fails.py

from __future__ import print_function

import sys

from twisted.internet import defer
from twisted.internet.endpoints import TCP4ClientEndpoint, connectProtocol
from twisted.internet.task import react
from twisted.conch.ssh.transport import SSHClientTransport
from twisted.python import log


log.startLogging(sys.stdout, setStdout=False)


class ForgeSSHClientTransport(SSHClientTransport):

    # def verifyHostKey(self, hostKey, fingerprint):
    #     print("WAT")
    #     return defer.succeed(True)

    def connectionSecure(self):
        print("Testing")


def main(reactor, hostname, port):
    port = int(port)

    sshClient = TCP4ClientEndpoint(reactor, hostname, port)

    d = connectProtocol(sshClient, ForgeSSHClientTransport())

    return d


if __name__ == "__main__":
    react(main, sys.argv[1:])

works.py

from __future__ import print_function

import sys

from twisted.internet import defer
from twisted.internet.endpoints import TCP4ClientEndpoint, connectProtocol
from twisted.internet.task import react
from twisted.conch.ssh.transport import SSHClientTransport
from twisted.python import log


log.startLogging(sys.stdout, setStdout=False)


class ForgeSSHClientTransport(SSHClientTransport):

    def __init__(self):
        self.connectionReady = defer.Deferred(
            lambda d: self.transport.abortConnection()
        )
        # Clear the reference to that deferred to help the garbage collector
        # and to signal to other parts of this implementation (in particular
        # connectionLost) that it has already been fired and does not need to
        # be fired again.

        def readyFired(result):
            self.connectionReady = None
            return result

        self.connectionReady.addBoth(readyFired)

    # def verifyHostKey(self, hostKey, fingerprint):
    #     print("WAT")
    #     return defer.succeed(True)

    def connectionSecure(self):
        print("Testing")


def main(reactor, hostname, port):
    port = int(port)

    sshClient = TCP4ClientEndpoint(reactor, hostname, port)

    d = connectProtocol(sshClient, ForgeSSHClientTransport())
    d.addCallback(lambda protocol: protocol.connectionReady)

    return d


if __name__ == "__main__":
    react(main, sys.argv[1:])