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Comment by sigjuice | Hacker News Reader

Resolving a .local domain name works like this.

  $ dig -p 5353 raspberrypi.local @224.0.0.251
   
  ; <<>> DiG 9.8.3-P1 <<>> -p 5353 raspberrypi.local @224.0.0.251
  ;; global options: +cmd
  ;; Got answer:
  ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 21640
  ;; flags: qr aa; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0
   
  ;; QUESTION SECTION:
  ;raspberrypi.local.		IN	A
   
  ;; ANSWER SECTION:
  raspberrypi.local.	10	IN	A	192.168.1.175

A DNS query packet is to the multicast address 224.0.0.251 and UDP port 5353. All devices on your network that are listening on that address will see the query (e.g. Linux systems with avahi-daemon or Macs running mDNSResponder). The device whose name is raspberryi will respond. RFC6762 should have all the details.

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This is the best explanation of how mDNS works that I've ever seen. Thank you!

mDNS is cool and all, but I wish they would've picked a slightly-more-specific TLD. ".local" is the natural suffix for LANs, and making computers confused about whether they should resolve that via multicast or standard DNS techniques wasn't very nice. Something like ".mdns" or ".zeroconf" would've been a lot better.

mDNS is roughly the equivalent of going door-to-door in the LAN and asking anybody if they've heard of "Greg", so it I don't think it is that confusing that it gets .local, especially given the circumstances that lead to relying on mDNS in that most LANs don't have authoritative DNS servers.

It's not that the name doesn't make sense, it's just that it collides with a very common TLD for LANs. It shouldn't be squatting on a name humans want to use. Computers are unlikely to do the correct thing when you ping a .local on a network that uses that suffix, because they have to decide how to handle the potential name collision (which is still somewhat frequent, more than 15 years later).

It's the other way around, though? .local's usage on LANs prior to the RFC was squatting on a non-reserved TLD. The RFC finally reserved the TLD for an appropriate usage. The name applies equally to both usages, the usages can live side-by-side, and the RFC is even kind enough to support that (though as you point out frequently don't always work as expected), even though it didn't need to take into account backwards compatibility with "squatters".

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