Packet Structure
The mDNS Ethernet frame is a multicast UDP packet to:
- MAC address
01:00:5E:00:00:FB - IPv4 address
224.0.0.251or IPv6 addressFF02::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
0for a query and1for 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
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