[Snort-users] Vlan Tagging Issue with Snort

infosec posts infosec.posts at ...11827...
Fri Sep 10 10:38:36 EDT 2010


On Fri, Sep 10, 2010 at 9:05 AM, Joel Esler <jesler at ...1935...> wrote:
> So, just to clarify, if you don't run the bpf instances, does everything get analyzed properly?
> Joel
> On Sep 9, 2010, at 7:05 PM, infosec posts wrote:
>> Greetings Snort Gurus,
>> Our network folks were recently forced to make some architectural and
>> equipment changes that are creating an issue with our snort
>> monitoring.  The core of the problem is that on the switch that sends
>> our IDS sensor traffic via a monitor port, outbound traffic is not
>> tagged, but inbound traffic (from the Internet) come across at trunk
>> and have 802.1Q vlan tags.  This cannot be changed due to
>> architectural restrictions and (sad/ridiculous/stupid) limitations in
>> the network swtiches.  I know that snort supports and understands vlan
>> tagging, but because only one direction of the traffic is tagged, it
>> is creating a problem for us.
>> Our IDS box runs two snort instances, with filters like this:
>> BPF="not port 80 and not port 443"
>> BPF="port 80 and port 443"
>> In order to see the inbound traffic, I include the "vlan" filter, like so:
>> BPF="vlan and not port 80 and not port 443"
>> ...but then I *only* see inbound traffic, and not outbound.
>> If I include no BPF filters at all, I can see all inbound and outbound
>> traffic, but then I can't filter ports I don't want to see on a given
>> snort instance.  Tcpdump exhibits the same issue on this box, so I've
>> been testing with it, and have had the following results:
>> tcpdump –nni eth1 : all inbound and outbound traffic (no filter applied)
>> tcpdump –nni eth1 port 80 : only outbound port 80; tagged traffic is excluded
>> tcpdump –nni eth1 vlan and port 80  : only inbound port 80; un-tagged
>> traffic is excluded
>> tcpdump –nni eth1 vlan or port 80  :  only inbound traffic; not sure
>> why this doesn’t catch outbound 80 as well
>> tcpdump –nni eth1 vlan or not vlan :  all inbound and outbound traffic
>> tcpdump –nni eth1 vlan and not vlan :  all inbound and outbound traffic
>> tcpdump –nni eth1 vlan or not vlan and port 80 :  no traffic
>> tcpdump -nni eth1 port 80 and vlan : “expression rejects all packets”
>> A couple of other notes:
>> -The platform is RHEL5.
>> -We are limited to one monitor session on the swtich feeding the sensor.
>> At this point, I think I'm down to a last resort of running separate
>> snort instances for inbound and outbound, but I'd really prefer not to
>> add the resource and administrative overhead.  We plan to test feeding
>> the monitor session into an access port on a second switch to strip
>> the vlan tags on the inbound packets, but I don't have high hopes for
>> that being successful.
>> Am I missing something?  Any other ideas or suggestions?
>> Thanks in advance.
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