Title: Routing a specific user on a specific network interface on Linux

	Title: Routing a specific user on a specific network interface on Linux
	Author: Solène
	Date: 23 April 2022
	Tags: linux networking security
	Description: This guide explains how to configure a network interface
	to be used by default by a single user without affecting the other
	systems users.

	# Introduction

	I have a special network need on Linux, I must have a single user going
	through specific VPN tunnel. This can't be done using a different
	metric for the VPN or by telling the program to bind on a specific
	interface.

	# How does it work

	The setup is easy once you find how to proceed on Linux: we define a
	new routing table named 42 and add a rule assigning user with uid 1002
	to this routing table. It's important to declare the VPN default route
	on the exact same table to make it work.

	```shell script
	#!/bin/sh

	REMOTEGW=YOUR_VPN_REMOTE_GATEWAY_IP
	LOCALIP=YOUR_VPN_LOCAL_IP
	INTERFACE=tun0

	ip route add table 42 $REMOTEGW dev tun0
	ip route add table 42 default via $REMOTEGW dev tun0 src $LOCALIP
	ip rule add pref 500 uidrange 1002-1002 lookup 42
	ip rule add from $LOCALIP table 42
	```

	# Conclusion

	It's quite complicated to achieve this on Linux because there are many
	ways to proceed like netns (network namespace), iptables or vrf but the
	routing solution is quite elegant, and the documentation are never
	obvious for this use case.

	I'd like to thank @[email protected] from the Fediverse for
	giving me the first bits about ip rules and using a different route
	table.