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From "Simon Tatham (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (SSHD-256) SSH_MSG_CHANNEL_REQUEST for Putty handled incorrectly
Date Tue, 24 Sep 2013 10:29:05 GMT

     [ https://issues.apache.org/jira/browse/SSHD-256?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Simon Tatham updated SSHD-256:
------------------------------

    Attachment: 0001-Consistently-send-replies-to-channel-requests.patch

I've been investigating this same issue (having had it reported initially as a bug against
PuTTY). I agree that Michael Heemskerk's patch is sensible, but I also think that the problem
with ignoring the "want reply" flag goes considerably further than just this one request type.
I looked through the request handling in the various channel classes and discovered that "want
reply" is often ignored in other situations, which is a source of further latent bugs just
waiting for a peer implementation to trigger them.

I've attached my own suggested patch which centralises the handling of responses to channel
requests, so as to make sure a response is _always_ sent if "want reply" is true, and _never_
sent if it is not, regardless of the channel type or whether the request is recognised. I
haven't been conveniently able to check the behaviour in previously failing cases, but I have
run the result through "mvn test" which reported no failures.

I would recommend applying this patch _in addition_ to Michael Heemskerk's patch.
                
> SSH_MSG_CHANNEL_REQUEST for Putty handled incorrectly
> -----------------------------------------------------
>
>                 Key: SSHD-256
>                 URL: https://issues.apache.org/jira/browse/SSHD-256
>             Project: MINA SSHD
>          Issue Type: Bug
>    Affects Versions: 0.7.0, 0.8.0, 0.9.0
>            Reporter: Michael Heemskerk
>         Attachments: 0001-Consistently-send-replies-to-channel-requests.patch
>
>
> Symptom:
> SSH connection is kept open after the command completes when using Putty, Plink or TortoisePlink,
or rather Putty does not finish.
> See:
> * https://code.google.com/p/tortoisegit/issues/detail?id=1880
> * https://jira.atlassian.com/browse/STASH-3824
> Cause:
> ChannelSession has special handling for SSH_MSG_CHANNEL_REQUEST messages that are sent
by Putty:
>         if (type != null && type.endsWith("@putty.projects.tartarus.org")) {
>             // Ignore but accept, more doc at
>             // http://tartarus.org/~simon/putty-snapshots/htmldoc/AppendixF.html
>             return true;
>         }
> There are really two problems with this workaround that was introduced in SSHD-77:
> 1) This code ignores the "want reply" flag on the field and simply ignores the request.
According to the SSH protocol specification, the server MUST send back a reply (success or
failure) if the "want reply" flag is set to true.
> 2) Putty _intentionally_ sends an invalid request and _expects_ to receive a SSH_MSG_CHANNEL_FAILURE
response (see http://tartarus.org/~simon/putty-snapshots/htmldoc/AppendixF.html), which is
the default behaviour of ChannelSession.
> I don't know whether changes in Putty or changes in MinaSSHD have made the workaround
obsolete, but it is obsolete. I've verified that without the bit of code I quoted above, the
Putty terminates normally.

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