ipv6 source address selection

Mikael Abrahamsson swmike at swm.pp.se
Sat Oct 19 17:12:38 CEST 2013

I'm trying to influence my source address selection. First I thought I'd 
figure out how it works by default.

I have a /48. Let's call it 2001:db8:1::/48

I created three /64s on the same LAN with A-bit set so clients would do 
SLAAC within these:


Then I set up loopback addresses on my router:


Then I tried pinging each loopback address from a host which has 2 
addresses out of each /64. It now picked a source address within the same 
/56. I consistently both on a Ubuntu 13.04 and OSX 10.8.5 machine get the 
same behaviour.

So above means that pinging 2001:db8:1:1fff::1 it would use the :1000: 
address, and pinging :2fff::1 would use the :2000::/64 address.

If I ping outside my /48 it will consistently use the last created address 
(I tried adding a 4th lan, 8000, and it then uses that one), which I 
perfectly understand.

When I ping :5000: and so on, it will sometimes use the :0000: address and 
not the :8000: that is used for the rest of global traffic.

I have nothing /56 or /48 magic in routing table or "ip addrlabel list", 
but it still seems to be something special when it comes to the same /48 
as the machine has addresses in.

Any help understanding what is going on is appreciated.

Mikael Abrahamsson    email: swmike at swm.pp.se

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