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# The Evolution of Finger: building a simple finger service

## Introduction

This is the first part of the Twisted tutorial Twisted from Scratch, or The Evolution of Finger .

If you’re not familiar with ‘finger’ it’s probably because it’s not used as much nowadays as it used to be. Basically, if you run `finger nail` or `finger nail@example.com` the target computer spits out some information about the user named `nail` . For instance:

    Login: nail                           Name: Nail Sharp
    Directory: /home/nail                 Shell: /usr/bin/sh
    Last login Wed Mar 31 18:32 2004 (PST)
    New mail received Thu Apr  1 10:50 2004 (PST)
         Unread since Thu Apr  1 10:50 2004 (PST)
    No Plan.

If the target computer does not have the `fingerd` daemon running you’ll get a “Connection Refused” error. Paranoid sysadmins keep `fingerd` off or limit the output to hinder crackers and harassers. The above format is the standard `fingerd` default, but an alternate implementation can output anything it wants, such as automated responsibility status for everyone in an organization. You can also define pseudo “users”, which are essentially keywords.

This portion of the tutorial makes use of factories and protocols as introduced in the Writing a TCP Server howto and deferreds as introduced in Using Deferreds and Generating Deferreds . Services and applications are discussed in Using the Twisted Application Framework .

By the end of this section of the tutorial, our finger server will answer TCP finger requests on port 1079, and will read data from the web.

## Refuse Connections

`finger01.py`

    from twisted.internet import reactor
    reactor.run()

This example only runs the reactor. It will consume almost no CPU resources. As it is not listening on any port, it can’t respond to network requests — nothing at all will happen until we interrupt the program. At this point if you run `finger nail` or `telnet localhost 1079` , you’ll get a “Connection refused” error since there’s no daemon running to respond. Not very useful, perhaps — but this is the skeleton inside which the Twisted program will grow.

As implied above, at various points in this tutorial you’ll want to observe the behavior of the server being developed. Unless you have a finger program which can use an alternate port, the easiest way to do this is with a telnet client. `telnet localhost 1079` will connect to the local host on port 1079, where a finger server will eventually be listening.

### The Reactor

You don’t call Twisted, Twisted calls you. The reactor is Twisted’s main event loop, similar to the main loop in other toolkits available in Python (Qt, wx, and Gtk). There is exactly one reactor in any running Twisted application. Once started it loops over and over again, responding to network events and making scheduled calls to code.

Note that there are actually several different reactors to choose from; `from twisted.internet import reactor` returns the current reactor. If you haven’t chosen a reactor class yet, it automatically chooses the default. See the Reactor Basics HOWTO for more information.

## Do Nothing

`finger02.py`

    from twisted.internet import protocol, reactor

    class FingerProtocol(protocol.Protocol):
        pass

    class FingerFactory(protocol.ServerFactory):
        protocol = FingerProtocol

    reactor.listenTCP(1079, FingerFactory())
    reactor.run()

Here, `reactor.listenTCP` opens port 1079. (The number 1079 is a reminder that eventually we want to run on port 79, the standard port for finger servers.) The specified factory, `FingerFactory` , is used to handle incoming requests on that port. Specifically, for each request, the reactor calls the factory’s `buildProtocol` method, which in this case causes `FingerProtocol` to be instantiated. Since the protocol defined here does not actually respond to any events, connections to 1079 will be accepted, but the input ignored.

A Factory is the proper place for data that you want to make available to the protocol instances, since the protocol instances are garbage collected when the connection is closed.

## Drop Connections

`finger03.py`

    from twisted.internet import protocol, reactor

    class FingerProtocol(protocol.Protocol):
        def connectionMade(self):
            self.transport.loseConnection()

    class FingerFactory(protocol.ServerFactory):
        protocol = FingerProtocol

    reactor.listenTCP(1079, FingerFactory())
    reactor.run()

Here we add to the protocol the ability to respond to the event of beginning a connection — by terminating it. Perhaps not an interesting behavior, but it is already close to behaving according to the letter of the standard finger protocol. After all, there is no requirement to send any data to the remote connection in the standard. The only problem, as far as the standard is concerned, is that we terminate the connection too soon. A client which is slow enough will see his `send()` of the username result in an error.

