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Commands using agent forwarding hang #399

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dylanahsmith opened this Issue Sep 22, 2014 · 4 comments

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dylanahsmith commented Sep 22, 2014

Version 1.15.0 introduced a regression with ssh agent forwarding which causes commands to hang when they use the forwarded agent. This bug wasn't present in version 1.14.1

We were experiencing this problem with fabric, which can be reproduced with the following script executed with fab -H $HOST test

from fabric.api import run, env
env.forward_agent = True
def test():
    run('ssh-add -L')

Fabric is using paramiko.agent.AgentRequestHandler(channel) to enable ssh agent forwarding, and I managed to reproduce the regression using paramiko directly:

import sys
import paramiko

host = sys.argv[1]
client = paramiko.SSHClient()
client.load_system_host_keys()
client.connect(host)
channel = client.get_transport().open_session()
forward = paramiko.agent.AgentRequestHandler(channel)
channel.exec_command('ssh-add -L')
stdout = channel.makefile('r')
stderr = channel.makefile_stderr('r')
for line in stdout:
    sys.stdout.write(line)
for line in stderr:
    sys.stderr.write(line)
channel.close()
forward.close()
client.close()
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dylanahsmith commented Sep 22, 2014

@bitprophet I found commit 752507a was the first failing commit using git bisect.

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dylanahsmith commented Sep 22, 2014

The regression came when resolving merge conflicts in that merge commit. The problem is that Transport.init changed the position of the gss_kex and gss_deleg_creds arguments, and SSHClient.connect constructs a Transport with positional arguments for gss_kex and gss_deleg_creds.

I'll write a pull request that creates the Transport with keyword arguments.

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bitprophet commented Sep 22, 2014

@dylanahsmith Thanks for the awesomely detailed bug report - I'll get right on this!

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bitprophet commented Sep 22, 2014

Rolling into the PR.

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