KVM: Configure Mirrored Port’s Traffic to Be Visible in Guest (SNORT)

This one was tricky. If you want SNORT to run as a guest, getting the port mirrored traffic to it wasn’t exactly trivial.

My Host Setup:

NIC 1: Handles all bridges and VLANs

NIC2: Dedicated to SNORT, plugged directly into mirrored port on switch.


Before we start, make sure your SNORT guest is off and the bridge interface that you want to use for packet sniffing is down. Getting the packets to flow properly requires a very specific sequence and failure to do that will only result in frustration and confusion.

First Step (on KVM Host):

Edit /etc/network/interfaces to reflect for NIC2 (eth3 in my case):

auto eth3
iface eth3 inet manual

#SNORT Interface
auto br3
iface br3 inet manual
bridge_ports eth3
bridge_stp off

Bring up br3

ifup br3

I’ll need to test it, but you may ensure the host doesn’t get an address on that interface by adding in a line: address

Now, the tricky part is to get the traffic to show in the guest. On the host, just to make sure your port mirroring is working properly, do:

sudo tcpdump -i br3

You should see a ton of traffic if configured properly.

Now, to get the traffic not destined for the host to go to the guest (SNORT), type in:

brctl setageing br3 0
brctl setfd br3 0

Now, if you boot up your guest (it should have two interfaces), you should be able to tcpdump -i eth# and it will show the same large amount of traffic. The above is manual and will not persist across reboots.

To make it persistent in Ubuntu (replace br3 with your bridge interface name):

cd /etc/network/if-up.d

touch br3-mirror

chmod +x br3-mirror

nano br3-mirror

if [ "$IFACE" = br3 ]; then
brctl setageing br3 0
brctl setfd br3 0


3 thoughts on “KVM: Configure Mirrored Port’s Traffic to Be Visible in Guest (SNORT)

  1. Hi

    Your post was extremely helpful and informative! Would you also be able to elaborate a little more on this post on what to do once we have completed the mirrored port configuration on the KVM Host? As in, once I am done mirroring the traffic on my KVM guest, how can I revert back to the original settings on the KVM Host?

    I have multiple KVM guests. Rebooting the KVM Host is not an option for me. I am a newbie to KVM and I’m sorry if my question seems very trivial.



  2. A massive thank you for posting this – I’d been struggling with why traffic was visible on the host but not the KVM guest, now it works perfectly.

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