There's a fairly lengthy tutorial on disk91.com that looks promising. It's a little dated and based on Wheezy rather than Jessie, but I think it should at least provide some pointers.
A USB ethernet dongle was connected to an original model B Pi. The network configuration was then changed to allow traffic to flow between the built in network adapter and the dongle. They used dnsmasq to allow the Pi to act as a DNS server to a device on the local network.
To create given conditions on the network they used tc. They chose to manipulate connection bandwidth and latency, but from the look of the man page you can play with it to create a wide range of disturbances. To adjust the parameters they built a simple locally hosted web page which passed form inputs to a CGI script.
At the time of writing the tutorial page includes a downloadable script which should automagically set up their limited bandwidth/latency scenario.