This is a Ruby interface to Free/NetBSD's filemon(4) device, which allows for tracing of file operations of a process and its children.

It is not a security tool, but intended for auditing processes for determining file dependencies.

One example of real-world practical use is for accelerating FreeBSD/NetBSD world building using bmake's meta mode.

Add this line to your application's Gemfile:

gem 'filemon'

And then execute:

$ bundle

Or install it yourself as:

$ gem install filemon

The filemon device works by writing tracing data to a file descriptor. For the time being, this interface only provides a means of configuring that, what you do with the result is up to you.

You may need to kldload filemon before any of this works.

To monitor a forked process, this mirrors the code documented in the FreeBSD man page:

monitor = Filemon::Device.new
monitor.fd = File.new('filemon.out', 'w')

pid = fork do
  monitor.pid = $$
  # Do something here.
end

Process.waitpid(pid)
monitor.close

But nothing stops you from simply monitoring the current process:

monitor = Filemon::Device.new(fd: STDERR, pid: $$)
# Do something here.
monitor.close

Or indeed any pid your user has permission to trace.

A simple command-line tool is provided for tracing commands:

% bin/filemon sleep 1
# filemon version 5
# Target pid 53942
# Start 1497269126.786684
V 5
E 65204 /bin/sleep
R 65204 /etc/libmap.conf
R 65204 /usr/local/etc/libmap.d
R 65204 /var/run/ld-elf.so.hints
R 65204 /lib/libc.so.7
X 65204 0 0
# Stop 1497269127.857683
# Bye bye

And one for monitoring pids:

% bin/filemonpid PID [PID2 [...]]

After checking out the repo, run bin/setup to install dependencies. Then, run rake test to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and tags, and push the .gem file to rubygems.org.

Bug reports and pull requests are welcome on GitHub at https://github.com/Freaky/ruby-filemon.

The gem is available as open source under the terms of the MIT License.