You can also run commands when a user authenticates, grab their keys from github.com/username.keys, validate they're a user in a specific github group, then let them connect by outputing the keys, otherwise nothing to deny them access.
It's really great for ops teams where you want to give ssh access and manage it from github teams without needing a complex system.
Honest question, why is ProxyCommand `fun`? What do I get out of ProxyCommand that i do not get out of setting the correct order for ProxyJump and doing an ssh finalhost -- domy --bidding?
ProxyJump is a newer functionality. There used to be only ProxyCommand. ProxyJump is a shortcut for the usual way to use ProxyCommand to connect through a bastion host but ProxyCommand is more flexible. For example with ProxyCommand you can run any command to connect to the remote host. ProxyJump only connects over ssh. I think I replaced all my ProxyCommand with ProxyJump because I don't need much else than the normal use case.
You can get a lot more out of ProxyCommand. For example, you can run SSH over non-IP protocols, such as serial, Bluetooth RFCOMM for embedded boards, or vsock for virtual machines without networking set up at all. The latter is built into and setup up automatically by systemd:
ProxyCommand allows you to use any command to setup a connection. Not necessarily an ssh command, like ProxyJump. It can be any command, as long as it receives on stdin and produces on stdout, it can act like a TCP connection.
ProxyJump is a special case of `ProxyCommand ssh -p <port> <user>@<host>`. Can't replace the `ssh` in there when using ProxyJump.
I came across ProxyCommand earlier this week, funnily enough. I have Cloudflare Zero Trust set up with an SSH service[0], and have the server firewall drop all incoming traffic. That helps reduce my attack surface, since I don't have any incoming ports open.
I use ProxyCommand in edge-case devices where key auth is not an option and the password is not controlled by me. ProxyCommand points to a script the retrieves the password from the vault, puts it on the clipboard for pasting, reminds me via stderr it's done so, and then proxies the connection.
Interesting. I might have such a use case. Do you have anything about best practices on how to automate grabbing passwords from vaults? Cuz it seems to me that the vault needs to be kept open or keep the vault password somewhere on disk.
ProxyCommand is fun