Simple Python SSDP discovery

ssdp.py

import socket
import struct
import sys
from httplib import HTTPResponse
from BaseHTTPServer import BaseHTTPRequestHandler
from StringIO import StringIO

import gtk
import gobject

LIB_ID = 'my_library'
MCAST_GRP = '239.255.255.250'
MCAST_PORT = 1900
SERVICE_LOCS = {'id1': '127.0.0.1:7766', 'id2': '127.0.0.1:7766'}

DISCOVERY_MSG = ('M-SEARCH * HTTP/1.1\r\n' +
                 'ST: %(library)s:%(service)s\r\n' +
                 'MX: 3\r\n' +
                 'MAN: "ssdp:discover"\r\n' +
                 'HOST: 239.255.255.250:1900\r\n\r\n')

LOCATION_MSG = ('HTTP/1.1 200 OK\r\n' +
                'ST: %(library)s:%(service)s\r\n'
                'USN: %(service)s\r\n'
                'Location: %(loc)s\r\n'
                'Cache-Control: max-age=900\r\n\r\n')


class Request(BaseHTTPRequestHandler):
    def __init__(self, request_text):
        self.rfile = StringIO(request_text)
        self.raw_requestline = self.rfile.readline()
        self.error_code = self.error_message = None
        self.parse_request()

    def send_error(self, code, message):
        self.error_code = code
        self.error_message = message


class Response(HTTPResponse):
    def __init__(self, response_text):
        self.fp = StringIO(response_text)
        self.debuglevel = 0
        self.strict = 0
        self.msg = None
        self._method = None
        self.begin()


def interface_addresses(family=socket.AF_INET):
    for fam, _, _, _, sockaddr in socket.getaddrinfo('', None):
        if family == fam:
            yield sockaddr[0]


def client(timeout=1, retries=5):
    socket.setdefaulttimeout(timeout)

    for _ in xrange(retries):
        for addr in interface_addresses():
            sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM, socket.IPPROTO_UDP)
            sock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
            sock.setsockopt(socket.IPPROTO_IP, socket.IP_MULTICAST_TTL, 2)
            sock.bind((addr, 0))

            msg = DISCOVERY_MSG % dict(service='id1', library=LIB_ID)
            for _ in xrange(2):
                # sending it more than once will
                # decrease the probability of a timeout
                sock.sendto(msg, (MCAST_GRP, MCAST_PORT))

            try:
                data = sock.recv(1024)
            except socket.timeout:
                pass
            else:
                response = Response(data)
                print response.getheader('Location')
                return


def server(timeout=5):

    socket.setdefaulttimeout(timeout)

    sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM, socket.IPPROTO_UDP)
    sock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
    sock.setsockopt(socket.IPPROTO_IP, socket.IP_MULTICAST_TTL, 2)
    sock.bind(('', MCAST_PORT))

    mreq = struct.pack('4sl', socket.inet_aton(MCAST_GRP), socket.INADDR_ANY)
    sock.setsockopt(socket.IPPROTO_IP, socket.IP_ADD_MEMBERSHIP, mreq)

    cond = gobject.IO_IN | gobject.IO_HUP
    gobject.io_add_watch(sock, cond, handle_requests)

    gtk.main()


def handle_requests(sock, _):

    data, addr = sock.recvfrom(4096)
    request = Request(data)
    if not request.error_code and \
            request.command == 'M-SEARCH' and \
            request.path == '*' and \
            request.headers['ST'].startswith(LIB_ID) and \
            request.headers['MAN'] == '"ssdp:discover"':

        service = request.headers['ST'].split(':', 2)[1]
        if service in SERVICE_LOCS:
            loc = SERVICE_LOCS[service]
            msg = LOCATION_MSG % dict(service=service, loc=loc, library=LIB_ID)
            sock.sendto(msg, addr)

    return True


if __name__ == '__main__':
    if len(sys.argv) > 1 and 'client' in sys.argv[1]:
        client()
    else:
        server()

I'm trying to adapt this code to write a simple SSDP client and I've run into a couple of issues.

The line socket.getaddrinfo('', None) throws the exception socket.gaierror: [Errno 8] nodename nor servname provided, or not known. It looks like getaddrinfo isn't happy with passing in a blank string as the hostname.

So I changed it to socket.getaddrinfo('localhost', None) and it happily returns "127.0.0.1" which allows the client to carry on.

The next issue I run into is this statement sock.sendto(msg, (MCAST_GRP, MCAST_PORT)) throws an exception socket.error: [Errno 51] Network is unreachable and this has me totally lost.