Sep 042012
 

A new Suricata IPS mode

Suricata IPS capabilities are not new. It is possible to use Suricata with Netfilter or ipfw to build a state-of-the-art IPS. On Linux, this system has not the best throughput performance. Patrick McHardy’s work on netlink: memory mapped I/O should bring some real improvement but this is not yet available.

I’ve thus decided to do an implementation of IPS based on AF_PACKET (read raw socket). The idea is based on one of the snort’s running mode. It peers two network interfaces and all packets received from one interface are sent to the other interface (if a signature with drop keyword does not fired on the packet). This requires to dedicate two network interfaces for Suricata but this provide a simple bridge system. As suricata is using latest AF_PACKET features (read load balancing), it was possible to build something really promising.

Victor Julien has commited my implementation which is now available in current git tree and will be part of Suricata 1.4beta1.

Configuration

You need to dedicate two network interfaces for this mode. The configuration is made via some new configuration variable available in the description of an AF_PACKET interface.

For example, the following configuration will create a Suricata acting as IPS between interface eth0 and eth1:

af-packet:
  - interface: eth0
    threads: 1
    defrag: yes
    cluster-type: cluster_flow
    cluster-id: 98
    copy-mode: ips
    copy-iface: eth1
    buffer-size: 64535
    use-mmap: yes
  - interface: eth1
    threads: 1
    cluster-id: 97
    defrag: yes
    cluster-type: cluster_flow
    copy-mode: ips
    copy-iface: eth0
    buffer-size: 64535
    use-mmap: yes

Basically, we’ve got an af-packet configuration with two interfaces. Interface eth0 will copy all received packets to eth1 because of the copy-* configuration variable:

    copy-mode: ips
    copy-iface: eth1

The configuration on eth1 is symetric:

    copy-mode: ips
    copy-iface: eth0

There is some important facts to consider when setting up this mode:

  • The implementation of this mode is dependent of the zero copy mode of AF_PACKET. Thus you need to set use-mmap to yes on both interface.
  • MTU on both interfaces have to be equal: the copy from one interface to the other is direct and packet bigger then the MTU will be dropped by kernel.
  • Set different values of cluster-id on both interfaces to avoid conflict.

The copy-mode variable can take the following values:

  • ips: the drop keyword is honored and matching packet are dropped.
  • tap: no drop occurs, Suricata act as a bridge

Please note, that mode can be different on the peered interfaces.

Current limitation

One other important point is that the threads variable must be equal on both interface. This is due to the fact the peering is done at the socket level: each packet received on a socket is sent to a peered socket listening to the peered interface.

At last but not least, you need at least Linux kernel 3.6 (or 3.4.12) to be able to set a threads value higher than 1. There is a bug in prior kernel that causes an infinite loop (packet being sent from an interface and reread, …). If you can’t wait or can’t made a full upgrade of your kernel you can use my patch which will be part of Linux 3.6 and of Linux 3.4.12.

Running it

Running Suricata in AF_PACKET IPS mode is straight forward. Once the configuration has been updated, you can simply run:

suricata -c /etc/suricata/suricata.yaml --af-packet

Please note that is possible to have normal IDS interface running simultaneously. For example, eth3 could be added to the af-packet configuration and used a regular interface.

Some implementation details

You can find some additional technical information in the commit.

As mentioned below, the peering is done at the socket level. In Suricata, each capture thread is opening a socket and is binding it to the cluster-id of the interface. Once a thread is started, it is peered with an thread listening to the copy-iface interface which socket will be used as a send interface. This system permit to avoid locking issue in workers running mode as only one thread will write to the peered socket at a time because we have only one packet treated by a capture thread at a time.

We need to reuse the interface for a simple reason: if a new dedicated socket was to be used, the packet sent to this socket would be received by Suricata read socket instead of being ignored. Linux kernel implement the natural feature of avoiding to send back packet to the sending socket.

This behavior was implemented in Linux kernel for regular socket but this was not the case for clustered sockets. This is why at least Linux 3.6 or my patch is needed to be able to use a value of threads bigger than one.

Conclusion

This mode is new in Suricata and is is really promising. It has suffered limited testing in high bandwidth environment. So, as usual, feedback is more than welcome. Don’t hesitate to send me a mail or to ask question on Suricata Mailing lists.