Unable to issue Let's Encrypt certificate: cURL error 7: Failed to connect to

Symptoms

• Unable to issue SSL certificate for any domain in Plesk with the error:
  

  PLESK_ERROR: Error: Let's Encrypt SSL certificate installation failed:
  Could not obtain directory: cURL error 7: Failed to connect to 2a02:26f0:f6:1ad::3d5: Network is unreachable (see http://curl.haxx.se/libcurl/c/libcurl-errors.html)

• Output of the following command is empty:
  

  # lsmod | grep v6

• IPv6 address is assigned to network interface:
  

  # ip a
  1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN qlen 1
  link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
  inet 127.0.0.1/8 scope host lo
  valid_lft forever preferred_lft forever
  2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP qlen 1000
  link/ether 2001:db8:f61:a1ff:0:0:0:80 brd ff:ff:ff:ff:ff:ff
  inet 203.0.113.2/24 brd 203.0.113.255 scope global eth0
  valid_lft forever preferred_lft forever

Cause

IPv6 kernel module is not loaded. Therefore Let's Encrypt can not validate domains, because IPv6 is preferred protocol.

Resolution

The solution has two options:

• completely remove IPv6 addresses from the network interfaces in order to perform token validation through IPv4 only
• properly configure IPv6.