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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:

[[email protected] ~] telnet mailserver 25
Connected to mailserver.
Escape character is '^]'.
220 ESMTP Sendmail 8.11.6/8.11.6; 200302281626
ehlo Hello [], pleased to meet you
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
354 Enter mail, end with "." on a line by itself
250 2.0.0 g2MA1R619237 Message accepted for delivery
221 2.0.0 closing connection
Connection closed by foreign host.

Checking that a web server answers to basic requests:

[[email protected] ~] telnet 80
Connected to
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.

[[email protected] ~]

This is perfectly safe, because you never have to give a username and/or password for getting the data you want, so nobody can snoop that important information off the cable.

Introducing Linux
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