Acquire::http::ProxyAutoDetect "/usr/share/squid-deb-proxy-client/apt-avahi-discover";

apt-avahi-discover.py

#!/usr/bin/python
#
# use avahi to find a _apt_proxy._tcp provider and return
# a http proxy string suitable for apt

import asyncore
import functools
import os
import socket
import sys
import time
from subprocess	import Popen, PIPE, call

DEFAULT_CONNECT_TIMEOUT_SEC = 2

def DEBUG(msg):
    if "--debug" in sys.argv:
        sys.stderr.write(msg + "\n")


def get_avahi_discover_timeout():
    APT_AVAHI_TIMEOUT_VAR = "APT::Avahi-Discover::Timeout"
    p = Popen(
        ["/usr/bin/apt-config", "shell", "TIMEOUT", APT_AVAHI_TIMEOUT_VAR], 
        stdout=PIPE)
    stdout, stderr = p.communicate()
    if not stdout:
        DEBUG(
            "no timeout set, using default '%s'" % DEFAULT_CONNECT_TIMEOUT_SEC)
        return DEFAULT_CONNECT_TIMEOUT_SEC
    if not stdout.startswith("TIMEOUT="):
        raise ValueError("got unexpected apt-config output: '%s'" % stdout)
    varname, sep, value = stdout.strip().partition("=")
    timeout = int(value.strip("'"))
    DEBUG("using timeout: '%s'" % timeout)
    return timeout

@functools.total_ordering
class AptAvahiClient(asyncore.dispatcher):
    def __init__(self, addr):
        asyncore.dispatcher.__init__(self)
        if is_ipv6(addr[0]):
            self.create_socket(socket.AF_INET6, socket.SOCK_STREAM)
            self.connect( (addr[0], addr[1], 0, 0) )
        else:
            self.create_socket(socket.AF_INET, socket.SOCK_STREAM)
            self.connect(addr)
        self._time_init = time.time()
        self.time_to_connect = sys.maxint
        self.address = addr
    def handle_connect(self):
        self.time_to_connect = time.time() - self._time_init
        self.close()
    def __eq__(self, other):
        return self.time_to_connect == other.time_to_connect
    def __lt__(self, other):
        return self.time_to_connect < other.time_to_connect
    def __repr__(self):
        return "<%s> %s: %s" % (
            self.__class__.__name__, self.addr, self.time_to_connect)


def is_ipv6(a):
    return ':' in a

def is_linklocal(addr):
    # Link-local should start with fe80 and six null bytes
    return addr.startswith("fe80::")

def get_proxy_host_port_from_avahi():
    service = '_apt_proxy._tcp'

    # Obtain all of the services addresses from avahi, pulling the IPv6
    # addresses to the top.
    addr4 = []
    addr6 = []
    p = Popen(['avahi-browse', '-kprtf', service], stdout=PIPE)
    DEBUG("avahi-browse output:")
    for line in p.stdout:
        DEBUG(" '%s'" % line)
        if line.startswith('='):
            tokens = line.split(';')
            addr = tokens[7]
            port = int(tokens[8])
            if is_ipv6(addr):
                # We need to skip ipv6 link-local addresses since 
                # APT can't use them
                if not is_linklocal(addr):
                    addr6.append((addr, port))
            else:
                addr4.append((addr, port))

    # Run through the offered addresses and see if we we have a bound local
    # address for it.
    addrs = []
    for (ip, port) in addr6 + addr4:
        try:
            res = socket.getaddrinfo(ip, port, 0, 0, 0, socket.AI_ADDRCONFIG)
            if res:
                addrs.append((ip, port))
        except socket.gaierror:
            pass
    if not addrs:
        return None
    
    # sort by answering speed
    hosts = []
    for addr in addrs:
        hosts.append(AptAvahiClient(addr))
    # 2s timeout, arbitray
    timeout = get_avahi_discover_timeout()
    asyncore.loop(timeout=timeout)
    DEBUG("sorted hosts: '%s'" % sorted(hosts))

    # No host wanted to connect
    if (all(h.time_to_connect == sys.maxint for h in hosts)):
        return None

    fastest_host = sorted(hosts)[0]
    fastest_address = fastest_host.address
    return fastest_address


if __name__ == "__main__":
    # Dump the approved address out in an appropriate format.
    address = get_proxy_host_port_from_avahi()
    if address:
        (ip, port) = address
        if is_ipv6(ip):
            print "http://[%s]:%s/" % (ip, port)
        else:
            print "http://%s:%s/" % (ip, port)