[Gllug] Oddity in IPv6 routing

Anthony Newman anthony.newman at ossified.net
Wed Mar 26 10:27:38 UTC 2008

John Winters wrote:
> I have set up a gateway machine on my LAN (called knight) which uses an 
> IPv6 tunnel over my normal IPv4 connection to connect to the outside 
> world.  (ISP is A&A who provide the IPv6 tunnel.)  knight is also 
> running radvd which means all the other machines on the LAN pick up 
> global IPv6 addresses and see knight as their IPv6 gateway.  IPv6 
> forwarding is turned on on knight.

Unless you have another block of addresses routed over your tunnel, or 
you bridge your tunnel device with your LAN segment, this is not really 
expected to work anyway. Despite a /64 being used typically for a tunnel 
and having almost 2^64 addresses within it, it isn't routable as such 
because you are using it as a /64 on the end-point. This is one of the 
paradoxes of IPv6 that I've never come to terms with; the design 
effectively throws away half the address space by making the default 
host allocation a /64, and NAT-PT is specifically discouraged because it 
was a nasty hack for IPv4, but we have so many bits that we apparently 
don't care :)

If you have bodged something that works by deliberately lengthening the 
network prefix on your tunnel to make routing work, it's not really the 
right way to go although you appear to have made it appear functional. 
Tunnel providers will usually allocate you a routed /56 or even the 
generally accepted end-user /48 for this sort of internal routing, which 
is expected.

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