TCP DNS server test case

tcp_dns_client.py

# Sends a TCP DNS request in two packets.
# First sends the message size, then sends the message.
# This behavior is similar to that of the Windows nslookup command.
import socket, binascii, sys, struct

def main(dns_server):
    s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    # disable Nagle's algorithm, to allow request to be sent in two pieces.
    s.setsockopt(socket.IPPROTO_TCP, socket.TCP_NODELAY, True)
    s.connect((dns_server, 53))
    # an A record lookup of eecs340.com, captured with wireshark:
    dns_request_hex = "4ba901000001000000000000076565637333343003636f6d0000010001"
    size_bytes = struct.pack(">H", len(dns_request_hex)//2)
    s.send(size_bytes) 
    s.send(binascii.unhexlify(dns_request_hex))
    
    # get response
    response = b'';
    while 1:
        rcv_data = s.recv(2048)
        if not rcv_data: break
        #print("received data of size ", len(rcv_data))
        #print("got bytes:", binascii.hexlify(rcv_data))
        response += rcv_data
        # check size header
        if len(response) > 2:
            size, = struct.unpack(">H", response[0:2])
            # check that full response was received
            if len(response) >= size + 2:  # plus two is for the length header
                break
    print("binary response is: ", binascii.hexlify(response))
    print("ASCII response is: ", response)


if __name__ == "__main__":
    if len(sys.argv) < 2:
        print("please pass the dns server hostname/IP as a parameter")
        sys.exit(-1)
    dns_server = sys.argv[1]
    main(dns_server)

Fixed to work on Python 2