Prevent SSH client passing TERM environment variable to server?

Question

I'm currently using Fedora 18 gnome-terminal, then started tmux multiplexer in it. After I connected to a CentOS 5 server via ssh command, I find:

• ls result has no color
• tmux, screen, hexedit, htop all failed to start with error message like:

  open terminal failed: missing or unsuitable terminal: screen-256color

It seems that ssh passes the $TERM environment variable to the server, but I can't find it in /etc/ssh/ssh_config file of Fedora 18.

Although I can manually change the $TERM variable on the server, each time I connect, it happens again. So how to prevent it?

Answer 1

$TERM is to tell applications what terminal they're talking to so they know how to talk to it.

Change it to a value supported by the remote host and that matches as closely as possible your terminal (screen).

Most Linux systems should at least have a screen terminfo entry. If not, screen implements a superset of vt100 and vt100 is universal. So:

TERM=screen ssh host

or

TERM=vt100 ssh host

If you do need the 256 color support, you could try xterm-256color which should be close enough (screen supports 256 colors the same way xterm does) and tell applications your terminal application supports 256 colors and tell them how to use them.

Or you can install the terminfo entry on the remote host.

infocmp | ssh root@remote-host 'cat > "$TERM.info" && tic "$TERM.info"'

Answer 2

See man ssh_config:

 SendEnv
         Specifies what variables from the local environ(7) should be sent
         to the server.  Note that environment passing is only supported
         for protocol 2.  The server must also support it, and the server
         must be configured to accept these environment variables.  Refer
         to AcceptEnv in sshd_config(5) for how to configure the server.
         Variables are specified by name, which may contain wildcard char‐
         acters.  Multiple environment variables may be separated by
         whitespace or spread across multiple SendEnv directives. The
         default is not to send any environment variables.

and man sshd_config:

 AcceptEnv
         Specifies what environment variables sent by the client will be
         copied into the session's environ(7).  See SendEnv in
         ssh_config(5) for how to configure the client.  Note that envi-
         ronment passing is only supported for protocol 2.  Variables are
         specified by name, which may contain the wildcard characters `*'
         and `?'.  Multiple environment variables may be separated by
         whitespace or spread across multiple AcceptEnv directives.  Be
         warned that some environment variables could be used to bypass
         restricted user environments.  For this reason, care should be
         taken in the use of this directive.  The default is not to accept
         any environment variables.

According to that, the default should be not to send any variables, but TERM seems to be special. It is send anyway.

Therefore you can either change TERM when calling ssh (like TERM=xterm ssh ...), change it after login (like in .bash_profile), or define the unknown TERM type on server side (assuming you have root access there). See other answer for details.