Webmasters Asked on December 5, 2021
Let’s say I need to change the CNAME for my subdomain test.mysite.example.
I want it to go to a load balancer endpoint aws-my-endpoint.example.
But in creating the CNAME, I type it wrong. e.g. aw-my-endpoint.example.
Since it’s possible that a client can cache DNS for up to 48hrs (despite what the TTL setting is), could this cause test.mysite.example to be down for 48hours? Even though I immediately fix the CNAME after noticing the typo?
As soon as you change your records, one recursive nameserver can query for it, and hence grab the "bad" value and then cache it, for either its TTL or the negative TTL depending on what is the record and the response exactly.
If you did in fact really reload the nameservers with the bad value.
Also, there are offline checker. named-checkzone for example is capable of verifying multiple things on a zone before it is loaded for live queries (but not really for CNAME records, mostly for MX, SRV and NS ones). In the same way, a smart provider giving you some kind of UI for your DNS records could detect things like this beforehand (but it should be mostly a warning and not necessarily a block, you could want to enter such invalid CNAME in preparation of further operations later).
Answered by Patrick Mevzek on December 5, 2021
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