess more aggressively. Nmap will
still tell you when an imperfect match is printed and display its confidence level
(percentage) for each guess.

--max-os-tries (Set the maximum number of OS detection tries against a target) .
When Nmap performs OS detection against a target and fails to find a perfect match, it
usually repeats the attempt. By default, Nmap tries five times if conditions are
favorable for OS fingerprint submission, and twice when conditions aren't so good.
Specifying a lower --max-os-tries value (such as 1) speeds Nmap up, though you miss
out on retries which could potentially identify the OS. Alternatively, a high value
may be set to allow even more retries when conditions are favorable. This is rarely
done, except to generate better fingerprints for submission and integration into the
Nmap OS database.

NMAP SCRIPTING ENGINE (NSE)

The Nmap Scripting Engine (NSE) is one of Nmap's most powerful and flexible features. It
allows users to write (and share) simple scripts (using the Lua programming language[11].
) to automate a wide variety of networking tasks. Those scripts are executed in parallel
with the speed and efficiency you expect from Nmap. Users can rely on the growing and
diverse set of scripts distributed with Nmap, or write their own to meet custom needs.

Tasks we had in mind when creating the system include network discovery, more
sophisticated version detection, vulnerability detection. NSE can even be used for
vulnerability exploitation.

To reflect those different uses and to simplify the choice of which scripts to run, each
script contains a field associating it with one or more categories. Currently defined
categories are auth, broadcast, default. discovery, dos, exploit, external, fuzzer,
intrusive, malware, safe, version, and vuln. These are all described at
https://nmap.org/book/nse-usage.html#nse-categories.

Scripts are not run in a sandbox and thus could accidentally or maliciously damage your
system or invade your privacy. Never run scripts from third parties unless you trust the
authors or have carefully audited the scripts yourself.

The Nmap Scripting Engine is described in detail at https://nmap.org/book/nse.html and is
controlled by the following options:

-sC .
Performs a script scan using the default set of scripts. It is equivalent to
--script=default. Some of the scripts in this category are considered intrusive and
should not be run against a target network without permission.

--script filename|category|directory|expression[,...] .
Runs a script scan using the comma-separated list of filenames, script categories, and
directories. Each element in the list may also be a Boolean expression describing a
more complex set of scripts. Each element is interpreted first as an expression, then
as a category, and finally as a file or directory name.

There are two special features for advanced users only. One is to prefix script names
and expressions with + to force them to run even if they normally wouldn't (e.g. the
relevant service wasn't detected on the target port). The other is that the argument
all may be used to specify every script in Nmap's database. Be cautious with this
because NSE contains dangerous scripts such as exploits, brute force authentication
crackers, and denial of service attacks.

File and directory names may be relative or absolute. Absolute names are used
directly. Relative paths are looked for in the scripts of each of the following places
until found: --datadir
$NMAPDIR.
~/.nmap (not searched on Windows).
HOME\AppData\Roaming\nmap (only on Windows).
the directory containing the nmap executable
the directory containing the nmap executable, followed by ../share/nmap
NMAPDATADIR.
the current directory.

When a directory name is given, Nmap loads every file in the directory whose name ends
with .nse. All other files are ignored and directories are not searched recursively. When
a filename is given, it does not have to have the .nse extension; it will be added
automatically if necessary. Nmap scripts are stored in a scripts subdirectory of the Nmap
data directory by default (see https://nmap.org/book/data-files.html). For efficiency,
scripts are indexed in a database stored in scripts/script.db,. which lists the category
or categories in which each script belongs. When referring to scripts from script.db by
name, you can use a shell-style ‘*’ wildcard.

nmap --script "http-*"
Loads all scripts whose name starts with http-, such as http-auth and http-open-proxy.
The argument to --script had to be in quotes to protect the wildcard from the shell.

More complicated script selection can be done using the and, or, and not operators to
build Boolean expressions. The operators have the same precedence[12] as in Lua: not is
the highest, followed by and and then or. You can alter precedence by using parentheses.
Because expressions contain space characters it is necessary to quote them.

nmap --script "not intrusive"
Loads every script except for those in the intrusive category.

nmap --script "default or safe"
This is functionally equivalent to nmap --script "default,safe". It loads all scripts
that are in the default category or the safe category or both.

nmap --script "default and safe"
Loads those scripts that are in both the default and safe categories.

nmap --script "(default or safe or intrusive) and not http-*"
Loads scripts in the default, safe, or intrusive categories, except for those whose
names start with http-.

--script-args n1=v1,n2={n3=v3},n4={v4,v5} .
Lets you provide arguments to NSE scripts. Arguments are a comma-separated list of
name=value pairs. Names and values may be strings not containing whitespace or the
characters ‘{’, ‘}’, ‘=’, or ‘,’. To include one of these characters in a string,
enclose the string in single or double quotes. Within a quoted string, ‘\’ escapes a
quote. A backslash is only used to escape quotation marks in this special case; in all
other cases a backslash is interpreted literally. Values may also be tables enclosed
in {}, just as in Lua. A table may contain simple string values or more name-value
pairs, including nested tables. Many scripts qualify their arguments with the script
name, as in xmpp-info.server_name. You may use that full qualified version to affect
just the specified script, or you may pass the unqualified version (server_name in
this case) to affect all scripts using that argument name. A script will first check
for its fully qualified argument name (the name specified in its documentation) before
it accepts an unqualified argument name. A complex example of script arguments is
--script-args
'user=foo,pass=",{}=bar",whois={whodb=nofollow+ripe},xmpp-info.server_name=localhost'.
The online NSE Documentation Portal at https://nmap.org/nsedoc/ lists the arguments
that each script accepts.

--script-args-file filename .
Lets you load arguments to NSE scripts from a file. Any arguments on the command line
supersede ones in the file. The file can be an absolute path, or a path relative to
Nmap's usual search path (NMAPDIR, etc.) Arguments can be comma-separated or
newline-separated, but otherwise follow the same rules as for --script-args, without
requiring special quoting and escaping, since they are not parsed by the shell.

--script-help filename|category|directory|expression|all[,...] .
Shows help about scripts. For each script matching the given specification, Nmap
prints the script name, its categories, and its description. The specifications are
the same as those accepted by --script; so for example if you want help about the
ftp-anon script, you would run nmap --script-help ftp-anon. In addition to getting
help for individual scripts, you can use this as a preview of what scripts will be run
for a specification, for example with nmap --script-help default.

--script-trace .
This option does what --packet-trace does, just one ISO layer higher. If this option
is specified all incoming and outgoing communication performed by a script is printed.
The displayed information includes the communication protocol, the source, the target
and the transmitted data. If more than 5% of all transmitted data is not printable,
then the trace output is in a hex dump format. Specifying --packet-trace enables
script tracing too.

--script-updatedb .
This option updates the script database found in scripts/script.db which is used by
Nmap to determine the available default scripts and categories. It is only necessary
to update the database if you have added or removed NSE scripts from the default
scripts directory or if you have changed the categories of any script. This option is
generally used by itself: nmap --script-updatedb.

