Nemu (Netwok EMUlator) is a small Python library to create emulated networks and run and test programs in them.
Different programs, or copies of the same program, can run in different emulated nodes, using only the emulated network to communicate, without ever noticing they all run in the same computer.
Nemu provides a very simple interface to create nodes, connect them arbitrarily with virtual interfaces, configure IPv4 and IPv6 addresses and routes, and start programs in the nodes. The virtual interfaces also support emulation of delays, loss, and reordering of packets, and bandwidth limitations.
You can even start interactive sessions by opening xterms on different nodes, Nemu has special support for forwarding X sessions to the emulated nodes.
More advanced configurations, like setting up netfilter (iptables) rules, starting VPN tunnels, routing daemons, etc, are simply supported by executing the appropriate commands in the emulated nodes, exactly as if they were executed in real machines in a real network.
All this is achieved with very small overhead, thanks to the Linux kernel's network name spaces capabilities, part of the bigger Linux containers project.
To get a feeling of what you can do with Nemu, take a peek at this sample script that creates 3 interconnected nodes, runs some tests and then starts xterms running tcpdump so you can see the packets flowing from one node to the other.
Nemu was started as a research project at INRIA (Institut de Recheche en Informatique et Automatique, a French research institution) and now it is developed jointly by INRIA staff and external developers.
Nemu is now part of Debian! You can just apt-get install python-nemu and
start using it.