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?

Solution 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 -x | ssh -t root@remote-host '
  cat > "$TERM.info" && tic -x "$TERM.info"'

Solution 2

In my case I simply added an alias to my .zshrc (.bashrc if using bash) on my local desktop:

alias ssh='TERM=xterm ssh'

If you already use an alias, adjust it to include the Environment assignment.

Solution 3

Changing $TERM might work, but I don't suggest this, it's only a workaround instead of a solution.

When I encounter this problem on my systems, I fix it by installing support for the most common terminal types to the remote system:

• yum install ncurses-base for screen-256color on CentOS
• yum install ncurses-term for screen-256color-bce on CentOS
• apt install ncurses-base for both screen-256color and screen-256color-bce on Debian, Ubuntu and Mint

The ncurses-related packages also provide support for a lot of other terminals, and they are also available on all other large distributions. (But for my use-case and your question this should be enough info)

Solution 4

The best solution is to end up with full terminal support during SSH sessions. To achieve this, upload your terminfo files to your home directory on the remote server.

Locally: Check your working TERM value:

 echo $TERM

In the following examples alacritty is used. Next, find local the terminfo files for your terminal:

 locate alacritty | grep terminfo

I found results under /usr/share/terminfo. Finally, upload the files to `~/.terminfo on the remote host.

 # On the remote host
 mkdir -p ~/.terminfo/a

 # Locally
 scp /usr/share/terminfo/a/alacritty* remotehost:.terminfo/a/

The terminfo format is a compiled format so simply copying the files may not work. If it doesn't work and infocmp and tic are available locally and remotely, you can use this method instead:

 infocmp -x | ssh -t root@remote-host 'cat > "$TERM.info" && tic -x "$TERM.info"'

View more solutions

Comments

[michas]

The literal question was, how to prevent sending TERM. - Answer: You cannot. - What he should do is setting it to a suitable value, yes.

[LiuYan 刘研]

change $TERM temporarily may be a workaround, but i need to do it each time. by the way, it seems that both CentOS 5 and Fedora 18 AcceptEnv all locale environment variables (LANG, LC_*, ...)

[LiuYan 刘研]

good to know infocmp and tic, once compiled, no need to temporarily change $TERM again. by the way, i just copied (rsync) /usr/share/terminfo/s/screen-256color from Fedora 18 to CentOS, it seems works ok (rsync -tv /usr/share/terminfo/s/screen-256color root@the_host:/usr/share/terminfo/s).

[LiuYan 刘研]

forgot to mentioned, i ran tmux in gnome-terminal of Fedora 18, tmux changed $TERM value to screen-256color from xterm-256color.

[LiuYan 刘研]

btw, can ssh works in this way: get the terminfo which host/server supported (not pushed into), then pickup one the client terminal can support?

[Dmitri DB]

I found it amusing that this infocmp piped through ssh thing left a file called dumb.info in my home dir.

[Alan Jenkins]

infocmp | ssh -t root@remote-host 'cat > "$TERM.info" && tic "$TERM.info"' can be shortened to infocmp | ssh root@remote-host "tic -". This works because when you have a pipe it can be accessed as a file by using - and luckily pipes work across SSH.

[starfry]

I copy the terminfo file from /usr/share/terminfo to the same place on the remote host. Is there any reason why using infocmp and tic would be preferable?

[Stéphane Chazelas]

@starfry, I can't give you any guarantee that the binary format of the tic (terminfo compiler) generated files will be the same from one system to the next or that their location will be the same. tic may also make sure the permissions are correct or create intermediary directories where needed.

[miken32]

Or, don't start a subshell and another process: export TERM=${TERM%%-256color}

[mazunki]

The original command gave me Pseudo-terminal will not be allocated because stdin is not a terminal. error, while the shortened version actually worked for me. Alacritty works on ssh now =)

[Gringo Suave]

This may be the best answer, assuming desire/permission to install packages remotely.

[Mark Stosberg]

If you are willing to use a solution that involves modifying files on every remote server you ssh into, you could instead install the desired terminal support files in ~/.terminfo on all the remote hosts.

[Mark Stosberg]

This solution is less desirable as it dumbs down the terminal support on the remote host, while uploading the terminal defintiions to your home directory on the server allows full terminal support in the remote connection.

[Mark Stosberg]

Besides that you might not have permission to install packages remotely, such a package might not be available. For example Ubuntu 16.04 and 18.04 are still active LTS lines, but the terminal info for the alacritty terminal is not packaged until 20.04.

[zaTricky]

This is a really cool solution. The only negative is that it involves a bit of effort for a server you might only connect to once. For the other usecases it's robust and less guesswork.

[Phantomwhale]

Ended up being the winner for me - I'm using kitty terminal, and none of the machines I SSH into have any support for that, so setting the TERM value back to a common base value is probably the best option in almost all cases