Telnet
Telnet (short for «teletype network») is a client/server application protocol that provides access to virtual terminals of remote systems on local area networks or the Internet.
Today, telnet is rarely used due to security concerns and is often only used for legacy software and hardware, for information on how to mitigate the security issues see #Security concerns.
Installation
Install the inetutils package which includes a telnet client, a telnet server with systemd service and sockets.
To configure a telnet server with xinetd, install xinetd as well.
Configuration
To enable telnet server connections in systemd, enable telnet.socket (if the telnet server should be started on every boot), and start telnet.socket to test connectivity.
To enable telnet server connections in xinetd, edit /etc/xinetd.d/telnet , change disable = yes to disable = no and restart the xinetd service.
Enable systemd xinetd service if you wish to start it at boot time.
Testing the setup
Try opening a telnet connection to your server:
Try a root login to see if your configuration permits it and the security implications that implies.
If the session disconnects before you receive a login prompt, try installing inetutils-git AUR in place of the current inetutils and restarting telnet.socket.
Tip: If you receive junk codes from a remote telnet server sending non-ascii chars with a non-unicode encoding, you might want to try luit to solve this problem.
Tips and tricks
Security concerns
Telnet is a plain text protocol which does not support TLS encryption. Data can be easily intercepted and modified therefore should not be used on production servers, or to transmit sensitive data.
There are attempts to mitigate said security concerns, see #Netkit and #Tunneling for mitigations.
Netkit
You can use netkit-telnet-ssl AUR which modifies the protocol to support TLS encryption.
Tunneling
You can use a VPN or SSH to create an encrypted tunnel for the telnet traffic between two devices, or networks.
Note: The telnet traffic will only be encrypted while it is within the encrypted tunnel, ensure the network or device it is tunneled to is secure.
telnetd(8) — Linux man page
The telnetd program is a server which supports the DARPA telnet interactive communication protocol. Telnetd is normally invoked by the internet server (see inetd(8)) for requests to connect to the telnet port as indicated by the /etc/services file (see services(5)). The -debug option may be used to start up telnetd manually, instead of through inetd(8). If started up this way, port may be specified to run telnetd on an alternate TCP port number.
The telnetd program accepts the following options:
This option may be used for specifying what mode should be used for authentication. Note that this option is only useful if telnetd has been compiled with support for authentication, which is not available in the current version. The following values of authmode are understood:
debug
Turns on authentication debugging code.
user
Only allow connections when the remote user can provide valid authentication information to identify the remote user, and is allowed access to the specified account without providing a password.
valid
Only allow connections when the remote user can provide valid authentication information to identify the remote user. The login(1) command will provide any additional user verification needed if the remote user is not allowed automatic access to the specified account.
other
Only allow connections that supply some authentication information. This option is currently not supported by any of the existing authentication mechanisms, and is thus the same as specifying valid.
none
This is the default state. Authentication information is not required. If no or insufficient authentication information is provided, then the login(1) program will provide the necessary user verification.
off’ This disables the authentication code. All user verification will happen through the login(1) program.
-D debugmode
This option may be used for debugging purposes. This allows telnetd to print out debugging information to the connection, allowing the user to see what telnetd is doing. There are several possible values for debugmode:
options
Prints information about the negotiation of telnet options.
report‘ Prints the options information, plus some additional information about what processing is going on.
netdata
Displays the data stream received by telnetd.
ptydata
Displays data written to the pty.
exercise
Has not been implemented yet.
-edebug‘ If telnetd has been compiled with support for encryption, then the -edebug option may be used to enable encryption debugging code.
-h‘ Disables the printing of host-specific information before login has been completed.
-L loginprg
This option may be used to specify a different login program. By default, /bin/login is used.
-n‘ Disable TCP keep-alives. Normally telnetd enables the TCP keep-alive mechanism to probe connections that have been idle for some period of time to determine if the client is still there, so that idle connections from machines that have crashed or can no longer be reached may be cleaned up.
-N‘ Do not use DNS to canonicalize the local hostname; gethostname(2) returns a fully qualified name.
-s‘ This option is only enabled if telnetd is compiled with support for SecurID cards. It causes the -s option to be passed on to login(1), and thus is only useful if login(1) supports the -s flag to indicate that only SecurID validated logins are allowed. This is usually useful for controlling remote logins from outside of a firewall.
-S tos‘ Sets the IP type-of-service (TOS) option for the telnet connection to the value tos.
-X authtype
This option is only valid if telnetd has been built with support for the authentication option. It disables the use of authtype authentication, and can be used to temporarily disable a specific authentication type without having to recompile telnetd.
If the file /etc/issue.net is present, telnetd will display its contents before the login prompt of a telnet session (see issue.net(5)).
Telnetd operates by allocating a pseudo-terminal device (see pty(4)) for a client, then creating a login process which has the slave side of the pseudo-terminal as stdin, stdout, and stderr. 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 is started up, telnetd sends telnet options to the client side indicating a willingness to do the following telnet options, which are described in more detail below:
DO AUTHENTICATION
WILL ENCRYPT
DO TERMINAL TYPE
DO TSPEED
DO XDISPLOC
DO NEW-ENVIRON
DO ENVIRON
WILL SUPPRESS GO AHEAD
DO ECHO
DO LINEMODE
DO NAWS
WILL STATUS
DO LFLOW
DO TIMING-MARK
The pseudo-terminal allocated to the client is configured to operate in cooked mode, and with XTABS CRMOD enabled (see tty(4)).
