From HEXONET Wiki
xDNS (extended DNS) - X-ALIAS-<RR> (Alias xDNS record sets)
The following three X-ALIAS-<RR> Records are supported:
You can use any of them to have all respective records of the reference zone copied over to your zone. Any changes to the respective records of the reference zone will be constantly monitored and your zone will be automatically updated accordingly if there are any changes. No additional lookups are made when your zone is queried so this is the high performance solution if you want to manage only one dedicated authoritative DNS zone but utilize the respective A, AAAA or TXT records for as many other DNS zones as needed.
X-ALIAS-<RR> Records have the following syntax:
yourdomain.com. <TTL> IN X-ALIAS-<RR> <REFERENCE-DNS-ZONE>
PLEASE NOTE: The secluding dot after the domain name (www.yourdomain.com.) is required in any case!
yourdomain.com. 3600 IN X-ALIAS-A zenfolio.webapp.ispapi.net.
Result: All A records of the DNS zone zenfolio.webapp.ispapi.net. will be copied to the DNS zone yourdomain.com. and the root domain will resolve to the same A records as zenfolio.webapp.ispapi.net. does.
subdomain.yourdomain.com. 3600 IN X-ALIAS-AAAA <REFERENCE-DNS-ZONE>
Result: All AAAA records of the reference DNS zone will be copied to your DNS zone and your subdomain will resolve to the same AAAA records as the reference domain.
yourdomain.com. 3600 IN X-ALIAS-TXT <REFERENCE-DNS-ZONE>
Result: All TXT records of the reference DNS zone will be copied to your DNS zone and your root domain will return the respective TXT records as the reference domain does.