How forward and reverse DNS works

Question

What I understand about how DNS works is like this: first let's assume mydomain.com has the IP address 12.34.56.78. Now when I put the url mydomain.com in the browser, the browser sends a dns lookup to its local dns server, asking, hey, do you know the ip address for mydomain.com. If the local dns server does not know about it, it will ask the parent dns servers, if the parent also does not know, then it keeps asking all the way up until the root dns server. The root dns server will ask some server in charge of the .com tld. The dns server in charge of the .com will have knowledge about mydomain.com because mydomain.com is the .com family. Then the answer will be returned back to the initial asker. Also the answer quite likely will be cached in the dns servers involved in the asking process. Would anyone correct my understanding if it is wrong.

So my real question is about how reverse dns lookup works. Let's say if I want to find out what domain name is for the ip 12.34.56.78. I run the command dig -x 12.34.56.78. If my local dns server does not know the answer, which server does it further ask? Is it 12.in-addr.arpa., or 34.12.in-addr.arpa.? If this is the case, are these names like 12.in-addr.arpa. valid domain names? And where should they be deployed so that the reverse lookup requests will know whom to ask?

Answer 1

How a reverse DNS lookup is accomplished:

• The DNS resolver reverses the IP, and adds it to ".in-addr.arpa" (or ".ip6.arpa" for IPv6 lookups), turning 192.0.2.25 into 25.2.0.192.in-addr.arpa.
• The DNS resolver then looks up the PTR record for 25.2.0.192.in-addr.arpa.
  • The DNS resolver asks the root servers for the PTR record for 25.2.0.192.in-addr.arpa.
  • The root servers refer the DNS resolver to the DNS servers in charge of the Class A range (192.in-addr.arpa, which covers all IPs that begin with 192).
  • In almost all cases, the root servers will refer the DNS resolver to a "RIR" ("Regional Internet Registry"). These are the organizations that allocate IPs. In general, ARIN handles North American IPs, APNIC handles Asian-Pacific IPs, and RIPE handles European IPs.
  • The DNS resolver will ask the ARIN DNS servers for the PTR record for 25.2.0.192.in-addr.arpa.
  • The ARIN DNS servers will refer the DNS resolver to the DNS servers of the organization that was originally given the IP range. These are usually the DNS servers of your ISP, or their bandwidth provider.
  • The DNS resolver will ask the ISP's DNS servers for the PTR record for 25.2.0.192.in-addr.arpa.
  • The ISP's DNS servers will refer the DNS resolver to the organization's DNS servers.
  • The DNS resolver will ask the organization's DNS servers for the PTR record for 25.2.0.192.in-addr.arpa.
  • The organization's DNS servers will respond with "host.example.com".

Source here.