Limit number of connections per user?

[Scott Neugroschl]

I'm working from modified 5.0p1 codebase.

What I'm looking for is a mechanism to limit the number of simultaneous
connections on a per-user/IP basis.
That is, disallow multiple simultaneous logins/authentication of the
same user from different IP addresses.

e.g.:

fred from 10.1.1.1 - accept
fred from 10.1.1.2 -- reject while fred is still connected from 10.1.1.1
fred from 10.1.1.1 - OK (same IP)
--- all freds log out
fred from 10.1.1.2 -- OK (fred not logged in)

Is this doable, or not?  I realize that the sshd architecture may make
this difficult or impossible.

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Scott Neugroschl