MacOS Protocols

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Bonjour

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. mDNS services listen in port 5353/UDP, use regular DNS queries and use the multicast address 224.0.0.251 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---
12:23:20.361  ...STARTING...
Timestamp     A/R    Flags  if Domain               Service Type         Instance Name
12:23:20.362  Add        3   1 local.               _ssh._tcp.           M-C02C934RMD6R
12:23:20.362  Add        3  10 local.               _ssh._tcp.           M-C02C934RMD6R
12:23:20.362  Add        2  16 local.               _ssh._tcp.           M-C02C934RMD6R

#Announce HTTP service
dns-sd -R "Index" _http._tcp . 80 path=/index.html

#Search HTTP services
dns-sd -B _http._tcp

When a new service is started the new service mulitcasts its presence to everyone on the subnet. The listener didn’t have to ask; it just had to be listening.

You ca use this tool to see the offered services in your current local network.
Or you can write your own scripts in python with python-zeroconf:

from zeroconf import ServiceBrowser, Zeroconf


class MyListener:

    def remove_service(self, zeroconf, type, name):
        print("Service %s removed" % (name,))

    def add_service(self, zeroconf, type, name):
        info = zeroconf.get_service_info(type, name)
        print("Service %s added, service info: %s" % (name, info))


zeroconf = Zeroconf()
listener = MyListener()
browser = ServiceBrowser(zeroconf, "_http._tcp.local.", listener)
try:
    input("Press enter to exit...\n\n")
finally:
    zeroconf.close()

If you feel like Bonjour might be more secured disabled, you can do so with:

sudo launchctl unload -w /System/Library/LaunchDaemons/com.apple.mDNSResponder.plist

References