TIMING AND PERFORMANCE

One of my highest Nmap development priorities has always been performance. A default scan
(nmap hostname) of a host on my local network takes a fifth of a second. That is barely
enough time to blink, but adds up when you are scanning hundreds or thousands of hosts.
Moreover, certain scan options such as UDP scanning and version detection can increase
scan times substantially. So can certain firewall configurations, particularly response
rate limiting. While Nmap utilizes parallelism and many advanced algorithms to accelerate
these scans, the user has ultimate control over how Nmap runs. Expert users carefully
craft Nmap commands to obtain only the information they care about while meeting their
time constraints.

Techniques for improving scan times include omitting non-critical tests, and upgrading to
the latest version of Nmap (performance enhancements are made frequently). Optimizing
timing parameters can also make a substantial difference. Those options are listed below.

Some options accept a time parameter. This is specified in seconds by default, though you
can append ‘ms’, ‘s’, ‘m’, or ‘h’ to the value to specify milliseconds, seconds, minutes,
or hours. So the --host-timeout arguments 900000ms, 900, 900s, and 15m all do the same
thing.

--min-hostgroup numhosts; --max-hostgroup numhosts (Adjust parallel scan group sizes) .
Nmap has the ability to port scan or version scan multiple hosts in parallel. Nmap
does this by dividing the target IP space into groups and then scanning one group at a
time. In general, larger groups are more efficient. The downside is that host results
can't be provided until the whole group is finished. So if Nmap started out with a
group size of 50, the user would not receive any reports (except for the updates
offered in verbose mode) until the first 50 hosts are completed.

By default, Nmap takes a compromise approach to this conflict. It starts out with a
group size as low as five so the first results come quickly and then increases the
groupsize to as high as 1024. The exact default numbers depend on the options given.
For efficiency reasons, Nmap uses larger group sizes for UDP or few-port TCP scans.

When a maximum group size is specified with --max-hostgroup, Nmap will never exceed
that size. Specify a minimum size with --min-hostgroup and Nmap will try to keep group
sizes above that level. Nmap may have to use smaller groups than you specify if there
are not enough target hosts left on a given interface to fulfill the specified
minimum. Both may be set to keep the group size within a specific range, though this
is rarely desired.

These options do not have an effect during the host discovery phase of a scan. This
includes plain ping scans (-sn). Host discovery always works in large groups of hosts
to improve speed and accuracy.

The primary use of these options is to specify a large minimum group size so that the
full scan runs more quickly. A common choice is 256 to scan a network in Class C sized
chunks. For a scan with many ports, exceeding that number is unlikely to help much.
For scans of just a few port numbers, host group sizes of 2048 or more may be helpful.

--min-parallelism numprobes; --max-parallelism numprobes (Adjust probe parallelization) .
These options control the total number of probes that may be outstanding for a host
group. They are used for port scanning and host discovery. By default, Nmap calculates
an ever-changing ideal parallelism based on network performance. If packets are being
dropped, Nmap slows down and allows fewer outstanding probes. The ideal probe number
slowly rises as the network proves itself worthy. These options place minimum or
maximum bounds on that variable. By default, the ideal parallelism can drop to one if
the network proves unreliable and rise to several hundred in perfect conditions.

The most common usage is to set --min-parallelism to a number higher than one to speed
up scans of poorly performing hosts or networks. This is a risky option to play with,
as setting it too high may affect accuracy. Setting this also reduces Nmap's ability
to control parallelism dynamically based on network conditions. A value of 10 might be
reasonable, though I only adjust this value as a last resort.

The --max-parallelism option is sometimes set to one to prevent Nmap from sending more
than one probe at a time to hosts. The --scan-delay option, discussed later, is
another way to do this.

--min-rtt-timeout time, --max-rtt-timeout time, --initial-rtt-timeout time (Adjust probe
timeouts) .
Nmap maintains a running timeout value for determining how long it will wait for a
probe response before giving up or retransmitting the probe. This is calculated based
on the response times of previous probes.

If the network latency shows itself to be significant and variable, this timeout can
grow to several seconds. It also starts at a conservative (high) level and may stay
that way for a while when Nmap scans unresponsive hosts.

Specifying a lower --max-rtt-timeout and --initial-rtt-timeout than the defaults can
cut scan times significantly. This is particularly true for pingless (-Pn) scans, and
those against heavily filtered networks. Don't get too aggressive though. The scan can
end up taking longer if you specify such a low value that many probes are timing out
and retransmitting while the response is in transit.

If all the hosts are on a local network, 100 milliseconds (--max-rtt-timeout 100ms) is
a reasonable aggressive value. If routing is involved, ping a host on the network
first with the ICMP ping utility, or with a custom packet crafter such as Nping. that
is more likely to get through a firewall. Look at the maximum round trip time out of
ten packets or so. You might want to double that for the --initial-rtt-timeout and
triple or quadruple it for the --max-rtt-timeout. I generally do not set the maximum
RTT below 100 ms, no matter what the ping times are. Nor do I exceed 1000 ms.

--min-rtt-timeout is a rarely used option that could be useful when a network is so
unreliable that even Nmap's default is too aggressive. Since Nmap only reduces the
timeout down to the minimum when the network seems to be reliable, this need is
unusual and should be reported as a bug to the nmap-dev mailing list..

--max-retries numtries (Specify the maximum number of port scan probe retransmissions) .
When Nmap receives no response to a port scan probe, it could mean the port is
filtered. Or maybe the probe or response was simply lost on the network. It is also
possible that the target host has rate limiting enabled that temporarily blocked the
response. So Nmap tries again by retransmitting the initial probe. If Nmap detects
poor network reliability, it may try many more times before giving up on a port. While
this benefits accuracy, it also lengthen scan times. When performance is critical,
scans may be sped up by limiting the number of retransmissions allowed. You can even
specify --max-retries 0 to prevent any retransmissions, though that is only
recommended for situations such as informal surveys where occasional missed ports and
hosts are acceptable.

The default (with no -T template) is to allow ten retransmissions. If a network seems
reliable and the target hosts aren't rate limiting, Nmap usually only does one
retransmission. So most target scans aren't even affected by dropping --max-retries to
a low value such as three. Such values can substantially speed scans of slow (rate
limited) hosts. You usually lose some information when Nmap gives up on ports early,
though that may be preferable to letting the --host-timeout expire and losing all
information about the target.

--host-timeout time (Give up on slow target hosts) .
Some hosts simply take a long time to scan. This may be due to poorly performing or
unreliable networking hardware or software, packet rate limiting, or a restrictive
firewall. The slowest few percent of the scanned hosts can eat up a majority of the
scan time. Sometimes it is best to cut your losses and skip those hosts initially.
Specify --host-timeout with the maximum amount of time you are willing to wait. For
example, specify 30m to ensure that Nmap doesn't waste more than half an hour on a
single host. Note that Nmap may be scanning other hosts at the same time during that
half an hour, so it isn't a complete loss. A host that times out is skipped. No port
table, OS detection, or version detection results are printed for that host.

--scan-delay time; --max-scan-delay time (Adjust delay between probes) .
This option causes Nmap to wait at least the given amount of time between each probe
it sends to a given host. This is particularly useful in the case of rate limiting..
Solaris machines (among many others) will usually respond to UDP scan probe packets
with only one ICMP message per second. Any more than that sent by Nmap will be
wasteful. A --scan-delay of 1s will keep Nmap at that slow rate. Nmap tries to detect
rate limiting and adjust the scan delay accordingly, but it doesn't hurt to specify it
explicitly if you already know what rate works best.

