nginx cannot bind to IPv6 interface on Plesk server: Cannot assign requested address

Symptoms

1. The website is not accessible:

   CONFIG_TEXT: ERR_CONNECTION_REFUSED

2. Nginx service is crashed, error in /var/log/nginx/error.log :

   CONFIG_TEXT: [emerg] 1204#0: bind() to [2001:db8:f61:a1ff:0:0:0:80]:80 failed (99: Cannot assign requested address)

3. Entries in /var/log/messages regarding IPv6 interface not being enabled before nginx attempt to start:

   CONFIG_TEXT: network[268]: Bringing up interface eth0:
   dhclient[665]: DHCPREQUEST on eth0 to 255.255.255.255 port 67 (xid=0x4b684081)
   dhclient[665]: DHCPACK from 2001:db8:f61:a1ff:0:0:0:80 (xid=0x4b684081)
   network[268]: [ OK ]
   nginx[352]: nginx: [emerg] bind() to [2001:db8:f61:a1ff:0:0:0:80]:80 failed (99: Cannot assign requested address)
   systemd[1]: Failed to start Startup script for nginx service.
   systemd[1]: Unit nginx.service entered failed state.

Cause

Misconfiguration of IPv6 network interface.

Resolution

Contact network administrator in order to identify why IPv6 is not available after the system reboot.

As a workaround:

1. Connect to server over SSH.
2. Add the following string to file /etc/sysctl.conf:

   CONFIG_TEXT: net.ipv6.ip_nonlocal_bind = 1

   This directive allows daemons to bind to non-existing IPv6 network interfaces.

3. Restart nginx service:

   # service nginx restart