Fix slow SSH and DNS in CentOS 6

On a DigitalOcean or Vultr VPS, with IPv6 enabled, you may find that fresh installation of CentOS 6 would have slow SSH connectivity. DNS requests are slow. The solution is easy: you have to allow ICMP traffic for both v4 and v6 in iptables.

iptables -A INPUT -p icmp --icmp-type echo-request -j ACCEPT
ip6tables -I INPUT -p icmpv6 -j ACCEPT

You can run the above command to affect the current firewall state.

After this, save the current rules:

service iptables save