SSH Server with the Python Twisted Library

Installing the library

Assuming you have Python installed on your system:

pip install twisted
pip install pyOpenSSL
pip install service_identity

Testing the installation

$ python
>>> import twisted
>>> twisted.__version__
'15.2.1'
>>> import OpenSSL
>>> import twisted.internet.ssl
>>> twisted.internet.ssl.SSL
<module 'OpenSSL.SSL' from '/usr/local/lib/python2.7/site-packages/OpenSSL/SSL.pyc'>

Implementing your own ssh-sever with Twisted

Put this script into a file sshserver.py

from twisted.conch import avatar, recvline
from twisted.conch.interfaces import IConchUser, ISession
from twisted.conch.ssh import factory, keys, session
from twisted.conch.insults import insults
from twisted.cred import portal, checkers
from twisted.internet import reactor
from zope.interface import implements
 
class SSHDemoProtocol(recvline.HistoricRecvLine):
    def __init__(self, user):
       self.user = user
 
    def connectionMade(self):
        recvline.HistoricRecvLine.connectionMade(self)
        self.terminal.write("Welcome to my test SSH server.")
        self.terminal.nextLine()
        self.do_help()
        self.showPrompt()
 
    def showPrompt(self):
        self.terminal.write("$ ")
 
    def getCommandFunc(self, cmd):
        return getattr(self, 'do_' + cmd, None)
 
    def lineReceived(self, line):
        line = line.strip()
        if line:
            print line
            f = open('logfile.log', 'w')
            f.write(line + '\n')
            f.close
            cmdAndArgs = line.split()
            cmd = cmdAndArgs[0]
            args = cmdAndArgs[1:]
            func = self.getCommandFunc(cmd)
            if func:
                try:
                    func(*args)
                except Exception, e:
                    self.terminal.write("Error: %s" % e)
                    self.terminal.nextLine()
            else:
                self.terminal.write("No such command.")
                self.terminal.nextLine()
        self.showPrompt()
 
    def do_help(self):
        publicMethods = filter(
            lambda funcname: funcname.startswith('do_'), dir(self))
        commands = [cmd.replace('do_', '', 1) for cmd in publicMethods]
        self.terminal.write("Commands: " + " ".join(commands))
        self.terminal.nextLine()
 
    def do_echo(self, *args):
        self.terminal.write(" ".join(args))
        self.terminal.nextLine()
 
    def do_whoami(self):
        self.terminal.write(self.user.username)
        self.terminal.nextLine()
 
    def do_quit(self):
        self.terminal.write("Thanks for playing!")
        self.terminal.nextLine()
        self.terminal.loseConnection()
 
    def do_clear(self):
        self.terminal.reset()
 
class SSHDemoAvatar(avatar.ConchUser):
    implements(ISession)
 
 
    def __init__(self, username):
        avatar.ConchUser.__init__(self)
        self.username = username
        self.channelLookup.update({'session': session.SSHSession})
 
 
    def openShell(self, protocol):
        serverProtocol = insults.ServerProtocol(SSHDemoProtocol, self)
        serverProtocol.makeConnection(protocol)
        protocol.makeConnection(session.wrapProtocol(serverProtocol))
 
 
    def getPty(self, terminal, windowSize, attrs):
        return None
 
 
    def execCommand(self, protocol, cmd):
        raise NotImplementedError()
 
 
    def closed(self):
        pass
 
 
class SSHDemoRealm(object):
    implements(portal.IRealm)
     
    def requestAvatar(self, avatarId, mind, *interfaces):
        if IConchUser in interfaces:
            return interfaces[0], SSHDemoAvatar(avatarId), lambda: None
        else:
            raise NotImplementedError("No supported interfaces found.")
def getRSAKeys():
 
 
    with open('id_rsa') as privateBlobFile:
        privateBlob = privateBlobFile.read()
        privateKey = keys.Key.fromString(data=privateBlob)
 
 
    with open('id_rsa.pub') as publicBlobFile:
        publicBlob = publicBlobFile.read()
        publicKey = keys.Key.fromString(data=publicBlob)
 
 
    return publicKey, privateKey
 
 
if __name__ == "__main__":
    sshFactory = factory.SSHFactory()
    sshFactory.portal = portal.Portal(SSHDemoRealm())
 
 
users = {'admin': 'aaa', 'guest': 'bbb'}
sshFactory.portal.registerChecker(
    checkers.InMemoryUsernamePasswordDatabaseDontUse(**users))
pubKey, privKey = getRSAKeys()
sshFactory.publicKeys = {'ssh-rsa': pubKey}
sshFactory.privateKeys = {'ssh-rsa': privKey}
reactor.listenTCP(22222, sshFactory)
reactor.run()

Create an RSA key for encryption

ssh-keygen -t rsa -b 4096 -C "[email protected]"

ATTENTION: as a standard, the key gets written into the .ssh directory of the current user - make sure not to override your keys!!!

Start the server

python sshserver.py

Connect to the server

ssh admin@localhost -p 22222
 
((password 'aaa'))
 
>>> Welcome to my test SSH server.
Commands: clear echo help quit whoami
$

Testing commands

The above server is implemented in such a way, that it outputs all commands to STDOUT. So you can redirect the output into a file and after a ssh session assure that the expected commands where called on the server. This is neat for functional testing of programs using an ssh connection.

That's it - hope you like it!

neerajkanhere/twisted.md