DNS authentication and timings

I've successfully set up a custom certbot configuration and due to specifics of my certificate (several domains, some of which also should contain wildcards) I have to use DNS authentication since it is the only available option.

And here s the caveat: I create the needed domain through my server's, API, and I check that domain is accessible through some DNS server, but certbot anyway shows an error "DNS resolution error" or "wrong challenge domain".

I think it's because of the DNS latency. I set TTL in my challenge record to the smallest available value 30 seconds, and introduced a delay of 15 minutes between creation of a domain and exit of the authenticator script, and anyway still often get those errors.

So my question is: what should be my strategy to ensure DNS record is already propagated to the DNS server Letsencrypt use to resolve challenge domains?

Should wait for (documentation suggest an hour!) in the authenticator script, or it is better to run the script once without the delay, and then re-run it once again after an hour passed?

You will probably need to wait for more of a DNS expert than me.

But, when checking the TXT record for a DNS challenge the Let's Encrypt servers check the authoritative name servers. Typically the delay is short (often just seconds) for the DNS provider to sync those. You can use the unboundtest.com test site which mimics what LE servers do to lookup TXT records.

Knowing your DNS provider and an example domain name is very helpful when trying to assist with such questions. And, are you using one of the DNS challenge plug-ins or is that part of your custom scripting too?


Who is your DNS provider?

Several providers are notorious for using read-through caches on their public servers, but not implementing a write-through cache on their internal API. That means edits you make on their systems can usually take anywhere from 301 seconds (5minutes + 1 second) to 3601 seconds (60 minutes + 1 second) to clear [longer and shorter are possible, those are just the most typical settings]. These are application/infrastructure caches, and not related to TTL settings – so customers have no control over this behavior aside from switching providers.


I use Yandex 360 DNS , I don't know if it helps.

And yes, it is my own script, AFAIK there is no plugin for Yandex

SO, I can see I can use GET request with unboundtest.com and it gives me the results in a text format (is it like a dig?)
So is the format documented, should I parse it somehow or there are some more reliable way to get the content of TXT records?

Well, what you'd want to do in a script, rather than parsing unboundtest, is to query your authoritative DNS servers directly yourself, just like Let's Encrypt would do.


You might want to check out the post below. We recently helped someone with some Yandex problems. It has some unusual behavior


Honestly, for this particular requirement, it is doing the opposite of helping.

Are you able to switch to any other DNS service provider (DSP)?
[any DSP that can handle TXT updates via API; and do them quickly will work better]

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As shown by the other post (No TXT records found - #2 by rg305), Yandex DNS isn't working as advertised.


Sorry but this was of course my first guess. And of course it didn't work, so that's when I introduced the delay.

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Nope, this is not the reason to switch to another provider, this is just an inconvenience. The certificate is updated anyway, only the script works not every time which is not ideal.

So AFAICS the problem is that two Yandex DNS servers are out of sync? So can I for example query both of them and do not exit the script until both servers answer the correct record for, say, three times in a row with some delay?

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We don't know.

In theory, yes. In practice they could be using clusterization or anycast (both, probably) so each "server" is actually several machines and you never know which is answering.


You can try such ideas but I was getting the wrong response from their authoritative dns servers even after a long delay. The dns1 was wrong about 1/3 of the time and the other only about 1/10.

After such a wait you might still get a failure when Let's Encrypt checks them. So, just start renewing 30 days before expiry and try several times a day. Many attempts may fail but you only need one to work in the 30 day period.

A certbot renew does very little for "fresh enough" certs. Certbot just looks at your local cert and if it is fresh it does nothing else. Just be sure not to use "force" option - that won't help and is actually worse.

You could try switching DNS providers too. Ideally to one with a support API for DNS challenge


In my book, any reason is good enough to switch.
DNS is extremely simple - its's either done right or it isn't.
At least consider adding more DNS servers into the equation.
[more hits and less misses on any given test]


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