Snort mailing list archives
Re: Multiple "sniffing" interfaces
From: Bennett Todd <bet () rahul net>
Date: Wed, 23 Jul 2003 14:07:22 -0400
2003-07-23T12:00:10 Bryan Miller:
I was curious as to the list member's experiences with using multiple interfaces in "sniffing" mode.
Lots of folks do it. There are two fundamentally diffent models. When there are multiple logically distinct nets, and the incentive is simply to save hardware costs by letting one sensor device act as multiple logical sensors, then you run multiple snort instances. Note that none of the nets should be "high-traffic" (i.e. multiples of tens of Mbps total aggregate of the lot); the aggregate capacity of N snorts on one box is likely to be dramatically less than the 1/N times the capacity of a single snort on that box, if only due to context switching. The alternate model applies when you're using multiple interfaces simply because you don't want to buy a separate box to aggregate the traffic, but you are happy to have all the aggregated traffic examined by one snort. Sometimes --- e.g. when nets have high-availability with load-balancing and the packets associated with a single connection might traverse different links, or when you're using a network tap and the traffic in two directions is presented on two different nics --- you really _need_ to aggregate all the traffic into one snort instance. Other times it's not conceptually important, you could use one snort or many, but you don't mind aggregating it --- they all run the same config, and you don't need the alerts to distinguish which NIC the traffic arrived on --- and you don't want to pay the afore-mentioned performance hit for multiple snorts on one box. In this case, you use some OS or platform specific feature to aggregate the traffic. On some platforms, if you want snort to monitor _all_ interfaces including in particular the mgmt interface if any, you can tell snort to listen to "any". I believe the snort FAQ at www.snort.org mentions this. On some platforms, there are platform-specific drivers to aggregate traffic from multiple NICs and present it as one logical NIC. I've done this using the bonding driver on Linux, works great; performance is not measureably degraded relative to having an external device aggregate the traffic and present it on one NIC. -Bennett
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Current thread:
- Multiple "sniffing" interfaces Bryan Miller (Jul 23)
- Re: Multiple "sniffing" interfaces Bryan Irvine (Jul 23)
- Re: Multiple "sniffing" interfaces Bennett Todd (Jul 23)
- Re: Multiple "sniffing" interfaces Derya Sezen (Jul 24)