Dots in a hostname

Dns, Networking

Yesterday I had a very interesting discussion with our senior administrator.

I was configuring reverse DNS records for our /24 block and I decided to use format - IP.static.domain.com thus if for IP 1.2.3.4, I pointed reverse DNS (PTR) to 1.2.3.4.static.domain.com

When I got chance to show my work to my senior administrator, he said - It’s wrong to use 1.2.3.4.static.domain.com in a hostname. Too many dots will make DNS resolution very slow (forward - reverse - again forward). And I should have used 1-2-3-4.static.domain.com

A very interesting point (and indeed a confusion!)

At first instance I was totally stunned myself and was thinking same as he said. He is senior admin and knows over a million more things then me and such feelings usually pushes you in case when you believe in someone without checking on facts.

(Disclaimer: No offence to my senior admin. He is a very smart person, and a good friend too).

After a while, I gave it a DEEP thought and discussed with a couple of more friends. I got clear idea about it. 

Let’s understand how 1.2.3.4.static.domain.com will be resolved.Here, first of all dns resolver will query com (root) servers for getting NS delegation of domain.com. Now as domain.com will reply with some NS (let’s call them ns1 & ns2). Next, DNS resolver will query ns1 for 1.2.3.4.static.domain.com and done - it will get back reply with A record - 1.2.3.4 (game over!).  :)

Same thing happens even if we use 1-2-3-4.static.domain.com So what “could have” caused slowdown?

There can be a significant slowdown IF for say in 1.2.3.4.static.domain.com

4.static.domain.com acts as a separate DNS zone rather then acting as nothing (when we create A record for 1.2.3.4.static.domain.com). If 4.static.domain.com has separate NS, and assume 3.4.static.domain.com is a further sub zone defined by some other NS …the one which are different then parent nameservers. But in our case 4.static.domain.com or 3.4.static.domain.com or 2.3.4.static.domain.com is just nothing. The only thing was 1.2.3.4.static.domain.com which was simply an A record on parent domain name.

Hence 1.2.3.4.static.domain.com is same as 1-2-3-4.static.domain.com and I assume it’s rather an issue of convention admins prefer rather then any technical reason behind it.