If you have a recent distribution of Linux, you might find the message “IPv6 addrconf: prefix with wrong length 48” repeated a lot in syslog. If you Google this error message you’ll quickly find that this is because IPv6 auto configuration (sort of like DHCP) is failing. Now if you don’t want to bother with IPv6 yet or if you use static IPv6 (like my servers do) you don’t need IPv6 auto configuration.

A quick fix to solve the problem (as mentioned on sites like these) is to run the following commands:

echo 0 > /proc/sys/net/ipv6/conf/eth0/autoconf
echo 0 > /proc/sys/net/ipv6/conf/eth0/accept_ra

And yes, that solves the problem – until the next reboot that is. The permanent solution mentioned on that site however, does not work (as also confirmed by this IPv6 howto). The reason is that referring to all network interfaces using “all” in the following lines in /etc/sysctl.conf somehow doesn’t work:

net.ipv6.conf.all.autoconf = 0
net.ipv6.conf.all.accept_ra = 0

The simple solution is to refer to each network interface specifically. My servers have both eth0 and eth1 (2 NICs) so I setup /etc/sysctl.d/ipv6.conf as follows:

net.ipv6.conf.default.autoconf = 0
net.ipv6.conf.default.accept_ra = 0
net.ipv6.conf.all.autoconf = 0
net.ipv6.conf.all.accept_ra = 0
net.ipv6.conf.eth0.autoconf = 0
net.ipv6.conf.eth0.accept_ra = 0
net.ipv6.conf.eth1.autoconf = 0
net.ipv6.conf.eth1.accept_ra = 0

If you have only one network interface, you can omit the “eth1” lines. Alternatively you can use pre-up commands as described in the IPv6 howto, though I think my solution is prettier.