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When does the Cisco ASA IPS module inspect traffic?

with 2 comments

Sure we can put an IPS module inside an Cisco ASA firewall but the first you may ask or will be asked is at what point does that traffic get inspected by the IPS Module? Which might not seem like a tough question but it can make you think if you got VPNs terminating on your ASA, does it inspect the traffic before encryption, after encryption, or is encrypted traffic bypassed? I did a post about the packet flow through a Cisco ASA some time ago and it did mention the IPS is involved after NATs, application inspection, and even ACLs however there was no mention of how encrypted VPN traffic.

Well, as it turns out the traffic is redirected to the IPS module after it enters the ASA on the ingress interface, the VPN traffic is decrypted, and the firewall inspection policies have been applied. Now for outgoing VPN traffic the IPS inspects that traffic and then it is encrypted and sent through the IPSec connection. Makes perfect sense.

Here is a quick visual from

ASA IPS Inspection

You’ll notice once the traffic enters from the outside interface, if it is VPN traffic it is then decrypted sent though the firewall policies and is then diverted to the IPS module. Once the IPS module has had a chance to inspect the traffic it is then sent back to the ASA to be sent out the inside interface.

Something I want to point out the IPS Modules work in 2 different modes, inline mode which is shown above where the traffic is sent through the IPS module and back into the ASA this mode may degrade throughput depending on the model of the module and the amount of traffic being inspected. We also have Promiscuous mode where the traffic is not sent directly though the IPS Module but the IPS module instead receives a copy of the traffic to inspect. The upside of promiscuous mode is the fact throughput is not effected however it is less secure than inline mode since the traffic sent to its destination at the same time it sent to the IPS module, the IPS module however can sent a RST packet ending connection or ‘shun’ the offensive IP address. When an address is ‘shunned’ any traffic from that source is dropped.

Here is a quick visual of promiscuous mode:

ASA IPS Inspection Promiscuous

You can read more about the IPS ASA module inspection process here.

Written by Stephen J. Occhiogrosso

December 28, 2012 at 12:35 PM

2 Responses

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  1. […] ago before and I wanted to continue the trend into a few more IPv6 related posts but that last IPS post spiked my interest, so I had to publish that one.  Now we know the addressing scheme is different […]

  2. […] Source: When does the Cisco ASA IPS module inspect traffic? […]

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