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 @loweel@bbs.keinpfusch.net from the Fediverse for
giving me the first bits about ip rules and using a different route
table.