“Let’s Encrypt™” HTTP DCV error: 400 urn:ietf:params:acme:error:connection Timeout during connect (likely firewall problem)

Symptoms

You encounter an error similar to the following when running AutoSSL.

CONFIG_TEXT: 11:41:18 PM WARN “Let’s Encrypt™” HTTP DCV error (domain.tld: 400 urn:ietf:params:acme:error:connection (The server could not connect to a validation target) (203.0.113.2: Fetching http://domain.tld/.well-known/acme-challenge/3RJ3W8AGIGxAjBXJQcQGKD-8k-JQF3p9hmzWuFKyZds: Timeout during connect (likely firewall problem))

or

CONFIG_TEXT: “Let’s Encrypt™” HTTP DCV error (domain.tld): 400 urn:ietf:params:acme:error:connection (The server could not connect to a validation target) (203.0.113.2  Fetching http://domain.tld/.well-known/acme-challenge/t7KGY62Dh5gaqB6Cz8XXVNJ2D2T4ucq_uGj_tR7CXj8: Connection reset by peer)
[2024-03-11T21:09:36Z]          Net::ACME2::X::HTTP::Network: The system failed to send an HTTP “POST” request to “https://acme-v02.api.letsencrypt.org/acme/chall-v3/325168804607/tYVNfw” because of an error: Timed out while waiting for socket to become ready for reading

Cause

The error occurs when Let's Encrypt is unable to reach the site on TCP port 80 for the HTTP DCV check. For Let's Encrypt to issue a certificate, inbound and outbound TCP ports 80 and 443 must be open through all firewalls and applicable security devices.

Workaround

Open inbound and outbound TCP ports 80 and 443 through all firewalls and applicable security devices.