Multicast DNS - Packet Structure

Packet Structure

The mDNS Ethernet frame is a multicast UDP packet to:

  • MAC address 01:00:5E:00:00:FB
  • IPv4 address 224.0.0.251 or IPv6 address FF02::FB
  • UDP port 5353

Its structure is based on the DNS packet format. It is built out of 2 parts: header and data.

offset (bytes) 0 1
0 ID = 0x0000
2 Flags
4 QDCOUNT
6 ANCOUNT
8 NSCOUNT
10 ARCOUNT
12
Data

The flags are all 0 except for:

  • the first bit, Query/Response (QR), which is 0 for a query and 1 for a response
  • the sixth bit, Authoritative Answer (AR), which is equal to the bit QR

The data is the UTF-8 list of the parts of the name preceded by the length of the item. In a question, the name is terminated with a 0x00 and followed by QTYPE which is 0x0001 for a host address, and QCLASS which is 0x0001 for Internet.

Read more about this topic:  Multicast DNS

Famous quotes containing the words packet and/or structure:

    “The captain was a duck
    With a packet on his back,
    And when the ship began to move
    The captain said, Quack! Quack!”
    —Mother Goose (fl. 17th–18th century. I saw a ship a-sailing (l. 13–16)

    “Women over fifty already form one of the largest groups in the population structure of the western world. As long as they like themselves, they will not be an oppressed minority. In order to like themselves they must reject trivialization by others of who and what they are. A grown woman should not have to masquerade as a girl in order to remain in the land of the living.”
    —Germaine Greer (b. 1939)