FTP service cannot be started by xinetd on Plesk server if IPv6 is disabled

Symptoms

FTP service fails to start on a Plesk server with the following error:

xinetd[503]: socket creation failed (Address family not supported by protocol (errno = 97)). service = ftp
xinetd[503]: Service ftp failed to start and is deactivated.

Cause

The issue is caused by disabled IPv6 support on the host:

# grep -r ipv6 /etc/modprobe.d/
alias ipv6 off
options ipv6 disable=1

# grep -i v6 /etc/sysconfig/network
NETWORKING_IPV6=no
IPV6FORWARDING=yes

Resolution

1. Connect to the server via SSH.

2. Replace the IPv6 value by IPv4 in the /etc/xinetd.d/ftp_psa file as shown in the example below. For this, edit the file in any text editor, for example, in the vi editor:

   service ftp
   {
   flags = IPv6 <== REMOVE
   flags = IPv4 <== ADD
   disable = no
   socket_type = stream
   protocol = tcp
   wait = no
   user = root
   instances = UNLIMITED
   server = /usr/sbin/in.proftpd
   server_args = -c /etc/proftpd.conf
   }

   Note: this change could be rewritten after the Plesk upgrade

3. Save changes and close the file.

4. Start the xinetd service:

   # systemctl start xinetd