I'm posting this question in more than one forum to see if someone can
help me. Please excuse my bad manners but this is a very hot topic
now, at least for me ;)

Hello

I haveto block SSH connections from host A to host B if, and only if,
a certain effective user was used to login into host A.

In details:
1) Host B contains an account named user1.
2) Host A contains many accounts and an account named user2.
3) In host A, account xxx is allowed to sudo to account user2.
4) Someone logs into host B as user1. In that case this authentication
should be accepted.
5) Someone logs into host A using account user2 and then ssh to host B
as user1. In that case this authentication should be accepted.
6) Someone logs into host A using account xxx and then, using sudo,
becomes account user2 and then ssh to host B as user1. In that case,
the tentative to ssh to host B needs to be blocked.

Any ideas?

Cheers,
Andreas