IPv6 addrconf: prefix with wrong length 96

If you get this error message in your console (dmesg) logs on Debian or Ubuntu:

IPv6 addrconf: prefix with wrong length 96

Then what is happening is that your server is autoconfiguring an ipv6 address that is also in use somewhere else.

You can fix this by doing the following:

sysctl net.ipv6.conf.all.accept_ra=0
sysctl net.ipv6.conf.all.autoconf=0
sysctl net.ipv6.conf.lo.autoconf=0
sysctl net.ipv6.conf.eth0.autoconf=0
sysctl net.ipv6.conf.eth1.autoconf=0

You will also need to manually add those entries to /etc/sysctl.conf also to have them survive a reboot.