With all the IPv6 configuration work som ... / 2010-04-11

With all the IPv6 configuration work something bothers me in ifconfig, it can still decide to stop showing IPv6 (inet6) addresses:

# ifconfig eth0.1
eth0.1    Link encap:Ethernet  HWaddr 00:1f:c6:59:76:f6  
          inet addr:10.42.2.1  Bcast:10.42.2.255  Mask:255.255.255.0
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:17245335 errors:0 dropped:0 overruns:0 frame:0
          TX packets:17342308 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
          RX bytes:10403720060 (9.6 GB)  TX bytes:17783892122 (16.5 GB)

The addresses are there:

# ip -6 addr ls dev eth0.1
3: eth0.1@eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 
    inet6 2001:888:1011::694/64 scope global 
       valid_lft forever preferred_lft forever
    inet6 fe80::21f:c6ff:fe59:76f6/64 scope link 
       valid_lft forever preferred_lft forever

Filed as Ubuntu bug 560807: ifconfig does not display inet6 addresses.
Update 2010-04-13: Problem source found, diff added to the bugreport.