Sep 262013
 
When doing some tests on Suricata, I needed to setup a small IPv6 network. The setup is simple with one laptop which is Ethernet connected to a desktop. And the desktop host a Virtualbox system. This way, the desktop can act as a router with laptop on eth0 and Vbox on vboxnet0. To setup the desktop/router, I’ve used:
ip a a 4::1/64 dev eth0
ip a a 2::1/64 dev vboxnet0
echo "1">/proc/sys/net/ipv6/conf/all/forwarding
To setup the laptop who already has a IPv6 public address on eth0, I’ve done:
ip a a 4::4/64 dev wlan0
ip -6 r a 2::2/128 via 4::1 src 4::2 metric 128
Almost same thing on the Vbox:
ip a a 2::2/64 dev eth0
ip -6 r a default via 2::1
This setup should be enough but when I tried to do from the laptop:
ping6 2::2
I got a failure. I then checked the routing on the laptop:
# ip r g 2::2
2::2 via 4::1 dev wlan0  src 2a01:e35:1394:5bd0:f8b3:5a98:2715:6c8d  metric 128
A public IPv6 address is used as source address and this is confirmed by a tcpdump on the desktop:
# tcpdump -i eth0 icmp6 -nv
10:54:48.841761 IP6 (hlim 64, next-header ICMPv6 (58) payload length: 64) 2a01:e35:1394:5bd0:f8b3:5a98:2715:6c8d > 4::1: [icmp6 sum ok] ICMP6, echo request, seq 11
And the desktop does not know how to reach this IP address because it does not have a public IPv6 address. On the laptop, I’ve dumped wlan0 config to check the address:
# ip a l dev wlan0
3: wlan0:  mtu 1500 qdisc mq state UP group default qlen 1000
    link/ether c4:85:08:33:c4:c8 brd ff:ff:ff:ff:ff:ff
    inet 192.168.1.137/24 brd 192.168.1.255 scope global wlan0
       valid_lft forever preferred_lft forever
    inet6 4::4/64 scope global
       valid_lft forever preferred_lft forever
    inet6 2a01:e35:1434:5bd0:f8b3:5a98:2715:6c8d/64 scope global temporary dynamic
       valid_lft 86251sec preferred_lft 84589sec
    inet6 2a01:e35:1434:5bd0:c685:8ff:fe33:c4c8/64 scope global dynamic
       valid_lft 86251sec preferred_lft 86251sec
    inet6 fe80::c685:8ff:fe33:c4c8/64 scope link
       valid_lft forever preferred_lft forever
And, yes, 2a01:e35:1394:5bd0:f8b3:5a98:2715:6c8d is a dynamic IPv6 address which is used by default to get out (and bring a little privacy). Deleting the address did fix the ping issue:
# ip a d 2a01:e35:1394:5bd0:f8b3:5a98:2715:6c8d/64 dev wlan0
# ping6 2::2
PING 2::2(2::2) 56 data bytes
64 bytes from 2::2: icmp_seq=1 ttl=63 time=5.47 ms
And getting the route did confirm the fix was working:
# ip r g 2::2
2::2 via 4::1 dev wlan0  src 4::4  metric 128 
All that to say, that it can be useful to desactivate temporary IPv6 address before setting up a test network:
echo "0" > /proc/sys/net/ipv6/conf/wlan0/use_tempaddr
Apr 292011
 

IPv6 global address

The global address is used in IPv6 to communicate with the outside world. This is thus the one that is used as source for any communication and thus in a way identify you on Internet. Below is a dump of an interface configuration:
eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000
    link/ether 00:22:15:64:42:bd brd ff:ff:ff:ff:ff:ff
    inet6 2a01:f123:1234:5bd0:222:15ff:fe64:42bd/64 scope global dynamic 
       valid_lft 86314sec preferred_lft 86314sec
    inet6 fe80::222:15ff:fe64:42bd/64 scope link 
       valid_lft forever preferred_lft forever
The global address is here 2a01:f123:1234:5bd0:222:15ff:fe64:42bd/64. It is build by using the prefix and adding an identifier build with the hardware address. For example, here the hardware address is 00:22:15:64:42:bd and the global IPv6 address is ending with 22:15ff:fe64:42bd. It is thus easy to go from the IPv6 global address to the hardware address. To fix this issue and increase the privacy of network user, privacy extensions have been developed.

