Setting up a OpenVPN server

From Alpine Linux

This article describes how to set up an OpenVPN server with the Alpine Linux. This is an ideal solution for allowing single users or devices to remotely connect to your network. \\ It is recommended you have a publicly routable static IP address in order for this to work. This means that your IP address cannot be a bogon IP address (list IPv4, list IPv6). If your Internet-connected machine doesn't have a static IP address, No-ip can be used for resolving DNS names to IP addresses.

Set up Alpine

Initial Set up

Follow Installation to set up Alpine Linux.

Install programs

Install openvpn

apk add openvpn

Prepare autostart of OpenVPN

rc-update add openvpn default

modprobe tun echo "tun" >> /etc/modules-load.d/tun.conf

Enable IP Forwarding

echo "net.ipv4.ip_forward = 1" >> /etc/sysctl.d/ipv4.conf

sysctl -p /etc/sysctl.d/ipv4.conf

Certificates

One of the first things that needs to be done is to make sure you have secure keys to work with. Alpine makes this easy by having a web interface to manage the certificates. Documentation for it can be found here: Generating SSL certs with ACF. It is a best practice not to have your certificate server be on the same machine as the router being used for remote connectivity.

You will need to create a server (ssl_server_cert) certificate for the server and one client certificate (ssl_client_cert) for each client. To use the certificates, you should download the .pfx file and extract it.

To extract the three parts of each .pfx file, use the following commands:

To get the ca cert out:

openssl pkcs12 -in PFXFILE -cacerts -nokeys -out ca.pem

To get the cert file out:

openssl pkcs12 -in PFXFILE -nokeys -clcerts -out cert.pem

To get the private key file out: (Make sure the key stays private)

openssl pkcs12 -in PFXFILE -nocerts -nodes -out key.pem

On the VPN server, you can also install the acf-openvpn package, which contains a web page to automatically upload and extract the server certificate. There is also a button to automatically generate the Diffie-Hellman parameters.

If you would prefer to generate your certificates using OpenVPN utilities, see #Alternate Certificate Method

Configure OpenVPN server

Example configuration file for server. Place the following content in /etc/openvpn/openvpn.conf: 

local "Public Ip address"
port 1194
proto udp
dev tun
ca /etc/openvpn/easy-rsa/keys/ca.crt 
cert /etc/openvpn/easy-rsa/keys/Server.crt # SWAP WITH YOUR CRT NAME
key /etc/openvpn/easy-rsa/keys/Server.key # SWAP WITH YOUR KEY NAME
dh /etc/openvpn/easy-rsa/keys/dh1024.pem # If you changed to 2048, change that here!
server 10.0.0.0 255.255.255.0
ifconfig-pool-persist ipp.txt
push "route 10.0.0.0 255.0.0.0"
push "dhcp-option DNS 10.0.0.1"
keepalive 10 120
comp-lzo
user nobody
group nobody
persist-key
persist-tun
status /var/log/openvpn-status.log
log-append  /var/log/openvpn.log
verb 3

(Instructions are based on openvpn.net/howto.html#server)

Test your configuration

Test configuration and certificates

openvpn --config /etc/openvpn/openvpn.conf

Configure OpenVPN client

Example client.conf: 

client
dev tun
proto udp
remote "public IP" 1194
resolv-retry infinite
nobind
ns-cert-type server # This means the certificate on the openvpn server needs to have this field. Prevents MitM attacks
persist-key
persist-tun
ca client-ca.pem
cert client-cert.pem
key client-key.pem
comp-lzo
verb 3

(Instructions are based on openvpn.net/howto.html#client)

Save settings

Don't forget to save all your settings if you are running a RAM-based system.

lbu commit

More than one server or client

If you want more than one server or client running on the same Alpine box, use the standard Multiple Instances of Services process.

For example, to create a config named "AlphaBravo":

  • Create an approriate /etc/openvpn/openvpn.conf file, but name it "/etc/openvpn/AlphaBravo.conf"
  • create a new symlink of the init.d script:

ln -s openvpn /etc/init.d/openvpn.AlphaBravo

  • Have the new service start automatically

rc-update add openvpn.AlphaBravo

Alternate Certificate Method

Manual Certificate Commands

(Instructions are based on openvpn.net/howto.html#pki)

Initial setup for administrating certificates

The following instructions assume you want to save your configs, certs and keys in /etc/openvpn/keys.
Start by moving to the /usr/share/openvpn/easy-rsa folder to execute commands

apk add easy-rsa # from the community repo cd /usr/share/easy-rsa

If not already done, create a folder where you will save your certificates and save a copy of your /usr/share/easy-rsa/vars for later use.

mkdir /etc/openvpn/keys cp ./vars.example ./vars #easy-rsa v3 cp ./vars /etc/openvpn/keys #easy-rsa v2

For EasyRSA v3 see: https://community.openvpn.net/openvpn/wiki/EasyRSA

The instructions below are for EasyRSA v2:

If not already done, edit /etc/openvpn/keys/vars
(This file is used for defining paths and other standard settings)

vim /etc/openvpn/keys/vars

  • Change KEY_DIR= from "$EASY_RSA/keys" to "/etc/openvpn/keys"
  • Change KEY_SIZE, CA_EXPIRE, KEY_EXPIRE, KEY_COUNTRY, KEY_PROVINCE, KEY_CITY, KEY_ORG, KEY_EMAIL to match your system.

source the vars to set properties

source /etc/openvpn/keys/vars

touch /etc/openvpn/keys/index.txt echo 00 > /etc/openvpn/keys/serial

Set up a 'Certificate Authority' (CA)

Clean up the keys folder.

./clean-all

Generate Diffie-Hellman parameters

./build-dh

To make the CA certificates and keys

./build-ca

Set up an 'OpenVPN Server'

Create server certificates

./build-key-server <commonname>

Set up an 'OpenVPN Client'

Create client certificates

./build-key <commonname>

Revoke a certificate

To revoke a certificate

./revoke-full <commonname>

The revoke-full script will generate a CRL (certificate revocation list) file called crl.pem in the keys subdirectory.
The file should be copied to a directory where the OpenVPN server can access it, then CRL verification should be enabled in the server configuration:

crl-verify crl.pem

OpenVPN and LXC

Let's call this LXC "mylxc"...

On the host 

modprobe tun
mkdir /var/lib/lxc/mylxc/rootfs/dev/net
mknod /var/lib/lxc/mylxc/rootfs/dev/net/tun c 10 200
chmod 666 /var/lib/lxc/mylxc/rootfs/dev/net/tun

In /var/lib/lxc/mylxc/config 

lxc.cgroup.devices.allow = c 10:200 rwm

On the guest 

apk add openvpn

Then config as usual.

This should work both as server and as client.

persistent devices

lxc guest have their dev recreated on each restart in a tmpfs. This means all devices are reset and are not read from the rootfs dev directory. To make it persistent you can use an autodev script by adding the following to your lxc guest config: 

# tun (openvpn)
lxc.cgroup.devices.allow = c 10:200 rwm
# audodev script to add devices
lxc.hook.autodev=/var/lib/lxc/CONTAINER/autodev

The autodev script: 

#!/bin/sh
# dev is populated on earch container start.
# to make devices persistence we need to recreate them on each start.

cd ${LXC_ROOTFS_MOUNT}/dev
mkdir net
mknod net/tun c 10 200
chmod 0666 net/tun
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