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?
6 Answers
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
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1Um, if the user doesn't use
-N
, they have shell access. This really doesn't solve the problem and is dangerous.– mlissnerMar 29, 2017 at 18:46 -
15@mlissner: No, if you have
/bin/false
as shell, you won't have shell access, as every login session will terminate immediately.– SvenMar 30, 2017 at 16:26 -
1
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@Rémi Yes, but the question was asking for "only permit tunneling". If you want SCP then you can set up the account with the
scponly
shell instead offalse
.scponly
will immediately exit if no SCP is initiated, but a user can still use-N
to do port forwarding. Usingscponly
would allow tunnels and scp.– bobpaulFeb 27 at 17:06
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.
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9This 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 thecommand="..."
option in.ssh/authorized_keys
if you want to restrict shell access from there. Jan 1, 2015 at 19:58 -
2@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. Dec 10, 2018 at 16:37
Give the user a shell that only allows them to log out such as /usr/local/bin/press_to_exit.sh
#!/bin/bash
read -n 1 -p "Press any key to exit" key
This way they can stay logged in as long as they want, with tunnels active, but not run any commands. Ctrl-C
closes the connection.
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7I 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. Jan 21, 2013 at 9:03
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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5Most linux installs already come with something for that. /sbin/nologin, or /bin/false or similar. Aug 20, 2009 at 11:27
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1
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2I 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.– jrgAug 20, 2009 at 17:06
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4@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
.– dr_Aug 23, 2018 at 8:00 -
1There are some things that are dangerous in shell scripts, but this isn’t one of them (The original implementation of nologin - in BSD 4.4 - being a script is quite telling.) The main reason for my custom script suggestion was to support a tunnel being added after connection, not the message, hence why I personally still like Earl’s -N approach. These days I’d probably use a container with nothing else in it, as well.– jrgAug 26, 2018 at 19:51
##Create a new user
sudo useradd -m [user]
##Edit /etc/ssh/sshd_config
Match User [user]
#AllowTcpForwarding yes
#X11Forwarding no
#PermitTunnel no
GatewayPorts yes ##Enable listening on 0.0.0.0
AllowAgentForwarding no
PermitOpen localhost:2888 ##specify port
ForceCommand echo 'This account can only be used for Tunneling'
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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2The 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?– dr_Aug 23, 2018 at 7:56
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2@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. Aug 24, 2018 at 7:26