[clue-tech] Fwd: proper setting for TTL used on MX records

[David L. Anselmi]

David L. Anselmi wrote:
> WFT Electronics, Gus Salvatore Calabrese wrote:
> 
>>
>> What is the proper value to set TTL to on a MX record ?
>> Do lower numbers impact other nameservers ?
> 
> It depends.

But seriously.

dnsreport.com says "RFC1912 2.2 recommends a value between 1200 to 43200 
seconds (20 minutes to 12 hours)".  I'd tend to go for the long side of 
that.  There's no reason your MX records should be different than other 
records.

The impact to other nameservers is small, unless you get misbehaved 
caches.  The impact to your nameservers is direct.  If you're worried 
about it, pick a number and measure your load.  Then half and double it, 
measure, and compare.  Then you'll know what you want.

Note that if your record is changing (which should be less frequent for 
an MX than an A), you want something short to minimize impact of the 
change.  Assuming your record never changes, pick something long.  Then 
1 TTL before changing it, set it to something short (minutes).  After 
the change, set it back to the long value.

If (e.g., dynamic IPs) things change frequently, you have to figure out 
how often they change and how much outage is acceptable and set things 
based on that.  (Outage time is more important than frequency of change.)

What's the load on your DNS servers?  Probably it doesn't matter at all 
what you pick, especially if you can't answer off the top of your head.

HTH,
Dave
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