29

Is it possible to configure ssh (on linux) to allow access for tunneling only? Ie user can setup tunnels but cannot get a shell/access files?

39

Yes, just use /bin/false as shell and instruct the user to start the tunneling SSH process without executing any remote command (i.e. the -N flag for OpenSSH):

ssh -N -L 1234:target-host:5678 ssh-host
  • Um, if the user doesn't use -N, they have shell access. This really doesn't solve the problem and is dangerous. – mlissner Mar 29 '17 at 18:46
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    @mlissner: No, if you have /bin/false as shell, you won't have shell access, as every login session will terminate immediately. – Sven Mar 30 '17 at 16:26
10

In the user's .ssh/authorized_keys file, put something like the following:

permitopen="192.168.1.10:3306",permitopen="10.0.0.16:80",no-pty ssh-rsa AAAAB3N...

So, basically, you the controls would be in front of the user's ssh public key separated by a space. In the example, connections using the specific public key will be allowed to do SSH port forwarding only to 192.168.1.10's MySQL server and 10.0.0.16's web server, and will not be assigned a shell (no-pty). You're specifically asking about the "no-pty" option, but the others may also be useful if the user is only supposed to tunnel to specific servers.

Look at the man page for sshd for more options for the authorized_keys file.

Note that the user's experience may look a little odd: when they ssh in, it will look like the session is hanging (as they are not getting a pty). That's OK. If the user has specified port forwarding with, for example, "-L3306:192.168.1.10:3306", the port forwarding will still be in effect.

In any case, give it a try.

  • 6
    This is dangerous advice; no-pty doesn't prevent shell access, it just doesn't give the shell a pty. It doesn't display the prompt (ie. "appears to hang"), but you can still give commands just fine. You need the command="..." option in .ssh/authorized_keys if you want to restrict shell access from there. – Aleksi Torhamo Jan 1 '15 at 19:58
  • @AleksiTorhamo is correct, but this is a start; you also must set shell to /usr/sbin/nologin or /bin/false in /etc/passwd to fully restrict the shell. I edited the above entry to reflect that, along with Aleksi's advice. – Jamieson Becker Dec 10 '18 at 16:37
4

Give the user a shell that only allows them to log out such as /bin/press_to_exit.sh

#!/bin/bash
read -n 1 -p "Press any key to exit" key

This way he can stay logged in as long as he wants, with tunnels active, but not run any commands. Ctrl-c closes the connection.

  • 3
    I think that at least in theory, the user will be able to hit CTRL+C at the right moment (between lines 1 and 2) and end up with a full bash shell. – andreas-h Jan 21 '13 at 9:03
2

Assign a shell that doesn't let the user log in.

e.g.

#!/bin/sh
echo "No interactive login available."
sleep 60
exit 0

would prevent them from getting a shell prompt, and give them a time-out of 60 seconds - if there's no connection active for 60 seconds then it will exit and thereby disconnect them completely (increase the number according to requirements).

They can't execute a remote command, either, because that shell won't let them.

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    Most linux installs already come with something for that. /sbin/nologin, or /bin/false or similar. – Rory Aug 20 '09 at 11:27
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    What would Ctrl-C do in the above example? – Arjan Aug 20 '09 at 12:00
  • Arjan: Since the script is used as the shell, Ctrl-C would have the same effect as "logout" on a normal one. – grawity Aug 20 '09 at 12:47
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    I actually like earl's response, with the "-N" option, but if you want to give your user a helpful, friendly, informative message - and stop them leaving SSH connections all over the place - then a custom script is nice and clear about what it's doing. – jrg Aug 20 '09 at 17:06
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    @jrg This discussion is old and you might have changed your mind :) but IMHO it's dangerous to come out with handmade shell scripts. There is already /sbin/nologin, which you can customize with an user-friendly message in /etc/nologin.txt. – dr01 Aug 23 '18 at 8:00
0

My solution is to provide the user who only may be tunneling, without an interactive shell, to set that shell in /etc/passwd to /usr/bin/tunnel_shell.

Just create the executable file /usr/bin/tunnel_shell with an infinite loop.

Additionally make use of the AllowGroups and Match Group option.

Fully explained here: http://blog.flowl.info/2011/ssh-tunnel-group-only-and-no-shell-please/

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    The user might be able to exit the executable somehow and escape a shell. Are you perfectly confident that your executable won't allow to do this? – dr01 Aug 23 '18 at 7:56
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    @dr01 To be honest, I am not sure if this is 100% secure. I guess when you exit the executable, the SSH session is quit, too. I will research this more and might comment again here. – Daniel W. Aug 24 '18 at 7:26

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