How to see what a bonjour service resolves to

Register with

dns-sd -R "myapp" _http._tcp . 8000  

Browse with

dns-sd -B _http._tcp  

Resolve with

dns-sd -L "myapp" _http._tcp  

Also check out the App Store product “Discover - DNS-SD Browser”
A quick alternative with less detailed information is:

dns-sd -B _services._dns-sd._udp  

How to find my MacBook’s bonjour name

hostname  
# => Lous-MacBook-Air.local  

How to see dns-sd packets on the wire

Start tcpdump:

  sudo tcpdump -i lo0 udp -vv -X -nn  
                      ^           ^  
                      |           |- Don't resolve hostnames or port names  
                      |-- dns uses UDP  

Register the service:

dns-sd -R "Joypad Service" _joypad local 5500  

Notice in tcpdump the destination address 224.0.0.251 - this is the multicast address.
Notice also the destination port 5353

sudo lsof -i -P | grep 5353  

Shows:

mDNSRespo   187 _mdnsresponder    7u  IPv4 0x4d1ba82cb627e84f      0t0    UDP *:5353  
mDNSRespo   187 _mdnsresponder    8u  IPv6 0x4d1ba82cb627952f      0t0    UDP *:5353  

Can query the responder directly for dns records with dig (update 2024-01-27, this no longer works):

dig @224.0.0.251 -p 5353 -t ptr _joypad._tcp.local  

Browse for Joypad:

dns-sd -B _joypad local  

Where on MacOS is the dns_sd.h header

As of Sonoma 14.2.1:

/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/dns_sd.h

or:

/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/dns_sd.h

From the email with subject: “UDP Broadcast uses random source ports” Aug 31:

The most robust implementations I've seen use the API provided by this  
header file:  

 "/usr/include/dns_sd.h"  

...  

The "dns_sd.h" API is pretty straightforward and gets the job done.  

...  

There is also a ruby gem named "dnssd" which compiles its C extension  
against "dns_sd.h". You can browse the source code for that on github.