Feb 232012
 

Victor Julien has just pushed a new feature to suricata’s git tree. It brings improvements to the AF_PACKET capture mode.

This capture mode can be used on Linux. It is the native way to capture packet. Suricata is able to use the interesting new multithreading feature provided by AF_PACKET on recent kernels: it is possible to have multiple capture threads receiving the packet of a single interface.

The commits add mmaped ring buffer support to AF_PACKET capture and also provide a zero copy mode. Mmaped ring buffer is mechanism similar to the one used by PF_RING. The kernel allocates some memory to store the packets and share this memory with the capture process. Instead of sending messages, the kernel just write to the shared memory and the process capture reads it. This is less consuming in term of CPU ressource and helps to increase the capture rate. But the main avantage of this technique is that the capture process can treat the packets without making a copy and this saves a lot of time

To activate this features, you need a Suricata compiled from latest git and you need to modify some entries in your suricata.yaml file. You have to tell suricata that you want to activate the mmap feature. For example to activate the feature on eth0, you have to add ‘use-mmap’ to your configuration:

af-packet:
  - interface: eth0
    use-mmap: yes

You can then run Suricata with the command:

suricata -c suricata.yaml --af-packet=eth0

This setup will not activate the zero copy feature which is currently dependant of the running mode. You will need to activate the worker mode to enable zero copy. To do so, run Suricata with a command similar to this one:

suricata -c suricata.yaml --af-packet=eth0 --runmode=workers

This code should provide an interesting performance boost to the AF_PACKET capture system. I’ve no number to provide now but I will be happy to hear some if you make some tests

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