In part 1 of this series, we discussed what TTL is, and provided a few exercises to help strengthen your understanding of TTL. We'll pick up where we left off with our same DNS zone for domain1.com:
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.
As we discussed, the NS records will expire 1 day after they were last cached, the MX records will expire 4 hours after they were last cached, and the A record will expire 60 minutes after it was last cached. What does this mean for you and your planned migration?
A common misconception about TTL is that it's the amount of time the Name Server requires to update the record. This is absolutely not the case. Do you remember what RFC 1034 said a TTL is? A TTL is:
...primarily used by resolvers when they cache RRs. The TTL describes how long a RR can be cached before it should be* discarded.
Where in there does it say that a TTL is how long a name server takes to update? What does it say? It says that a DNS resolver (which is anything that resolves DNS, your computer has a DNS resolver in it) caches the records, and the TTL describes how long the DNS resolver should cache the record before discarding it (and then re-requesting it).
What this means is that (for the sake of simplicity) every computer connected to the internet has it's own local cache of websites it's visited, mail servers it's talked to, etc. and will not re-request those records until it's own, local, cache expires. TTL is how long it should* keep the records until it discards them and re-requests them.
Object lesson time...
UserPC1.ValuedReader.com visits www.TheEmailAuthority.com at 2:30 PM. www.TheEmailAuthority.com has a 1 hour TTL for it's A record. If I move the website at 3 PM, and the old IP stops serving the site, how long would it be until UserPC1.ValuedReader.com can read my site again?
UserPC-2.ValuedReader.com visits www.TheEmailAuthority.com at 3:10 PM How long will it be until they can read the site?
The answers and explanations will be in part three.
Friday, December 7, 2012
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...
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...
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