blog/content/posts/wireguard-vpn-with-2-or-more-subnets.adoc

---
title: "WireGuard VPN with 2 or more subnets"
description: "How to connect 2 subnets with WireGuard."
date: 2019-02-14T21:38:28+01:00
draft: false
tags:
- wireguard
- vpn
comtodon: 9fqBQKGZa6qMf98wIC
---

:source-highlighter: pygments

I wanted to create a https://en.wikipedia.org/wiki/WireGuard[WireGuard] VPN with
2 subnets in different physical places, each with their own server. I couldn't
find an example how to do that, so I wrote this one.

== Introduction

This HowTo is Linux specific.

I'm going to use the IP range `fd69::/48` for the VPN, `fd69:0:0:1::/64` for
subnet 1 and `fd69:0:0:2::/64` for subnet 2. I'm going to call the server of
subnet 1 `server1`, its first client `client1a`, the second one `client1b` and
so on.

All clients in subnet 1 will connect to `server1` and all clients in subnet 2
will connect to `server2`. `server1` and `server2` will be connected. If
`client1a` wants to connect to `client2a`, the route will be:
`client1a → server1 → server2 → client2a`.

== Preparations

https://www.wireguard.com/install/[Install WireGuard], create `/etc/wireguard`
and generate a key-pair on each participating peer.

[source,shell]
----
mkdir /etc/wireguard
cd /etc/wireguard
umask 077
wg genkey | tee privatekey | wg pubkey > publickey
----

== Configure servers

.Turn on IP forwarding:
[source,shell]
----
echo "net.ipv6.conf.all.forwarding = 1" > /etc/sysctl.d/ip-forward.conf
sysctl -p /etc/sysctl.d/ip-forward.conf
----

[NOTE]
http://strugglers.net/~andy/blog/2011/09/04/linux-ipv6-router-advertisements-and-forwarding/[IP forwarding will put your computer into "router-mode"],
it will no longer autoconfigure via https://en.wikipedia.org/wiki/SLAAC[SLAAC].
If you need SLAAC, add `net.ipv6.conf.DEVICE.accept_ra = 2` to `ip-forward.conf`.

.`server1:/etc/wireguard/wg0.conf`:
[source,cfg]
----
# This peer
[Interface]
Address = fd69:0:0:1::1/48
PrivateKey = <PRIVATE KEY OF server1>
ListenPort = 51820

# Server of subnet 2
[Peer]
PublicKey = <PUBLIC KEY OF server2>
Endpoint = server2:51820
AllowedIPs = fd69:0:0:2::/64

# Clients of subnet 1
[Peer]
PublicKey = <PUBLIC KEY OF client1a>
AllowedIPs = fd69:0:0:1::a/128

[Peer]
PublicKey = <PUBLIC KEY OF client1b>
AllowedIPs = fd69:0:0:1::b/128
----

.`server2:/etc/wireguard/wg0.conf`:
[source,cfg]
----
# This peer
[Interface]
Address = fd69:0:0:2::1/48
PrivateKey = <PRIVATE KEY OF server2>
ListenPort = 51820

# Server of subnet 1
[Peer]
PublicKey = <PUBLIC KEY OF server1>
Endpoint = server1:51820
AllowedIPs = fd69:0:0:1::/64

# Clients of subnet 2
[Peer]
PublicKey = <PUBLIC KEY OF client2a>
AllowedIPs = fd69:0:0:2::a/128
----

== Configure clients

.`client1a:/etc/wireguard/wg0.conf`:
[source,cfg]
----
[Interface]
Address = fd69:0:0:1::a/48
PrivateKey = <PRIVATE KEY OF client1a>

[Peer]
PublicKey = <PUBLIC KEY OF server1>
Endpoint = server1:51820
AllowedIPs = fd69::/48
PersistentKeepalive = 25
----

.`client1b:/etc/wireguard/wg0.conf`:
[source,cfg]
----
[Interface]
Address = fd69:0:0:1::b/48
PrivateKey = <PRIVATE KEY OF client1b>

[Peer]
PublicKey = <PUBLIC KEY OF server1>
Endpoint = server1:51820
AllowedIPs = fd69::/48
PersistentKeepalive = 25
----

.`client2a:/etc/wireguard/wg0.conf`:
[source,cfg]
----
[Interface]
Address = fd69:0:0:2::a/48
PrivateKey = <PRIVATE KEY OF client2a>

[Peer]
PublicKey = <PUBLIC KEY OF server2>
Endpoint = server1:51820
AllowedIPs = fd69::/48
PersistentKeepalive = 25
----

The `AllowedIPs` setting acts as a routing table. When a peer tries to send a
packet to an IP, it will check `AllowedIPs`, and if the IP appears in the list,
it will send it through the WireGuard interface.

The `PersistentKeepalive` setting ensures that the connection is maintained and
that the peer continues to be reachable, even behind a NAT.

== Start VPN

Run `wg-quick up wg0` on each peer.

== Further reading

The article https://www.stavros.io/posts/how-to-configure-wireguard/[How to easily configure WireGuard]
by Stavros Korokithakis helped me a great deal in understanding WireGuard.

== Updates

* Updated 2019-02-16 to include IP forwarding.
* Updated 2019-02-16 with information on how to turn SLAAC back on.