Vaurien: The Chaos Proxy

I had a need to test out a program's behavior when its backend web server returned errors. Unit testing would have been difficult in this case, because I specifically wanted to test the program's macro result when encountering, say, 10% server response errors.

After searching around, I came across Vaurien, the Chaos TCP Proxy. It's seriously cool. As its name suggests, it's a TCP proxy server that you can route a local connection through to a backend server. In TCP mode, it can delay packets, insert bad data, or drop packets, testing the socks (pun intended) off your application.

It also supports the application-level protocols: HTTP, Memcache, MySQL, Redis, and SMTP, and its architecture is very modular, so it's easy to plug in new protocols.

For example, here's how I can run it in HTTP mode:

$ vaurien --protocol http --proxy 127.0.0.1:8888 \
          --backend www.google.com:80 --behavior 50:error

This says to run an HTTP proxy that connects to google.com on the backend and return a 5xx HTTP error code 50% of the time. Testing it out with curl:

$ curl --head -H "Host: www.google.com" http://127.0.0.1:8888/
HTTP/1.1 200 OK
...

$ curl --head -H "Host: www.google.com" http://127.0.0.1:8888/
HTTP/1.1 500 Internal Server Error
Content-Type: text/html; charset=UTF-8

$ curl --head -H "Host: www.google.com" http://127.0.0.1:8888/
HTTP/1.1 502 Bad Gateway
Content-Type: text/html; charset=UTF-8

Filed this away as a super useful tool.