Flooded with UDP bad checksum on port 53 (but port is closed and checksum offloaded)

I have a virtual server that I run a web server and openvpn on.

It is not running bind, and port 53 is closed.

I am recently getting flooded with errors in syslog (about 1/sec) such as:

Sep 29 15:06:59 kernel: [4579319.118505] UDP: bad checksum. From 128.47.17.191:4127 to <<my server IP>>:53 ulen 49
Sep 29 15:07:31 kernel: [4579350.981024] UDP: bad checksum. From 69.161.227.72:27788 to <<my server IP>>:53 ulen 39

I've tried checksum offloading with:

ethtool --offload eth0 rx off tx off

Finally I simply blocked all UDP traffic with iptables. I had to remove a rule that was allowing all UDP traffic, which is a bit concerning. iptables -A INPUT -p udp --dport 53 -j DROP

So that leaves two questions:

1. Why was I receiving these errors - was this an attack of some sort?
2. What UDP ports do I need to allow in my iptables to make sure my VPN and web server continue to function properly? Do I need to allow UDP port 53 to my DNS servers?