Is it safe to use /bin/cat as shell for restrited user?

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http://unix.stackexchange.com – I need to remontely log on to few peoples machines to do maintanance tasks, but sometimes the machines can be behind firewalls or NAT's. The way I have been doing it is to give the user advice on how to log in to my system with SSH so that reverse tunneling is enabled, like "use command 'ssh -R 12345:localhost:22 [email protected]' and login with password xyzzy" Now, this works fine, but for the sake of safety I need to go and disable the ssh login for user 'uremonte' when I am done, and remember to re-enable it when I need it again, or else someone might log into my administation fro (HowTos)