[Snort-users] Trigger anomalies (on LXC container versus host)

Doug Burks doug.burks at ...11827...
Sun May 3 13:19:49 EDT 2015

Hi Chris,

Have you verified that NIC offloading features are disabled?

On Sun, May 3, 2015 at 9:31 AM, Chris <berzerkatives at ...11827...> wrote:
> I'm observing a problematic difference in behaviour between two
> instances of Snort that are configured identically (recursive diff'ed
> their config dirs, and compared their initialisation outputs) aside
> from the required differences (interfaces names) as one is running
> inside an LXC container, listening to its single virtual interface, and
> the other instance is on the hypervisor/base OS listening to the bridge
> interface that all the containers are attached to. The container
> receives traffic through NAT'ing rules on the hypervisor.
> What I see is that certain rules aren't being triggered on the
> container instance of Snort, but are being triggered on the hypervisor.
> This is despite being able to see the packets that trigger these rules
> appear on both machines (hypervisor and container) using tcpdump to
> view the respective interfaces that Snort is configured to listen on.
> Specifically, the rules that I've noticed are being ignored are those
> that involve HTTP header inspection, like GET /test.cgi.
> Like I said, I can see what look like the EXACT SAME packets on these
> respective interfaces, so I've tried the following troubleshooting
> without any luck.
>  * Switching off Snort on the hypervisor in case it was interfering.
>  * Creating a rule that triggers for any packet that is considered to
>    be web traffic (i.e. EXTERNAL any -> HTTP HTTP_PORT) and this
>    triggers for those packets without issue, so it's not a problem with
>    those variables being misconfigured.
>  * Wondering whether LXC doesn't properly isolate the interfaces
>    somehow, so I tried configuring the container Snort to use the
>    bridge interface on the hypervisor, however it correctly wasn't able
>    to use it (as it didn't exist inside the container, of course).
> So I'm stuck as to where to go next. The container is where I want Snort
> to be running, as it's my load balancer (including SSL termination) so
> that's where I would like to detect and block rogue traffic. The only
> reason that I run it on the hypervisor is to just see whether any
> concerning traffic is bypassing the load balancer, and whether
> undesirable traffic is being generated by services behind it.
> Thanks for your time, I really hope someone can shed some light on this
> frustrating situation. Very happy to answer any questions about the
> setup, including configuration specifics, though they're essentially
> vanilla installions on Debian Wheezy straight out of apt.
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Doug Burks
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