stutterfuzz

stutterfuzz is a fuzzer & stress tester for TCP servers. Using a directory of input files to be sent, it rapidly opens many connections in parallel to a single server and sends one file over each connection. It randomly varies packet boundaries each time to attempt to trigger buffering & parsing issues on the server.

Input files

To generate input files for your server, you can start with some hand-crafted client -> server protocol data, or dumps from existing connections. To expand this test set to exercise more paths in your server code, consider using american fuzzy lop. Note that you may need to modify your server to accept data from stdin to use afl. It's worth the effort.

Running the server

Connecting over loopback is best (for speed), so it's better if you can run stutterfuzz and your server on the same machine. Consider making your server valgrind clean and running it under strict valgrind options:

valgrind --track-fds=yes --show-leak-kinds=all --leak-check=full ./yourserver --flag1 --flag2=value --dont-fork

This enables you to detect more cases of problems triggered by stutterfuzz.

Running stutterfuzz

sudo apt-get -y install build-essential git clang
git clone https://github.com/flamingcowtv/stutterfuzz
cd stutterfuzz
make
./stutterfuzz --blob-dir=/path/to/source/directory --host=::1 --port=6789

stutterfuzz will print statistics about source files (check these for sanity), then continously print runtime statistics.

Stats

rounds: number of passes through all input files
mean_cycles_to_connect: number of cycle-ms length cycles we make to connect, average. > 1.00 here may indicate network latency, or that the server is overloaded and you should increase --cycle-ms or lower --num-conns
ready_to_send: portion of attempts that a connection has received acks for all outgoing data and is ready to send more. < 1.00 here indicates network latency in excess of --cycle-ms, or delayed ack sending from the receiver.

Good statistics look like this:

Stats: rounds=7400, mean_cycles_to_connect=1.00, ready_to_send=1.00

Bad statistics look like this:

Stats: rounds=2400, mean_cycles_to_connect=68.08, ready_to_send=0.48

When will it stop?

stutterfuzz never exits normally; the search space is too large to be deterministic. Hit ctrl-c to exit manually. It will also exit with an error if any connections to the server fail, assuming that it has triggered a problem (or is being blocked).

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