how to open an SSH tunnel through another host:

# ssh -l <user> -L <local_port>:<remote_host>:<remote_port> <host_to_connect>

this can be chained through multiple hosts, e.g.

# ssh -l user1 -L 80:localhost.1234 server1 (type on local machine)
# ssh -l user2 -L 1234:localhost:80 server2 (type on server1)

now server2's port can be reached through http://localhost