The AskUbuntu question was: How do I SSH to machine A via B in one command?
This response worked for me:
Host <visible hostname alias> Controlmaster auto User <user> hostname <visible hostname> port <port> IdentityFile ~/.ssh/<id file> Host <private LAN hostname alias> ProxyCommand ssh -q -W <private LAN hostname>:<private LAN port> <visible hostname alias>
Or in my particular case:
Host externalmachine.ca Controlmaster auto User externalmachineusername hostname externalmachine.ca port 22 IdentityFile ~/.ssh/id_dsa.pub Host internalmachinehostname ProxyCommand ssh -q -W internalmachineusername@internalmachinehostname:22 externalmachine.ca