BitTorrent Client : Getting Peer List From Tracker

Question

I am trying to get the peer list from the trackers using https://wiki.theory.org/BitTorrent_Tracker_Protocol

But most of the torrent files available on the net use a udp tracker. The above website is for HTTP trackers. The torrent files with HTTP based trackers give an error.(Passkey error , or hash error or user error or 404 not found response)

Right now my request code for all HTTP trackers look like :

payload = urllib.urlencode(payload)
        for ann in self.torrent.announce_list:
            if("udp" not in ann[0]):
                url = ann[0]
                print "Encoded Request URL :",(url + "?" + payload)
                print "Requesting Tracker for Peer List"
                try:
                    response = urllib.urlopen(url + "?" + payload).read()
                    print "RESPONSE :",response
                except Exception, e:
                    print e

I have crosschecked my request URL against the requests being made by uTorrent using wireshark. They all match. The info-hash is being generated correctly.

How do I retrieve the peer list from UDP as well as HTTP trackers ? I am trying not to use any external library like lib-torrent. Some guidance would be really helpful.

Answer 1

The following code worked for me. Calling announce_udp() with the tracker URL and the payload dictionary with all the URL parameeters.

import binascii, urllib, socket, random, struct
from bcode import bdecode
from urlparse import urlparse, urlunsplit

def announce_udp(tracker,payload):
    tracker = tracker.lower()
    parsed = urlparse(tracker)

    # Teporarly Change udp:// to http:// to get hostname and portnumbe
    url = parsed.geturl()[3:]
    url = "http" + url
    hostname = urlparse(url).hostname
    port = urlparse(url).port


    sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
    sock.settimeout(8)
    conn = (socket.gethostbyname(hostname), port)
    #sock.bind((socket.gethostname(),s_port))

    #Get connection ID
    req, transaction_id = udp_create_connection_request()
    sock.sendto(req, conn);
    buf = sock.recvfrom(2048)[0]
    connection_id = udp_parse_connection_response(buf, transaction_id)

    #Annoucing
    s_port = sock.getsockname()[1] #get port number to which socket is connected
    req, transaction_id = udp_create_announce_request(connection_id, payload,s_port)
    sock.sendto(req, conn)
    print "Announce Request Sent"
    buf = sock.recvfrom(2048)[0]
    print "Response received"
    return udp_parse_announce_response(buf, transaction_id)

def udp_create_announce_request(connection_id, payload, s_port):
    action = 0x1 #action (1 = announce)
    transaction_id = udp_get_transaction_id()
    # print "2.Transaction ID :", transaction_id
    buf = struct.pack("!q", connection_id)                                  #first 8 bytes is connection id
    buf += struct.pack("!i", action)                                        #next 4 bytes is action 
    buf += struct.pack("!i", transaction_id)                                #followed by 4 byte transaction id
    buf += struct.pack("!20s", urllib.unquote(payload['info_hash']))        #the info hash of the torrent we announce ourselves in
    buf += struct.pack("!20s", urllib.unquote(payload['peer_id']))          #the peer_id we announce
    buf += struct.pack("!q", int(urllib.unquote(payload['downloaded'])))    #number of bytes downloaded
    buf += struct.pack("!q", int(urllib.unquote(payload['left'])))          #number of bytes left
    buf += struct.pack("!q", int(urllib.unquote(payload['uploaded'])))      #number of bytes uploaded
    buf += struct.pack("!i", 0x2)                                           #event 2 denotes start of downloading
    buf += struct.pack("!i", 0x0)                                           #IP address set to 0. Response received to the sender of this packet
    key = udp_get_transaction_id()                                          #Unique key randomized by client
    buf += struct.pack("!i", key)
    buf += struct.pack("!i", -1)                                            #Number of peers required. Set to -1 for default
    buf += struct.pack("!i", s_port)                                        #port on which response will be sent
    return (buf, transaction_id)

def udp_parse_announce_response(buf, sent_transaction_id):
    #print "Response is:"+str(buf)  
    if len(buf) < 20:
        raise RuntimeError("Wrong response length while announcing: %s" % len(buf)) 
    action = struct.unpack_from("!i", buf)[0] #first 4 bytes is action
    res_transaction_id = struct.unpack_from("!i", buf, 4)[0] #next 4 bytes is transaction id    
    if res_transaction_id != sent_transaction_id:
        raise RuntimeError("Transaction ID doesnt match in announce response! Expected %s, got %s"
            % (sent_transaction_id, res_transaction_id))
    print "Reading Response"
    if action == 0x1:
        print "Action is 3"
        ret = dict()
        offset = 8; #next 4 bytes after action is transaction_id, so data doesnt start till byte 8      
        ret['interval'] = struct.unpack_from("!i", buf, offset)[0]
        print "Interval:"+str(ret['interval'])
        offset += 4
        ret['leeches'] = struct.unpack_from("!i", buf, offset)[0]
        print "Leeches:"+str(ret['leeches'])
        offset += 4
        ret['seeds'] = struct.unpack_from("!i", buf, offset)[0]
        print "Seeds:"+str(ret['seeds'])
        offset += 4
        peers = list()
        x = 0
        while offset != len(buf):
            peers.append(dict())
            peers[x]['IP'] = struct.unpack_from("!i",buf,offset)[0]
            print "IP: "+socket.inet_ntoa(struct.pack("!i",peers[x]['IP']))
            offset += 4
            if offset >= len(buf):
                raise RuntimeError("Error while reading peer port")
            peers[x]['port'] = struct.unpack_from("!H",buf,offset)[0]
            print "Port: "+str(peers[x]['port'])
            offset += 2
            x += 1
        return ret,peers
    else:
        #an error occured, try and extract the error string
        error = struct.unpack_from("!s", buf, 8)
        print "Action="+str(action)
        raise RuntimeError("Error while annoucing: %s" % error)

def udp_create_connection_request():
    connection_id = 0x41727101980                   #default connection id
    action = 0x0                                    #action (0 = give me a new connection id)   
    transaction_id = udp_get_transaction_id()
    print "1.Transaction ID :", transaction_id
    buf = struct.pack("!q", connection_id)          #first 8 bytes is connection id
    buf += struct.pack("!i", action)                #next 4 bytes is action
    buf += struct.pack("!i", transaction_id)        #next 4 bytes is transaction id
    return (buf, transaction_id)

def udp_parse_connection_response(buf, sent_transaction_id):
    if len(buf) < 16:
        raise RuntimeError("Wrong response length getting connection id: %s" % len(buf))            
    action = struct.unpack_from("!i", buf)[0] #first 4 bytes is action

    res_transaction_id = struct.unpack_from("!i", buf, 4)[0] #next 4 bytes is transaction id
    if res_transaction_id != sent_transaction_id:
        raise RuntimeError("Transaction ID doesnt match in connection response! Expected %s, got %s"
            % (sent_transaction_id, res_transaction_id))

    if action == 0x0:
        connection_id = struct.unpack_from("!q", buf, 8)[0] #unpack 8 bytes from byte 8, should be the connection_id
        return connection_id
    elif action == 0x3:     
        error = struct.unpack_from("!s", buf, 8)
        raise RuntimeError("Error while trying to get a connection response: %s" % error)
    pass

def udp_get_transaction_id():
    return int(random.randrange(0, 255))