I haven’t been asked this question yet, but I assume I will be soon and I don’t know the answer - how often does a device on the internet actually query DNS when sending/receiving packets? Does it send a request every single time before sending a message or just before the initial packet is sent? What about if it is a long back and forth between the devices - is there some protocol rule that it should query DNS every x packets sent?
Thanks!
The short answer is it depends. DNS lookups are typically cached on the requesting PC in order to limit the stress on the servers, and the DNS records have a time to live (TTL) associated with them. This is an amount of seconds that the record is good for, and a new one won’t be looked up until it expires. This could be a few seconds to weeks depending on the record - the TTL reported by the authoritative DNS can be configured to be any value. A typical default value for the TTL is 86,400 seconds (1 day), although some modern systems use much lower values like 300 seconds due to the internet infrastructure now being so critical for some things that it would be unacceptable to have downstream propagation times of a day or more.
If you’re on a Windows machine you can actually look at your DNS cache and the TTL associated with each domain by typing
ipconfig /displaydns
in command prompt. AFAIK it’s more complicated to do this on a Mac but should be possible with a little poking around.
Hope that helps 
That’s great - thanks for the response!