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There are a couple of different ways to execute commands or run programs on a remote machine and have the output, be it text or graphics, sent to your workstation. The connections can be secure or insecure. While it is of course advised to use secure connections instead of transporting your password over the network unencrypted, we will discuss some practical applications of the older (unsafe) mechanisms, as they are still useful in a modern networked environment, such as for troubleshooting or running exotic programs.
10.4.2. Rsh, rlogin and telnet
The rlogin and rsh commands for remote login and remote execution of commands are inherited from UNIX. While seldom used because they are blatantly insecure, they still come with almost every Linux distribution for backward compatibility with UNIX programs.
Telnet, on the other hand, is still commonly used, often by system and network administrators. Telnet is one of the most powerful tools for remote access to files and remote administration, allowing connections from anywhere on the Internet. Combined with an X server, remote graphical applications can be displayed locally. There is no difference between working on the local machine and using the remote machine.
Because the entire connection is unencrypted, allowing telnet connections involves taking high security risks. For normal remote execution of programs, Secure SHell or ssh is advised. We will discuss the secure method later in this section.
However, telnet is still used in many cases. Below are some examples in which a mail server and a web server are tested for replies:
Checking that a mail server works:
[jimmy@blob ~] telnet mailserver 25
Trying 192.168.42.1...
Connected to mailserver. Escape character is '^]'.
220 m1.some.net ESMTP Sendmail 8.11.6/8.11.6; 200302281626
ehlo some.net
250-m1.some.net Hello blob.some.net [10.0.0.1], pleased to meet you 250-ENHANCEDSTATUSCODES
250-8BITMIME
250-SIZE
250-DSN
[jimmy@blob ~] telnet mailserver 25
Trying 192.168.42.1...
Connected to mailserver. Escape character is '^]'.
220 m1.some.net ESMTP Sendmail 8.11.6/8.11.6; 200302281626
ehlo some.net
250-m1.some.net Hello blob.some.net [10.0.0.1], pleased to meet you 250-ENHANCEDSTATUSCODES
250-8BITMIME
250-SIZE
250-DSN
250-ONEX
250-ETRN
250-XUSR
250 HELP
mail from: [email protected]
250 2.1.0 [email protected]... Sender ok
rcpt to: [email protected]
250 2.1.5 [email protected]... Recipient ok
data
354 Enter mail, end with "." on a line by itself
test
.
250 2.0.0 g2MA1R619237 Message accepted for delivery
quit
221 2.0.0 m1.some.net closing connection Connection closed by foreign host.
250-ONEX
250-ETRN
250-XUSR
250 HELP
mail from: [email protected]
250 2.1.0 [email protected]... Sender ok
rcpt to: [email protected]
250 2.1.5 [email protected]... Recipient ok
data
354 Enter mail, end with "." on a line by itself
test
.
250 2.0.0 g2MA1R619237 Message accepted for delivery
quit
221 2.0.0 m1.some.net closing connection Connection closed by foreign host.
Checking that a web server answers to basic requests:
[jimmy@blob ~] telnet www.some.net 80
Trying 64.39.151.23...
Connected to www.some.net. Escape character is '^]'. HEAD / ;HTTP/1.1
HTTP/1.1 200 OK
Date: Fri, 22 Mar 2002 10:05:14 GMT
Server: Apache/1.3.22 (UNIX) (Red-Hat/Linux) mod_ssl/2.8.5 OpenSSL/0.9.6
DAV/1.0.2 PHP/4.0.6 mod_perl/1.24_01
Last-Modified: Fri, 04 Jan 2002 08:21:00 GMT ETag: "70061-68-3c3565ec"
Accept-Ranges: bytes Content-Length: 104 Connection: close Content-Type: text/html
Connection closed by foreign host. [jimmy@blob ~]
[jimmy@blob ~] telnet www.some.net 80
Trying 64.39.151.23...
Connected to www.some.net. Escape character is '^]'. HEAD / ;HTTP/1.1
HTTP/1.1 200 OK
Date: Fri, 22 Mar 2002 10:05:14 GMT
Server: Apache/1.3.22 (UNIX) (Red-Hat/Linux) mod_ssl/2.8.5 OpenSSL/0.9.6
DAV/1.0.2 PHP/4.0.6 mod_perl/1.24_01
Last-Modified: Fri, 04 Jan 2002 08:21:00 GMT ETag: "70061-68-3c3565ec"
Accept-Ranges: bytes Content-Length: 104 Connection: close Content-Type: text/html