TTL in DNS part 1

TTL, or Time To Live, is a confusing subject for many people. Due to DNS's distributed nature, there are many different scenarios to consider. In this post we will explain what TTL is, and what it is not, using a real world example.

First, I suppose we'll answer why TTL is important. TTL can impact your email infrastructure any time you make changes to a Name Server that hosts your DNS, the A record that your mail server resolves to (for instance, when you change to a new hosting provider), or when you change your MX records (for a new hosting provider, or a new mail relay). By no means is this a comprehensive list, but it will suffice for this example.  Since DNS records can be cached for days sometimes, it's important to understand how they work when planning a migration so you can account for several day's worth of traffic.

How it works:
According to RFC 1034, "DOMAIN NAMES - CONCEPTS AND FACILITIES", each DNS record (or "Resource Record", "RR" for short) has a TTL or Time To Live.   It describes TTL as:

TTL which is the time to live of the RR. This field is a 32 bit integer in units of seconds, an is primarily used by resolvers when they cache RRs. The TTL describes how long a RR can be cached before it should be* discarded. 

A "Resolver" is anything that resolves a DNS record. This could be your ISP's name servers, or your local computer. Anything that queries and resolves DNS records is a "DNS Resolver".

Let's take the below zone into consideration:

domain1.com. 86400 IN NS ns1.domain1s-ISP.com. 
domain1.com. 86400 IN NS ns2.domain1s-ISP.com. 

domain1.com. 14400 IN MX mail.domain1.com. 

mail.domain1.com. 3600  IN A  10.11.12.13. 

So, let's take our three examples above and do some "real world" exercises.  Remember, TTL is expressed in seconds.  You might want to open your calculator app now ;-)

If domain1.com wanted to switch their Name Server records to a different ISP, how many days would it be until the NS TTL expired?

If domain1.com wanted to add a hosted anti-spam service (which would require them to change the MX records), how many hours would it take for the MX TTL to expire?

If domain1.com wanted to upgrade their mail server and point the A record to a new IP, how many minutes would it take for the A record's TTL to expire?

Ok, so we've gotten through the math portion of this post.  Pencils on the table please. Now we know that records can take DAYS to update depending on their TTL.  How does this impact our migration?  I'll answer in part two...