Configure a Wireguard interface (wg)

WireGuard is a very promising VPN technology available in the community repository since Alpine 3.10.

There are several ways to install and configure an interface.

In order to load the WireGuard kernel module, you need a compatible kernel:

linux-lts
linux-virt

Bringing up an interface using wg-tools

The most straightforward method, and the one recommended in WireGuard documentation, is to use wg-quick.

Install wireguard-tools

apk add wireguard-tools

Reboot and then load the module

modprobe wireguard

Add it to /etc/modules to automatically load it on boot.

Then, we need to create a private and a public key:

wg genkey | tee privatekey | wg pubkey > publickey

Then, we create a new config file /etc/wireguard/wg0.conf using those keys:

[Interface]
Address = 10.123.0.1/24
ListenPort = 45340
PrivateKey = SG1nXk2+kAAKnMkL5aX3NSFPaGjf9SQI/wWwFj9l9U4= # the key from the previously generated privatekey file
PostUp = iptables -A FORWARD -i %i -j ACCEPT; iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE;iptables -A FORWARD -o %i -j ACCEPT
PostDown = iptables -D FORWARD -i %i -j ACCEPT; iptables -t nat -D POSTROUTING -o eth0 -j MASQUERADE;iptables -D FORWARD -o %i -j ACCEPT

The PostUp and PostDown steps are there to ensure the interface wg0 will accept and forward traffic to eth0. The postrouting and forward to %i is not required, but it will enable "VPN mode" where users can access the internet via this server if desired. Note that this requires iptables installed and enabled: apk add iptables && rc-update add iptables. Reference this WireGuard documentation for information on adding peers to the config file.

To bring up the new interface we use:

wg-quick up wg0

To take it down, we can use wg-quick down wg0 which will clean up the interface and remove the iptables rules. Note: If running in a Docker container, you will need to run with --cap-add=NET_ADMIN to modify your interfaces.

Bringing up an interface using ifupdown-ng

The official documents from WireGuard show examples of how to set up an interface with the use of wg-quick. In this how-to, we are not going to use that utility. We'll use the plain wg command and ifupdown-ng.

apk add wireguard-tools-wg

Now that all the tools are installed, you can setup the interface. The setup of your interface config is out of the scope of this document. You should consult the manual page of wg.

After you have finished setting up your wgX interface config, you can add it to your /etc/network/interfaces:

auto wg0
iface wg0 inet static
       requires eth0
       use wireguard
       address 192.168.42.1
       post-up iptables -A FORWARD -i wg0 -j ACCEPT; iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE;iptables -A FORWARD -o wg0 -j ACCEPT
       post-down iptables -D FORWARD -i wg0 -j ACCEPT; iptables -t nat -D POSTROUTING -o eth0 -j MASQUERADE;iptables -D FORWARD -o wg0 -j ACCEPT

This config will automatically:

bring the WireGuard interface up after the eth0 interface
assign a config to this interface (which you have previously created)
setup the interface address and netmask
add the route once the interface is up
remove the interface when it goes down
enable traffic forwarding (the post-up and post-down lines; requires iptables) (note that this is not required unless you want peers to be able to access external resources like the internet)

Note: If you are using the same config (/etc/wireguard/wg0.conf) from a wg-quick setup, you must comment out the Address line in the [Interface] section. Otherwise, the interface will not come up.

To start and stop the interface, you execute:

ifup wg0
ifdown wg0

If your interface config is not stored under /etc/wireguard/ you need to specify a wireguard-config-path as well.

Enable IP Forwarding

If you intend for peers to be able to access external resources (including the internet), you will need to enable forwarding. Edit the file /etc/sysctl.conf (or a .conf file under /etc/sysctl.d/) and add the following line.

net.ipv4.ip_forward = 1

Then either reboot or run sysctl -p /etc/sysctl.conf to reload the settings. To ensure forwarding is turned on, run sysctl -a | grep ip_forward and ensure net.ipv4.ip_forward is set to 1. To make the change permanent across reboots, you may need to enable the sysctl service: rc-update add sysctl.

Running with modloop

If you are running from a RAM disk, you can't modify the modloop.

You can get around it by unpacking the modloop, mounting the unpacked modules folder, then installing WireGuard.

#!/bin/sh
apk add squashfs-tools # install squashfs tools to unpack modloop
unsquashfs -d /root/squash /lib/modloop-lts # unpack modloop to root dir
umount /.modloop # unmount existing modloop
mount /root/squash/ /.modloop/ # mount unpacked modloop
apk del wireguard-lts # uninstall previous WireGuard install
apk add wireguard-lts
apk add wireguard-tools

You can repack the squash filesystem or put this script in the /etc/local.d/ path so it runs at boot-up.