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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