[Snort-users] Snort Flex response layer 2 address issue

Jeff Nathan jeff at ...950...
Wed Feb 3 09:01:05 EST 2010

Nope, I don't think you're missing anything.  As written, sp_respond2
does exactly what you say it does when you configure link-layer
responses.  One solution would be to support the specification of the
mac address of the next hop router when using link-layer injection.

I'm actually surprised that no one spotted this sooner - it's been 6 years. :)


On Tue, Feb 2, 2010 at 11:37 PM, Carl <carl at ...14760...> wrote:
> We have snort running on a box with two nics, eth0 is configured as the
> management interface and eth2 is configured as the monitoring interface.
> We are mirroring all interesting traffic to eth2 of the snort box. We
> are trying to test the flex response, however, we never see the resets
> generated by snort reach the client or server. I ran a tcpdump on the
> snort boxes management interface and we see flex response is trying to
> send the resets, however, it seems to pull they layer2 info from the
> alert that triggered the flex response and uses it when sending the reset.
> Here is a capture of the traffic snort saw that generated the flex response
> 21:30:50.703838 00:1d:b5:c4:ac:fe > 00:0b:45:17:78:00, ethertype IPv4
> (0x0800), length 66: (tos 0x0, ttl 58, id 52741, offset 0, flags [DF],
> proto TCP (6), length 52)
>     x.x.x.163.39485 > y.y.y.20.80: Flags [F.], cksum 0x194a (correct),
> seq 2068258091, ack 3628601739, win 92, options [nop,nop,TS val
> 529950849 ecr 2742540099], length 0
> Here is a capture of the reset that snort tries to send out
> 21:07:24.702620 00:1d:b5:c4:ac:fe > 00:0b:45:17:78:00, ethertype IPv4
> (0x0800), length 54: (tos 0x0, ttl  64, id 44352, offset 0, flags
> [none], proto: TCP (6), length: 40) x.x.x.163.35652 > y.y.y.20.http: R,
> cksum 0xa867 (correct), 1675:1675(0) ack 1495 win 0
> 21:07:24.702601 00:0b:45:17:78:00 > 00:1d:b5:c4:ac:fe, ethertype IPv4
> (0x0800), length 54: (tos 0x0, ttl  64, id 17445, offset 0, flags
> [none], proto: TCP (6), length: 40) y.y.y..20.http > x.x.x.163.35652: R,
> cksum 0xa6f7 (correct), 1679:1679(0) ack 1859 win 0
> Notice that the mac-addresses are the same as the original traffic.
> This will not work since the client and server on different subnets -
> the reset has to be routed so the destination mac-address of the reset
> should be set to the mac-address of snorts default gateway otherwise the
> router would never pull the packets off the wire for it to be routed.
> Are we missing a setting to get flex response working when the client
> and server exist on separate networks?
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