Sniffing (network wiretap, sniffer) FAQ
2.3 How can I configure my local network to make sniffing harder?
Replacing your hub with a switch will provide a simple, yet effective defense against casual sniffing.
While this solution is extremely effective in practice (and should be strongly considered), it shouldn't be relied upon as a complete defense against sniffing. A switch still creates a "broadcast domain", providing an attacker the ability to spoof ARP packets.
The easiest such exploit is the "router redirection". ARP queries contain the correct IP-to-MAC mapping for the sender. In order to reduce ARP traffic, most machines will cache this information that they read from the query broadcasts. Therefore, a malicious attacker can redirect nearby machines to forward traffic through it by sending out regular ARP packets containing the router's IP address mapped to its own MAC address. All the machines on the local wire will believe the hacker is the router, and therefore pass their traffic through him/her.
A similar attack would be to DoS a target victim and force it off the network, then begin using its IP address. If a hacker does this carefully, s/he can inherit connections already established without dropping them. Windows machines are even so polite that when they come onto the network and see someone else using their address, they will kindly shut down their own TCP/IP stacks and allow this to continue. SMB (the Windows file sharing protocol) is also kind enough to allow predictable identifiers, allowing cr/hackers to predict enough information to keep the connection going.
Most intrusion detection systems and even network management tools like the Expert Sniffer(tm) will detect these shenanigans. For example, putting the BlackICE IDS on all the Windows end-nodes or hooked to a normal port (to receive broadcasts) will alert the security admin that such things are taking place (but, will generate false positives when DHCP reassigns addresses. Sigh.)
Most Ethernet adapters allow the MAC address to be manually configured. Thus a hacker can spoof MAC addresses by reassigning the address on the adapter, or by bypassing the built-in stack and hand-crafting frames. The hacker must maintain a a constant stream of outgoing frames in order to convince the auto-learning switch that they are the legitimate owner of the MAC address.
Many (most??) switches allow MAC addresses to be configured statically in order to prevent this sort of thing. While it may be a difficult management burden to do this for all end-nodes, it may prove useful for the router, restricting the hacker to wiretapping individual end-nodes instead of everyone all at once.
Some switches can be kicked out of "bridging" mode into "repeating" mode where all frames are broadcast on all ports all the time. This is done by overflowing the address tables with lots of false MAC addresses. This can be done with a simple traffic generation phase, or by sending a continual stream of random garbage through the switch