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Author(s) Olivier Bonaventure
Deadline No deadline
Submission limit No limitation
Category tags DNS

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DNS and the UDP port numbers


The UDP port numbers

DNS messages are carried inside UDP segments. Could you predict the source and destination port numbers of the DNS response knowing the DNS request ?

# Length Summary Status
0 41 bytes Domain Name System (query)
1 69 bytes Domain Name System (response)  

0000  f696003500294709  16eb012000010000  
0010  0000000003777777  076578616d706c65  
0020  036f726700001c00  01                

0000  ö..5.)G..ë. ....
0010  .....www.example
0020  .org.....       
  • User Datagram Protocol
    • Source Port: 63126
      • Destination Port: 53
        • Length: 41
          • Checksum: 0x4709 [unverified]
          • Domain Name System (query)
            • Transaction ID: 0x16eb
              • Flags: 0x0120 Standard query
                • 0... .... .... .... = Response: Message is a query
                  • .000 0... .... .... = Opcode: Standard query (0)
                    • .... ..0. .... .... = Truncated: Message is not truncated
                      • .... ...1 .... .... = Recursion desired: Do query recursively
                        • .... .... .0.. .... = Z: reserved (0)
                          • .... .... ..1. .... = AD bit: Set
                            • .... .... ...0 .... = Non-authenticated data: Unacceptable
                            • Questions: 1
                              • Answer RRs: 0
                                • Authority RRs: 0
                                  • Additional RRs: 0
                                    • Queries
                                      • www.example.org: type AAAA, class IN
                                        • Name: www.example.org
                                          • Name Length: 15
                                            • Label Count: 3
                                              • Type: AAAA (IPv6 Address) (28)
                                                • Class: IN (0x0001)
                                            0000  ????????0045a528  16eb81a000010001  
                                            0010  0000000003777777  076578616d706c65  
                                            0020  036f726700001c00  01c00c001c000100  
                                            0030  0147d10010260628  0002200001024818  
                                            0040  9325c81946                          

                                            0000  ????.E¥(.ë......
                                            0010  .....www.example
                                            0020  .org.....À......
                                            0030  .GÑ..&.(.. ...H.
                                            0040  .%È.F           
                                            • User Datagram Protocol
                                              • Source Port: ?
                                              • Destination Port: ?
                                              • Length: 69
                                                • Checksum: 0xa528 [unverified]
                                                • Domain Name System (response)
                                                  • Transaction ID: 0x16eb
                                                    • Flags: 0x81a0 Standard query response, No error
                                                      • 1... .... .... .... = Response: Message is a response
                                                        • .000 0... .... .... = Opcode: Standard query (0)
                                                          • .... .0.. .... .... = Authoritative: Server is not an authority for domain
                                                            • .... ..0. .... .... = Truncated: Message is not truncated
                                                              • .... ...1 .... .... = Recursion desired: Do query recursively
                                                                • .... .... 1... .... = Recursion available: Server can do recursive queries
                                                                  • .... .... .0.. .... = Z: reserved (0)
                                                                    • .... .... ..1. .... = Answer authenticated: Answer/authority portion was authenticated by the server
                                                                      • .... .... ...0 .... = Non-authenticated data: Unacceptable
                                                                        • .... .... .... 0000 = Reply code: No error (0)
                                                                        • Questions: 1
                                                                          • Answer RRs: 1
                                                                            • Authority RRs: 0
                                                                              • Additional RRs: 0
                                                                                • Queries
                                                                                  • www.example.org: type AAAA, class IN
                                                                                    • Name: www.example.org
                                                                                      • Name Length: 15
                                                                                        • Label Count: 3
                                                                                          • Type: AAAA (IPv6 Address) (28)
                                                                                            • Class: IN (0x0001)
                                                                                          • Answers
                                                                                            • www.example.org: type AAAA, class IN, addr 2606:2800:220:1:248:1893:25c8:1946
                                                                                              • Name: www.example.org
                                                                                                • Type: AAAA (IPv6 Address) (28)
                                                                                                  • Class: IN (0x0001)
                                                                                                    • Time to live: 83921
                                                                                                      • Data length: 16
                                                                                                        • AAAA Address: 2606:2800:220:1:248:1893:25c8:1946