A simple DNS server instance written in Python

I had a wicked idea to build a DNS server on my Android tablet, because I had installed SL4A and Python's interpreter on the tablet, and I wanted to continue learning Python, so I decided to implement it in Python.

After searching on Google, the DNS implemented in Python didn't find the answer I was looking for, so I decided to implement it myself.

Nothing fancy is being implemented, just the ability to simply match and reply to A record query.

The implementation code is as follows:

'''
Created on 2012-10-15
@author: RobinTang
'''
import socketserver
import struct
# DNS Query
class SinDNSQuery:
    def __init__(self, data):
        i = 1
        self.name = ''
        while True:
            d = data[i]
            if d == 0:
                break;
            if d < 32:
                self.name = self.name + '.'
            else:
                self.name = self.name + chr(d)
            i = i + 1
        self.querybytes = data[0:i + 1]
        (self.type, self.classify) = struct.unpack('>HH', data[i + 1:i + 5])
        self.len = i + 5
    def getbytes(self):
        return self.querybytes + struct.pack('>HH', self.type, self.classify)
# DNS Answer RRS
# this class is also can be use as Authority RRS or Additional RRS 
class SinDNSAnswer:
    def __init__(self, ip):
        self.name = 49164
        self.type = 1
        self.classify = 1
        self.timetolive = 190
        self.datalength = 4
        self.ip = ip
    def getbytes(self):
        res = struct.pack('>HHHLH', self.name, self.type, self.classify, self.timetolive, self.datalength)
        s = self.ip.split('.')
        res = res + struct.pack('BBBB', int(s[0]), int(s[1]), int(s[2]), int(s[3]))
        return res
# DNS frame
# must initialized by a DNS query frame
class SinDNSFrame:
    def __init__(self, data):
        (self.id, self.flags, self.quests, self.answers, self.author, self.addition) = struct.unpack('>HHHHHH', data[0:12])
        self.query = SinDNSQuery(data[12:])
    def getname(self):
        return self.query.name
    def setip(self, ip):
        self.answer = SinDNSAnswer(ip)
        self.answers = 1
        self.flags = 33152
    def getbytes(self):
        res = struct.pack('>HHHHHH', self.id, self.flags, self.quests, self.answers, self.author, self.addition)
        res = res + self.query.getbytes()
        if self.answers != 0:
            res = res + self.answer.getbytes()
        return res
# A UDPHandler to handle DNS query
class SinDNSUDPHandler(socketserver.BaseRequestHandler):
    def handle(self):
        data = self.request[0].strip()
        dns = SinDNSFrame(data)
        socket = self.request[1]
        namemap = SinDNSServer.namemap
        if(dns.query.type==1):
            # If this is query a A record, then response it

            name = dns.getname();
            if namemap.__contains__(name):
                # If have record, response it
                dns.setip(namemap[name])
                socket.sendto(dns.getbytes(), self.client_address)
            elif namemap.__contains__('*'):
                # Response default address
                dns.setip(namemap['*'])
                socket.sendto(dns.getbytes(), self.client_address)
            else:
                # ignore it
                socket.sendto(data, self.client_address)
        else:
            # If this is not query a A record, ignore it
            socket.sendto(data, self.client_address)
# DNS Server
# It only support A record query
# user it, U can create a simple DNS server
class SinDNSServer:
    def __init__(self, port=53):
        SinDNSServer.namemap = {}
        self.port = port
    def addname(self, name, ip):
        SinDNSServer.namemap[name] = ip
    def start(self):
        HOST, PORT = "0.0.0.0", self.port
        server = socketserver.UDPServer((HOST, PORT), SinDNSUDPHandler)
        server.serve_forever()
# Now, test it
if __name__ == "__main__":
    sev = SinDNSServer()
    sev.addname('www.aa.com', '192.168.0.1')    # add a A record
    sev.addname('www.bb.com', '192.168.0.2')    # add a A record
    sev.addname('*', '0.0.0.0') # default address
    sev.start() # start DNS server
# Now, U can use "nslookup" command to test it
# Such as "nslookup www.aa.com"