agent-forwarding when SSHing

Use ssh -A, or specify this in ~/.ssh/config:

ForwardAgent Yes

agent-forwarding when SUDOing

Put this in your /etc/sudoers file:

Defaults    env_keep=SSH_AUTH_SOCK

agent-forwarding when SUing

I'm not sure. Here are some options that were suggested:

keychain?
use sudo -i instead of su, and then use the env_keep=SSH_AUTH_SOCK trick

agent-forwarding from inside tmux/screen

See one of these: (TODO: which one is the clearest / most succinct?)