Cannot set reverse DNS

@KevinM1:

I've selected picus.illuminati.com as my FQDN in my hosts file (I'm a Deus Ex fan :D ). Whenever I try to use that for both my A and AAAA DNS record entries, they just stay blank. When I use just the hostname, picus, they get saved properly.

When I try setting the reverse DNS, regardless of whether I try searching for just picus or picus.illuminati.com, I get an error saying there's no matching forward DNS entry.

So, I guess my question is if I need a domain name pointed at the nameservers for all of this to work, because I'm trying to setup fail2ban according to the guide, and gmail isn't accepting my test sendmail messages because there's no reverse DNS set.

From a long term perspective, I'm anticipating hosting several sites on my account, so I'd like to be able to send things like fail2ban notices to my email from a 'central' server account not tied to any particular site that I'm hosting. Is that possible without me needing to purchase another domain name to represent the server itself?

The Linode Manager requires that whatever name you wish to set for reverse DNS have a corresponding forward record for that address. So if your Linode's IP is 192.0.2.1 (this is just an example, that is not a usable IP address), and you wanted the reverse DNS to be picus.example.com, picus.example.com would need an A record for 192.0.2.1. The same rules apply for IPv6 (obviously with the required forward record being AAAA). If none of the sites you host represent you or your business (if you're running a business, however small or large), it's a good idea to purchase another domain to represent you, in case the sites you're hosting decide to go somewhere else for whatever reason. For example, Google uses 1e100.net for the DNS of their infrastructure (1*10^100 is a googol, which is what the founders wanted to call the company, but they misspelled it Google and the name stuck).

But can I only set the A/AAAA records for incoming domain names, then?

In other words, the quick start guide tells me to make a hostname and FQDN in my hosts file, which can be anything, and then set my DNS. But when I attempt to enter my made up FQDN in my A/AAAA records, they don't get set. It's a blank hostname followed by the IP address in the summary table.

Sorry, it's just that the guides are a little confusing on what I can actually set for my A/AAAA records.

@KevinM1:

But can I only set the A/AAAA records for incoming domain names, then?

In other words, the quick start guide tells me to make a hostname and FQDN in my hosts file, which can be anything, and then set my DNS. But when I attempt to enter my made up FQDN in my A/AAAA records, they don't get set. It's a blank hostname followed by the IP address in the summary table.

Sorry, it's just that the guides are a little confusing on what I can actually set for my A/AAAA records.

Right, you can only set A/AAAA records for domains you have control over. The hostname can be whatever you want (picus, bananas, combfish, etc), but the domain name really should be something you control, as that makes up the other half of the FQDN, which is just the hostname you picked plus the domain name. If you don't have a domain name you can use, you can skip specifying the FQDN in the /etc/hosts file, and just list the hostname. Later when you do have a domain name, you just add the FQDN to /etc/hosts. Some software will complain if it can't figure out the FQDN, but will continue to work. Until you have a domain that you control that you can set the DNS records for the FQDN to point to your Linode, and then set the reverse DNS to that name, your email tests are going to fail.

Okay, thanks for the clarification. It makes a lot more sense now. :)