Nested SSH session with Paramiko.

Question

I'm rewriting a Bash script I wrote into Python. The crux of that script was

ssh -t first.com "ssh second.com very_remote_command"

I'm having a problem with the nested authentication with paramiko. I wasn't able to find any examples dealing with my precise situation, but I was able to find examples with sudo on a remote host.

The first method writes to stdin

ssh.connect('127.0.0.1', username='jesse', password='lol')
stdin, stdout, stderr = ssh.exec_command("sudo dmesg")
stdin.write('lol\n')
stdin.flush()

The second creates a channel and uses the socket-like send and recv.

I was able to get stdin.write to work with sudo, but it doesn't work with ssh on the remote host.

import paramiko

ssh = paramiko.SSHClient()
ssh.set_missing_host_key_policy(paramiko.AutoAddPolicy())
ssh.connect('first.com', username='luser', password='secret')
stdin, stdout, stderr = ssh.exec_command('ssh [email protected]')
stdin.write('secret')
stdin.flush()
print '---- out ----'
print stdout.readlines()
print '---- error ----'
print stderr.readlines()

ssh.close()

...prints...

---- out ----
[]
---- error ----
['Pseudo-terminal will not be allocated because stdin is not a terminal.\r\n', 'Permission denied, please try again.\r\n', 'Permission denied, please try again.\r\n', 'Permission denied (publickey,password,keyboard-interactive).\r\n']

The pseudo-terminal error reminded me of the -t flag in my original command, so I switched to the second method, using a Channel. Instead of ssh.exec_command and later, I have:

t = ssh.get_transport()
chan = t.open_session()
chan.get_pty()
print '---- send ssh cmd ----'
print chan.send('ssh [email protected]')
print '---- recv ----'
print chan.recv(9999)
chan = t.open_session()
print '---- send password ----'
print chan.send('secret')
print '---- recv ----'
print chan.recv(9999)

...but it prints '---- send ssh cmd ----' and just hangs until I kill the process.

I'm new to Python and none too knowledgeable about networks. In the first case, why does sending the password work with sudo but not with ssh? Are the prompts different? Is paramiko even the right library for this?

Answer 1

I managed to find a solution, but it requires a little manual work. If anyone have a better solution, please tell me.

ssh = paramiko.SSHClient()
ssh.set_missing_host_key_policy(paramiko.AutoAddPolicy())
ssh.connect('first.com', username='luser', password='secret')

chan = ssh.invoke_shell()

# Ssh and wait for the password prompt.
chan.send('ssh second.com\n')
buff = ''
while not buff.endswith('\'s password: '):
    resp = chan.recv(9999)
    buff += resp

# Send the password and wait for a prompt.
chan.send('secret\n')
buff = ''
while not buff.endswith('some-prompt$ '):
    resp = chan.recv(9999)
    buff += resp

# Execute whatever command and wait for a prompt again.
chan.send('ls\n')
buff = ''
while not buff.endswith('some-prompt$ '):
    resp = chan.recv(9999)
    buff += resp

# Now buff has the data I need.
print 'buff', buff

ssh.close()

The thing to note is that instead of this

t = ssh.get_transport()
chan = t.open_session()
chan.get_pty()

...you want this

chan = ssh.invoke_shell()

It reminds me of when I tried to write a TradeWars script when I was a kid and gave up coding for ten years. :)

Answer 2

For a ready made solution check out pxssh from the pxpect project. Look at the sshls.py and ssh_tunnel.py examples.

http://www.noah.org/wiki/Pexpect