openssh bugs - Mar 2020 - [Bug 3143] New: Add the ability to capture wildcard portion of Host keyword in ssh_config

https://bugzilla.mindrot.org/show_bug.cgi?id=3143

            Bug ID: 3143
           Summary: Add the ability to capture wildcard portion of Host
                    keyword in ssh_config
           Product: Portable OpenSSH
           Version: -current
          Hardware: Other
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P5
         Component: ssh
          Assignee: unassigned-bugs at mindrot.org
          Reporter: lavneetpuri at gmail.com

Use Case:
`ssh pc-1` would connect to remote-cpu-1.domain.com

Currently we can use `%h` token but it captures the whole argument to
Host keyword (i.e pc-1). This feature would enable capturing just the
pc number (ie. 1) by using a new `%m` token for example. It is useful
to simplify connection to complex ssh hostnames

For the above use-case, the ssh config can be specified as follows:
Host pc-*
  Hostname remote-cpu-%m.domain.com

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Darren Tucker <dtucker at dtucker.net> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |dtucker at dtucker.net

--- Comment #1 from Darren Tucker <dtucker at dtucker.net> ---
There's a couple of problems with this:
 - the current matching code doesn't lend itself to supporting this
kind of backreference
 - it does however currently support multiple wildcards (eg
"*-*-?.domain.*") which don't map well onto the token you
suggested.
It'd probably need to be regex-style digits %1 - %9.
 - "Host" specifically supports multiple matches.  You could have, eg,
"Host pc-? *-* *.*.*", at which point you could have 1, 2 or 3
backrefs.  In that case, what does "%3" evaluate to and when?

I'm not sure it's useful enough to be worth the extra complexity in the
parser, which is already pretty complex.

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--- Comment #2 from Lavneet <lavneetpuri at gmail.com> ---
I ended up writting a wrapper script to account for this special case -
one that is not very neat - so I realize the complexity this may add to
the parser. However, in terms of usefulness, ssh configs are among the
best features that boost productivity. I wonder if this logic (and more
advanced parsing feature to follow) could be offloaded from the main
build - as a plugin of sorts. I'm not aware if this has been done
before for any other components of the program.

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