[Snort-users] tranparent proxy client IP not showing in alert

Trembly.MaryEtta Trembly.MaryEtta at ...15614...
Tue May 1 10:40:09 EDT 2012

We have an issue with configuring snort to alert on the client side of a transparently proxied web connection. We are mirroring the client side of the connection to an interface on our snort sensor.

It seems we should see alerts showing the client IP but the alerts we see are only showing the proxy IP.  If we tell snort to ignore the proxy IP, we do not get any alerts. Using tcpdump we are able to extract packets that show the source IP as the client and dest IP as the external web server...the same way the client sees the traffic. Shouldn't snort be able to alert on these packets while ignoring the web proxy IP?

We are aware of an enable_xff option for snort to extract the X-Forward record; unfortunately barnyard2 is not able to extract the Original Client info to send to the database on any version of snort newer than This version of snort is too old for our use.

I believe snort is actually following the MAC Address and matching it to the web proxy IP, and ignoring the packets because it can match the MAC to the proxy. The captured packets that show the client talking "directly" to the external web server IP have the mac address of the web proxy, that is how the packets get transferred through the web proxy.

Is there a way to tell snort not to try to match the MAC to IP?


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