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Please tell me if what I'm trying to do is feasible.

I have a router named "R" which is connected to WAN. R allows adding rules to the routing table. There are numerous of machines connected to the LAN port of R, they all have ip addresses 192.168.1.* assigned with DHCP on R. Among those machines, there's a machine C with ip address 192.168.1.100. I want all traffic of other machines in the subnet to pass-through machine C where some filtering and logging will be done.

Is this possible? Is there a name for what I'm trying to do? (so i can do more googling later)

EDIT: C is running linux

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1 Answer 1

This can't be easily done with a plain static routing table because when C forwards the traffic on to R, it will be redirected back.

The simplest solution is to set C as the default gateway on all other machines. Some routers allows you to change the default gateway field in the DHCP message. If your don't, you can still disable the DHCP featuer on the router and use a software implementation on C.

An alternative is to do monitering on the second layer. There are some software that will do ARP spoofing and allow your computer to get traffic that were going to the router, however this does not work with all setups and sometimes cause firewall alerts. If you have some budget, the simplest solution is to buy a switch and connect the uplink to a hub which is also connected to C and the router. But you may as well get a second NIC and use C as the router.

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I can change the default gateway field in the DHCP message, What should I do to pass all the traffic from C back to R? ( C is running linux ) Because, eventually R is connected to WAN. –  xiamx Nov 29 '12 at 23:23

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