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Here's the setup:

Host A : 192.168.0.5/24
Host B : 192.168.0.6/24
Host C : 192.168.0.1/24

All connected via switch.

I want to monitor traffic between A and B by setting a route on A and B to use C as their GW for each other.

I can get this to work fine if A and B are on different subnets, but was hoping there would be a way to do it with all on the same subnet.

S0 first I setup routes on both A and B by issuing

route add -host B gw C

(and vice versa for B)

On C i enable forwarding. When i run wireshark on C, and then ping from A to B, I see the ping request come in from A, with A's MAC sent to C's MAC. then C retransmits using C's MAC to B's MAC.

A and B both have 'accept_redirects' off so that they are forced to use C.

Problem: pinging from A to B never works. Wireshark on B never shows an incoming ping request.

Wireshark on C shows its being sent to B's ip/mac, but it looks like B never gets it, or its being filtered out somewhere below wireshark.

any hints? what could be filtering out the packet?

  • Proxy ARP. Or Ettercap to implement ARP poisoning. Best option is to mirror the switch ports for A and B to C, but that isn't usually a feature on consumer grade networks. – roaima Jan 20 at 16:50
  • Dont think proxy_arp will make a difference since A and B already know to send their packets to C. C is forwarding them (according to Wireshark), but the receiving end does not show their receipt in wireshark. – patrick h Jan 20 at 17:49
  • Currently, can B ping C? – A.B Jan 20 at 20:30
  • Yes, both A and B can ping C, and C and ping both. I'm wondering if there is some filtering happening down under somewhere in B ???? Firewalld is disabled. Rp_filter off (but shouldn't be an issue here anyway I don't think) – patrick h Jan 20 at 20:37
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Try arpspoof from the dsniff package.

It won't work if the two hosts can protect against ARP Poisoning since this is exactly how it works.

Shell 1

# Enable packet forwarding
echo 1 > /proc/sys/net/ipv4/ip_forward

# Tell A we are B
arpspoof -t hostA hostB

Shell 2

# Tell B we are A
arpspoof -t hostB hostA

Shell 3

# Watch the traffic
tshark -f 'host hostA and host hostB and not arp'
  • Not sure about this fix in my situation, because my routing box already sees the ping from A-to-B and forwards it to B's MAC, and vice verse. The problem is wireshark on B never shows receipt of the request from A (even tho wireshark on router shows forwarding to B's mac) – patrick h Jan 20 at 18:49
  • You said there is no routing, so the device is acting as a switch, and therefore susceptible to ARP spoofing per this solution. Without this your Host C won't see anything - that's exactly what switches are supposed to do. – roaima Jan 20 at 22:32
  • There is an explicit static route on A and B to each other via C. – patrick h Jan 21 at 15:35
  • That is completely contradictory to the information provided in your question, where it says, "Host A: 192.168.0.5/24 / Host B: 192.168.0.6/24 / Host C: 192.168.0.1/24". Which is correct? – roaima Jan 21 at 16:14
  • I'm sorry, I think you might have missed this part, "S0 first I setup routes on both A and B by issuing route add -host B gw C (and vice versa for B)" – patrick h Jan 21 at 21:17

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