I will explain my solution to this, if you have input and/or improvements i would be glad to hear them. This approach is based on using proxy_ndp and can only route specific IPv6 addresses to a domU. We currently have no need for subnet routing and i did not look into that. Our setup is based on Ubuntu 8.04.3 LTS (Codename Hardy) and so is the configuration here and the patch.

First we add a IPv6 address to our physical interface in the dom0:

/etc/network/interfaces
iface eth0 inet6 static
 address 2a01:4f8:100:4463:1::1
 netmask 64
 up ip -6 route add 2a01:4f8:100:4460::1 dev eth0
 up ip -6 route add default via 2a01:4f8:100:4460::1

Enable routing and proxy_ndp on the interfaces:

/etc/sysctl.conf
net.ipv6.conf.all.forwarding=1
net.ipv6.conf.all.proxy_ndp=1

Patch BenV’s vif-route script to enable the use of proxy_ndp:

/etc/xen/scripts/vif-route
--- vif-route.orig    2009-09-28 13:45:40.000000000 +0200
+++ vif-route 2010-07-08 22:36:32.000000000 +0200
@@ -55,6 +55,7 @@
         ifconfig ${vif} ${main_ip} netmask 255.255.255.255 up
        if [ ! -z "${main_ip6}" ]; then
                ip -6 addr add ${main_ip6} dev ${vif}
+               echo 1 >/proc/sys/net/ipv6/conf/${vif}/proxy_ndp
        fi
         echo 1 >/proc/sys/net/ipv4/conf/${vif}/proxy_arp
         ipcmd='add'
@@ -80,6 +81,10 @@
                log info "[vif-route] Adding IPv6 address ${addr} with src ${main_ip6} for $vif."
              result=`${cmdprefix} ip -6 route ${ipcmd} ${addr} dev ${vif} src ${main_ip6} 2>&1`
                log info "[vif-route] Result: ${result}"
+
+               log info "[vif-route] Adding IPv6 proxy ${addr} on ${netdev:-eth0}."
+             result=`${cmdprefix} ip -6 neighbor ${ipcmd} proxy ${addr} dev ${netdev:-eth0} 2>&1`
+               log info "[vif-route] Result: ${result}"
        fi
     done
 fi

You can download the original or the patched version here too in case the link above doesn’t work for you.

Now to give a domU a specific IPv6 address you’ll have to adjust the vif statement of that guest configuration:

/etc/xen/<domu>.cfg
vif = [ 'ip=188.40.226.34 2a01:4f8:100:4463:2::1,mac=00:16:3E:EB:42:59' ]

Inside that domU you’ll need to configure this:

/etc/network/interfaces
iface eth0 inet6 static
 address 2a01:4f8:100:4463:2::1
 netmask 64
 gateway 2a01:4f8:100:4463:1::1