TXT records in dnsPython

Question

I implemented a simple DNS server. It just responds with a TXT record. I accept the script as an NS server for example.com. NS Server - xyzk This works fine when I output something like:

$ dig demo.example.com @x.y.z.k

; <<>> DiG 9.3.6-P1-RedHat-9.3.6-4.P1.el5_4.1 <<>> demo.example.com @x.y.z.k
;; global options:  printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 10028
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 2, ADDITIONAL: 2

;; QUESTION SECTION:
;demo.example.com. IN A

;; ANSWER SECTION:
demo.example.com. 1000 IN TXT "test message"

but this will not work when I ask the same question with one of the public name servers (for example, Sun DNS in 4.2.2.1):

$ dig demo.example.com @4.2.2.1(I get the same when I give out $ dig demo.example.com @4.2.2.1 TXT)

; <<>> DiG 9.3.6-P1-RedHat-9.3.6-4.P1.el5_4.1 <<>> demo.example.com
;; global options:  printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: SERVFAIL, id: 10905
;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 0

;; QUESTION SECTION:
;demo.example.com. IN A

;; Query time: 5837 msec
;; SERVER: 172.16.1.1#53(172.16.1.1)
;; WHEN: Tue Jul 20 04:01:27 2010
;; MSG SIZE  rcvd: 75

Do you guys know what's wrong? Interestingly, if I change the response type to something like CNAME, not TXT, it works fine.

def dns_respond(resp, q, data_type):
    rrset = dns.rrset.from_text(q.name, 1000,
           dns.rdataclass.IN, dns.rdatatype.TXT, 'test message')
    resp.answer.append(rrset)
    return resp

def dns_ok(resp, q, data = None, msg = ''):
    return dns_respond(resp = resp, q = q, data_type = 'OK')

def dns_error(resp, q):
    return dns_respond(resp = resp, q = q, data_type = 'Error')

def requestHandler(address, message):
    resp = None
    message_id = ord(message[0]) * 256 + ord(message[1])
    logging.debug('msg id = ' + str(message_id))
    if message_id in serving_ids:
        # the request is already taken, drop this message
        logging.debug('I am already serving this request.')
        return
    serving_ids.append(message_id)
    msg = dns.message.from_wire(message)
    op = msg.opcode()
    if op == 0:
        # standard and inverse query
        qs = msg.question
        if len(qs) > 0:
            q = qs[0]
            logging.debug('request is ' + str(q))
            if q.rdtype == dns.rdatatype.A:
                resp = std_qry(msg)
            else:
                # not implemented
                #resp = std_qry(msg)
                resp = make_response(qry=msg, RCODE=4)   
    else:
        # not implemented
        resp = make_response(qry=msg, RCODE=4)   

    if resp:
        s.sendto(resp.to_wire(), address)

def std_qry(msg):
    qs = msg.question
    logging.debug(str(len(qs)) + ' questions.')

    answers = []
    nxdomain = False
    for q in qs:
        resp = make_response(qry=msg)
        logging.critical('Sending...')
        return dns_ok(resp, q)

def make_response(qry=None, id=None, RCODE=0):
    if qry is None and id is None:
        raise Exception, 'bad use of make_response'
    if qry is None:
        resp = dns.message.Message(id)
        # QR = 1
        resp.flags |= dns.flags.QR
        if RCODE != 1:
            raise Exception, 'bad use of make_response'
    else:
        resp = dns.message.make_response(qry)
    #resp.flags |= dns.flags.AA
    resp.flags |= dns.flags.RA
    resp.set_rcode(RCODE)
    return resp
s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
s.bind(('', 53))
logging.debug('binded to UDP port 53.')
serving_ids = []

while True:
    logging.debug('waiting requests.')
    message, address = s.recvfrom(1024)
    logging.debug('serving a request.')
    requestHandler(address, message)

Answer 1

I see some real errors that will stop this:

• You return the record TXTto the record request A- this is probably the show stopper.
• AA ( ) RA
• , ,
• ,

, , , :

• NS SOA ,
• ( IP-, , queryID), (queryID)
• , , NXDOMAIN (rcode = 3)
• , , NOERROR (rcode = 0) NOTIMPL
• , REFUSED (rcode = 5)