FTP service cannot be started by xinetd if IPv6 is disabled

Symptoms

• FTP service fails to start 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.`

• IPv6 support is disabled:

  # grep v6 /etc/modprobe.conf
  alias ipv6 off
  options ipv6 disable=1
  # grep -i v6 /etc/sysconfig/network
  NETWORKING_IPV6=no
  IPV6FORWARDING=yes

Cause

The issue is caused by disabled IPv6 support on the host

Resolution

Connect to the server via SSH and remove the line in /etc/xinetd.d/ftp_psa
as shown in the example below:

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 lost after Plesk upgrade.