When Nmap adjusts the scan delay upward to cope with rate limiting, the scan slows
down dramatically. The --max-scan-delay option specifies the largest delay that Nmap
will allow. A low --max-scan-delay can speed up Nmap, but it is risky. Setting this
value too low can lead to wasteful packet retransmissions and possible missed ports
when the target implements strict rate limiting.

Another use of --scan-delay is to evade threshold based intrusion detection and
prevention systems (IDS/IPS)..

--min-rate number; --max-rate number (Directly control the scanning rate) .
Nmap's dynamic timing does a good job of finding an appropriate speed at which to
scan. Sometimes, however, you may happen to know an appropriate scanning rate for a
network, or you may have to guarantee that a scan will be finished by a certain time.
Or perhaps you must keep Nmap from scanning too quickly. The --min-rate and --max-rate
options are designed for these situations.

When the --min-rate option is given Nmap will do its best to send packets as fast as
or faster than the given rate. The argument is a positive real number representing a
packet rate in packets per second. For example, specifying --min-rate 300 means that
Nmap will try to keep the sending rate at or above 300 packets per second. Specifying
a minimum rate does not keep Nmap from going faster if conditions warrant.

Likewise, --max-rate limits a scan's sending rate to a given maximum. Use --max-rate
100, for example, to limit sending to 100 packets per second on a fast network. Use
--max-rate 0.1 for a slow scan of one packet every ten seconds. Use --min-rate and
--max-rate together to keep the rate inside a certain range.

These two options are global, affecting an entire scan, not individual hosts. They
only affect port scans and host discovery scans. Other features like OS detection
implement their own timing.

There are two conditions when the actual scanning rate may fall below the requested
minimum. The first is if the minimum is faster than the fastest rate at which Nmap can
send, which is dependent on hardware. In this case Nmap will simply send packets as
fast as possible, but be aware that such high rates are likely to cause a loss of
accuracy. The second case is when Nmap has nothing to send, for example at the end of
a scan when the last probes have been sent and Nmap is waiting for them to time out or
be responded to. It's normal to see the scanning rate drop at the end of a scan or in
between hostgroups. The sending rate may temporarily exceed the maximum to make up for
unpredictable delays, but on average the rate will stay at or below the maximum.

Specifying a minimum rate should be done with care. Scanning faster than a network can
support may lead to a loss of accuracy. In some cases, using a faster rate can make a
scan take longer than it would with a slower rate. This is because Nmap's adaptive
retransmission algorithms will detect the network congestion caused by an excessive
scanning rate and increase the number of retransmissions in order to improve accuracy.
So even though packets are sent at a higher rate, more packets are sent overall. Cap
the number of retransmissions with the --max-retries option if you need to set an
upper limit on total scan time.

--defeat-rst-ratelimit .
Many hosts have long used rate limiting. to reduce the number of ICMP error messages
(such as port-unreachable errors) they send. Some systems now apply similar rate
limits to the RST (reset) packets they generate. This can slow Nmap down dramatically
as it adjusts its timing to reflect those rate limits. You can tell Nmap to ignore
those rate limits (for port scans such as SYN scan which don't treat non-responsive
ports as open) by specifying --defeat-rst-ratelimit.

Using this option can reduce accuracy, as some ports will appear non-responsive
because Nmap didn't wait long enough for a rate-limited RST response. With a SYN scan,
the non-response results in the port being labeled filtered rather than the closed
state we see when RST packets are received. This option is useful when you only care
about open ports, and distinguishing between closed and filtered ports isn't worth the
extra time.

--nsock-engine epoll|kqueue|poll|select .
Enforce use of a given nsock IO multiplexing engine. Only the select(2)-based fallback
engine is guaranteed to be available on your system. Engines are named after the name
of the IO management facility they leverage. Engines currently implemented are epoll,
kqueue, poll, and select, but not all will be present on any platform. Use nmap -V to
see which engines are supported.

-T paranoid|sneaky|polite|normal|aggressive|insane (Set a timing template) .
While the fine-grained timing controls discussed in the previous section are powerful
and effective, some people find them confusing. Moreover, choosing the appropriate
values can sometimes take more time than the scan you are trying to optimize. So Nmap
offers a simpler approach, with six timing templates. You can specify them with the -T
option and their number (0–5) or their name. The template names are paranoid (0),
sneaky (1), polite (2), normal (3), aggressive (4), and insane (5). The first two are
for IDS evasion. Polite mode slows down the scan to use less bandwidth and target
machine resources. Normal mode is the default and so -T3 does nothing. Aggressive mode
speeds scans up by making the assumption that you are on a reasonably fast and
reliable network. Finally insane mode. assumes that you are on an extraordinarily
fast network or are willing to sacrifice some accuracy for speed.

These templates allow the user to specify how aggressive they wish to be, while
leaving Nmap to pick the exact timing values. The templates also make some minor speed
adjustments for which fine-grained control options do not currently exist. For
example, -T4. prohibits the dynamic scan delay from exceeding 10 ms for TCP ports and
-T5 caps that value at 5 ms. Templates can be used in combination with fine-grained
controls, and the fine-grained controls will you specify will take precedence over the
timing template default for that parameter. I recommend using -T4 when scanning
reasonably modern and reliable networks. Keep that option even when you add
fine-grained controls so that you benefit from those extra minor optimizations that it
enables.

If you are on a decent broadband or ethernet connection, I would recommend always
using -T4. Some people love -T5 though it is too aggressive for my taste. People
sometimes specify -T2 because they think it is less likely to crash hosts or because
they consider themselves to be polite in general. They often don't realize just how
slow -T polite. really is. Their scan may take ten times longer than a default scan.
Machine crashes and bandwidth problems are rare with the default timing options (-T3)
and so I normally recommend that for cautious scanners. Omitting version detection is
far more effective than playing with timing values at reducing these problems.

While -T0. and -T1. may be useful for avoiding IDS alerts, they will take an
extraordinarily long time to scan thousands of machines or ports. For such a long
scan, you may prefer to set the exact timing values you need rather than rely on the
canned -T0 and -T1 values.

The main effects of T0 are serializing the scan so only one port is scanned at a time,
and waiting five minutes between sending each probe. T1 and T2 are similar but they
only wait 15 seconds and 0.4 seconds, respectively, between probes. T3. is Nmap's
default behavior, which includes parallelization. -T4 does the equivalent of
--max-rtt-timeout 1250ms --min-rtt-timeout 100ms --initial-rtt-timeout 500ms
--max-retries 6 and sets the maximum TCP scan delay to 10 milliseconds. T5 does the
equivalent of --max-rtt-timeout 300ms --min-rtt-timeout 50ms --initial-rtt-timeout
250ms --max-retries 2 --host-timeout 15m as well as setting the maximum TCP scan delay
to 5 ms.

FIREWALL/IDS EVASION AND SPOOFING

Many Internet pioneers envisioned a global open network with a universal IP address space
allowing virtual connections between any two nodes. This allows hosts to act as true
peers, serving and retrieving information from each other. People could access all of
their home systems from work, changing the climate control settings or unlocking the doors
for early guests. This vision of universal connectivity has been stifled by address space
shortages and security concerns. In the early 1990s, organizations began deploying
firewalls for the express purpose of reducing connectivity. Huge networks were cordoned
off from the unfiltered Internet by application proxies, network address translation, and
packet filters. The unrestricted flow of information gave way to tight regulation of
approved communication channels and the content that passes over them.

