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I am trying to prevent my ssh users from having more than one session open at once, my system is Ubuntu 11.10.

I thought the setting was in /etc/security/limits.conf where I set the following:

betatest hard maxlogins 1

However, this has no effect and I can log in multiple times using user account name betatest from different machines.

The sshd_config is using PAM and the login and sshd configs in /etc/pam.d both have the setting:

session    required

If anyone knows how I can restrict the concurrent sessions on a per user basis I would really appreciate the help.

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Do you think it might be possible to have a PAM module written to check if the user attempting to login is already logged in and if they are then stop them logging in again? I don't have the skills for this but if it's possible then at least I have a starting point. – Kyoku Mar 9 '12 at 15:49

Per user setting is very tricky, you might have to write some wrapper script for this. SSHD config has an option MaxSessions which limits no of connections per ip.

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The risk is the user will share the login details with friends on different IP's, that's the main issue I'm trying to avoid. – Kyoku Mar 9 '12 at 5:11
Then proy answer about ssh MaxSessions is your answer – Gabor Vincze Mar 9 '12 at 7:16
MaxSessions only limits per ip basis; so it seems will not work – Ajo Augustine Mar 9 '12 at 8:38

It appears to me, from the MAN page for pam_limits (at least on rhel/centos), that adding a maxlogins parameter for the desired user(s) would do the trick... but I haven't tested it:

       pam_limits - PAM module to limit resources

SYNOPSIS [change_uid] [conf=/path/to/limits.conf] [debug] [utmp_early] [noaudit]

       The pam_limits PAM module sets limits on the system resources that can be obtained in a user-session. Users of uid=0 are affected by this limits, too.

       By default limits are taken from the /etc/security/limits.conf config file. Then individual *.conf files from the /etc/security/limits.d/ directory are read.
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