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dnswalk - A DNS database debugger


dnswalk [ -adilrfFm ] domain.


dnswalk is a DNS debugger. It performs zone transfers of specified domains, and checks
the database in numerous ways for internal consistency, as well as for correctness
according to accepted practices with the Domain Name System.

The domain name specified on the command line MUST end with a '.'. You can specify a
forward domain, such as dnswalk podunk.edu. or a reverse domain, such as dnswalk


-r Recursively descend sub-domains of the specified domain. Use with care.
-a Turn on warning of duplicate A records. (see below)
-d Print debugging and 'status' information to stderr. (Use only if redirecting
stdout) See DIAGNOSTICS section.
-m Perform checks only if the zone has been modified since the previous run.
-F perform "fascist" checking. When checking an A record, compare the PTR name for
each IP address with the forward name and report mismatches. (see below) I
recommend you try this option at least once to see what sorts of errors pop up -
you might be surprised!.
-i Suppress check for invalid characters in a domain name. (see below)
-l Perform "lame delegation" checking. For every NS record, check to see that the
listed host is indeed returning authoritative answers for this domain.
The following the list of error messages that dnswalk will return if it sees a
potential problem with the database. Duplicate messages will be suppressed
automatically for each zone. Error messages are prefixed by a keyword indiciating
the message type: "WARN" (possible data problem), "FAIL" (failure to access data),
or "BAD" (invalid data). dnswalk exits with a return code equal to the number of
"BAD" errors.
X PTR Y: unknown host
X is a PTR record to Y, but Y is not a valid host (no A record). These are often
left over from when someone deleted a host from the DNS and forgot to delete the
PTR record.
X PTR Y: A record not found
X is a PTR record to Y, but the IP address associated with the PTR record is not
listed as an address for Y. There should be an A record for every valid IP address
for a host. Many Internet services will not talk to you if you have mismatched PTR
X is a PTR record to Y, but Y is a CNAME to Z. PTR records MUST point to the
canonical name of a host, not an alias.
X CNAME Y: unknown host
X is aliased to Y, but Y is not a valid host (no A record).
X is aliased to Y, but Y is aliased to Z. CNAMEs should not be chained.
X MX Y: unknown host
X is an MX to Y, but Y is not a valid host (no A record).
X MX Y: CNAME (to Z)
X is an MX to Y, but Y is an alias for Z. MX records must point to the canonical
name, not an alias.
X A Y: no PTR record
X has an IP address Y, but there is no PTR record to map the IP address Y back to a
hostname (usually X). Many Internet servers (such as anonymous FTP servers) will
not talk to addresses that don't have PTR records.
warning: X has only one authoritative nameserver
Zones must have at least one authoritative nameserver, in case one is down or
unreachable. Make sure the parent and child domains list all authoritative
nameservers for a zone.
Cannot check X: no available nameservers!
The X zone was delegated with NS records but all the nameservers for the zone
are either unavailable or say that they have no data for the zone (are lame).
Verify that the X zone isn't a typo, and if so make sure that all the listed
nameservers are configured to answer with data for the zone.
X: invalid character(s) in name
Allowable characters in a domain name are the ASCII letters a through Z the digits
0 through 9, and the "-" character. A "." may be used only as a domain separator.
(checking can be suppressed with -i )
X: domain occurred twice, forgot trailing '.'?
A sanity check which looks for "dom.ain.dom.ain." in a name. This is often caused
by forgetting to put a trailing '.' on the end of a name.
(with -a switch)
X: possible duplicate A record (glue of Z?)
A duplicate A records is listed for X. NOTE: this is most often caused by the
practice of always putting A records for all secondaries after NS glue records.
While this is not an error, it is usually redundant and makes changing IP addresses
later more difficult, since they occur more than one time in the file (and in
multiple files). You may get spurious errors, mostly because of a quirk in BIND
releases before 4.9.x that reports cached glue A records in a zone transfer even
though they don't exist in the original zone file.
(with -F switch)
X A Y: points to Z
X has Y for an IP address, but the PTR record associated with Y returns "Z" as the
name associated with that host. This is not necessarily an error (for example if
you have an A record for your domain name), but can be useful to check for A
records which point to the wrong host, or PTR records that point to the wrong host.
Cannot find address for nameserver X
This error is generated if the address for a delegated nameserver X cannot be
resolved. This could be a lame delegation (due to a typo in delegation), or a
temporary DNS error.
(with -l switch)
X NS Y: lame NS delegation
Y is a listed nameserver for zone X, but Y is not returning authoritative data for
zone X. This is usually the result of a lack of communication on the part of the
respective hostmasters. Lame delegations are not fatal problems except in severe
cases, they just tend to create significant increases in DNS traffic. NS records
for the parent and child domains should be consistent, and each server listed in
the NS record MUST be able to answer with authoritative data, either by being a
primary or secondary for the zone.
Cannot get SOA record for X from Y (lame?)
This error is generated if dnswalk cannot get the SOA record for zone X from the
nameserver Y. This could mean a lame delegation, or simply that the host is
temporarily unreachable.

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