Bonjour is an Apple-designed technology that enables computers and devices located on the same network to learn about services offered by other computers and devices. It is designed such that any Bonjour-aware device can be plugged into a TCP/IP network and it will pick an IP address and make other computers on that network aware of the services it offers. Bonjour is sometimes referred to as Rendezvous, Zero Configuration, or Zeroconf.
Zero Configuration Networking, such as Bonjour provides:
Must be able to obtain an IP Address (even without a DHCP server)
Must be able to do name-to-address translation (even without a DNS server)
Must be able to discover services on the network
The device will get an IP address in the range 169.254/16 and will check if any other device is using that IP address. If not, it will keep the IP address. Macs keeps an entry in their routing table for this subnet: netstat -rn | grep 169
For DNS the Multicast DNS (mDNS) protocol is used. mDNSservices listen in port 5353/UDP, use regular DNS queries and use the multicast address 18.104.22.168 instead of sending the request just to an IP address. Any machine listening these request will respond, usually to a multicast address, so all the devices can update their tables.
Each device will select its own name when accessing the network, the device will choose a name ended in .local (might be based on the hostname or a completely random one).
For discovering services DNS Service Discovery (DNS-SD) is used.
The final requirement of Zero Configuration Networking is met by DNS Service Discovery (DNS-SD). DNS Service Discovery uses the syntax from DNS SRV records, but uses DNS PTR records so that multiple results can be returned if more than one host offers a particular service. A client requests the PTR lookup for the name <Service>.<Domain> and receives a list of zero or more PTR records of the form <Instance>.<Service>.<Domain>.
The dns-sd binary can be used to advertise services and perform lookups for services:
#Search ssh services
dns-sd -B _ssh._tcp
Browsing for _ssh._tcp
DATE: ---Tue 27 Jul 2021---
Timestamp A/R Flags if Domain Service Type Instance Name