Network obstructions such as firewalls can make mapping a network exceedingly difficult.
It will not get any easier, as stifling casual reconnaissance is often a key goal of
implementing the devices. Nevertheless, Nmap offers many features to help understand these
complex networks, and to verify that filters are working as intended. It even supports
mechanisms for bypassing poorly implemented defenses. One of the best methods of
understanding your network security posture is to try to defeat it. Place yourself in the
mind-set of an attacker, and deploy techniques from this section against your networks.
Launch an FTP bounce scan, idle scan, fragmentation attack, or try to tunnel through one
of your own proxies.

In addition to restricting network activity, companies are increasingly monitoring traffic
with intrusion detection systems (IDS). All of the major IDSs ship with rules designed to
detect Nmap scans because scans are sometimes a precursor to attacks. Many of these
products have recently morphed into intrusion prevention systems (IPS). that actively
block traffic deemed malicious. Unfortunately for network administrators and IDS vendors,
reliably detecting bad intentions by analyzing packet data is a tough problem. Attackers
with patience, skill, and the help of certain Nmap options can usually pass by IDSs
undetected. Meanwhile, administrators must cope with large numbers of false positive
results where innocent activity is misdiagnosed and alerted on or blocked.

Occasionally people suggest that Nmap should not offer features for evading firewall rules
or sneaking past IDSs. They argue that these features are just as likely to be misused by
attackers as used by administrators to enhance security. The problem with this logic is
that these methods would still be used by attackers, who would just find other tools or
patch the functionality into Nmap. Meanwhile, administrators would find it that much
harder to do their jobs. Deploying only modern, patched FTP servers is a far more powerful
defense than trying to prevent the distribution of tools implementing the FTP bounce
attack.

There is no magic bullet (or Nmap option) for detecting and subverting firewalls and IDS
systems. It takes skill and experience. A tutorial is beyond the scope of this reference
guide, which only lists the relevant options and describes what they do.

-f (fragment packets); --mtu (using the specified MTU) .
The -f option causes the requested scan (including ping scans) to use tiny fragmented
IP packets. The idea is to split up the TCP header over several packets to make it
harder for packet filters, intrusion detection systems, and other annoyances to detect
what you are doing. Be careful with this! Some programs have trouble handling these
tiny packets. The old-school sniffer named Sniffit segmentation faulted immediately
upon receiving the first fragment. Specify this option once, and Nmap splits the
packets into eight bytes or less after the IP header. So a 20-byte TCP header would be
split into three packets. Two with eight bytes of the TCP header, and one with the
final four. Of course each fragment also has an IP header. Specify -f again to use 16
bytes per fragment (reducing the number of fragments).. Or you can specify your own
offset size with the --mtu option. Don't also specify -f if you use --mtu. The offset
must be a multiple of eight. While fragmented packets won't get by packet filters and
firewalls that queue all IP fragments, such as the CONFIG_IP_ALWAYS_DEFRAG option in
the Linux kernel, some networks can't afford the performance hit this causes and thus
leave it disabled. Others can't enable this because fragments may take different
routes into their networks. Some source systems defragment outgoing packets in the
kernel. Linux with the iptables. connection tracking module is one such example. Do a
scan while a sniffer such as Wireshark. is running to ensure that sent packets are
fragmented. If your host OS is causing problems, try the --send-eth. option to bypass
the IP layer and send raw ethernet frames.

Fragmentation is only supported for Nmap's raw packet features, which includes TCP and
UDP port scans (except connect scan and FTP bounce scan) and OS detection. Features
such as version detection and the Nmap Scripting Engine generally don't support
fragmentation because they rely on your host's TCP stack to communicate with target
services.

-D decoy1[,decoy2][,ME][,...] (Cloak a scan with decoys) .
Causes a decoy scan to be performed, which makes it appear to the remote host that the
host(s) you specify as decoys are scanning the target network too. Thus their IDS
might report 5–10 port scans from unique IP addresses, but they won't know which IP
was scanning them and which were innocent decoys. While this can be defeated through
router path tracing, response-dropping, and other active mechanisms, it is generally
an effective technique for hiding your IP address.

Separate each decoy host with commas, and you can optionally use ME. as one of the
decoys to represent the position for your real IP address. If you put ME in the sixth
position or later, some common port scan detectors (such as Solar Designer's.
excellent Scanlogd). are unlikely to show your IP address at all. If you don't use
ME, Nmap will put you in a random position. You can also use RND. to generate a
random, non-reserved IP address, or RND:number to generate number addresses.

Note that the hosts you use as decoys should be up or you might accidentally SYN flood
your targets. Also it will be pretty easy to determine which host is scanning if only
one is actually up on the network. You might want to use IP addresses instead of names
(so the decoy networks don't see you in their nameserver logs).

Decoys are used both in the initial ping scan (using ICMP, SYN, ACK, or whatever) and
during the actual port scanning phase. Decoys are also used during remote OS detection
(-O). Decoys do not work with version detection or TCP connect scan. When a scan delay
is in effect, the delay is enforced between each batch of spoofed probes, not between
each individual probe. Because decoys are sent as a batch all at once, they may
temporarily violate congestion control limits.

It is worth noting that using too many decoys may slow your scan and potentially even
make it less accurate. Also, some ISPs will filter out your spoofed packets, but many
do not restrict spoofed IP packets at all.

-S IP_Address (Spoof source address) .
In some circumstances, Nmap may not be able to determine your source address (Nmap
will tell you if this is the case). In this situation, use -S with the IP address of
the interface you wish to send packets through.

Another possible use of this flag is to spoof the scan to make the targets think that
someone else is scanning them. Imagine a company being repeatedly port scanned by a
competitor! The -e option and -Pn are generally required for this sort of usage. Note
that you usually won't receive reply packets back (they will be addressed to the IP
you are spoofing), so Nmap won't produce useful reports.

-e interface (Use specified interface) .
Tells Nmap what interface to send and receive packets on. Nmap should be able to
detect this automatically, but it will tell you if it cannot.

--source-port portnumber; -g portnumber (Spoof source port number) .
One surprisingly common misconfiguration is to trust traffic based only on the source
port number. It is easy to understand how this comes about. An administrator will set
up a shiny new firewall, only to be flooded with complaints from ungrateful users
whose applications stopped working. In particular, DNS may be broken because the UDP
DNS replies from external servers can no longer enter the network. FTP is another
common example. In active FTP transfers, the remote server tries to establish a
connection back to the client to transfer the requested file.

Secure solutions to these problems exist, often in the form of application-level
proxies or protocol-parsing firewall modules. Unfortunately there are also easier,
insecure solutions. Noting that DNS replies come from port 53 and active FTP from port
20, many administrators have fallen into the trap of simply allowing incoming traffic
from those ports. They often assume that no attacker would notice and exploit such
firewall holes. In other cases, administrators consider this a short-term stop-gap
measure until they can implement a more secure solution. Then they forget the security
upgrade.

