Month: April 2011

IPv6 privacy extensions on Linux

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.

Joining the OISF coding staff

My collaboration with OISF has been announced today. This is an honor for me to join this excellent team on this wonderful project. I’ve taken a lot of pleasure in the past months contributing to the project and I’m sure the start of an official collaboration will lead to good things. The challenge is high and I will do my best to merit the trust.

A big thanks to all people who congrat me for this nomination.

Building Suricata under OpenBSD

Suricata 1.1beta2 has brought OpenBSD to the list of supported operating system. I’m a total newbie to OpenBSD so excuse me for the lack of respect of OpenBSD standards and usages in this documentation.

Here’s the different step, I’ve used to finalize the port starting from a fresh install of OpenBSD.

If you want to use source taken from git, you will need to install building tools:

pkg_add git libtool

automake and autoconf need to be installed to. For a OpenBSD 4.8, one can run:

pkg_add autoconf-2.61p3 automake-1.10.3

For a OpenBSD 5.[01], one can run:

pkg_add autoconf-2.61p3 automake-1.10.3p3

For OpenBDS 5.2:

pkg_add autoconf-2.61p3 automake-1.10.3p6

Autoconf 2.61 is know to work, some other versions triggers a compilation failure.

Then you can simply clone the repository and run autogen:

git clone git://phalanx.openinfosecfoundation.org/oisf.git
cd oisf
export AUTOCONF_VERSION=2.61
export AUTOMAKE_VERSION=1.10
./autogen.sh

Before running configure, you need to add the dependencies:

pkg_add gcc pcre libyaml libmagic libnet-1.1.2.1p0

Now, we’re almost done and we can run configure:

CPPFLAGS="-I/usr/local/include" CFLAGS="-L/usr/local/lib" ./configure --prefix=/opt/suricata

You can now run make and make install to build and install suricata.

Some new features of IPS mode in Suricata 1.1beta2

The IDS/IPS suricata has a native support for Netfilter queue. This brings IPS functionnalities to users running Suricata on Linux.

Suricata 1.1beta2 introduces a lot of new features related to the NFQ mode.

New stream inline mode

One of the main improvement of Suricata IPS mode is related with the new stream engine dedicated to inline. Victor Julien has a great blog post about it.

Multiqueue support

Suricata can now be started on multiple queue by using a comma separated list of queue identifier on the command line. The following syntax:

suricata -q 0 -q 1 -c /etc/suricata.yaml

will start a suricata listening to Netfilter queue 0 and 1.

The option has been added to improve performance of Suricata in NFQ mode. One of the observed limitation is the number of packets per second that can be sent to a single queue. By being able to specify multiple queues, this is possible to increase performances. The best impact is on multicore systems where it adds scalability.

This feature can be used with the queue‐balance option the NFQUEUE target which was added in Linux 2.6.31. When using −−queue−balance x:x+n instead of −−queue−num, packets are then balanced across the given queues and packets belonging to the same connection are put into the same nfqueue.

NFQ mode setting

Presentation

One of the difficulty of IPS usage is to build an adapted firewall ruleset. Following my blog post on this issue, I’ve decided to implement most of the existing mode in Suricata.
When running in NFQ inline mode, it is possible now to use a simulated non-terminal NFQUEUE verdict by using the ‘repeat’ mode.
This permit to send all needed packet to suricata via this a rule:

iptables -I FORWARD -m mark ! --mark $MARK/$MASK -j NFQUEUE

And below, you can have your standard filtering ruleset.

Configuration and other details

To activate this mode, you need to set mode to ‘repeat’ in the nfq section.

nfq:
  mode: repeat
  repeat_mark: 1
  repeat_mask: 1

This option uses the NF_REPEAT verdict instead of using a standard NF_ACCEPT verdict. The effect is that the packet is sent back at the start of the table where the queuing decision has been taken. As Suricata has put on the packet the mark $repeat_mark with respect to the mask $repeat_mask, when the packet reaches again the iptables NFQUEUE rules it does not match anymore because of the mark and the ruleset floowing this rule is used.

The ‘mode’ option in nfq section can have two other values:

  • accept (which is default simple mode)
  • route (which is little bit tricky)

The idea behind the route option is that a program using NFQ can issues a verdict that has for effect to send the packet to an another queue. This is thus possible to chain softwares using NFQ.
To activate this option, you have to use the following syntax:

nfq:
  mode: route
  route_queue: 2

There is not much usage of this option. One mad mind could think at chaining suricata and snort in IPS mode 😉

The nfq_set_mark keyword

Effect and usage of nfq_set_mark

A new keyword nfq_set_mark has been added to the rules option.
This is an enhancement of the NFQ mode. If a packet matches a rule using nfq_set_mark in NFQ mode, it is marked with the mark/mask specified in the option during the verdict.

The usage is really simple. In any rules, you can add the nfq_set_mark keyword to specify the mark to put on a packet and what is the the 

pass tcp any any -> any 80 (msg:"TCP port 80"; sid:91000004; nfq_set_mark:0x80/0x80; rev:1;)

That’s great but what can I do with that ?

If you are not familiar with the advanced network capabilities, you may wondering how it can be used. Nefilter mark can be used to modify the handling of a packet by the network stack, ie routing, quality of service, Netfilter. Thus, the concept is the following, you can use Suricata to detect a suspect packet and decide to change the way it is handled by the network stack.
Thanks to the CONNMARK target, you can modify the way all packets of the connection are handled.

Among the possibilities:

  • Degrade QoS for a connection when a suspect packet has been seen
  • Trigger Netfilter logging of all subsequent packets of the connection (logging could be done in pcap for instance)
  • Change routing to send the trafic to a dedicated equipment