Privacy extensions

The RFC 3041 describes how to build and use temporary addresses that will be used as source address for connection to the outside world. To activate this feature, you simply have to modify an entry in /proc. For example to activate the feature on eth0, you can do
echo "2">/proc/sys/net/ipv6/conff/eth0/use_tempaddr
The usage of the option is detailled in the must-read ip-sysctl.txt file:
use_tempaddr - INTEGER
        Preference for Privacy Extensions (RFC3041).
          <= 0 : disable Privacy Extensions
           == 1 : enable Privacy Extensions, but prefer public
                  addresses over temporary addresses.
           >  1 : enable Privacy Extensions and prefer temporary
                 addresses over public addresses.
        Default:  0 (for most devices)
                 -1 (for point-to-point devices and loopback devices)
After network restart (a simple ifdown, ifup of the interface is enough), the output of the ip a command looks like that:
eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000
    link/ether 00:22:15:64:42:bd brd ff:ff:ff:ff:ff:ff
    inet 192.168.1.129/24 brd 192.168.1.255 scope global eth0
    inet6 2a01:f123:1234:5bd0:21f1:f624:d2b8:3702/64 scope global temporary dynamic 
       valid_lft 86314sec preferred_lft 2914sec
    inet6 2a01:f123:1234:5bd0:222:15ff:fe64:42bd/64 scope global dynamic 
       valid_lft 86314sec preferred_lft 86314sec
    inet6 fe80::222:15ff:fe64:42bd/64 scope link 
       valid_lft forever preferred_lft forever
A new temporary address has been added. After preferred_lft seconds, it becomes deprecated and a new address is added:
eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000
    link/ether 00:22:15:64:42:bd brd ff:ff:ff:ff:ff:ff
    inet 192.168.1.129/24 brd 192.168.1.255 scope global eth0
    inet6 2a01:f123:1234:5bd0:55c3:7efd:93d1:5057/64 scope global temporary dynamic 
       valid_lft 85009sec preferred_lft 1672sec
    inet6 2a01:f123:1234:5bd0:21f1:f624:d2b8:3702/64 scope global temporary deprecated dynamic 
       valid_lft 82077sec preferred_lft 0sec
    inet6 2a01:f123:1234:5bd0:222:15ff:fe64:42bd/64 scope global dynamic 
       valid_lft 86398sec preferred_lft 86398sec
    inet6 fe80::222:15ff:fe64:42bd/64 scope link 
       valid_lft forever preferred_lft foreverr
The deprecated address is removed when the valid_lft counter reach zero second.

Some more tuning

The default duration for a prefered adress is of one day. This can be changed by modifying the temp_prefered_lft variable. For example, you can add to sysctl.conf:
net.ipv6.conf.eth0.temp_prefered_lft = 7200
The default validity length of the addresses can be changed via the temp_valid_lft variable. The max_desync_factor set the max random time to wait before asking a new address. This is used to avoid that all computers in network ask for an address at the same time. On side effect is that if you set the prefered or valid time to a low value, the max_desync_factor must also be decreased. If not, there will be long time period without temporary address. If temp_prefered_lft is multiple time lower than temp_valid_lft, then the deprecated addresses will accumulate. To avoid overloading the kernel, a maximum number of addresses is set. Equal to 16 by default, it can be changed by setting the max_addresses sysctl variable.

Known issues and problems

As the temporary address is used for connection to the outside and has a limited duration, some long duration connections (tink ssh) will be cut when the temporary address is removed. I’ve also observed a problem when the maximum number of addresses is reached:
ipv6_create_tempaddr(): retry temporary address regeneration.
ipv6_create_tempaddr(): regeneration time exceeded. disabled temporary address support.
The result was that the temporary address support was disabled and the standard global address was used again. When setting temp_prefered_lft to 3600 and keeping temp_valid_ft to default value, the problem is reproduced easily.

Conclusion

The support of IPv6 privacy extensions is correct but the lack of link with existing connection can cause the some services to be disrupted. A easy to use per-software selection of address could be really interesting to avoid these problems.