in-addr.arpa default search domains

Question

Andreas_CF OP

Created May ’24

Replies 1

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Hi,

I observed some unexpected behavior and hope that someone can enlighten me as to what this is about:

mDNSResponder prepends IP / network based default search domains that are checked before any other search domain. E.g. 0.1.168.192.in-addr.arpa. would be used for an interface with an address in the the 192.168.1.0/24 subnet. This is done for any configured non-link-local IP address.

I tried to find any mention of an approach like this in RFCs but couldn't spot anything.

Please note that this is indeed a search domain and different from reverse-DNS lookups.

Example output of tcpdump for ping devtest:

10:02:13.850802 IP (tos 0x0, ttl 64, id 43461, offset 0, flags [none], proto UDP (17), length 92)
    192.168.1.2.52319 > 192.168.1.1.53: 54890+ [1au] A? devtest.0.1.168.192.in-addr.arpa. (64)

I was able to identify the code that adds those default IP subnet based search domains but failed to spot any indication as to what this is about: https://github.com/apple-oss-distributions/mDNSResponder/blob/d5029b5/mDNSMacOSX/mDNSMacOSX.c#L4171-L4211

Does anyone here have an ideas as to what this might be about?

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Answer 1

Kentzo OP

Apr ’25

This is mentioned in RFC 6763 Section 11:

The <domain> part of the query name may also be derived a different way, from the host's IP address. The host takes its IP address and calculates the logical AND of that address and its subnet mask, to derive the 'base' address of the subnet (the 'network address' of that subnet, or, equivalently, the IP address of the 'all-zero' host address on that subnet). It then constructs the conventional DNS "reverse mapping" name corresponding to that base address, and uses that as the <domain> part of the name for the queries described above. For example, if a host has the address 192.168.12.34, with the subnet mask 255.255.0.0, then the 'base' address of the subnet is 192.168.0.0, and to discover the recommended automatic browsing domain(s) for devices on this subnet, the host issues a DNS PTR query for the name "lb._dns-sd._udp.0.0.168.192.in-addr.arpa."

I suppose mDNSResponder (stub DNS resolver on macOS) uses this approach not just for DNS-SD but in general.

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