I had been discussing IPv6 address precedence recently and realized that I’d never really messed with it.  I have a FreeBSD host that has multiple IPv6 addresses, an EUI-64 address as well as a statically assigned address.  If you don’t know what IPv6 or EUI-64 is, I suggest you brush up because IPv6 is coming and it’s coming sooner than you thing.

Using the ip6addrctl command I can manipulate which  address is preferred for outbound connections.

[buraglio@synack:~ ] ip6addrctl show                                                                                        
Prefix                          Prec Label      Use
::¹⁄₁₂₈                           50     0        0
::/0                              40     1  1165005
2002::/16                         30     2        0
::/96                             20     3        0
::ffff:0.0.0.0/96                 10     4        0
2620:0:e00:4000::/64               1     0       63

This is the default configuration on my FreeBSD 7.1 machine.  Here we can see that I have several IPv6 addresses:

[buraglio@synack:~ ] ifconfig                                                                                               <1215>

le0: flags=8843 metric 0 mtu 1500

options=8

ether 00:0c:29:b6:96:ba

inet6 fe80::20c:29ff:feb6:96ba%le0 prefixlen 64 scopeid 0x1 

inet6 2620:0:e00:4000::146 prefixlen 64 

inet6 2620:0:e00:4000:20c:29ff:feb6:96ba prefixlen 64 

inet 128.174.43.146 netmask 0xffffff80 broadcast 128.174.43.255

media: Ethernet autoselect

status: active

Obviously the fe80 address is my link local.  The 2620:0:e00:4000::146 is a manually assigned address and 2620:0:e00:4000:20c:29ff:feb6:96ba is my EUI-64 address.I believe the default behavior on FreeBSD is to prefer the manually configured address, which is the behavior that is exhibited when I test it.  

buraglio@synack:~ ] ping6 remote.buraglio.com                                                                              <1218>

PING6(56=40+8+8 bytes) 2620:0:e00:4000::146 –> 2607:f2f8:4980::2

16 bytes from 2607:f2f8:4980::2, icmp_seq=0 hlim=47 time=44.454 ms

16 bytes from 2607:f2f8:4980::2, icmp_seq=1 hlim=47 time=64.172 ms

results in

[buraglio@collector:~ ] sudo tcpdump ip6 proto 58

08:30:17.381695 IP6 2620:0:e00:4000::146 > 2607:f2f8:4980::2: ICMP6, echo request, seq 0, length 16

08:30:17.381781 IP6 2607:f2f8:4980::2 > 2620:0:e00:4000::146: ICMP6, echo reply, seq 0, length 16

08:30:18.951558 IP6 2620:0:e00:4000::146 > 2607:f2f8:4980::2: ICMP6, echo request, seq 1, length 16

08:30:18.951641 IP6 2607:f2f8:4980::2 > 2620:0:e00:4000::146: ICMP6, echo reply, seq 1, length 16

If I change the precedence to prefer the EUI-64 address, it will source that traffic from there, as such:

sudo ip6addrctl add 2620:0:e00:4000:20c:29ff:feb6:96ba/64 1 primary                              

and do the experiment again, I see different results.