[CentOS] Server used in DOS attack on UDP port 0

Hi,

One of our AWS machines was used in an DOS attack last night and I am
looking for possible attack vectors. AWS tells me it was sending UDP port 0
traffic to a cloudflare address.

This instance had an incorrectly configured AWS security group exposing all
ports.

The server in question is a Centos 7 based FreeIPA server, OpenVPN
concentrator and DNS server.

With a brief inspection before the instance was stopped no evidence of
intrusion could be detected in the obvious places and the machine is
protected by standard SELinux policies.

On this machine Firewalld is currently configured with a single zone with
masquerade enabled

firewalld config.
public (default, active)
  interfaces: eth0
  sources:
  services: dhcpv6-client dns http https kerberos kpasswd ldap ldaps ntp
openvpn ssh
  ports: 81/tcp
  masquerade: yes
  forward-ports:
  icmp-blocks:
  rich rules:

Thanks,

Andrew