Overworked network administrators are not the only ones to fall into this trap.
Numerous products have shipped with these insecure rules. Even Microsoft has been
guilty. The IPsec filters that shipped with Windows 2000 and Windows XP contain an
implicit rule that allows all TCP or UDP traffic from port 88 (Kerberos). In another
well-known case, versions of the Zone Alarm personal firewall up to 2.1.25 allowed any
incoming UDP packets with the source port 53 (DNS) or 67 (DHCP).

Nmap offers the -g and --source-port options (they are equivalent) to exploit these
weaknesses. Simply provide a port number and Nmap will send packets from that port
where possible. Most scanning operations that use raw sockets, including SYN and UDP
scans, support the option completely. The option notably doesn't have an effect for
any operations that use normal operating system sockets, including DNS requests, TCP
connect scan,. version detection, and script scanning. Setting the source port also
doesn't work for OS detection, because Nmap must use different port numbers for
certain OS detection tests to work properly.

--data hex string (Append custom binary data to sent packets) .
This option lets you include binary data as payload in sent packets. hex string may
be specified in any of the following formats: 0xAABBCCDDEEFF..., AABBCCDDEEFF... or
\xAA\xBB\xCC\xDD\xEE\xFF.... Examples of use are --data 0xdeadbeef and --data
\xCA\xFE\x09. Note that if you specify a number like 0x00ff no byte-order conversion
is performed. Make sure you specify the information in the byte order expected by the
receiver.

--data-string string (Append custom string to sent packets) .
This option lets you include a regular string as payload in sent packets. string can
contain any string. However, note that some characters may depend on your system's
locale and the receiver may not see the same information. Also, make sure you enclose
the string in double quotes and escape any special characters from the shell.
Examples: --data-string "Scan conducted by Security Ops, extension 7192" or
--data-string "Ph34r my l33t skills". Keep in mind that nobody is likely to actually
see any comments left by this option unless they are carefully monitoring the network
with a sniffer or custom IDS rules.

--data-length number (Append random data to sent packets) .
Normally Nmap sends minimalist packets containing only a header. So its TCP packets
are generally 40 bytes and ICMP echo requests are just 28. Some UDP ports. and IP
protocols. get a custom payload by default. This option tells Nmap to append the
given number of random bytes to most of the packets it sends, and not to use any
protocol-specific payloads. (Use --data-length 0 for no random or protocol-specific
payloads.. OS detection (-O) packets are not affected. because accuracy there
requires probe consistency, but most pinging and portscan packets support this. It
slows things down a little, but can make a scan slightly less conspicuous.

--ip-options S|R [route]|L [route]|T|U ... ; --ip-options hex string (Send packets with
specified ip options) .
The IP protocol[13] offers several options which may be placed in packet headers.
Unlike the ubiquitous TCP options, IP options are rarely seen due to practicality and
security concerns. In fact, many Internet routers block the most dangerous options
such as source routing. Yet options can still be useful in some cases for determining
and manipulating the network route to target machines. For example, you may be able to
use the record route option to determine a path to a target even when more traditional
traceroute-style approaches fail. Or if your packets are being dropped by a certain
firewall, you may be able to specify a different route with the strict or loose source
routing options.

The most powerful way to specify IP options is to simply pass in values as the
argument to --ip-options. Precede each hex number with \x then the two digits. You may
repeat certain characters by following them with an asterisk and then the number of
times you wish them to repeat. For example, \x01\x07\x04\x00*36\x01 is a hex string
containing 36 NUL bytes.

Nmap also offers a shortcut mechanism for specifying options. Simply pass the letter
R, T, or U to request record-route,. record-timestamp,. or both options together,
respectively. Loose or strict source routing. may be specified with an L or S
followed by a space and then a space-separated list of IP addresses.

If you wish to see the options in packets sent and received, specify --packet-trace.
For more information and examples of using IP options with Nmap, see
http://seclists.org/nmap-dev/2006/q3/52.

--ttl value (Set IP time-to-live field) .
Sets the IPv4 time-to-live field in sent packets to the given value.

--randomize-hosts (Randomize target host order) .
Tells Nmap to shuffle each group of up to 16384 hosts before it scans them. This can
make the scans less obvious to various network monitoring systems, especially when you
combine it with slow timing options. If you want to randomize over larger group sizes,
increase PING_GROUP_SZ. in nmap.h. and recompile. An alternative solution is to
generate the target IP list with a list scan (-sL -n -oN filename), randomize it with
a Perl script, then provide the whole list to Nmap with -iL..

--spoof-mac MAC address, prefix, or vendor name (Spoof MAC address) .
Asks Nmap to use the given MAC address for all of the raw ethernet frames it sends.
This option implies --send-eth. to ensure that Nmap actually sends ethernet-level
packets. The MAC given can take several formats. If it is simply the number 0, Nmap
chooses a completely random MAC address for the session. If the given string is an
even number of hex digits (with the pairs optionally separated by a colon), Nmap will
use those as the MAC. If fewer than 12 hex digits are provided, Nmap fills in the
remainder of the six bytes with random values. If the argument isn't a zero or hex
string, Nmap looks through nmap-mac-prefixes to find a vendor name containing the
given string (it is case insensitive). If a match is found, Nmap uses the vendor's OUI
(three-byte prefix). and fills out the remaining three bytes randomly. Valid
--spoof-mac argument examples are Apple, 0, 01:02:03:04:05:06, deadbeefcafe, 0020F2,
and Cisco. This option only affects raw packet scans such as SYN scan or OS detection,
not connection-oriented features such as version detection or the Nmap Scripting
Engine.

--proxies Comma-separated list of proxy URLs (Relay TCP connections through a chain of
proxies) .
Asks Nmap to establish TCP connections with a final target through supplied chain of
one or more HTTP or SOCKS4 proxies. Proxies can help hide the true source of a scan or
evade certain firewall restrictions, but they can hamper scan performance by
increasing latency. Users may need to adjust Nmap timeouts and other scan parameters
accordingly. In particular, a lower --max-parallelism may help because some proxies
refuse to handle as many concurrent connections as Nmap opens by default.

This option takes a list of proxies as argument, expressed as URLs in the format
proto://host:port. Use commas to separate node URLs in a chain. No authentication is
supported yet. Valid protocols are HTTP and SOCKS4.

Warning: this feature is still under development and has limitations. It is
implemented within the nsock library and thus has no effect on the ping, port scanning
and OS discovery phases of a scan. Only NSE and version scan benefit from this option
so far—other features may disclose your true address. SSL connections are not yet
supported, nor is proxy-side DNS resolution (hostnames are always resolved by Nmap).

--badsum (Send packets with bogus TCP/UDP checksums) .
Asks Nmap to use an invalid TCP, UDP or SCTP checksum for packets sent to target
hosts. Since virtually all host IP stacks properly drop these packets, any responses
received are likely coming from a firewall or IDS that didn't bother to verify the
checksum. For more details on this technique, see https://nmap.org/p60-12.html

--adler32 (Use deprecated Adler32 instead of CRC32C for SCTP checksums) .
Asks Nmap to use the deprecated Adler32 algorithm for calculating the SCTP checksum.
If --adler32 is not given, CRC-32C (Castagnoli) is used. RFC 2960[14] originally
defined Adler32 as checksum algorithm for SCTP; RFC 4960[7] later redefined the SCTP
checksums to use CRC-32C. Current SCTP implementations should be using CRC-32C, but in
order to elicit responses from old, legacy SCTP implementations, it may be preferable
to use Adler32.

