External hosts are not resolved from a Plesk server: "Name or service not known" or "Could not resolve host"

Symptoms

• nginx fails to start with the error message:

  CONFIG_TEXT: nginx[58503]: nginx: [warn] "ssl_stapling" ignored, host not found in OCSP responder "r3.o.lencr.org" in the certificate "/usr/local/psa/var/certificates/scfO1mntc"

• When securing a domain with a Let's Encrypt certificate, the operation fails with:

  PLESK_ERROR: Could not issue a Let's Encrypt SSL/TLS certificate for example.com
  Failed to connect to the Let's Encrypt server https://acme-v01.api.letsencrypt.org.
  Please try again later or report the issue to support.
  
  Details
  
  Could not obtain directory:

• When installing a Plesk license, the operation fails with:

  PLESK_ERROR: cURL cannot communicate with license server https://id-00.kaid.plesk.com:443/ (): Couldn't resolve host name(6) cURL cannot communicate with license server https://id-00.kaid.plesk.com:443/ (): Couldn't resolve host name(6) (Error code: 2)

  ---

  PLESK_ERROR: Could not update the license. Make sure that connections to the license server ka.plesk.com on TCP port 443 are not blocked. See how to do so.

• On attempt to connect to any external host using the cURL utility, the command shows:

  # curl -I http://example.com
  curl: (6) Could not resolve host: example.com

Cause

The DNS resolver specified in the file /etc/resolv.conf cannot resolve domain names.

Resolution

1. Connect to the Plesk server via SSH.

2. Open the file /etc/resolv.conf in a text editor. In this example, we are using the vi editor:

   # vi /etc/resolv.conf

3. Comment out the invalid resolver by placing a # (hash) character at the beginning of the line:

   CONFIG_TEXT: #nameserver 203.0.113.2

4. Add a different DNS resolver, for example Cloudflare DNS:

   CONFIG_TEXT: nameserver 1.1.1.1

5. Save the change and close the file.

   Note: /etc/resolv.conf can be overwritten on Ubuntu 18.04 and higher by netplan and systemd-resolved.services. To learn more, see this KB article.

6. Retry the previously failed operation.

   Note: If the above solution does not help, contact your service provider in order to check the network setup.