Tweaking promiscous mode sniffing performance

[Martin Olsson <elof () sentor se>]

I'm trying to get the most out of my FreeBSD 4.x machine, and now I have a
question:


When an interface is put into promiscous mode, it forwards all received
packets to the kernel. The kernel (the IP stack) will check each packet to
see if it is addressed to the local machine.

My snort is listening to a Span-port which generate about 600 Mbps of
traffic. Not a single one of these billions of packets are addressed to
the sniffer machine.

I think it is pretty unneccesary for the kernel to waste CPU on all these
packets. (not only is the IP stack checking them, maybe you have a
firewall and ipsec configuration that the billions of packets are matched
against)

Is there any way to tell the kernel not to process the incoming packets on
a given interface, while sniffer programs still can read the raw data?


(My sniffer interface have no IP address.
Another interface, the management interface, do have an IP address and on
that interface I want the kernel to process all packets.)

/Elof



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