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Can a Cisco router act as an IDS for LAN traffic that does not pass through it? Specifically, can traffic passing through a switch be port mirrored to an interface on the router with the router acting as an IDS?

I am looking for a way to use an ISR G2 router like Snort to monitor traffic between devices on a LAN.

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  • what model/ feature set do you have? regardless i don't think there is anything that will work exactly like you think that it may work. – John Kennedy Nov 25 '13 at 16:14
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I am looking for a way to use an ISR G2 router like Snort [snip]

Your entire question starts and stops right there. The IPS/IDS within IOS is not designed for out-of-band traffic inspection. The router must be inline to inspect it.

Put simply, IOS is not SNORT. If you want SNORT, run SNORT.

(Also, the IPS/IDS within IOS is very slow -- being done entirely in the way under powered router CPU -- and often incomplete. You'll need a support contract to keep it remotely up to date.)

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No,

But what you CAN do is set up a box with SNORT.

Or you can apply an access-list to your Span session port that is just "permit ip any any log" and that will log traffic source and destination addresses (along with TCP/UDP port info). Then if you can redirect these log messages to a syslog server you could export them in realtime to a CSV and have an application like SNORT alalyze the traffic flows. Or you can log them locally on the device and then extract/parse them manually to look for anomalous traffic flows.

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I think John Kennedy is right, there isn't anything out there that seems like it fits the bill on your questions; but I'll give a wag at it anyhow.

Can a Cisco router act as an IDS for LAN traffic that does not pass through it?

No, although there are certain Cisco devices that can act as an IDS/IPS. Cisco has an entire line of products specializing in this, specifically.

These will likely be massive overkill for your situation though.

Specifically, can traffic passing through a switch be port mirrored to an interface on the router with the router acting as an IDS?

This has always been possible with port-mirroring (SPAN ports). They functionally do exactly what you're looking for. They replicate traffic ingressing/egressing an interface/VLAN and spit it out on a pre-defined port/VLAN.

I am looking for a way to use an ISR G2 router like Snort to monitor traffic between devices on a LAN.

I'm not so sure this model supports custom code (like snort) in the way you're expecting it to.

Although not as elegant, you could always set up a standalone machine to run your IDS applications on.

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  • Cisco IOS routers cannot inspect traffic not passing through them - PERIOD. (Cisco IDS appliances are not routers) Again, the ability to "span" traffic isn't the question; an IOS router cannot inspect traffic that did not pass through it. – Ricky Nov 25 '13 at 21:54
  • @RickyBeam You are correct, IOS cannot inspect traffic and the models listed won't act as a routing device. However, how is SPAN'ing not part of the question? He is asking about port-mirroring (i.e. Cisco SPAN technology)? – Ryan Foley Nov 25 '13 at 22:05
  • short version: "can I SPAN traffic into the router acting as an IDS" Answer: still no. (the span isn't the question) Traffic must naturally flow through the router for the IOS IDS engine to work. – Ricky Nov 25 '13 at 22:24

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