I'm having trouble reproducing this. I started a few thousand /bin/sleep commands and was able to log in via ssh as another user; the error message when logging in again as my normal user account showed an error message that looked appropriate.
In one ssh: sarnold@sec-xenial-amd64:~$ for i in `seq 1 6000` ; do /bin/sleep 10 & done ... [15813] 24043 [15814] 24044 [15815] 24045 -bash: fork: retry: Resource temporarily unavailable -bash: fork: retry: Resource temporarily unavailable In another terminal: $ ssh -oControlPath=none root@sec-xenial-amd64 Welcome to Ubuntu Xenial Xerus (development branch) (GNU/Linux 4.4.0-16-generic x86_64) * Documentation: https://help.ubuntu.com/ 0 packages can be updated. 0 updates are security updates. Last login: Thu Mar 31 15:42:23 2016 from 192.168.122.1 root@sec-xenial-amd64:~# $ ssh -oControlPath=none sec-xenial-amd64 shell request failed on channel 0 -- You received this bug notification because you are a member of Ubuntu Touch seeded packages, which is subscribed to openssh in Ubuntu. https://bugs.launchpad.net/bugs/1564451 Title: User processes are counted towards systemd limit for sshd processes Status in openssh package in Ubuntu: New Bug description: When running Xenial, user processes are counted towards the limit for the ssh.service, with a limit of 512. So if I login as a normal user via ssh and start 512 processes, nobody will be able to login any more and even all other users currently logged in will not be able to start any new tasks. I'm not certain whether this behaviour is by design, but to me it looks like a critical DOS possibility, so tagging as security bug. To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/openssh/+bug/1564451/+subscriptions -- Mailing list: https://launchpad.net/~touch-packages Post to : touch-packages@lists.launchpad.net Unsubscribe : https://launchpad.net/~touch-packages More help : https://help.launchpad.net/ListHelp
