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('loln')
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.rn', 'Permission denied, please try again.rn', 'Permission denied, please try again.rn', 'Permission denied (publickey,password,keyboard-interactive).rn']

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?

Asked By: mqsoh

||

Source

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.comn')
buff = ''
while not buff.endswith(''s password: '):
    resp = chan.recv(9999)
    buff += resp

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

# Execute whatever command and wait for a prompt again.
chan.send('lsn')
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. 🙂

Answered By: mqsoh

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

Answered By: snies

Answer 3

Here is a small example using paramiko only (and port forwarding):

import paramiko as ssh

class SSHTool():
    def __init__(self, host, user, auth,
                 via=None, via_user=None, via_auth=None):
        if via:
            t0 = ssh.Transport(via)
            t0.start_client()
            t0.auth_password(via_user, via_auth)
            # setup forwarding from 127.0.0.1:<free_random_port> to |host|
            channel = t0.open_channel('direct-tcpip', host, ('127.0.0.1', 0))
            self.transport = ssh.Transport(channel)
        else:
            self.transport = ssh.Transport(host)
        self.transport.start_client()
        self.transport.auth_password(user, auth)

    def run(self, cmd):
        ch = self.transport.open_session()
        ch.set_combine_stderr(True)
        ch.exec_command(cmd)
        retcode = ch.recv_exit_status()
        buf = ''
        while ch.recv_ready():
            buf += ch.recv(1024)
        return (buf, retcode)

# The example below is equivalent to
# $ ssh 10.10.10.10 ssh 192.168.1.1 uname -a
# The code above works as if these 2 commands were executed:
# $ ssh -L <free_random_port>:192.168.1.1:22 10.10.10.10
# $ ssh 127.0.0.1:<free_random_port> uname -a
host = ('192.168.1.1', 22)
via_host = ('10.10.10.10', 22)

ssht = SSHTool(host, 'user1', 'pass1',
    via=via_host, via_user='user2', via_auth='pass2')

print ssht.run('uname -a')

Answered By: Sinas

Answer 4

Sinas’s answer works well but didn’t provide all the output from very long commands for me. However, using chan.makefile() allows me to retrieve all the output.

The below works on a system that requires tty and also prompts for sudo password

ssh = paramiko.SSHClient()
ssh.load_system_host_keys()
ssh.set_missing_host_key_policy(paramiko.WarningPolicy())
ssh.connect("10.10.10.1", 22, "user", "password")
chan=ssh.get_transport().open_session()
chan.get_pty()
f = chan.makefile()
chan.exec_command("sudo dmesg")
chan.send("passwordn")
print f.read()
ssh.close()

Answered By: user2945126

Answer 5

You can create ssh connection using channel from another ssh connection. See here for more detail.

Answered By: David Lim

Nested SSH session with Paramiko

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