I'm configuring a dual stack network on a KVM server using static IPv4 and IPv6 addresses furnished by my provider.

I input all addresses, nameservers and gateways as required when installing Ubuntu. After that, I checked the /etc/network/interfaces file and noticed that the IPv6 stanza was absent (an ifconfig execution confirmed this), so I added the relevant lines. This is the final file:

# This file describes the network interfaces available on your system

# and how to activate them. For more information, see interfaces(5).



# The loopback network interface

auto lo

iface lo inet loopback



# The primary network interface

auto eth0

iface eth0 inet static

        address 151.236.18.86

        netmask 255.255.255.0

        network 151.236.18.0

        broadcast 151.236.18.255

        gateway 151.236.18.1

        # dns-* options are implemented by the resolvconf package, if installed

        dns-nameservers 91.227.204.227 91.227.205.227

        dns-search mydomainname.com

iface eth0 inet6 static

        pre-up modprobe ipv6

        address 2001:b60:1000:151:236:18:86:0

        netmask 112

        gateway 2001:b60:1000::1

        dns-nameservers 2001:4860:4860::8888 2001:4860:4860::8844

        dns-search mydomainname.com

I then restarted networking via sudo /etc/init.d/networking stop && sudo /etc/init.d/networking restart and noticed that, while IPv4 was working, outbound IPv6 connectivity was not available (I did not check for inbound connectivity yet).

ifconfig and ip -6 addr show the IPv6 address is recognized:

eth0      Link encap:Ethernet  HWaddr 52:54:00:b1:27:87  

          inet addr:151.236.18.86  Bcast:151.236.18.255  Mask:255.255.255.0

          inet6 addr: fe80::5054:ff:feb1:2787/64 Scope:Link

          inet6 addr: 2001:b60:1000:151:236:18:86:0/112 Scope:Global

          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1

          RX packets:16409 errors:0 dropped:0 overruns:0 frame:0

          TX packets:1178 errors:0 dropped:0 overruns:0 carrier:0

          collisions:0 txqueuelen:1000 

          RX bytes:1126656 (1.1 MB)  TX bytes:763658 (763.6 KB)



2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qlen 1000

    inet6 2001:b60:1000:151:236:18:86:0/112 scope global 

       valid_lft forever preferred_lft forever

    inet6 fe80::5054:ff:feb1:2787/64 scope link 

       valid_lft forever preferred_lft forever

On the contrary, no default route exists for the IPv6 stack:

$ ip -6 route

2001:b60:1000:151:236:18:86:0/112 dev eth0  proto kernel  metric 256 

fe80::/64 dev eth0  proto kernel  metric 256 

Trying to add the missing route leads to "No route to host" error:

$ sudo ip -6 route add default via 2001:b60:1000::1

RTNETLINK answers: No route to host

What could be wrong, and how can I fix the network configuration so that I can get the IPv6 stack working?