What is netshim?

Netshim is a lightweight diagnostic tool that lets you see exactly which network connections a program is making while it runs. When you launch an application with it, every time that program tries to reach out to a server or resolve a web address, the tool prints a log line showing where it's connecting. This gives you a clear, real-time picture of an application's network activity without having to dig through its source code or use a complex network monitor. It works through a clever Linux feature called LD_PRELOAD. When you start a program normally, it uses built-in system functions to handle networking, things like looking up a domain name or opening a connection. By using LD_PRELOAD, this tool steps in front of those built-in functions. Every time the application tries to connect to something, the tool briefly intercepts that request, writes down the destination address, and then passes the request along normally. The application runs just as it always would, but now you get a running transcript of its network activity. This is useful for developers, system administrators, or anyone curious about what a program is doing behind the scenes. For example, if you are running a web browser and want to see which tracking servers it quietly contacts in the background, you can launch it through this tool and watch the log scroll by. The README shows exactly this scenario, where starting a browser reveals connections to system communication channels, Google's DNS servers, and various Google-hosted domains, making visible connections you would never otherwise see. The tool is written in C and intentionally minimal. It does not require you to modify the application you are monitoring, which makes it easy to use with existing software. You simply compile the small C file into a shared library, and then prepend a single environment variable command to whatever program you want to inspect. The README does not go into detail beyond the basic usage and an example log, so there is no discussion of advanced configuration or filtering, but its straightforward approach makes network tracing accessible with almost no setup.