Freedom251 › Network Simulation Software

Shadow

Free plan Linux

What Shadow is

Shadow is a free Linux tool for researching, developing, testing, and evaluating real networked applications inside simulated networks. It runs unmodified application binaries as native Linux processes within a discrete-event simulation, then connects them through a private virtual network supporting protocols such as TCP and UDP. A configurable network graph can model path latency, packet loss, and routing conditions. Because experiments are deterministic, users can rerun a simulation to reproduce a bug and control variables. Shadow says it can represent distributed systems with thousands of network-connected processes on a Linux laptop, desktop, or server. Simulations are segregated from the Internet, but the project warns that Shadow is not intended to isolate malicious programs, which may access host files or make native system calls. More than 150 system-call API functions are implemented, though unsupported or complex application features may fail. Large-scale Tor simulations are particularly well supported. The software is open source, primarily written in Rust and C.

Who it suits

Shadow suits Linux researchers and developers who need controlled, repeatable network simulations, including large-scale Tor studies. It is also relevant for testing distributed applications against configurable latency and packet loss.

What it does well

Runs unmodified Linux application binaries
Deterministic simulations support bug reproduction
Models latency and packet loss
Can simulate thousands of processes
100% open-source software

What to know first

Linux-only platform
Not designed to isolate malicious programs
Unsupported system calls may cause failures
Building requires several Linux dependencies

Limits

Shadow implements over 150 system-call API functions, but applications using unsupported or complex features may not work correctly. It has no real-time emulation.

Getting started

Use a Linux laptop, desktop, or server
Install development dependencies including GCC, Python, GLib, CMake, and Rustup
Build or install Shadow from its open-source project
Configure a simulated network graph with desired conditions
Run an experiment with managed application processes

Verdict

Shadow combines real application execution with a private, configurable simulated network. Its deterministic model and scale fit research and testing, but users must account for security limitations and incomplete system-call coverage.

Questions

Is Shadow free and open source?

Yes. Shadow is free and described as 100% open-source software written primarily in Rust and C.

Which platform does Shadow support?

Shadow runs on Linux, including laptops, desktops, and servers.

What applications does it run?

It runs real, unmodified application binaries as native Linux processes inside a discrete-event simulation.

Does Shadow connect simulations to the Internet?

No. Simulations use a private virtual network segregated from the Internet.

What support channels are available?

The project directs users to GitHub Discussions for community support and GitHub Issues for bug reports.

Facts from Shadow's own pages

Configurable networks: A configurable network graph models characteristics such as path latency and packet loss. · shadow.github.io, 8 Oct 2026
Dependencies: Building Shadow requires Linux development dependencies including GCC, Python, GLib, CMake, and Rustup. · shadow.github.io, 8 Oct 2026
Deterministic experiments: Experiments can be scientifically controlled and deterministically replicated to help reproduce bugs and reduce confounding factors. · shadow.github.io, 8 Oct 2026
Example workflow: The basic example simulates an HTTP server and three clients making requests across virtual hosts. · shadow.github.io, 8 Oct 2026
Execution model: Shadow runs real, unmodified application binaries as native Linux processes in a discrete-event simulation. · shadow.github.io, 8 Oct 2026
Network controls: Shadow models routing characteristics such as path latency and packet loss using a configurable network graph. · shadow.github.io, 8 Oct 2026
Network model: Shadow connects managed applications through an internal simulated network with protocols such as TCP and UDP. · shadow.github.io, 8 Oct 2026
Network simulation: It connects managed processes through a private virtual network and simulates protocols including TCP and UDP. · shadow.github.io, 8 Oct 2026
Open source: The project describes Shadow as 100% open-source software written primarily in Rust and C. · shadow.github.io, 8 Oct 2026
Privacy: Shadow says simulations are private and segregated from the Internet. · shadow.github.io, 8 Oct 2026
Purpose: Shadow is a scientific experimentation tool for research, development, testing, and evaluation of real networked applications on simulated networks. · shadow.github.io, 8 Oct 2026
Real application execution: It runs unmodified application binaries as native Linux processes inside a discrete-event simulation. · shadow.github.io, 8 Oct 2026
Reproducibility: Shadow experiments are deterministic, so bugs can be reproduced by rerunning a simulation. · shadow.github.io, 8 Oct 2026
Scale: Shadow says it can simulate distributed systems with thousands of network-connected processes on a Linux laptop, desktop, or server. · shadow.github.io, 8 Oct 2026
Security caveat: Shadow's security policy warns that it is not designed to isolate potentially malicious programs, which may access host files or issue native system calls. · github.com, 8 Oct 2026
Source code: Shadow is 100% open-source software written primarily in Rust and C. · shadow.github.io, 8 Oct 2026
Support: The project directs users to GitHub Discussions for community support and GitHub Issues for bug reports. · github.com, 8 Oct 2026
Supported use case: The documentation says large-scale Tor network simulations are a particularly well-supported use case. · shadow.github.io, 8 Oct 2026
System call limits: Shadow implements over 150 system call API functions but does not yet fully support all API features, so applications using more complex features may not work correctly. · shadow.github.io, 8 Oct 2026
System call support: Shadow implements over 150 system call API functions, but applications using unsupported or complex features may not work correctly. · shadow.github.io, 8 Oct 2026
Testing and debugging: The project describes using large, diverse test networks to exercise corner cases and improve test coverage. · shadow.github.io, 8 Oct 2026
Tor use case: The documentation says large-scale Tor network simulations are a particularly well-supported use case. · shadow.github.io, 8 Oct 2026

Where Shadow stands on Freedom251

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