SSH ProxyCommand for jumping servers

January 24, 2022 - Reading time: ~1 minute

                                         +--+
                                         |  |
                                         |  |            +--------------------------+
+--------------------------+             |  |            |                          |
|                          |             |  |            |                          |
|                          |       +-----+--+-----+      |                          |
|                          |       |              |      |   Remote Machine (C)     |
|  Local Machine (A)       |       |              |      |                          |
|                          |       |              |      |                          |
|                          |       | Jump Server  |      |                          |
|                          |       |              |      |                          |
|                          |       |     (B)      |      |                          |
|                          |       |              |      |                          |
|                          |       |              |      |                          |
|                          |       |              |      |                          |
|                          |       |              |      +--------------------------+
+--------------------------+       |              |
                                   +-----+--+-----+
                                         |  |
                                         |  |
                                         |  |
                                         +--+

Getting to a server behind a firewall

Machine A can get to Machine B

Machine B can get to Machine C

Machine A CAN NOT get to Machine C directly.

Most people will use SSH to connect from Machine A to Machine B, then use SSH again to connect from Machine B to Machine C.

Another option: ProxyCommand

Add the following to SSH config

Host hostb
    User myself
    Hostname machineB

Host hostc
    User myself
    Hostname machineC
    Port 22
    ProxyCommand ssh -q -W %h:%p hostb

With this configuration, SSH can be used to connect directly from Machine A to Machine C. SSH will automatically direct the connection through Machine B.