OUTPUT

Any security tool is only as useful as the output it generates. Complex tests and
algorithms are of little value if they aren't presented in an organized and comprehensible
fashion. Given the number of ways Nmap is used by people and other software, no single
format can please everyone. So Nmap offers several formats, including the interactive mode
for humans to read directly and XML for easy parsing by software.

In addition to offering different output formats, Nmap provides options for controlling
the verbosity of output as well as debugging messages. Output types may be sent to
standard output or to named files, which Nmap can append to or clobber. Output files may
also be used to resume aborted scans.

Nmap makes output available in five different formats. The default is called interactive
output,. and it is sent to standard output (stdout).. There is also normal output,.
which is similar to interactive except that it displays less runtime information and
warnings since it is expected to be analyzed after the scan completes rather than
interactively.

XML output. is one of the most important output types, as it can be converted to HTML,
easily parsed by programs such as Nmap graphical user interfaces, or imported into
databases.

The two remaining output types are the simple grepable output. which includes most
information for a target host on a single line, and sCRiPt KiDDi3 0utPUt. for users who
consider themselves |<-r4d.

While interactive output is the default and has no associated command-line options, the
other four format options use the same syntax. They take one argument, which is the
filename that results should be stored in. Multiple formats may be specified, but each
format may only be specified once. For example, you may wish to save normal output for
your own review while saving XML of the same scan for programmatic analysis. You might do
this with the options -oX myscan.xml -oN myscan.nmap. While this chapter uses the simple
names like myscan.xml for brevity, more descriptive names are generally recommended. The
names chosen are a matter of personal preference, though I use long ones that incorporate
the scan date and a word or two describing the scan, placed in a directory named after the
company I'm scanning.

While these options save results to files, Nmap still prints interactive output to stdout
as usual. For example, the command nmap -oX myscan.xml target prints XML to myscan.xml and
fills standard output with the same interactive results it would have printed if -oX
wasn't specified at all. You can change this by passing a hyphen character as the argument
to one of the format types. This causes Nmap to deactivate interactive output, and instead
print results in the format you specified to the standard output stream. So the command
nmap -oX - target will send only XML output to stdout.. Serious errors may still be
printed to the normal error stream, stderr..

Unlike some Nmap arguments, the space between the logfile option flag (such as -oX) and
the filename or hyphen is mandatory. If you omit the flags and give arguments such as -oG-
or -oXscan.xml, a backwards compatibility feature of Nmap will cause the creation of
normal format output files named G- and Xscan.xml respectively.

All of these arguments support strftime-like. conversions in the filename. %H, %M, %S,
%m, %d, %y, and %Y are all exactly the same as in strftime. %T is the same as %H%M%S, %R
is the same as %H%M, and %D is the same as %m%d%y. A % followed by any other character
just yields that character (%% gives you a percent symbol). So -oX 'scan-%T-%D.xml' will
use an XML file with a name in the form of scan-144840-121307.xml.

Nmap also offers options to control scan verbosity and to append to output files rather
than clobbering them. All of these options are described below.

Nmap Output Formats

-oN filespec (normal output) .
Requests that normal output be directed to the given filename. As discussed above,
this differs slightly from interactive output.

-oX filespec (XML output) .
Requests that XML output be directed to the given filename. Nmap includes a document
type definition (DTD) which allows XML parsers to validate Nmap XML output. While it
is primarily intended for programmatic use, it can also help humans interpret Nmap XML
output. The DTD defines the legal elements of the format, and often enumerates the
attributes and values they can take on. The latest version is always available from
https://svn.nmap.org/nmap/docs/nmap.dtd.

XML offers a stable format that is easily parsed by software. Free XML parsers are
available for all major computer languages, including C/C++, Perl, Python, and Java.
People have even written bindings for most of these languages to handle Nmap output
and execution specifically. Examples are Nmap::Scanner[15]. and Nmap::Parser[16]. in
Perl CPAN. In almost all cases that a non-trivial application interfaces with Nmap,
XML is the preferred format.

The XML output references an XSL stylesheet which can be used to format the results as
HTML. The easiest way to use this is simply to load the XML output in a web browser
such as Firefox or IE. By default, this will only work on the machine you ran Nmap on
(or a similarly configured one) due to the hard-coded nmap.xsl filesystem path. Use
the --webxml or --stylesheet options to create portable XML files that render as HTML
on any web-connected machine.

-oS filespec (ScRipT KIdd|3 oUTpuT) .
Script kiddie output is like interactive output, except that it is post-processed to
better suit the l33t HaXXorZ who previously looked down on Nmap due to its consistent
capitalization and spelling. Humor impaired people should note that this option is
making fun of the script kiddies before flaming me for supposedly “helping them”.

-oG filespec (grepable output) .
This output format is covered last because it is deprecated. The XML output format is
far more powerful, and is nearly as convenient for experienced users. XML is a
standard for which dozens of excellent parsers are available, while grepable output is
my own simple hack. XML is extensible to support new Nmap features as they are
released, while I often must omit those features from grepable output for lack of a
place to put them.

Nevertheless, grepable output is still quite popular. It is a simple format that lists
each host on one line and can be trivially searched and parsed with standard Unix
tools such as grep, awk, cut, sed, diff, and Perl. Even I usually use it for one-off
tests done at the command line. Finding all the hosts with the SSH port open or that
are running Solaris takes only a simple grep to identify the hosts, piped to an awk or
cut command to print the desired fields.

Grepable output consists of comments (lines starting with a pound (#)). and target
lines. A target line includes a combination of six labeled fields, separated by tabs
and followed with a colon. The fields are Host, Ports, Protocols, Ignored State, OS,
Seq Index, IP ID, and Status.

The most important of these fields is generally Ports, which gives details on each
interesting port. It is a comma separated list of port entries. Each port entry
represents one interesting port, and takes the form of seven slash (/) separated
subfields. Those subfields are: Port number, State, Protocol, Owner, Service, SunRPC
info, and Version info.

As with XML output, this man page does not allow for documenting the entire format. A
more detailed look at the Nmap grepable output format is available from
https://nmap.org/book/output-formats-grepable-output.html.

-oA basename (Output to all formats) .
As a convenience, you may specify -oA basename to store scan results in normal, XML,
and grepable formats at once. They are stored in basename.nmap, basename.xml, and
basename.gnmap, respectively. As with most programs, you can prefix the filenames with
a directory path, such as ~/nmaplogs/foocorp/ on Unix or c:\hacking\sco on Windows.

Verbosity and debugging options

-v (Increase verbosity level) .
Increases the verbosity level, causing Nmap to print more information about the scan
in progress. Open ports are shown as they are found and completion time estimates are
provided when Nmap thinks a scan will take more than a few minutes. Use it twice or
more for even greater verbosity: -vv, or give a verbosity level directly, for example
-v3..

Most changes only affect interactive output, and some also affect normal and script
kiddie output. The other output types are meant to be processed by machines, so Nmap
can give substantial detail by default in those formats without fatiguing a human
user. However, there are a few changes in other modes where output size can be reduced
substantially by omitting some detail. For example, a comment line in the grepable
output that provides a list of all ports scanned is only printed in verbose mode
because it can be quite long.