## Read Username, Drop Connections

`finger04.py`

    from twisted.internet import protocol, reactor
    from twisted.protocols import basic

    class FingerProtocol(basic.LineReceiver):
        def lineReceived(self, user):
            self.transport.loseConnection()

    class FingerFactory(protocol.ServerFactory):
        protocol = FingerProtocol

    reactor.listenTCP(1079, FingerFactory())
    reactor.run()

Here we make `FingerProtocol` inherit from LineReceiver , so that we get data-based events on a line-by-line basis. We respond to the event of receiving the line with shutting down the connection.

If you use a telnet client to interact with this server, the result will look something like this:

    $ telnet localhost 1079
    Trying 127.0.0.1...
    Connected to localhost.localdomain.
    alice
    Connection closed by foreign host.

Congratulations, this is the first standard-compliant version of the code. However, usually people actually expect some data about users to be transmitted.

## Read Username, Output Error, Drop Connections

`finger05.py`

    from twisted.internet import protocol, reactor
    from twisted.protocols import basic

    class FingerProtocol(basic.LineReceiver):
        def lineReceived(self, user):
            self.transport.write("No such user\r\n")
            self.transport.loseConnection()

    class FingerFactory(protocol.ServerFactory):
        protocol = FingerProtocol

    reactor.listenTCP(1079, FingerFactory())
    reactor.run()

Finally, a useful version. Granted, the usefulness is somewhat limited by the fact that this version only prints out a “No such user” message. It could be used for devastating effect in honey-pots (decoy servers), of course.

## Output From Empty Factory

`finger06.py`

    # Read username, output from empty factory, drop connections

    from twisted.internet import protocol, reactor
    from twisted.protocols import basic

    class FingerProtocol(basic.LineReceiver):
        def lineReceived(self, user):
            self.transport.write(self.factory.getUser(user)+"\r\n")
            self.transport.loseConnection()

    class FingerFactory(protocol.ServerFactory):
        protocol = FingerProtocol

        def getUser(self, user):
            return "No such user"

    reactor.listenTCP(1079, FingerFactory())
    reactor.run()

The same behavior, but finally we see what usefulness the factory has: as something that does not get constructed for every connection, it can be in charge of the user database. In particular, we won’t have to change the protocol if the user database back-end changes.

## Output from Non-empty Factory

`finger07.py`

    # Read username, output from non-empty factory, drop connections

    from twisted.internet import protocol, reactor
    from twisted.protocols import basic

    class FingerProtocol(basic.LineReceiver):
        def lineReceived(self, user):
            self.transport.write(self.factory.getUser(user)+"\r\n")
            self.transport.loseConnection()

    class FingerFactory(protocol.ServerFactory):
        protocol = FingerProtocol

        def __init__(self, **kwargs):
            self.users = kwargs

        def getUser(self, user):
            return self.users.get(user, "No such user")

    reactor.listenTCP(1079, FingerFactory(moshez='Happy and well'))
    reactor.run()

Finally, a really useful finger database. While it does not supply information about logged in users, it could be used to distribute things like office locations and internal office numbers. As hinted above, the factory is in charge of keeping the user database: note that the protocol instance has not changed. This is starting to look good: we really won’t have to keep tweaking our protocol.

