Before we go into how to set up your r DNS, let’s have a brief discussion of how email receivers use it, at the other end, to identify who is sending them email.
NOTE: The below discussion is highly technical, and you can skip it if you like, and go straight to the section ‘How to Configure r DNS‘.
Then, you simply flip around the IP address at the dots. Of course, that is a lot of typing, which is why there are options that cut right to the chase: system_prompt$ dig -x .226 … Instead of flipping the IP address at the dots, we’ll do that at each hexadecimal digit of the uncompressed IPv6 address.
So, if you want to find out the r DNS for the IP address `.226`, you would simply turn it around on the dots (this is actually referred to as an “octet boundary”) and tack the `in-addr.arpa` at the end before issuing your lookup for a matching PTR record, like so: system_prompt$ dig ptr 226.2. So, in order to look up the name corresponding to the IPv6 address `2607:f8b02:c01::6a`, one would have to type in this command: system_prompt$ dig ptr \ a.184.108.40.206.0.0.0.0.0.0.0.0.0.0.0.1.0.c.0.2.0.0.4.0.b.8.f.220.127.116.11.ip6
This is true for businesses that send out their own email, as well. There has been a lot of discussion about the value — or lack of — checking r DNS as part of the checks that mailbox providers perform routinely.
Being pragmatic, your role is actually about getting your email to the inbox, so let’s focus on passing that test.
It is impossible to continuously test if this guide still works because I can’t possibly afford to buy a new Fire TV or Fire TV Stick every time Amazon updates the software version that their devices leave the factor with.
Now you can thank the developers of `dig` for adding the `-x` shortcut, thus shortening it to system_prompt$ dig -x 2607:f8b02:c01::6a A note about Time to Live (TTL) DNS is a worldwide, distributed and highly available database.
Its scalability — its ability to operate at the scale it does — depends on the extensive use of caching.
That is, client computers query “recursive caching name servers” that in turn query other servers until they are able to provide a suitable response.
At that point, they store a copy of such response, just in case the same query comes later.
For example, a TTL of 258835 seconds, or almost 3 days, would mean that if you were to change that PTR record, you would have to wait for as many as 3 days before the change would be visible everywhere on the Internet. For email deliverability purposes, you’ll want to make sure that your r DNS — or reverse resolution — matches the forward resolution.