openssh bugs - May 2022 - [Bug 3419] regular expression patterns in Host directive

https://bugzilla.mindrot.org/show_bug.cgi?id=3419

            Bug ID: 3419
           Summary: regular expression patterns in Host directive
           Product: Portable OpenSSH
           Version: v9.0p1
          Hardware: Other
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P5
         Component: Miscellaneous
          Assignee: unassigned-bugs at mindrot.org
          Reporter: calestyo at scientia.org

Hey.

One thing that could be really nice was, the Host directive (not sure
whether it would make sense for Match, too) would allow to match not
only the current patterns (which basically have "just" '*',
'?', "[?]"
and '!') but general regular expressions.

The idea is that one is able to more thoroughly specify hostnames, e.g.
right now, in order to match:
server1.example.org
server2.example.org
server3.example.org
one can do:
a) server*.example.org
b) server?.example.org
c) server[1-3].example.org

(a) and (b) are not really exact (as they'd also match e.g.
serverX.example.org). (c) is difficult if one has more digits (servers
1-45) and really wants to match only those.

The above is still a very simple example of course, but with regular
expressions one could do much more nice things like matching:
(www\.)?[^.]+\.(intranet|public)\.example\.(com|biz)

Or (at least if PCRE were chosen as RE language) one could do
exclusions (i.e. all but a certain pattern).


So the first motivation for such feature is "simply" to better select
the actually desired hostnames only (which * and ? may not always help
with).


And I guess that level is what this enhancement idea is about.


Possible future ideas:

However, this could be made even much more useful, if one would allow
to use the matched subexpressions via back-references in the
sub-directives of a given Host block.
IIRC Apache httpd, can do such things in its
<(Location|Directory|File)Match> blocks.

I think of use-cases like:
Host ^[^.]+\.(intranet|public)\.example\.com$
     Identity ~/.ssh/example.com_\1_id_rsa

where different keys are used with different groups of hosts.

The above is obviously something, which one could do also now (just
with a bit more configuration lines.


One could however think to even further extend syntax:

We at the university run many servers for the LHC Computing Grid.
These are all named like server<n>.example.org
We buy new ones every 2-3 years, and these are then all the same (so
.e.g. n = 1 - 34 are one model and 35-53 another or so)

These have service processors (like HP's ILO or Dell's iDrac) whose SSH
we use for serial console access... and typically their SSH is not so
well maintained and after a while upgrades stop, so they don't support
modern algorithms (e.g. just RSA keys, but not ED25519) or need things
like:
        KexAlgorithms                  
+diffie-hellman-group-exchange-sha256
        HostkeyAlgorithms               +ssh-rsa
        PubkeyAcceptedAlgorithms        +ssh-rsa


If one would now have something like an <If> directive that matches
TOKENS or the above back-references one could do things like:

Host ^server(\d+)\.service\.example\.com$
     ProxyJump ...
     User ...
     If %1 >= 1 AND %1 <= 34
     {
        KexAlgorithms                  
+diffie-hellman-group-exchange-sha256
        HostkeyAlgorithms               +ssh-rsa
        PubkeyAcceptedAlgorithms        +ssh-rsa
     }
     If %1 >= 35 AND %1 <= 52
     {
         # possible other non default settings
     }
The backref would be handled like a TOKEN.

Obviously, one would then need a powerful If directive with many
operators, perhaps again with regexp string matching and perhaps also
with conversion of the matched strings to integers... or operators like
"in".


Not saying that this would really be worth all the (considerable)
effort... it would be just something to keep in mind for possible
future developments.


Cheers,
Chris.

