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How to capture a sequential stream of data between two machines, making no assumptions about network layer protocols? On wired network, one can do it by ARP spoofing and then sniff the network. One wireless network, could it be possible? Or could one disguise its MAC and receiving data from both S and D at the same time?

In addition, some WiFi has password on it, and some doesn't. It is said that using a WiFi that doesn't have a password is significantly insecure. Why? How could others attack or monitor my activities with a network without password?

One more question. It is said that facilities like airpcap can be used to do such things like monitoring the WiFi network. What is its principles? Could I achieve similar goals without such kind of facilities? (PS: airpcap is fairly not cheap :/ )

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The first step in any sort of MITM attack on a network is connecting to the network. With a wired network, that involves somehow connecting your machine to the network through the use of an Ethernet cable. With a wireless network, you just need to connect to the network.. well, wirelessly. Without a requiring a password to connect to a wireless network, anyone can simply sonnect to the network and start sniffing away.

Once you are on the network itself, all the same principles of a MITM attack on a wired network applies. You do not need any expensive hardware to perform the attack, just a laptop with a wireless card that supports monitor mode.

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    If the network does not require a password (an open network), then actually you can sniff all of the traffic without even connecting to the network, which is an added bonus for the attacker. None of the traffic is encrypted, so the attacker can simply see all of the plaintext data in the air. This means they do not even have to be a MITM or perform any sort of MITM attack (ARP spoofing or otherwise), unless they wish to modify in-transit data. That is one of many reasons why open networks are not secure. – Anorov May 4 '13 at 4:26
  • "without even connecting to the network" this means that I should probably using some facilities to sniff the network. Is that true? – stcheng May 5 '13 at 1:50
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You can technically start sniffing away without "connecting" to the network. Terry is correct, if the network is open (no encryption, WEP/WPA/WPA2) then you can just "Join" the network and sniff the traffic. However, you do not need to join the network to sniff the traffic. WLANs use radio frequencies, all you have to do is match the freq (channel) and have a wireless card that you can put into monitor mode.

If the network is using encryption you can still sniff all the packets. The only problem is that it will be encrypted. However, if you manage to capture specific packets you can break the encryption (WEP) or get the key (WPA/2) and then decrypt all the data you captured while still not "Joined" to the network.

Like Terry said, Wireless is wireless, it's like broadcast television. If you tune to the right frequency (channel) you pick up data.

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