## Use Deferreds

`finger08.py`

    # Read username, output from non-empty factory, drop connections
    # Use deferreds, to minimize synchronicity assumptions

    from twisted.internet import protocol, reactor, defer
    from twisted.protocols import basic

    class FingerProtocol(basic.LineReceiver):
        def lineReceived(self, user):
            d = self.factory.getUser(user)

            def onError(err):
                return 'Internal error in server'
            d.addErrback(onError)

            def writeResponse(message):
                self.transport.write(message + '\r\n')
                self.transport.loseConnection()
            d.addCallback(writeResponse)

    class FingerFactory(protocol.ServerFactory):
        protocol = FingerProtocol

        def __init__(self, **kwargs):
            self.users = kwargs

        def getUser(self, user):
            return defer.succeed(self.users.get(user, "No such user"))

    reactor.listenTCP(1079, FingerFactory(moshez='Happy and well'))
    reactor.run()

But, here we tweak it just for the hell of it. Yes, while the previous version worked, it did assume the result of getUser is always immediately available. But what if instead of an in-memory database, we would have to fetch the result from a remote Oracle server? By allowing getUser to return a Deferred, we make it easier for the data to be retrieved asynchronously so that the CPU can be used for other tasks in the meanwhile.

As described in the Deferred HOWTO , Deferreds allow a program to be driven by events. For instance, if one task in a program is waiting on data, rather than have the CPU (and the program!) idly waiting for that data (a process normally called ‘blocking’), the program can perform other operations in the meantime, and waits for some signal that data is ready to be processed before returning to that process.

In brief, the code in `FingerFactory` above creates a Deferred, to which we start to attach callbacks . The deferred action in `FingerFactory` is actually a fast-running expression consisting of one dictionary method, `get` . Since this action can execute without delay, `FingerFactory.getUser` uses `defer.succeed` to create a Deferred which already has a result, meaning its return value will be passed immediately to the first callback function, which turns out to be `FingerProtocol.writeResponse` . We’ve also defined an errback (appropriately named `FingerProtocol.onError` ) that will be called instead of `writeResponse` if something goes wrong.

## Run ‘finger’ Locally

`finger09.py`

    # Read username, output from factory interfacing to OS, drop connections

    from twisted.internet import protocol, reactor, defer, utils
    from twisted.protocols import basic

    class FingerProtocol(basic.LineReceiver):
        def lineReceived(self, user):
            d = self.factory.getUser(user)

            def onError(err):
                return 'Internal error in server'
            d.addErrback(onError)

            def writeResponse(message):
                self.transport.write(message + '\r\n')
                self.transport.loseConnection()
            d.addCallback(writeResponse)

    class FingerFactory(protocol.ServerFactory):
        protocol = FingerProtocol

        def getUser(self, user):
            return utils.getProcessOutput("finger", [user])

    reactor.listenTCP(1079, FingerFactory())
    reactor.run()

This example also makes use of a Deferred. `twisted.internet.utils.getProcessOutput` is a non-blocking version of Python’s `commands.getoutput` : it runs a shell command (`finger` , in this case) and captures its standard output. However, `getProcessOutput` returns a Deferred instead of the output itself. Since `FingerProtocol.lineReceived` is already expecting a Deferred to be returned by `getUser` , it doesn’t need to be changed, and it returns the standard output as the finger result.

Note that in this case the shell’s built-in `finger` command is simply run with whatever arguments it is given. This is probably insecure, so you probably don’t want a real server to do this without a lot more validation of the user input. This will do exactly what the standard version of the finger server does.

## Read Status from the Web

The web. That invention which has infiltrated homes around the world finally gets through to our invention. In this case we use the built-in Twisted web client via `twisted.web.client.getPage` , a non-blocking version of Python’s `urllib2.urlopen(URL).read` . Like `getProcessOutput` it returns a Deferred which will be called back with a string, and can thus be used as a drop-in replacement.

Thus, we have examples of three different database back-ends, none of which change the protocol class. In fact, we will not have to change the protocol again until the end of this tutorial: we have achieved, here, one truly usable class.