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https://bugzilla.mindrot.org/show_bug.cgi?id=3419

Darren Tucker <dtucker at dtucker.net> changed:

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

--- Comment #1 from Darren Tucker <dtucker at dtucker.net> ---
(In reply to Christoph Anton Mitterer from comment #0)
[...]
> The above is still a very simple example of course, but with regular
> expressions one could do much more nice things like matching:
> (www\.)?[^.]+\.(intranet|public)\.example\.(com|biz)
You can already do this via Match Exec, eg:

$ cat /tmp/ssh_config
Match Exec "egrep '^[^.]+\.(intranet|public)\.example\.com$'
<<<'%h'"
        Hostname matched
Host *
        Hostname notmatched

$ ssh -G -F /tmp/ssh_config foo.public.example.com | awk
'$1=="hostname"'
hostname matched

$ ssh -G -F /tmp/ssh_config somehost | awk '$1=="hostname"'
hostname notmatched

You can use any pattern matching language you have tooling for, not
just regular expressions.

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--- Comment #2 from Christoph Anton Mitterer <calestyo at scientia.org>
---
Nice, though the syntax is a bit ugly ;-)

But AFAIU, this would only work if the user's shell is bash, as it uses
the non-standard <<<, right?


And it gives some ugly errors, if the user accidentally has a ' in the
hostname.
In principle one could even think that this may cause accidental
execution an intended remote command, locally:

It's a bit constructed of curse, but consider something lile:
intended:
ssh -G  "foo.public.example.com" "'; echo 'foo'
>&2'" | awk
'$1=="hostname"'

written by accident:
ssh -G  "foo.public.example.com'; echo 'foo'
>&2'" | awk
'$1=="hostname"'
that actually prints:
foo
hostname matched

Now replace echo 'foo' with 'rm -rf /'.

But of course it's clear, that the same could just happen without using
the Match-exec at all... so it's not really an issue I think.




With %h, AFAIU, one really get's the same behaviour as with Host
<pattern>, i.e. after any substitutions via the Hostname or
CanonicalizeHostname options, right?
Could that be added to the description of %h? It already says for %n
that it's the one from the command line.

I could provide a patch if it helps you.


Since you've left the issue open,... do you still consider this? Or is
the Match+exec solution the way to go?
Cause if the latter, it would be nice if one could perhaps add that as
an example somewhere in the config.
Ideally with non-bash specific code, I guess printf '%s' '%s' |
egrep
... should do the job, too?!

One subtle remaining issue is perhaps, that this solution means that
the values of %-escapes appear in the process list.
I mean there is non like %p with p being the password, but it might
still be undesired by a user that others can see e.g. the true %h,
which may have been obfuscated by using a fake name on the command
line, and having ssh_config substitute that to the real one.
But again, only a very subtle thing, as usually there are other means
to find out that for another user.




Cheers,
Chris.

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Celeste Liu <CoelacanthusHex at gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |CoelacanthusHex at gmail.com

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Damien Miller <djm at mindrot.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |WONTFIX
                 CC|                            |djm at mindrot.org
             Status|NEW                         |RESOLVED

--- Comment #3 from Damien Miller <djm at mindrot.org> ---
We deliberately don't support regular expressions in OpenSSH because 1)
there are several syntaxes, 2) the good syntaxes aren't in system
libraries and 3) regexp implementations have a fairly spotty security
track record.

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--- Comment #4 from Christoph Anton Mitterer <calestyo at scientia.org>
---
(1) yes, though some (I'd say especially PCRE) have become like de
facto standards.

(2) At least all major Linuxes (I've checked Debian, Fedora, Arch,
OpenSUSE, Ubuntu, Alpine, Rocky, CentOS, CentOS Stream and Cygwin) have
their grep depend on libpcre2, so I'd be tempted to says that
effectively it's like a system lib.
But I guess the BSDs don't.

(3) For pcre2 I find 11 CVEs since 2015, which is considerably less
than what e.g. OpenSSH itself has (which is of course not meant as an
insult, but rather to put numbers into perspective). I did however not
check how serious all of these were.

I would however even intuitively guess, that e.g. for grep (which is
used in gazillions of scripts), security issues in pcre would be far
more problematic than for ssh, where we'd probably ever only match
against hostnames and usernames, which could be checked for the few
valid characters before even running pcre on them.


Aynway,... was just an idea which I've thought would make maintaining
complex sshd?_configs much simpler.

Cheers,
Chris :-)

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