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February 3, 2020

When Will Your DNS Record Be Published?

When publishing a DNS record through an API, it's often useful to know when the DNS record has been fully published and is visible to DNS resolvers. A perfect example which comes up at SSLMate is automatically validating a certificate request by publishing a DNS record. SSLMate must be sure that the DNS record is visible before it tells the certificate authority to validate it, or the certificate request may fail.

Unfortunately, I know of only one DNS provider that has an API to tell you when a change is published: Route 53. After submitting a DNS change request to Route 53, the API returns a ChangeInfo object which contains a status of either "PENDING" or "INSYNC". You can poll the change until its status becomes "INSYNC", which means the change has taken effect on all Route 53 servers. SSLMate has published a lot of DNS records through Route 53 and this API has never let me down, which makes me happy.

Other DNS providers offer absolutely nothing to help you determine when a DNS change is visible. In these cases, SSLMate can do nothing but sleep for 10-120 seconds (depending on the provider) and hope for the best. Unfortunately, it doesn't help for SSLMate to try to resolve the DNS record to see if the record has been published - modern authoritative DNS services use many different servers, often with anycast or load balancing, so just because SSLMate sees the record doesn't mean that others will.

And then there's Google Cloud DNS, which deserves a special mention because they offer an API that looks very similar to Route 53's: after submitting a change request, the API returns a change object with a status of "pending" that you can poll until the status becomes "done". Sounds perfect! Except if you read the fine print, it says:

A status of "done" means that the request to update the authoritative servers has been sent, but the servers might not be updated yet

Sure enough, I found that it often takes two minutes after a change becomes "done" for it to be fully visible. The change object also contains a very bizarre boolean called "isServing", which is documented as:

If the DNS queries for the zone will be served.

I'm not sure what this means, or why information about the zone's status would be present in a record change object. In my testing I never once saw a value besides false, even long after queries for both the individual record and the zone as a whole were being served.

So the change object API is completely useless, and I don't know why it exists - who cares if the "request to update the authoritative servers has been sent"? That's an internal implementation detail. It only matters to users of the API if the change has been fully applied everywhere. So, SSLMate doesn't use change objects. It sleeps for 2 minutes after adding the record and hopes for the best.

All of this is exasperated when requesting a certificate using the ACME protocol. With ACME, if you tell the server the DNS record is published, but the server doesn't see the record, your certificate order is invalidated. You have to create a new order, and you're given a different DNS record that you have to publish. That means your ACME client could potentially get in a situation where it never makes forward progress, because on each attempt it fails to wait long enough before telling the ACME server to check the record.

SSLMate has a workaround for this when talking to an ACME-using certificate authority such as Let's Encrypt. Instead of publishing the record returned by the ACME server, SSLMate publishes an NS record that delegates the record to a custom-built authoritative DNS server operated by SSLMate. SSLMate's authoritative server returns the record provided by the ACME server. The NS record never changes, so if checking the record fails and SSLMate has to create a new ACME order, it doesn't need to republish a DNS record in the customer's zone; instead it just has to update the record that SSLMate's authoritative server returns, which can be done instantaneously. Therefore, every retry is more likely to succeed than the previous one since more time has elapsed since publishing the NS record. All of this happens completely automatically and transparently to the user of SSLMate, and is one of the ways that SSLMate provides great dependability. (Another benefit is that if the customer's DNS provider doesn't provide an API, they can publish the NS record manually and never have to touch it again, even for renewals.)

Nevertheless, it would be really nice if more DNS providers offered an API like Route 53 to report when a DNS record has been published.


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