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:

    “we know our end
    A packet of worm-seed, a garden of spent tissues.”
    —Allen Tate (1899–1979)

    “The verbal poetical texture of Shakespeare is the greatest the world has known, and is immensely superior to the structure of his plays as plays. With Shakespeare it is the metaphor that is the thing, not the play.”
    —Vladimir Nabokov (1899–1977)