-d (Increase debugging level) .
When even verbose mode doesn't provide sufficient data for you, debugging is available
to flood you with much more! As with the verbosity option (-v), debugging is enabled
with a command-line flag (-d) and the debug level can be increased by specifying it
multiple times,. as in -dd, or by setting a level directly. For example, -d9 sets
level nine. That is the highest effective level and will produce thousands of lines
unless you run a very simple scan with very few ports and targets.

Debugging output is useful when a bug is suspected in Nmap, or if you are simply
confused as to what Nmap is doing and why. As this feature is mostly intended for
developers, debug lines aren't always self-explanatory. You may get something like:
Timeout vals: srtt: -1 rttvar: -1 to: 1000000 delta 14987 ==> srtt: 14987 rttvar:
14987 to: 100000. If you don't understand a line, your only recourses are to ignore
it, look it up in the source code, or request help from the development list
(nmap-dev).. Some lines are self explanatory, but the messages become more obscure as
the debug level is increased.

--reason (Host and port state reasons) .
Shows the reason each port is set to a specific state and the reason each host is up
or down. This option displays the type of the packet that determined a port or hosts
state. For example, A RST packet from a closed port or an echo reply from an alive
host. The information Nmap can provide is determined by the type of scan or ping. The
SYN scan and SYN ping (-sS and -PS) are very detailed, but the TCP connect scan (-sT)
is limited by the implementation of the connect system call. This feature is
automatically enabled by the debug option (-d). and the results are stored in XML log
files even if this option is not specified.

--stats-every time (Print periodic timing stats) .
Periodically prints a timing status message after each interval of time. The time is a
specification of the kind described in the section called “TIMING AND PERFORMANCE”; so
for example, use --stats-every 10s to get a status update every 10 seconds. Updates
are printed to interactive output (the screen) and XML output.

--packet-trace (Trace packets and data sent and received) .
Causes Nmap to print a summary of every packet sent or received. This is often used
for debugging, but is also a valuable way for new users to understand exactly what
Nmap is doing under the covers. To avoid printing thousands of lines, you may want to
specify a limited number of ports to scan, such as -p20-30. If you only care about the
goings on of the version detection subsystem, use --version-trace instead. If you only
care about script tracing, specify --script-trace. With --packet-trace, you get all of
the above.

--open (Show only open (or possibly open) ports) .
Sometimes you only care about ports you can actually connect to (open ones), and don't
want results cluttered with closed, filtered, and closed|filtered ports. Output
customization is normally done after the scan using tools such as grep, awk, and Perl,
but this feature was added due to overwhelming requests. Specify --open to only see
hosts with at least one open, open|filtered, or unfiltered port, and only see ports in
those states. These three states are treated just as they normally are, which means
that open|filtered and unfiltered may be condensed into counts if there are an
overwhelming number of them.

--iflist (List interfaces and routes) .
Prints the interface list and system routes as detected by Nmap. This is useful for
debugging routing problems or device mischaracterization (such as Nmap treating a PPP
connection as ethernet).

Miscellaneous output options

--append-output (Append to rather than clobber output files) .
When you specify a filename to an output format flag such as -oX or -oN, that file is
overwritten by default. If you prefer to keep the existing content of the file and
append the new results, specify the --append-output option. All output filenames
specified in that Nmap execution will then be appended to rather than clobbered. This
doesn't work well for XML (-oX) scan data as the resultant file generally won't parse
properly until you fix it up by hand.

--resume filename (Resume aborted scan) .
Some extensive Nmap runs take a very long time—on the order of days. Such scans don't
always run to completion. Restrictions may prevent Nmap from being run during working
hours, the network could go down, the machine Nmap is running on might suffer a
planned or unplanned reboot, or Nmap itself could crash. The administrator running
Nmap could cancel it for any other reason as well, by pressing ctrl-C. Restarting the
whole scan from the beginning may be undesirable. Fortunately, if normal (-oN) or
grepable (-oG) logs were kept, the user can ask Nmap to resume scanning with the
target it was working on when execution ceased. Simply specify the --resume option and
pass the normal/grepable output file as its argument. No other arguments are
permitted, as Nmap parses the output file to use the same ones specified previously.
Simply call Nmap as nmap --resume logfilename. Nmap will append new results to the
data files specified in the previous execution. Resumption does not support the XML
output format because combining the two runs into one valid XML file would be
difficult.

--stylesheet path or URL (Set XSL stylesheet to transform XML output) .
Nmap ships with an XSL. stylesheet. named nmap.xsl. for viewing or translating XML
output to HTML.. The XML output includes an xml-stylesheet directive which points to
nmap.xml where it was initially installed by Nmap. Run the XML file through an XSLT
processor such as xsltproc[17]. to produce an HTML file. Directly opening the XML
file in a browser no longer works well because modern browsers limit the locations a
stylesheet may be loaded from. If you wish to use a different stylesheet, specify it
as the argument to --stylesheet. You must pass the full pathname or URL. One common
invocation is --stylesheet https://nmap.org/svn/docs/nmap.xsl. This tells an XSLT
processor to load the latest version of the stylesheet from Nmap.Org. The --webxml
option does the same thing with less typing and memorization. Loading the XSL from
Nmap.Org makes it easier to view results on a machine that doesn't have Nmap (and thus
nmap.xsl) installed. So the URL is often more useful, but the local filesystem
location of nmap.xsl is used by default for privacy reasons.

--webxml (Load stylesheet from Nmap.Org) .
This is a convenience option, nothing more than an alias for --stylesheet
https://nmap.org/svn/docs/nmap.xsl.

--no-stylesheet (Omit XSL stylesheet declaration from XML) .
Specify this option to prevent Nmap from associating any XSL stylesheet with its XML
output. The xml-stylesheet directive is omitted.

MISCELLANEOUS OPTIONS

This section describes some important (and not-so-important) options that don't really fit
anywhere else.

-6 (Enable IPv6 scanning) .
Nmap has IPv6 support for its most popular features. Ping scanning, port scanning,
version detection, and the Nmap Scripting Engine all support IPv6. The command syntax
is the same as usual except that you also add the -6 option. Of course, you must use
IPv6 syntax if you specify an address rather than a hostname. An address might look
like 3ffe:7501:4819:2000:210:f3ff:fe03:14d0, so hostnames are recommended. The output
looks the same as usual, with the IPv6 address on the “interesting ports” line being
the only IPv6 giveaway.

While IPv6 hasn't exactly taken the world by storm, it gets significant use in some
(usually Asian) countries and most modern operating systems support it. To use Nmap
with IPv6, both the source and target of your scan must be configured for IPv6. If
your ISP (like most of them) does not allocate IPv6 addresses to you, free tunnel
brokers are widely available and work fine with Nmap. I use the free IPv6 tunnel
broker. service at http://www.tunnelbroker.net. Other tunnel brokers are listed at
Wikipedia[18]. 6to4 tunnels are another popular, free approach.

On Windows, raw-socket IPv6 scans are supported only on ethernet devices (not
tunnels), and only on Windows Vista. and later. Use the --unprivileged. option in
other situations.

