In the past (Blasco has mentioned this before) we said to include the zone-c of the parent zone, since he/she maintains the MX records. Sounds reasonable! Not at all: by the same token one could argue that the person maintaining a top level zone file should be registered as the zone-c for every primary subdomain, since (s)he maintains the NS records [in the top level zone file], just like in the case of MX-only domains (s)he maintains the MX records [in the top level zone file].
Well, this is an example of what I presently do with the few domains under "no" that are registered in the RIPE database (and which are of the "MX-only" variant): domain: fdata.no descr: Fellesdata A/S, Oslo admin-c: Geir Engebakken tech-c: Geir Engebakken zone-c: Uninett Hostmaster remarks: MX-only, maintained in "no" zone changed: Havard.Eidnes@runit.sintef.no 931124 source: RIPE
I've always just copied the admin-c into the zone-c in these cases, for the simple reason that there is no such thing as a zone for an MX-only domain and the admin contact can be held equally well responsible for the MX records, even though (s)he doesn't put/maintain them in the next higher zone file, as is the case with NS records.
Well, in case of DNS trouble (misconfiguration or similar lossage), the "zone-c" would in this case point in the wrong direction, no? Maybe the definition of "zone-c" should be refined to something like The zone contact for the zone where the authoritative data for the domain is registered. Note that in the case of a fully delegated domain, the NS records in the parent zone are actually not authoritative.
I would suggest to consider the presence of the *zc attribute mandatory only if *ns attributes are present in the domain entry.
Maybe worth discussing? (My initial reaction was "yes, this sounds great!", but after thinking about it for a while and reformulating the response to the previous paragraph, I'm not so sure anymore.) - Havard