Skip to main content

Easy packet traces using remotecap

I hate taking packet captures. I end up taking a ton of them at home and at work. Frankly, this workflow sucks:

# Oh shoot, I need to take a packet capture.
evan: ssh
host: tcpdump -i any -s 0 'not port 22' -w /tmp/foo.cap
# In another terminal after making the traffic I want...
evan: scp captures/foo.cap
evan: wireshark captures/foo.cap

There's all kinds of garbage that can happen along the way:
- /tmp/ (or wherever you're capturing to) could fill up
- Somebody else could clobber your capture file (that's a fun one)
- You might forget to copy the new capture file over

And the whole affair just takes forever. I want live updates! I don't want to fill up precious disk space on my server! I want my capture file to be on my machine!

Enter remotecap, something I built because I was sick of this.

Here's the workflow with remotecap:

evan: remotecap -w captures/foo.cap
# Screen is cleared and capture size and rate of growth is 
# printed.
# This can be disabled by using -q/--quiet

# In another terminal
evan: wireshark captures/foo.cap
# Now, make the traffic you want to view and simply hit <C-r> in Wireshark

remotecap will ssh into the target server, run tcpdump, and then pipe the outback back over ssh to a file on your system. That's not all it can do, however!

Here's a more complicated example:

remotecap -w captures/some-weird-problem --user notroot --sudo \
          --command-path /stupid/path/tcpdump \
          --filter 'port 80 and port 443 and port 1234' \
          --key ~/somerandomkey --packet-length 1234 \
          --known-hosts None \

This will run tcpdump on three machines simultaneously and put them in a folder named some-weird-problem as $HOST.cap, e.g. some-weird-problem/ It also logs in as a non-root user and then uses sudo to escalate privileges on these machines. The other options are pretty self-explanatory for anyone that has used ssh or tcpdump. My favorite thing is that you can specify the ssh listening port per host using : to separate the host from the port.

Internally, remotecap is built on top of asyncio, using asyncssh to do ssh connections and stream the output and aiofiles to allow file I/O that won't block the event loop. Because of this, it performs pretty well. I've been able to take captures on systems that are recieving 500 Mb/s or more.

To use remotecap, you need to have python >= 3.6 on your system. remotecap has been tested on Linux and should probably work on MacOS. I have no clue if this will work on Windows, so sorry there!

To install, simply do the following:

# You could do this with system Python, but that's not really a great idea.
# All file paths below are just examples. You can put this 
# virtualenv wherever you want.
evan: python3 -m venv ~/remotecap
evan: source ~/remotecap/bin/activate
# Make sure that you have libsodium installed. Install it using apt/dnf/pacman/zypper
# This also might need a compiler if the wheels don't work.
(remotecap) evan: pip install remotecap[recommends]
# Bunch of garbage prints
(remotecap) evan: remotecap --help

I may create packages for various distros using fpm at some point, but for now, just use pip.

If you run into any issues or just want to see the source, head on over to remotecap on Github.

Happy trails!


Comments powered by Disqus