Problem description: using DHCPv6 on SLC5 at CERN requires some tweaking of the network configuration files, due to a RedHat bug (https://bugzilla.redhat.com/show_bug.cgi?id=591630, https://bugzilla.redhat.com/show_bug.cgi?id=634857) and the way devices are authorized to connect to the network at CERN (i.e. by MAC address identification). There's also a problem with the default dhcpv6 client which uses a DUID not compatible with the CERN's need.

Solution: Follow these steps:

1) ip6tables drops the dhcpv6 lease packets from the DHCPv6 servers. In /etc/sysconfig/ip6table add this line (before the REJECTs):

-A RH-Firewall-1-INPUT -m state --state NEW -m udp -p udp --dport 546 --sport 547 -s fe80::/10 -d fe80::/10 -j ACCEPT

2) The default DUID is not good for CERN. The DUID  is in the file /var/lib/dhcpv6/dhcp6c_duid and can be created with this perl script. Execute these commands as root:

cd /var/lib/dhcpv6
rm client6.leases*
wget http://ipv6.web.cern.ch/sites/ipv6.web.cern.ch/files/mkduid.pl.txt
mv mkduid.pl.txt wide_mkduid_cern.pl
chmod 700 wide_mkduid_cern.pl
./wide_mkduid_cern.pl -t now -m 00:15:5D:FF:4B:37 (use the MAC addres of the eth interface registered in landb)

3) Enable the dhcpv6 client at boot time

To the file  /etc/sysconfig/network-scripts/ifcfg-eth0 add the line
 DHCPV6C=yes

Be sure to remove the line IPV6INIT=yes in case it's there

4) Enable IPv6 networking

To the file  /etc/sysconfig/network add these lines
NETWORKING_IPV6=yes
IPV6_AUTOCONF=yes

5) Restart ip6table and the network service, or reboot the server