Standards/Ethernet/Bonjour

Revision as of 14:51, 5 June 2012

Bonjour is Apple's protocol for service discovery. It is lightweight and works over UDP. Bonjour implements a Multicast Domain Name System (mDNS) service without requiring a conventional DNS server.

Bonjour comprises:

• host discovery: finding the IP addresses of .local devices
• service discovery: listing the capabilities of the devices

Bonjour is implemented in Linux systems under the name Avahi. There is also a Windows implementation of Bonjour.

Multicast frames

The mDNS frames are sent to the multicast address 01:00:5E:00:00:FB / 224.0.0.251, from and to port 5353.

Bonjour frames are filtered in Wireshark with the help of the expression:

ip.version==4 and udp.port==5353

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

Host discovery

Query

Typing in a terminal:

ping fpga1.diploma.local

causes the computer to send the following mDNS query:

01 00 5e 00 00 fb 00 23  32 b1 21 52 08 00 45 00
00 41 e6 9d 00 00 ff 11  00 00 99 6d 05 a0 e0 00
00 fb 14 e9 14 e9 00 2d  80 47 00 00 00 00 00 01
00 00 00 00 00 00 05 66  70 67 61 31 07 64 69 70
6c 6f 6d 61 05 6c 6f 63  61 6c 00 00 01 00 01 

This frame contains Ethernet header:

01 00 5e 00 00 fb 00 23  32 b1 21 52 08 00

IP header:

                                           45 00
00 41 e6 9d 00 00 ff 11  00 00 99 6d 05 a0 e0 00
00 fb

UDP header:

      14 e9 14 e9 00 2d  80 47

and mDNS payload:

                               00 00 00 00 00 01
00 00 00 00 00 00 05 66  70 67 61 31 07 64 69 70
6c 6f 6d 61 05 6c 6f 63  61 6c 00 00 01 00 01

The payload says:

• Transaction Id: 0000
• Flags: 0000 (standard query)
• Questions: 0001
• Answer RRs: 0000
• Authority RRs: 0000
• Additional RRs: 0000
• Query:
  • 05 bytes fpga1
  • 07 bytes diploma
  • 05 bytes local
• QTYPE: 0001 (host address)
• QCLASS: 0000

Answer

To this, the selected device should respond: