Is there a way to use a previously specified ssh-config entry in specifying `Hostname`? - ssh

Question

edit: I have seen cases where this works and cases where this doe not and I'm not sure I follow when/why it does.
Suppose I have a complicated enough entry where I specify multiple parameters to get to thathost:
Host thathost
ControlMaster auto
ServerAliveInterval 8
ConnectTimeout 8
Hostname 192.168.5.1
User mememe
ProxyJump thejumpbox
I want to re-use this definition in creating additional entries that provide different functionality by adding or overriding some configs. Specifically specifying an alternate port (no, I don't want it on the command-line).
Ideally I'd be able to do something like
Host theirhost
Hostname thathost
User themthem
or
Host my-remote-serialport
Hostname thathost
RequestTTY yes
RemoteCommand some-script-that-launches-kermit-on-a-specific-dev-tty
or
Host my-remote-serialport
Hostname thathost
Port 3004
I'm strictly looking to specify one host in terms of another existing one, I'm not looking to modify my Host entries to match some pattern "tricks".
Obviously I can utilize ProxyCommand ssh -q nc thathost... or ProxyJump thathost +Hostname localhost followed by all the other overrides (well for port override would pass that to nc) but that's both ugly and wasteful (an extra session) - please don't answer with that.
For me this has been the missing feature of ssh-config, but maybe I did not look hard enough.

Answer 1

It can't be used in the way you asked, like reuse a hostname definition, but the provided solution of ssh can solve much more problems.
A host rule can span multiple hosts.
Host thathost theirhost my-remote-serialport
ControlMaster auto
ServerAliveInterval 8
ConnectTimeout 8
Hostname 192.168.5.1
User mememe
ProxyJump thejumpbox
But obviously this doesn't solve your problem with modifying some properties.
The trick is, that the ssh config uses the first wins strategy for properties.
In your case you just has to add the modifications in front of the main config
Host theirhost
Hostname thathost
User themthem
Host my-remote-serialport
Hostname thathost
Port 3004
Host thathost theirhost my-remote-serialport
ControlMaster auto
ServerAliveInterval 8
ConnectTimeout 8
Hostname 192.168.5.1
User mememe
ProxyJump thejumpbox
theirhost is defined at two places, the properties Hostname and User are taken from the first definition, all other properties from the second definition.
The host part also accepts wildcards, example for a jumpbox with multiple reverse ssh endpoints:
HOST my_1
port 2001
HOST my_2
port 2002
HOST my_3
port 2003
HOST my_*
user pi
hostname localhost
ProxyJump thejumpbox

Answer 2

There's no way to reference a different block; however, you can include a specific configuration file that has a global configuration. For example, create a file meant for use by thathost, theirhost, and my-remote-serialport. Let's just call it foo-config.
ControlMaster auto
ServerAliveInterval 8
ConnectTimeout 8
Hostname 192.168.5.1
User mememe
ProxyJump thejumpbox
Then you can use the Include directive to read this in as needed:
Host theirhost
Include foo-config
User themthem
Host my-remote-serialport
Include foo-config
RequestTTY yes
RemoteCommand some-script-that-launches-kermit-on-a-specific-dev-tty
Host my-remote-serialport
Include foo-config
Port 3004
However, I suspect this approach is rarely necessary, and the approach given by jeb will probably be sufficient in most cases.