SSH authentication forwarding

[Michael ODonnell]

Heh.  It's funny now, looking back on it, but I experienced
several minutes of panic this morning as I connected to a remote
system via SSH and discovered that I was unexpectedly able to
connect back to the originating system at will without mentioning
any password.  It was definitely a WTF moment since my ~/.ssh
directory on the remote system has basically nothing in it,
certainly no SSH keys.  I was obliged to wonder if the SSH
server on the originating system had somehow been compromised
such that it no longer demanded keys for inbound connections,
but I now understand better what is really going on:

 - The originating system is rigged such that SSH connections
   to localhost work without passwords because ~/.ssh/id_dsa.pub
   is mentioned in ~/.ssh/authorized_keys

 - SSH on the remote system is rigged by default to forward
   SSH authentication agent traffic.

 - My SSH client config on the originating system is rigged
   to permit authentication agent forwarding.

...so once I'd authenticated to the remote system via password
all the plumbing was in place for the originating system to
pass my key to remote system which passed it back to originating
system, where it was found in the authorized_keys file and used
to authenticate me.

I note that the SSH man page warns:

   "Agent forwarding should be enabled with caution"

Uh-huh...