`finger10.py`

    # Read username, output from factory interfacing to web, drop connections

    from twisted.internet import protocol, reactor, defer, utils
    from twisted.protocols import basic
    from twisted.web import client

    class FingerProtocol(basic.LineReceiver):
        def lineReceived(self, user):
            d = self.factory.getUser(user)

            def onError(err):
                return 'Internal error in server'
            d.addErrback(onError)

            def writeResponse(message):
                self.transport.write(message + '\r\n')
                self.transport.loseConnection()
            d.addCallback(writeResponse)

    class FingerFactory(protocol.ServerFactory):
        protocol = FingerProtocol

        def __init__(self, prefix):
            self.prefix=prefix

        def getUser(self, user):
            return client.getPage(self.prefix+user)

    reactor.listenTCP(1079, FingerFactory(prefix='http://livejournal.com/~'))
    reactor.run()

## Use Application

Up until now, we faked. We kept using port 1079, because really, who wants to run a finger server with root privileges? Well, the common solution is “privilege shedding” : after binding to the network, become a different, less privileged user. We could have done it ourselves, but Twisted has a built-in way to do it. We will create a snippet as above, but now we will define an application object. That object will have `uid` and `gid` attributes. When running it (later we will see how) it will bind to ports, shed privileges and then run.

Read on to find out how to run this code using the twistd utility.

## twistd

This is how to run “Twisted Applications” — files which define an ‘application’. A daemon is expected to adhere to certain behavioral standards so that standard tools can stop/start/query them. If a Twisted application is run via twistd, the TWISTed Daemonizer, all this behavioral stuff will be handled for you. twistd does everything a daemon can be expected to — shuts down stdin/stdout/stderr, disconnects from the terminal and can even change runtime directory, or even the root filesystems. In short, it does everything so the Twisted application developer can concentrate on writing his networking code.

    root% twistd -ny finger11.tac # just like before
    root% twistd -y finger11.tac # daemonize, keep pid in twistd.pid
    root% twistd -y finger11.tac --pidfile=finger.pid
    root% twistd -y finger11.tac --rundir=/
    root% twistd -y finger11.tac --chroot=/var
    root% twistd -y finger11.tac -l /var/log/finger.log
    root% twistd -y finger11.tac --syslog # just log to syslog
    root% twistd -y finger11.tac --syslog --prefix=twistedfinger # use given prefix

There are several ways to tell twistd where your application is; here we show how it is done using the `application` global variable in a Python source file (a Twisted Application Configuration file).

`finger11.tac`

    # Read username, output from non-empty factory, drop connections
    # Use deferreds, to minimize synchronicity assumptions
    # Write application. Save in 'finger.tpy'

    from twisted.application import internet, service
    from twisted.internet import protocol, reactor, defer
    from twisted.protocols import basic

    class FingerProtocol(basic.LineReceiver):
        def lineReceived(self, user):
            d = self.factory.getUser(user)

            def onError(err):
                return 'Internal error in server'
            d.addErrback(onError)

            def writeResponse(message):
                self.transport.write(message + '\r\n')
                self.transport.loseConnection()
            d.addCallback(writeResponse)

    class FingerFactory(protocol.ServerFactory):
        protocol = FingerProtocol

        def __init__(self, **kwargs):
            self.users = kwargs

        def getUser(self, user):
            return defer.succeed(self.users.get(user, "No such user"))

    application = service.Application('finger', uid=1, gid=1)
    factory = FingerFactory(moshez='Happy and well')
    internet.TCPServer(79, factory).setServiceParent(
        service.IServiceCollection(application))

Instead of using `reactor.listenTCP` as in the above examples, here we are using its application-aware counterpart, `internet.TCPServer` . Notice that when it is instantiated, the application object itself does not reference either the protocol or the factory. Any services (such as TCPServer) which have the application as their parent will be started when the application is started by twistd. The application object is more useful for returning an object that supports the IService , IServiceCollection , IProcess , and sob.IPersistable interfaces with the given parameters; we’ll be seeing these in the next part of the tutorial. As the parent of the TCPServer we opened, the application lets us manage the TCPServer.

With the daemon running on the standard finger port, you can test it with the standard finger command: `finger moshez` .