openssh bugs - Nov 2014 - [Bug 2310] New: functionality to start process before ssh and/or to "wrap" such command around ssh

https://bugzilla.mindrot.org/show_bug.cgi?id=2310

            Bug ID: 2310
           Summary: functionality to start process before ssh and/or to
                    "wrap" such command around ssh
           Product: Portable OpenSSH
           Version: 6.7p1
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P5
         Component: sftp
          Assignee: unassigned-bugs at mindrot.org
          Reporter: calestyo at scientia.net

Hi.

This feature request basically evolved out of a post[0] on the
mailinglist, where however no one came up with a really clean solution.

What I basically would want is something like LocalCommand, just that
is run "before".

Now of course starting a command before ssh, can be done via shell
script wrapping and that like.
The disadvantage here is however, that I cannot easily start commands
on a per host basis, unless I write my own parser for SSH config files,
which also takes things like CanonicaliseHostnames into account.

Typical example for starting something *before* ssh would be, e.g.
kinit, that requests a kerberos ticket, or perhaps (for certain special
hosts) brining up some ppp network route or whatever.



But actually "just" starting something before ssh isn't the only
thing
I'd wish:
My thinking goes also into "wrapping" another command around ssh,
mainly something like k5start[1] or krenew[1], which would greatly
simply connecting to hosts from different(!) realms.


I'm not sure though, how easy the later can be done,...

If it would work, one might need to take security implications into
account, especially when this is used together with control channel
multiplexing.
I remember, that some things where then fixed for *all* further
connections via that control socket, even if the later ssh wasn't
invoked with such option.
If the same would e.g. apply to transmission of kerberos credentials,
than all further connections could accidentally inherit the credentials
from the first one, started with k5start wrapped around.



Cheers,
Chris.


[0]
https://lists.mindrot.org/pipermail/openssh-unix-dev/2014-October/033082.html
[1] http://www.eyrie.org/~eagle/software/kstart/

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https://bugzilla.mindrot.org/show_bug.cgi?id=2310

Damien Miller <djm at mindrot.org> changed:

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

--- Comment #1 from Damien Miller <djm at mindrot.org> ---
This functionality can easily be achieved using shell functions, shell
aliases, shell scripts or wrapper binaries. There is no need to add
more options to accomplish this. Each new option is a maintenance
burden for developers and a cognitive load for users, so they need to
be well justified.

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--- Comment #2 from Christoph Anton Mitterer <calestyo at scientia.net>
---
(In reply to Damien Miller from comment #1)
> This functionality can easily be achieved using shell functions,
> shell aliases, shell scripts or wrapper binaries.
Could you please elaborate on how this can be easily achieved the ways
you describe, especially when considering that people may use arbitrary
HostName definitions or features like CanonicaliseHostname, which would
basically make it necessary for any wrapper program to implement a full
ssh_config parser.

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Christoph Anton Mitterer <calestyo at scientia.net> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |REOPENED
         Resolution|WONTFIX                     |---

--- Comment #3 from Christoph Anton Mitterer <calestyo at scientia.net>
---
Actually I've made some further tests on this issue in the meantime and
it seems as if there is just some "timing issue", which is why I
reopen
the issue for now, perhaps you can have a look, whether that could be
fixed easily.



Remember, what I've basically tried to achieve is to get automated
(i.e. passwordless) login to ssh hosts that require a kerberos ticket
(+afs) for login and for the home dir access.
And not only this, the whole thing should also work for multiple reams,
i.e. when I connect to some nodes at CERN I want a ticket for CERN.CH
being automatically generated and used with ssh,... while when I
contact
to the university's nodes, I want the same for the PHYISIK.LMU.DE
realm.


Okay from the k5start side everything seems to work. What I do is
basically
$ k5start -q -f kt -K 10 -- user at REALM ssh ....

The keytab file (kt) works, and when I run ssh with the above, login
works fine as well and the I get an AFS ticket (it's probably created
on the server side).


Now I try to get the whole thing into ssh_config, because I want all be
done automatically and especially for different realms, so I do about
the following:

Host lxplus.cern.ch
        GSSAPITrustDns yes

Host *.cern.ch
        User mitterer
        GSSAPIAuthentication yes
        GSSAPIDelegateCredentials yes
        GSSAPIRenewalForcesRekey yes 
        ProxyCommand k5start -q -f kt -K 10 -- mitterer at CERN.CH nc %h
%p

The usage of ProxyCommand with k5start and nc is a hack here, which I
hoped to solve the problem that the ticket is generated before the
connection is done,... and nc %h %p simply passes everything through.
In other words, this is the trick which I've tried to get the feature
what I've asked for in the original submission of this ticket.


This however doesn't work, which makes sense, as the ssh process has no
way to know where the credential cache is that k5start creates randomly
for this program invocation (nc), e.g. at /tmp/krb5cc_1000_VZ7FI5.

It also exports KRB5CCNAME='FILE:/tmp/krb5cc_1000_CERN', of course, but
this only gets known to nc.

At that point, ssh already looked for KRB5CCNAME, didn't find any, and
never tries again.



Do you see any simple solution to get that working somehow? Maybe even
in a completely different way (extending ssh-agent and k5start to
interact?) or some other tricky hack?
If not, and you still think that adding such feature isn't worth to
make ssh nicely usable with kerberos, than just close it again if you
feel necessary.


Thanks,
Chris.

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--- Comment #4 from Christoph Anton Mitterer <calestyo at scientia.net>
---
Created attachment 2639
  --> https://bugzilla.mindrot.org/attachment.cgi?id=2639&action=edit
0001-document-the-group-fallback-behaviour-in-DH-GEX.patch

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--- Comment #5 from Christoph Anton Mitterer <calestyo at scientia.net>
---
Comment on attachment 2639
  --> https://bugzilla.mindrot.org/attachment.cgi?id=2639
0001-document-the-group-fallback-behaviour-in-DH-GEX.patch

wah... again wrong bug... this please delete that attachment.

I really hate this "wrap on to the next bug" mis-feature of
bugzilla...

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Damien Miller <djm at mindrot.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |WONTFIX
             Status|REOPENED                    |RESOLVED

--- Comment #6 from Damien Miller <djm at mindrot.org> ---
We don't plan in implementing this. In answer to comment #2, the helper
command/alias can use "ssh -G" to resolve the active configuration for
most cases.

It is true that a helper command/alias is would not be able to see the
results of hostname canonicalisation, but it could avoid this by doing
that step itself if necessary. I don't think this shortcoming is enough
to justify the maintenance burden of another command-execution feature,
sorry.

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Damien Miller <djm at mindrot.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |CLOSED

--- Comment #7 from Damien Miller <djm at mindrot.org> ---
closing resolved bugs as of 8.6p1 release

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