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Let's get straight to the point. This is my fictive network architecture:

enter image description here

My question is: Would it be possible for Laptop B to intercept traffic sent and received by Laptop A? Please note that B only has WiFi access and no physical control over Router B. Furthermore, Router A and B create different wireless networks with different encryption keys.

If both shared the same wireless network or Laptop B had LAN-access to Router B, the answer would be trivial, but I'm not sure how the answer turns out in this scenario :-/

Thanks for your valuable help!

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Is laptop B without reception range of the wireless signals transmitted by Laptop A? If it is then you will also need to look at a second danger in your fictional setup. (Even with different encryption keys) –  Hennes Sep 26 '12 at 21:04
    
Laptop A and B are in reception range, yes. They are also both in reception range with both wireless routers. Care to explain what would be the second danger here? –  Alex Sep 26 '12 at 21:12
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Your question included intercept traffic sent and received by Laptop A. Laptop A in your schematic does this via WiFi. Which means that anyone close enough can intercept this radio traffic. They will get the encrypted data. If they figure out the right key then they can decrypt it. How hard it is to find this key depends on the protocol used, the amount of data capture and how much CPU power you can throw at it. (WEP takes seconds to crack. WPA2 is hard, but using a cluster of cheap amazon instances you can often crack it in hours.) –  Hennes Sep 26 '12 at 21:25

2 Answers 2

If wifi is the same subnet as the LAN between routers you can try arp poisoning and advertise the LAN clients (router A) yourself as gateway which makes them send all traffic to you and you then send it to router (It depend on your Router whether it will work on not, does it act like a switch between WLAN and LAN or as a router).

On windows you can use Cain&Able (Check this link for some info about ARP poisoning)

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Thanks for sharing. Router A is, well, a wireless router to its wireless clients, and acts as a LAN-client to Router B. I understand the principle of ARP poisoning, but I'm not quite sure about the following: As Router A has to act as a client to Router B (or vice versa), both routers would be required to be in the same subnet. Router A however could broadcast another subnet to its wireless clients as Router B does to its clients. How would this affect the possibility of ARP poisoning? –  Alex Sep 26 '12 at 21:15
    
ARP poisoning can be only used for traffic that happens in same subnet. so with ARP poisoning you can only catch traffic that goes through the subnet that your laptop is(if you're inbetween routers LAN subnet), like the traffic from Laptop A to the Internet, because it has to pass through the LAN betwen the routers. You could not be able to catch the traffic from lets say Laptop A to some other device that is in Green WiFi (Laptop A wifi), because that traffic would never cross throuhg the Router A to the inbetween routers LAN –  Alen Sep 26 '12 at 21:24

It depends whether it's a true router or merely a hub. A true router will spot packets from A with addresses on the WAN and send them only out onto the WAN. A hub will likely also broadcast the packets locally, visible to B as well.

Here's a pretty decent article explaining, Router vs Switch vs Hub.

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Thank you for your answer. Both devices are true routers. So theoretically intercepting traffic should not be possible. But what about the ARP poisoning @Alen mentioned? –  Alex Sep 26 '12 at 21:18

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