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# Creating a custom server

The builtin DNS server plugin is useful, but the beauty of Twisted Names is that you can build your own custom servers and clients using the names components.

> - In this section you will learn about the components required to build a simple DNS server.
> - You will then learn how to create a custom DNS server which calculates responses dynamically.

## A simple forwarding DNS server

Lets start by creating a simple forwarding DNS server, which forwards all requests to an upstream server (or servers).

`simple_server.py`

    # Copyright (c) Twisted Matrix Laboratories.
    # See LICENSE for details.

    """
    An example of a simple non-authoritative DNS server.
    """

    from twisted.internet import reactor
    from twisted.names import client, dns, server

    def main():
        """
        Run the server.
        """
        factory = server.DNSServerFactory(
            clients=[client.Resolver(resolv='/etc/resolv.conf')]
        )

        protocol = dns.DNSDatagramProtocol(controller=factory)

        reactor.listenUDP(10053, protocol)
        reactor.listenTCP(10053, factory)

        reactor.run()

    if __name__ == '__main__':
        raise SystemExit(main())

In this example we are passing a client.Resolver instance to the DNSServerFactory and we are configuring that client to use the upstream DNS servers which are specified in a local `resolv.conf` file.

Also note that we start the server listening on both UDP and TCP ports. This is a standard requirement for DNS servers.

You can test the server using `dig`. For example:

    $ dig -p 10053 @127.0.0.1 example.com SOA +short
    sns.dns.icann.org. noc.dns.icann.org. 2013102791 7200 3600 1209600 3600

## A server which computes responses dynamically

Now suppose we want to create a bespoke DNS server which responds to certain hostname queries by dynamically calculating the resulting IP address, while passing all other queries to another DNS server. Queries for hostnames matching the pattern workstation{0-9}+ will result in an IP address where the last octet matches the workstation number.

We’ll write a custom resolver which we insert before the standard client resolver. The custom resolver will be queried first.

Here’s the code:

`override_server.py`

    # Copyright (c) Twisted Matrix Laboratories.
    # See LICENSE for details.

    """
    An example demonstrating how to create a custom DNS server.

    The server will calculate the responses to A queries where the name begins with
    the word "workstation".

    Other queries will be handled by a fallback resolver.

    eg
        python doc/names/howto/listings/names/override_server.py

        $ dig -p 10053 @localhost workstation1.example.com A +short
        172.0.2.1
    """

    from twisted.internet import reactor, defer
    from twisted.names import client, dns, error, server

    class DynamicResolver(object):
        """
        A resolver which calculates the answers to certain queries based on the
        query type and name.
        """
        _pattern = 'workstation'
        _network = '172.0.2'

        def _dynamicResponseRequired(self, query):
            """
            Check the query to determine if a dynamic response is required.
            """
            if query.type == dns.A:
                labels = query.name.name.split('.')
                if labels[0].startswith(self._pattern):
                    return True

            return False

        def _doDynamicResponse(self, query):
            """
            Calculate the response to a query.
            """
            name = query.name.name
            labels = name.split('.')
            parts = labels[0].split(self._pattern)
            lastOctet = int(parts[1])
            answer = dns.RRHeader(
                name=name,
                payload=dns.Record_A(address=b'%s.%s' % (self._network, lastOctet)))
            answers = [answer]
            authority = []
            additional = []
            return answers, authority, additional

        def query(self, query, timeout=None):
            """
            Check if the query should be answered dynamically, otherwise dispatch to
            the fallback resolver.
            """
            if self._dynamicResponseRequired(query):
                return defer.succeed(self._doDynamicResponse(query))
            else:
                return defer.fail(error.DomainError())

    def main():
        """
        Run the server.
        """
        factory = server.DNSServerFactory(
            clients=[DynamicResolver(), client.Resolver(resolv='/etc/resolv.conf')]
        )

        protocol = dns.DNSDatagramProtocol(controller=factory)

        reactor.listenUDP(10053, protocol)
        reactor.listenTCP(10053, factory)

        reactor.run()

    if __name__ == '__main__':
        raise SystemExit(main())

Notice that `DynamicResolver.query` returns a Deferred. On success, it returns three lists of DNS records (answers, authority, additional), which will be encoded by dns.Message and returned to the client. On failure, it returns a DomainError, which is a signal that the query should be dispatched to the next client resolver in the list.

Note

The fallback behaviour is actually handled by ResolverChain.

ResolverChain is a proxy for other resolvers. It takes a list of IResolver providers and queries each one in turn until it receives an answer, or until the list is exhausted.

Each IResolver in the chain may return a deferred DomainError, which is a signal that ResolverChain should query the next chained resolver.

The DNSServerFactory constructor takes a list of authoritative resolvers, caches and client resolvers and ensures that they are added to the ResolverChain in the correct order.

Let’s use `dig` to see how this server responds to requests that match the pattern we specified:

    $ dig -p 10053 @127.0.0.1 workstation1.example.com A +short
    172.0.2.1

    $ dig -p 10053 @127.0.0.1 workstation100.example.com A +short
    172.0.2.100

And if we issue a request that doesn’t match the pattern:

    $ dig -p 10053 @localhost www.example.com A +short
    93.184.216.119

## Further Reading

For simplicity, the examples above use the `reactor.listenXXX` APIs. But your application will be more flexible if you use the Twisted Application APIs, along with the Twisted plugin system and `twistd`. Read the source code of names.tap to see how the `twistd names` plugin works.