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djoproject
Asked: 2020-10-13 13:03:12 +0800 CST

How to make reverse mDNS through IPv6 on OSX?

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I am looking for a way to make reverse mDNS query on OSX over IPv6 only. I am not interested by solution using IPv4.

The following works on Linux with dig 9.16.1:

dig +short @ff02::fb -p 5353 -x 2001:db8::1

I've installed dig 9.16.7 with homebrew on OSX, that command with the exact same arguments produces a timeout. And there is no sign of the outgoing query on Wireshark.

The following works well on both Linux and OSX:

dig +short @224.0.0.251 -p 5353 -x 192.168.1.23

The binary avahi-resolve-address is not available on OSX and I didn't find a way to make a reverse DNS works with dns-sd.

Here is the operating system version used:

  • OSX version: Catalina 10.15.7
  • Linux: Ubuntu 20.04
ipv6 mdns multicast

1 Answers

  1. Best Answer

    I found a couple of ways to do this, but they're both a bit ugly. First, you can use dns-sd -q and specify the ptr record type... but you need to convert the address you want to look up into nibble-by-nibble reverse format by yourself. For example, to look up fe80::4e6:97e7:d892:d977 you'd use:

    dns-sd -q 7.7.9.d.2.9.8.d.7.e.7.9.6.e.4.0.0.0.0.0.0.0.0.0.0.0.0.0.0.8.e.f.ip6.arpa ptr

    Unfortunately, this only seems to work for link-local addresses, not public addresses. I have no idea why. Also, note that dns-sd doesn't exit automatically after doing the lookup, but keeps monitoring for changes. You need to kill it after a bit.

    You can also use dig, somewhat like your original command, but you need to tell it which interface to send the query out over, and in a rather weird format. The "standard" thing to do would be to add a zone index for the interface, e.g. to send it to ff02::fb over en1 you'd use @ff02::fb%en1. But dig doesn't like that format, so you have to be sneakier. BSD-based unixes, including macOS, allow you to put a numeric zone index in the second 16-bit word of some kinds of addresses, and that does work here. Here's a (anonymized) example where I use netstat -rn -f inet6 to find out that 2001:db8:1234:5678::/64 is on interface en1, that interface's link number is 5, and then use that to do a lookup (I added ^^^s to emphasize the relevant parts):

    % netstat -rn -f inet6
Routing tables

Internet6:
Destination                             Gateway                         Flags         Netif Expire
[...]
2001:db8:1234:5678::/64                 link#5                          UC              en1       
                                        ^^^^^^                                          ^^^
[...]
% dig +short @ff02:5::fb -p 5353 -x 2001:db8:1234:5678:109f:8688:b2d1:3f12
                   ^
Gordons-MacBook.local.
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