Telnetd has support for enabling locally the following telnet options:
WILL ECHO' When the LINEMODE option is enabled, a WILLECHO or WONT ECHO will be sent to the client toindicate the current state of terminal echoing. Whenterminal echo is not desired, a WILL ECHO is sent toindicate that telnetd will take care of echoing anydata that needs to be echoed to the terminal, and thennothing is echoed. When terminal echo is desired, aWONT ECHO is sent to indicate that telnetd will not bedoing any terminal echoing, so the client should doany terminal echoing that is needed.
WILL BINARY’ Indicates that the client is willing to send a 8 bits of data, rather than the normal 7 bits of the Network Virtual Terminal.
WILL SGA’ Indicates that it will not be sending IAC GA, go ahead, commands.
WILL STATUS’ Indicates a willingness to send the client, upon request, of the current status of all TELNET options.
WILL TIMING-MARK
Whenever a DO TIMING-MARK command is received, it is always responded to with a WILL TIMING-MARK
WILL LOGOUT’ When a DO LOGOUT is received, a WILL LOGOUT is sent in response, and the TELNET session is shut down.
WILL ENCRYPT’ Only sent if telnetd is compiled with support for data encryption, and indicates a willingness to decrypt the data stream.
Telnetd has support for enabling remotely the following TELNET options:
DO BINARY’ Sent to indicate that telnetd is willing to receive an 8 bit data stream.
DO LFLOW’ Requests that the client handle flow control characters remotely.
DO ECHO’ This is not really supported, but is sent to identify a 4.2BSD telnet(1) client, which will improperly respond with WILL ECHO. If a WILL ECHO is received, a DONT ECHO will be sent in response.
DO TERMINAL-TYPE
Indicates a desire to be able to request the name of the type of terminal that is attached to the client side of the connection.
DO SGA’ Indicates that it does not need to receive IAC GA, the go ahead command.
DO NAWS’ Requests that the client inform the server when the window (display) size changes.
DO TERMINAL-SPEED
Indicates a desire to be able to request information about the speed of the serial line to which the client is attached.
DO XDISPLOC’ Indicates a desire to be able to request the name of the X windows display that is associated with the telnet client.
DO NEW-ENVIRON’ Indicates a desire to be able to request environment variable information, as described in RFC 1572.
DO ENVIRON’ Indicates a desire to be able to request environment variable information, as described in RFC 1408.
DO LINEMODE’ Only sent if telnetd is compiled with support for linemode, and requests that the client do line by line processing.
DO TIMING-MARK’ Only sent if telnetd is compiled with support for both linemode and kludge linemode, and the client responded with WONT LINEMODE. If the client responds with WILL TM, the it is assumed that the client supports kludge linemode. Note that the [-k] option can be used to disable this.
DO AUTHENTICATION
Only sent if telnetd is compiled with support for authentication, and indicates a willingness to receive authentication information for automatic login.
DO ENCRYPT’ Only sent if telnetd is compiled with support for data encryption, and indicates a willingness to decrypt the data stream. issue.net(5)).
Files
/etc/services, /etc/issue.net
See Also
Standards
TELNET PROTOCOL SPECIFICATION
RFC-855
TELNET OPTION SPECIFICATIONS
RFC-856
TELNET BINARY TRANSMISSION
RFC-857
TELNET ECHO OPTION
RFC-858
TELNET SUPPRESS GO AHEAD OPTION
RFC-859
TELNET STATUS OPTION
RFC-860
TELNET TIMING MARK OPTION
RFC-861
TELNET EXTENDED OPTIONS — LIST OPTION
RFC-885
TELNET END OF RECORD OPTION
RFC-1073
Telnet Window Size Option
RFC-1079
Telnet Terminal Speed Option
RFC-1091
Telnet Terminal-Type Option
RFC-1096
Telnet X Display Location Option
RFC-1123
Requirements for Internet Hosts — Application and Support
RFC-1184
Telnet Linemode Option
RFC-1372
Telnet Remote Flow Control Option
RFC-1416
Telnet Authentication Option
RFC-1411
Telnet Authentication: Kerberos Version 4
RFC-1412
Telnet Authentication: SPX
RFC-1571
Telnet Environment Option Interoperability Issues
RFC-1572
Telnet Environment Option
Bugs
Some TELNET commands are only partially implemented.
Because of bugs in the original 4.2 BSD telnet(1), telnetd performs some dubious protocol exchanges to try to discover if the remote client is, in fact, a 4.2 BSD telnet(1).
Binary mode has no common interpretation except between similar operating systems (Unix in this case).
The terminal type name received from the remote client is converted to lower case.
Telnetd never sends TELNET IAC GA (go ahead) commands.
The source code is not comprehensible.
Linux NetKit (0.17) December 29, 1996 Linux NetKit (0.17)