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We use two DNS providers for redundancy. Since the DynDNS DDOS attack of 2016, I expect we are not alone in this practice.

I am attempting to use the Let's Encrypt certbot with DNS challenge. Having two DNS providers seems to pose a problem. Do both DNS providers need to be updated with identical TXT records as part of the challenge process?

The real question is, how does the Let's Encrypt ACME Certificate Authority (CA) validate DNS TXT entries? Does it simply query the public DNS like any client would, or does it query against the specific DNS provider that maps to the certbot plugin I'm using? I suspect the former.

I attempted to create a certificate using only one of the plugins. The TXT entries were created, but validation failed. I expect I will need to update both DNS providers (or figure out how to link the two providers so they synchronize changes).

The documentation doesn't seem to indicate that using multiple plugins of the same type is possible, although I haven't tried it yet. One of my providers requires that I upgrade to gain access to their API.

If it is not possible to chain multiple authentication plugins, then I believe my only option for automated renewal is using the pre- and post-hooks with a custom script to modify both DNS providers.

I'd appreciate confirmation of this before I undertake the work and expense required (or choose to manage the renewal process manually).

EDIT: I'm aware that StackExchange wrote and published its own open-source DNS updater, dnscontrol, which could be used in pre- and post-hook scripts. However, one of our providers is not supported.

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    I appreciate the possibility that a single DNS provider could have all their servers attacked, where-as using multiple providers gives the possibility of having one provider down while the other is working. I'm not sure how common it is for people to manage two completely independent DNS providers though. Usually you'd just want to have one master and let any other DNS servers pull data from that. If the master goes down, the slaves just don't update for a while – USD Matt Apr 6 '18 at 14:52
  • Let's Encrypt will just do a DNS lookup like any other client and could get either provider – USD Matt Apr 6 '18 at 14:52
  • @USDMatt Can you cite your evidence please? – Larry Silverman Apr 6 '18 at 14:55
  • specific DNS provider that maps to the certbot plugin I'm using not sure what you mean by that. The ACME clients all implement the same ACME protocol. They are given a token to insert in DNS, send a simple response to say it's ready to be checked, then the server tries to lookup that record via the normal DNS system. Trying to do anything else depending on the client/plugin would likely introduce methods to trick them into looking up records you control rather than those publicly available on the Internet. – USD Matt Apr 6 '18 at 15:13
  • If you can get your DNS providers supported by dnscontrol (do they have a decent API, or can you move providers?) then it'll soon handle actually doing the DNS challenge and issuing the certs: github.com/StackExchange/dnscontrol/pull/327 – Shane Madden Apr 6 '18 at 15:22
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There are plenty of 3rd party ACME (LetsEncrypt) clients out there that provide more features than the default certbot. They maintain a list on their site here: ACME Client Implementations

A lot of them have some sort of DNS plugin architecture and some allow you to specify different plugins for different names in a cert. But none that I know of allow you to combine plugins per-zone (such that more than one plugin would service a single zone's TXT record).

However, if you're already considering writing pre/post hooks to get this done. I'd suggest using something like acme.sh. It has a ton of pre-written DNS plugins. You could probably fairly easily combine two or more of them into a single merged plugin that would effectively accomplish your goal.

  • I'd like to avoid taking on another dependency if I can. There look to be dozens of 3rd party clients to evaluate. If I can solve this without looking outside Let's Encrypt's own utilities, I would prefer it. Minting certs requires a high degree of trust. I trust Let's Encrypt, and I don't have the resources to review and stay on top of changes to third-party utilities. – Larry Silverman Apr 6 '18 at 15:38
  • I might like this more than my answer. It does not add another special case, and perhaps more importantly, it gives a welcome excuse to drop "easy-to-use" certbot and start using something lightweight & security-focused. – anx Apr 6 '18 at 15:48
  • Think of it less as taking another dependency and more of trading one dependency for another. Some of those 3rd party clients are better maintained (IMHO) than certbot. For example, acme.sh had support for the ACME v2 specification long before certbot did. – Ryan Bolger Apr 6 '18 at 17:07
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Letsencrypt validates your TXT entries by just querying a random resolver, with no control whatsoever about which one of your name servers is going to be used.

I recommend you delegate the _acme-challenge.example.com. zone to just one dns provider you have automated access to. You will never have a DoS situation at any of your DNS providers for longer than it takes to change this - or for longer than letsencrypt certificates are valid for. Therefore, its okay to skip the redundancy for ACME only, so you can use all the automation just like before you added more name servers.

edit based on comments: This strategy will not allow to continue certbot, as the respective certbot plugins is not correctly handling aliased or delegated validation domains. It should look to modify the zone of the validation record, but attempts to modify the zone of the validated domain, disregarding that it is very valid and for them to differ.

Other acme clients may require to explicitely state the location of the validation record, as is the case with acme.sh

  • I like this solution. I'd use Route53 for the sub. I've never delegated a subdomain before. This looks like the relevant doc, do you agree? docs.aws.amazon.com/Route53/latest/DeveloperGuide/… – Larry Silverman Apr 6 '18 at 15:28
  • Yeah, that looks like what you want to read if you want to continue with certbot-dns-route53 plugin. Note that i have not actually tried it, it just should work. – anx Apr 6 '18 at 15:58
  • delegate the _acme-challenge ... zone - this would work for a single record, but that doesn't seem like it would scale very well if you had many zones, or many records and didn't want a wildcard. – Zoredache Apr 6 '18 at 16:44
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    @Zoredache you just have to delegate one and CNAME the other names you want verified (some people even register a completely sererate domain, because their dns provider wont let them configure API keys with appropriately restricted access). – anx Apr 6 '18 at 17:01
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    I attempted the subdomain solution and I think I hit a fatal blocker. The certbot-dns-route53 plugin expects to modify the primary domain, which lives in one Route53 hosted zone. I modified the IAM policy to allow it to modify the new subdomain, which is in a different hosted zone. The plugin died, complaining that it could not modify the hosted zone that it wanted to modify. It's opinionated about which hosted zone it wants to modify, and it doesn't appear to be smart enough to handle the subdomain solution. – Larry Silverman Apr 6 '18 at 17:26

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