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The reason may be so that services on private subnets don't need internet access to use Let's Encrypt. Just a guess though.

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You don't need direct Internet access to use Let's Encrypt, as long as you can arrange for the challenge response to appear in public DNS under the name you want to use.

Would you mind giving an example of what that might look like? Or linking to something? I've always struggled with needing to open ports temporarily on stuff behind my own reverse proxy to avoid passing the certs by hand, and it sounds like something that'd be useful to understand.

The magic phrase is “DNS-01” challenge. You place a DNS TXT record to validate control of the domain. There are lots of ACME clients that support a wide variety of DNS service providers. For example, I have a Home Assistant server which automatically issues certs using Gandi DNS and the HA Lets Encrypt support, all without being on the internet (except for the DNS entries)

It's the DNS-01 challenge[1]. This reduces the challenge to using some DNS provider with an API supported by a client[2] / [3], as well as the server needing to be able to reach the LE-API. We use this with the CNAME delegation into an irrelevant zone everywhere to get wildcard certificates for our LBs ( meaning: the _acme_challenge.example.com record is just a CNAME for _acme_challenge.dont.ever.use.this.example.com, and the servers just have credentials to modify records in the zone <dont.ever.use.this.example.com>)

1: https://letsencrypt.org/docs/challenge-types/#dns-01-challen...

2: https://eff-certbot.readthedocs.io/en/stable/using.html#dns-...

3: https://github.com/acmesh-official/acme.sh/wiki/dnsapi

The acme client needs to reach LE though. Or you need to do a dance where the client is outside of the private network and ships the certificate into the private network.

I think you're confusing internet access with reachable from the internet.

If I remember correctly, you don't need your server to be reachable from the internet, but you still need to be able to contact your DNS provider and the LE server, so you need internet access

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