In the original days of computing, rsh/rlogin were used to connect to remote computers and execute commands. These commands had the problem that the passwords and commands were sent in the clear. To solve this problem, the SSH protocol was created. Twisted.Conch implements the second version of this protocol.
Writing a client with Conch involves sub-classing 4 classes: twisted.conch.ssh.transport.SSHClientTransport, twisted.conch.ssh.userauth.SSHUserAuthClient, twisted.conch.ssh.connection.SSHConnection, and twisted.conch.ssh.channel.SSHChannel. We'll start out
with SSHClientTransport because it's the base
of the client.
from twisted.conch import error
from twisted.conch.ssh import transport
from twisted.internet import defer
class ClientTransport(transport.SSHClientTransport):
def verifyHostKey(self, pubKey, fingerprint):
if fingerprint != 'b1:94:6a:c9:24:92:d2:34:7c:62:35:b4:d2:61:11:84':
return defer.fail(error.ConchError('bad key'))
else:
return defer.succeed(1)
def connectionSecure(self):
self.requestService(ClientUserAuth('user', ClientConnection()))
See how easy it is? SSHClientTransport
handles the negotiation of encryption and the verification of keys
for you. The one security element that you as a client writer need to
implement is verifyHostKey(). This method
is called with two strings: the public key sent by the server and its
fingerprint. You should verify the host key the server sends, either
by checking against a hard-coded value as in the example, or by asking
the user. verifyHostKey returns a twisted.internet.defer.Deferred which gets a callback
if the host key is valid, or an errback if it is not. Note that in the
above, replace 'user' with the username you're attempting to ssh with,
for instance a call to os.getlogin() for the
current user.
The second method you need to implement is connectionSecure(). It is called when the
encryption is set up and other services can be run. The example requests
that the ClientUserAuth service be started.
This service will be discussed next.
from twisted.conch.ssh import keys, userauth
# these are the public/private keys from test_conch
publicKey = 'ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAGEArzJx8OYOnJmzf4tfBEvLi8DVPrJ3\
/c9k2I/Az64fxjHf9imyRJbixtQhlH9lfNjUIx+4LmrJH5QNRsFporcHDKOTwTTYLh5KmRpslkYHR\
ivcJSkbh/C+BR3utDS555mV'
privateKey = """-----BEGIN RSA PRIVATE KEY-----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-----END RSA PRIVATE KEY-----"""
class ClientUserAuth(userauth.SSHUserAuthClient):
def getPassword(self, prompt = None):
return
# this says we won't do password authentication
def getPublicKey(self):
return keys.getPublicKeyString(data = publicKey)
def getPrivateKey(self):
return defer.succeed(keys.getPrivateKeyObject(data = privateKey))
Again, fairly simple. The SSHUserAuthClient takes care of most
of the work, but the actual authentication data needs to be
supplied. getPassword() asks for a
password, getPublicKey() and getPrivateKey() get public and private keys,
respectively. getPassword() returns
a Deferred that is called back with
the password to use. getPublicKey()
returns the SSH key data for the public key to use. keys.getPublicKeyString() will take
keys in OpenSSH and LSH format, and convert them to the
required format. getPrivateKey()
returns a Deferred which is
called back with the key object (as used in PyCrypto) for
the private key. getPassword()
and getPrivateKey() return Deferreds because they may need to ask the user
for input.
Once the authentication is complete, SSHUserAuthClient takes care of starting the code
SSHConnection object given to it. Next, we'll
look at how to use the SSHConnection
from twisted.conch.ssh import connection
class ClientConnection(connection.SSHConnection):
def serviceStarted(self):
self.openChannel(CatChannel(conn = self))
SSHConnection is the easiest,
as it's only responsible for starting the channels. It has
other methods, those will be examined when we look at SSHChannel.
from twisted.conch.ssh import channel, common
class CatChannel(channel.SSHChannel):
name = 'session'
def channelOpen(self, data):
d = self.conn.sendRequest(self, 'exec', common.NS('cat'),
wantReply = 1)
d.addCallback(self._cbSendRequest)
self.catData = ''
def _cbSendRequest(self, ignored):
self.write('This data will be echoed back to us by "cat."\r\n')
self.conn.sendEOF(self)
self.loseConnection()
def dataReceived(self, data):
self.catData += data
def closed(self):
print 'We got this from "cat":', self.catData
Now that we've spent all this time getting the server and
client connected, here is where that work pays off. SSHChannel is the interface between you and the
other side. This particular channel opens a session and plays with the
'cat' program, but your channel can implement anything, so long as the
server supports it.
The channelOpen() method is
where everything gets started. It gets passed a chunk of data;
however, this chunk is usually nothing and can be ignored.
Our channelOpen() initializes our
channel, and sends a request to the other side, using the
sendRequest() method of the SSHConnection object. Requests are used to send
events to the other side. We pass the method self so that it knows to
send the request for this channel. The 2nd argument of 'exec' tells the
server that we want to execute a command. The third argument is the data
that accompanies the request. common.NS encodes
the data as a length-prefixed string, which is how the server expects
the data. We also say that we want a reply saying that the process has a
been started. sendRequest() then returns a
Deferred which we add a callback for.
Once the callback fires, we send the data. SSHChannel supports the
twisted.internet.interface.Transport interface, so
it can be given to Protocols to run them over the secure
connection. In our case, we just write the data directly. sendEOF() does not follow the interface,
but Conch uses it to tell the other side that we will write no
more data. loseConnection() shuts
down our side of the connection, but we will still receive data
through dataReceived(). The closed() method is called when both sides of the
connection are closed, and we use it to display the data we received
(which should be the same as the data we sent.)
Finally, let's actually invoke the code we've set up.
from twisted.internet import protocol, reactor
def main():
factory = protocol.ClientFactory()
factory.protocol = ClientTransport
reactor.connectTCP('localhost', 22, factory)
reactor.run()
if __name__ == "__main__":
main()
We call connectTCP() to connect to
localhost, port 22 (the standard port for ssh), and pass it an instance
of twisted.internet.protocol.ClientFactory.
This instance has the attribute protocol
set to our earlier ClientTransport
class. Note that the protocol is set to the class ClientFactory, not an instance of
ClientFactory! When the connectTCP call completes, the protocol will be
called to create a ClientTransport() object
- this then invokes all our previous work.
It's worth noting that in the example main()
routine, the reactor.run() call never returns.
If you want to make the program exit, call
reactor.stop() in the earlier
closed() method.
If you wish to observe the interactions in more detail, adding a call
to log.startLogging(sys.stdout, setStdout=0)
before the reactor.run() call will send all
logging to stdout.