Configuring IPv6-over-GRE in /etc/network/interfaces

[Matthew Hall <mhall@mhcomputing.net>]

[Note: For some reason the subscription request to this list is not working 
for me so please add me directly if you have a reply for this thread, thanks!]

Hi All,

In what can only be described as an act of supreme technical incompetence, 
AT&T released a firmware update this year for some of their Uverse routers 
which causes them to block "inet6 v4tunnel", aka IP protocol 41. The update 
finally appeared in my environment some time in the past few days and broke 
everything completely without warning.

Apparently the 6RD implementation is buggy and gets confused by other IPv6 
tunnels it does not expect and just drops all the packets.

I have high quality IPv6 connectivity of my own and I don't want them breaking 
or censoring it so I'm trying to get it working again. So I am thinking of 
working around it until I can get this fixed by using GRE.

On the local end I have a Cisco IOS 15 router with this configuration:

interface Tunnel0
 no ip address
 ip tcp adjust-mss 1416
 ipv6 address Y::2/64
 ipv6 enable
 ipv6 mtu 1456
 ipv6 traffic-filter bogons_ipv6 in
 ipv6 virtual-reassembly in
 tunnel source Vlan2
 tunnel destination X.X.X.X
...
ipv6 route ::/0 Tunnel0

(Note that despite not saying "IPv4 GRE", that's what this will do, as that's 
the Cisco default tunnel mode. Before AT&T broke it, this used "tunnel mode 
ipv6ip" instead.)

On the remote end I have a Debian system in a colocation facility.

I tried to configure the following sort of tunnel to match on the Debian system:

iface home6 inet tunnel
    address Y::1
    netmask 64
    local X.X.X.X
    endpoint Z.Z.Z.Z
    dstaddr Y::2
    mode gre
    ttl 64
    up ip link set mtu 1456 dev $IFACE
    up ip route add Y+1::/64 via Y::2

However, when trying to activate it with ifup home6, I receive the following 
mysterious netlink error:

RTNETLINK answers: Invalid argument
Failed to bring up home6.

In addition to that I get the following broken half-configured tunnel device:

$ ip addr show dev home6
4: home6: <POINTOPOINT,NOARP,UP,LOWER_UP> mtu 1476 qdisc noqueue state UNKNOWN group default 
    link/gre X.X.X.X peer Z.Z.Z.Z
    inet6 fe80::200:5efe:4ad0:2eba/64 scope link 
       valid_lft forever preferred_lft forever

What am I missing here? I could not find any IPv6-in-GRE interfaces file 
examples on the Internet, and the man page was very general, so I am out of 
ideas.

Thanks,
Matthew.