I had an interesting problem recently:

An application makes http requests on predefined events to a specified url and we need to verify what exactly is being sent.

There are many ways to deal with that, from dumping outgoing request objects to listening in on outgoing traffic with Wireshark. But I have been a fan of nmap and its toolset, so I though that I would be a good opportunity to employ ncat – an netcat reimplementation shipped with nmap.

ncat, is as it precursor nc, a “Swiss Army Knife” (see man nc) of network tools. One of the many things it can do is to listen for incoming traffic and print it to StdOut. So all I needed to do is to point the app to a url with ncat listening.

The setup I have for development is a Vagrant box within a *nix compatible system (the shiny one).

Setting up ncat on the host system is straightforward enough:

ncat -lk 0.0.0.0 7777

-l is for listen, -k is for keep-alive, 0.0.0.0 points to localhost in a way that will make it possible to connect to it from the outside world, 7777 is the host post number.

To access that from the VM, we need an address. I have Vagrant set up a private network, so my host machine acts as a gateway for the guest system. So I need the gateway address:

netstat -rn

and look for a gateway that is not pointing to localhost :). In my case it is 10.0.2.2, might be a standard for Vagrant boxes, but I am not sure. Now I know I need to ping 10.0.2.2:7777 from my guest system to reach my ncat listener on the host system:

curl -X POST 10.0.2.2:7777 --data '{"hello": "world"}'

ncat ouput on the host machine:

ncat -lk 0.0.0.0 7777
POST / HTTP/1.1
User-Agent: curl/7.38.0
Host: 10.0.2.2:7777
Accept: */*
Content-Length: 18
Content-Type: application/x-www-form-urlencoded

{"hello": "world"}

This way I know it works.

All that was left was to put that ncat listener address in place of the target url in the app and done!