in.telnetd is a server that supports the DARPA standard TELNET virtual terminal protocol. in.telnetd is normally invoked in the internet server (see inetd(1M)), for requests to connect to the TELNET port as indicated by the /etc/services file (see services(4)).

in.telnetd operates by allocating a pseudo-terminal device for a client, then creating a login process which has the slave side of the pseudo-terminal as its standard input, output, and error. in.telnetd manipulates the master side of the pseudo-terminal, implementing the TELNET protocol and passing characters between the remote client and the login process.

When a TELNET session starts up, in.telnetd sends TELNET options to the client side indicating a willingness to do remote echo of characters, and to suppress go ahead. The pseudo-terminal allocated to the client is configured to operate in "cooked" mode, and with XTABS, ICRNL and ONLCR enabled. See termio(7I).

in.telnetd is willing to do: echo, binary, suppress go ahead, and timing mark. in.telnetd is willing to have the remote client do: binary, terminal type, terminal size, logout option, and suppress go ahead.

in.telnetd also allows environment variables to be passed, provided that the client negotiates this during the initial option negotiation. The DISPLAY environment variable may be sent this way, either by the TELNET general environment passing methods, or by means of the XDISPLOC TELNET option. DISPLAY can be passed in the environment option during the same negotiation where XDISPLOC is used. Note that if you use both methods, use the same value for both. Otherwise, the results may be unpredictable.

These options are specified in Internet standards RFC 1096, RFC 1408, RFC 1571, and RFC 1572.

The banner printed by in.telnetd is configurable. The default is (more or less) equivalent to "`uname -sr`" and will be used if no banner is set in /etc/default/telnetd. To set the banner, add a line of the form

BANNER="..."

to /etc/default/telnetd. Nonempty banner strings are fed to shells for evaluation. The default banner may be obtained by

BANNER="\\r\\n\\r\\n`uname -s` `uname -r`\\r\\n\\r\\n"

and no banner will be printed if /etc/default/telnetd contains

BANNER=""

telnetd and in.telnetd are IPv6-enabled. See ip6(7P).

in.telnetd uses pam(3PAM) for authentication, account management, session management, and password management. The PAM configuration policy, listed through /etc/pam.conf, specifies the modules to be used for in.telnetd. Here is a partial pam.conf file with entries for the telnet command using the UNIX authentication, account management, session management, and password management modules.

telnet  auth requisite          pam_authtok_get.so.1
telent  auth required           pam_dhkeys.so.1
telent  auth required           pam_unix_auth.so.1

telnet  account requisite       pam_roles.so.1
telnet  account required        pam_projects.so.1
telnet  account required        pam_unix_account.so.1

telnet  session required        pam_unix_session.so.1

telnet  password required       pam_dhkeys.so.1
telent  password requisite      pam_authtok_get.so.1
telnet  password requisite      pam_authtok_check.so.1
telnet  password required       pam_authtok_store.so.1

If there are no entries for the telnet service, then the entries for the "other" service will be used. If multiple authentication modules are listed, then the user may be prompted for multiple passwords.

/etc/default/telnetd

See attributes(5) for descriptions of the following attributes:

telnet(1), inetd(1M), pam(3PAM), inetd.conf(4)pam.conf(4), services(4), attributes(5), pam_authtok_check(5), pam_authtok_get(5), pam_authtok_store(5), pam_dhkeys(5), pam_passwd_auth(5), pam_unix(5), pam_unix_account(5), pam_unix_auth(5), pam_unix_session(5), ip6(7P), termio(7I)

Alexander, S., TELNET Environment Option, RFC 1572, Network Information Center, SRI International, Menlo Park, Calif., January 1994.

Borman, Dave, TELNET Environment Option, RFC 1408, Network Information Center, SRI International, Menlo Park, Calif., January 1993.

Borman, Dave, TELNET Environment Option Interoperability Issues, RFC 1571, Network Information Center, SRI International, Menlo Park, Calif., January 1994.

Crispin, Mark, TELNET Logout Option, RFC 727, Network Information Center, SRI International, Menlo Park, Calif., April 1977.

Marcy, G., TELNET X Display Location Option. RFC 1096, Network Information Center, SRI International, Menlo Park, Calif., March 1989.

Postel, Jon, and Joyce Reynolds, TELNET Protocol Specification, RFC 854, Network Information Center, SRI International, Menlo Park, Calif., May 1983.

Waitzman, D., TELNET Window Size Option, RFC 1073, Network Information Center, SRI International, Menlo Park, Calif., October 1988.

Some TELNET commands are only partially implemented.

Binary mode has no common interpretation except between similar operating systems.

The terminal type name received from the remote client is converted to lower case.

The packet interface to the pseudo-terminal should be used for more intelligent flushing of input and output queues.

in.telnetd never sends TELNET go ahead commands.

The pam_unix(5) module might not be supported in a future release. Similar functionality is provided by pam_authtok_check(5), pam_authtok_get(5), pam_authtok_store(5), pam_dhkeys(5), pam_passwd_auth(5), pam_unix_account(5), pam_unix_auth(5), and pam_unix_session(5).