Solving the issue with IPv6 when DAD is failed

Symptoms:

1. A lot of entries in the system log looking like this:
   
   kernel: [xxxx.yyyy] eth0: IPv6 duplicate address aaaa:bbbb:cccc:d:eee:fff:aaaa:aaaa detected!
   
   Where last several numbers in IPv6 address are matching the network interface MAC address.
   
   
2. Public ETH interface shows "dadfailed" when you run "ip a":
   
   inet6 fe80::aaaa:bbbb:cccc:dddd/64 scope link dadfailed

Fix:

Edit the file /etc/sysctl.conf and insert following lines into it:

net.ipv6.conf.eth0.accept_dad = 0
net.ipv6.conf.default.accept_dad = 0
net.ipv6.conf.eth0.dad_transmits = 0
net.ipv6.conf.default.dad_transmits = 0

Where "eth0" is your Ethernet interface name.

Then run: 

sysctl -p

And restart networking:

service network restart

OR

service networking restart

Or just reboot the VPS.