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Linux in a Nutshell

This directory of Linux commands is from Linux in a Nutshell, 5th Edition.

Click on any of the 687 commands below to get a description and list of available options. All links in the command summaries point to the online version of the book on Safari Bookshelf.

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telnetd [options]

TCP/IP command. Telnet protocol server. telnetd is invoked by the Internet server for requests to connect to the Telnet port (port 23 by default). telnetd allocates a pseudo-terminal device for a client, thereby creating a login process that has the slave side of the pseudo-terminal serving as stdin, stdout, and stderr. telnetd manipulates the master side of the pseudo-terminal by implementing the Telnet protocol and by passing characters between the remote client and the login process.

The Telnet protocol is often criticized because it uses no encryption and makes it easy for snoopers to pick up user passwords. Most sites now use ssh instead.


-a type

When compiled with authentication support, this option sets the authentication type. Accepted values are:


Debug authentication code.


No authentication required, but accept it if offered. Use login for any further verification needed to access an account.


Disable authentication.


Allow only authenticated remote users with permission to access their accounts without giving a password.


Allow only authenticated remote users. Use login for any additional verification needed to access an account.

-debug [port]

Start telnetd manually instead of through inetd. port may be specified as an alternate TCP port number on which to run telnetd.

-D modifier(s)

Debugging mode. This allows telnet to print out debugging information to the connection, enabling the user to see what telnet is doing. Several modifiers are available for the debugging mode:


Display data stream received by telnetd.


Print information about the negotiation of the Telnet options.


Display data written to the pseudo-terminal device.


Print options information, as well as some additional information about what processing is going on.


When compiled with support for encryption, enable encryption debugging code.


Don't print host-specific information until after login is complete.


Refuse connections from IP addresses with no reverse DNS information.


Disable checking for lost connections with TCP keep-alives.

-X type

Disable authentication type.

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