Mosh-like port forwarding

As you mentioned, the environment variables are removed by sudo, for security reasons.

But fortunately sudo is quite configurable: you can tell it precisely which environment variables you want to keep thanks to the env_keep configuration option in /etc/sudoers.

For agent forwarding, you need to keep the SSH_AUTH_SOCK environment variable. To do so, simply edit your /etc/sudoers configuration file (always using visudo) and set the env_keep option to the appropriate users. If you want this option to be set for all users, use the Defaults line like this:

Defaults    env_keep+=SSH_AUTH_SOCK

man sudoers for more details.

You should now be able to do something like this (provided user1's public key is present in ~/.ssh/authorized_keys in user1@serverA and user2@serverB, and serverA's /etc/sudoers file is setup as indicated above):

user1@mymachine> eval `ssh-agent`  # starts ssh-agent
user1@mymachine> ssh-add           # add user1's key to agent (requires pwd)
user1@mymachine> ssh -A serverA    # no pwd required + agent forwarding activated
user1@serverA> sudo su - user2     # sudo keeps agent forwarding active :-)
user2@serverA> ssh serverB         # goto user2@serverB w/o typing pwd again...
user2@serverB>                     # ...because forwarding still works

DHCP and port forwarding are two very different things.

DHCP is for assigning an IP address, Subnet Mask, Gateway, DNS and other predefined network settings.

Port forwarding is the ability to publish (can't think of a better word) a port from an application or service from a machine running behind a firewall, router or other blocking device.

If you are however saying that you want to publish a DHCP server to another machine, you will need to forward ports 67 and 68 - however, DHCP also works through the broadcast address. This can cause a lot of problems when running through separate networks, so take a look to see if your router supports DHCP chaining, routing, forwarding or whatever your vendor calls it.