-A (Aggressive scan options) .
This option enables additional advanced and aggressive options. Presently this enables
OS detection (-O), version scanning (-sV), script scanning (-sC) and traceroute
(--traceroute).. More features may be added in the future. The point is to enable a
comprehensive set of scan options without people having to remember a large set of
flags. However, because script scanning with the default set is considered intrusive,
you should not use -A against target networks without permission. This option only
enables features, and not timing options (such as -T4) or verbosity options (-v) that
you might want as well. Options which require privileges (e.g. root access) such as OS
detection and traceroute will only be enabled if those privileges are available.

--datadir directoryname (Specify custom Nmap data file location) .
Nmap obtains some special data at runtime in files named nmap-service-probes,
nmap-services, nmap-protocols, nmap-rpc, nmap-mac-prefixes, and nmap-os-db. If the
location of any of these files has been specified (using the --servicedb or
--versiondb options), that location is used for that file. After that, Nmap searches
these files in the directory specified with the --datadir option (if any). Any files
not found there, are searched for in the directory specified by the NMAPDIR.
environment variable. Next comes ~/.nmap. for real and effective UIDs; or on Windows,
HOME\AppData\Roaming\nmap (where HOME is the user's home directory, like
C:\Users\user). This is followed by the location of the nmap executable and the same
location with ../share/nmap appended. Then a compiled-in location such as
/usr/local/share/nmap or /usr/share/nmap.

--servicedb services file (Specify custom services file) .
Asks Nmap to use the specified services file rather than the nmap-services data file
that comes with Nmap. Using this option also causes a fast scan (-F) to be used. See
the description for --datadir for more information on Nmap's data files.

--versiondb service probes file (Specify custom service probes file) .
Asks Nmap to use the specified service probes file rather than the nmap-service-probes
data file that comes with Nmap. See the description for --datadir for more information
on Nmap's data files.

--send-eth (Use raw ethernet sending) .
Asks Nmap to send packets at the raw ethernet (data link) layer rather than the higher
IP (network) layer. By default, Nmap chooses the one which is generally best for the
platform it is running on. Raw sockets (IP layer). are generally most efficient for
Unix machines, while ethernet frames are required for Windows operation since
Microsoft disabled raw socket support. Nmap still uses raw IP packets on Unix despite
this option when there is no other choice (such as non-ethernet connections).

--send-ip (Send at raw IP level) .
Asks Nmap to send packets via raw IP sockets rather than sending lower level ethernet
frames. It is the complement to the --send-eth option discussed previously.

--privileged (Assume that the user is fully privileged) .
Tells Nmap to simply assume that it is privileged enough to perform raw socket sends,
packet sniffing, and similar operations that usually require root privileges. on Unix
systems. By default Nmap quits if such operations are requested but geteuid is not
zero. --privileged is useful with Linux kernel capabilities and similar systems that
may be configured to allow unprivileged users to perform raw-packet scans. Be sure to
provide this option flag before any flags for options that require privileges (SYN
scan, OS detection, etc.). The NMAP_PRIVILEGED. environment variable may be set as an
equivalent alternative to --privileged.

--unprivileged (Assume that the user lacks raw socket privileges) .
This option is the opposite of --privileged. It tells Nmap to treat the user as
lacking network raw socket and sniffing privileges. This is useful for testing,
debugging, or when the raw network functionality of your operating system is somehow
broken. The NMAP_UNPRIVILEGED. environment variable may be set as an equivalent
alternative to --unprivileged.

--release-memory (Release memory before quitting) .
This option is only useful for memory-leak debugging. It causes Nmap to release
allocated memory just before it quits so that actual memory leaks are easier to spot.
Normally Nmap skips this as the OS does this anyway upon process termination.

-V; --version (Print version number) .
Prints the Nmap version number and exits.

-h; --help (Print help summary page) .
Prints a short help screen with the most common command flags. Running Nmap without
any arguments does the same thing.

RUNTIME INTERACTION

During the execution of Nmap, all key presses are captured. This allows you to interact
with the program without aborting and restarting it. Certain special keys will change
options, while any other keys will print out a status message telling you about the scan.
The convention is that lowercase letters increase the amount of printing, and uppercase
letters decrease the printing. You may also press ‘?’ for help.

v / V
Increase / decrease the verbosity level

d / D
Increase / decrease the debugging Level

p / P
Turn on / off packet tracing

?
Print a runtime interaction help screen

Anything else
Print out a status message like this:

Stats: 0:00:07 elapsed; 20 hosts completed (1 up), 1 undergoing Service Scan
Service scan Timing: About 33.33% done; ETC: 20:57 (0:00:12 remaining)

EXAMPLES

Here are some Nmap usage examples, from the simple and routine to a little more complex
and esoteric. Some actual IP addresses and domain names are used to make things more
concrete. In their place you should substitute addresses/names from your own network.
While I don't think port scanning other networks is or should be illegal, some network
administrators don't appreciate unsolicited scanning of their networks and may complain.
Getting permission first is the best approach.

For testing purposes, you have permission to scan the host scanme.nmap.org.. This
permission only includes scanning via Nmap and not testing exploits or denial of service
attacks. To conserve bandwidth, please do not initiate more than a dozen scans against
that host per day. If this free scanning target service is abused, it will be taken down
and Nmap will report Failed to resolve given hostname/IP: scanme.nmap.org. These
permissions also apply to the hosts scanme2.nmap.org, scanme3.nmap.org, and so on, though
those hosts do not currently exist.

nmap -v scanme.nmap.org.

This option scans all reserved TCP ports on the machine scanme.nmap.org . The -v option
enables verbose mode.

nmap -sS -O scanme.nmap.org/24.

Launches a stealth SYN scan against each machine that is up out of the 256 IPs on the
class C sized network where Scanme resides. It also tries to determine what operating
system is running on each host that is up and running. This requires root privileges
because of the SYN scan and OS detection.

nmap -sV -p 22,53,110,143,4564 198.116.0-255.1-127.

Launches host enumeration and a TCP scan at the first half of each of the 255 possible
eight-bit subnets in the 198.116 class B address space. This tests whether the systems run
SSH, DNS, POP3, or IMAP on their standard ports, or anything on port 4564. For any of
these ports found open, version detection is used to determine what application is
running.

nmap -v -iR 100000 -Pn -p 80.

Asks Nmap to choose 100,000 hosts at random and scan them for web servers (port 80). Host
enumeration is disabled with -Pn since first sending a couple probes to determine whether
a host is up is wasteful when you are only probing one port on each target host anyway.

nmap -Pn -p80 -oX logs/pb-port80scan.xml -oG logs/pb-port80scan.gnmap 216.163.128.20/20.

This scans 4096 IPs for any web servers (without pinging them) and saves the output in
grepable and XML formats.

NMAP BOOK

While this reference guide details all material Nmap options, it can't fully demonstrate
how to apply those features to quickly solve real-world tasks. For that, we released Nmap
Network Scanning: The Official Nmap Project Guide to Network Discovery and Security
Scanning. Topics include subverting firewalls and intrusion detection systems, optimizing
Nmap performance, and automating common networking tasks with the Nmap Scripting Engine.
Hints and instructions are provided for common Nmap tasks such as taking network
inventory, penetration testing, detecting rogue wireless access points, and quashing
network worm outbreaks. Examples and diagrams show actual communication on the wire. More
than half of the book is available free online. See https://nmap.org/book for more
information.

Use nmap online using onworks.net services