2

Let's say we have name server A who serves for the domain name example.com and its subdomains.

Then we have another name server B who would want to avail the custom-subdomain.example.com from name server A.

The purpose of the name server B is to serve A-records for the domain name custom-subdomain.example.com and its subdomains ex: www.custom-subdomain.example.com.

In other words, server A is the parent zone while server B is the child zone.

With the following setup, who needs to serve the SOA-record for the child zone? The parent zone (A) or the child zone (B) itself?

6

Every server that considers itself authoritative for a zone will have a SOA record for that zone. As such, the server for the child zone will serve its own SOA record.

  • If Server A considers itself authoritative for example.com, it will have a SOA record for example.com.

  • If Server B has been configured with an authoritative zone named custom-subdomain.example.com, that zone will contain a SOA record called custom-subdomain.example.com. It is not concerned with the SOA record for example.com that lives on Server A. It only considers itself authoritative for custom-subdomain.example.com.

Authoritative servers do not concern themselves with the records in the parent zone or communicate with its nameservers. That said, there is usually some administrative relevance to data in the parent zone that could have negative impacts on resolution for the child zone. (NS or glue record A/AAAA mismatches between the parent and child zone, broken chains of trust for DNSSEC signed zones, etc.)

For further reading, you might want to take a look at RFC 2181 § 6 and its subsections. These cover the topic of zone cuts and the authority relationship between parent and child zones.

  • Andrew, I have edited the question based on your first statement. I hope it poses a better argument now. – Abel Melquiades Callejo Dec 4 '18 at 1:04
  • @Abel That works. I've tweaked the beginning of the answer to match. – Andrew B Dec 4 '18 at 1:14

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