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Suppose I acquired both a.example and b.example two domains. The goal is when I config somepage.a.example to point at an IP address, somepage.b.example can also resolve to this address. That means any lookup to *.b.example will be the same to *.a.example
As I consider, maybe some DNS provider offers something like automatic DNS mirror? or is there a way to use *.a.example for *.b.example DNS resolve?
You can use the slightly uncommon RFC 6672 DNAME DNS record which provides redirection for a subtree of the domain name tree in the DNS. That is, all names that end with a particular suffix are redirected to another part of the DNS.
A DNS record:
example.com. IN DNAME example.net.
will redirect:
a.example.com. --> a.example.net.
b.example.com. --> b.example.net.
c.d.example.com. --> c.d.example.net.
etc.
You will only need to manually maintain every record for example.com. that exists at the same level as the DNAME record:
example.com. IN SOA ns1.example.com. username.example.com. (
20140218131405 ; Serial number YYYYMMDDHHMMSS
28800 ; Refresh 8 hours
7200 ; Retry 2 hours
604800 ; Expire 7 days
86400 ; Minimum TTL 1 day )
example.com. IN NS ns1.example.net. ; ns1.example.net is a primary nameserver
example.com. IN NS ns2.example.net. ; ns2.example.net is also a nameserver
example.com. IN MX 10 mail.example.net. ; mail.example.net is the mailserver
example.com. IN MX 20 mail2.example.net. ; the secondary mailserver
example.com. IN A 192.0.2.1 ; IPv4 address for the bare domain
example.com. IN AAAA 2001:db8:10::1 ; IPv6 address for the bare domain
example.com. IN DNAME example.net. ; every record <something>.example.com will
; get mapped to <the same something>.example.net.
Please note that the RFC lists some issues, which may of import to you too.
Also, I'm not sure if you can have exceptions in the example.com zone file for records which should not get mapped to the corresponding example.net record, i.e. if the presence of a record such as old-www.example.com. IN A 192.0.2.2 will continue to work and will be preferred to the DNAME mapping of old-www.example.com. --> old-www.example.net.
.CAT TLD heavily used them... before stopping altogether. See datatracker.ietf.org/meeting/97/materials/… for example.
Feb 13, 2019 at 17:03
I don't think a DNS mirror service exists because it has a limited use case however, with amazon route 53, you can write a powershell script to create domain records for all your zones. Check out this script which will create a cname for all your hosted zones. You could use this as a template where it inputs one A records and then creates that record for both domains.
There's no reason why two different domain names can't point to the same IP address.
Conceptually, just configure the dns server for b.example to have all the same data as the server for a.example. If it was a small domain and you were manually updating dns data files, it would be simple to add the same records to two different files.
Whether it can be done automatically depends on the details - are the same dns servers being used for both domains? who's running the servers? Assuming it's the same provider, it could be very easy to do, but only if the provider supports it.
