How To Set Up OpenVPN on Debian 12 Bookworm

Network diagram

• OpenVPN Server:
  • OS: Debian GNU/Linux 12 (bookworm)
  • Role: OpenVPN Server + Gateway
  • IP: 192.168.0.254

Server (Debian)

Installation

• Install the openvpn package:

root@host:~# apt update && apt install openvpn

• Enable automatic start-up of the OpenVPN service:

root@host:~# sed -i 's/#AUTOSTART="all"/AUTOSTART="all"/' /etc/default/openvpn ; systemctl daemon-reload

PKI

• Enter the /etc/openvpn/ directory:

root@host:~# cd /etc/openvpn/

• Initialise the pki:

root@host:~# /usr/share/easy-rsa/easyrsa clean-all

root@host:~# /usr/share/easy-rsa/easyrsa init-pki

• Type yes to initialise:

WARNING!!!

You are about to remove the EASYRSA_PKI at: /etc/openvpn/pki
and initialize a fresh PKI here.

Type the word 'yes' to continue, or any other input to abort.
  Confirm removal: yes

• If, like me, you don't want to regenerate certificates often, edit the file /etc/openvpn/pki/vars:

#Sets the validity period of the EasyRSA certificate authority to 10 years (3650 days).
set_var EASYRSA_CA_EXPIRE       3650
#Sets the validity period of EasyRSA-issued certificates to 10 years (3650 days).
set_var EASYRSA_CERT_EXPIRE     3650
#Sets the key size for the EasyRSA certificate authority to 4096 bits, ensuring high security without excessive processing load.
set_var EASYRSA_KEY_SIZE        4096
#Sets EasyRSA certificate revocation list validity to 10 years.
set_var EASYRSA_CRL_DAYS       3650

• Create the certificate authority in /etc/openvpn/pki/ca.crt:

root@host:~# /usr/share/easy-rsa/easyrsa build-ca nopass

• Give a Common Name:

* Notice:
Using Easy-RSA configuration from: /etc/openvpn/pki/vars

* Notice:
Using SSL: openssl OpenSSL 3.0.8 7 Feb 2023 (Library: OpenSSL 3.0.8 7 Feb 2023)

Using configuration from /etc/openvpn/pki/cca2af5c/temp.3299e061
.........+++++
............................+++++
-----
You are about to be asked to enter information that will be incorporated
into your certificate request.
What you are about to enter is what is called a Distinguished Name or a DN.
There are quite a few fields but you can leave some blank
For some fields there will be a default value,
If you enter '.', the field will be left blank.
-----
Common Name (eg: your user, host, or server name) [Easy-RSA CA]:openvpn-host

CA creation complete and you may now import and sign cert requests.
Your new CA certificate file for publishing is at:
/etc/openvpn/pki/ca.crt

Server certificates

• Generate a certificate and key for the server:

root@host:~# /usr/share/easy-rsa/easyrsa build-server-full server nopass

• Generate the Diffie Hellman parameters in /etc/openvpn/pki/dh.pem:

root@host:~# /usr/share/easy-rsa/easyrsa gen-dh

Client certificates

• Generate the client01 certificate:

Note: Client certificates are located in the /etc/openvpn/pki/private/ and /etc/openvpn/pki/issued/ directories.

root@host:~# /usr/share/easy-rsa/easyrsa build-client-full client01 nopass

• Generate 10 clients certificates in a single command:

root@host:~# for i in $(seq -w 1 10);do /usr/share/easy-rsa/easyrsa build-client-full client"$i" nopass; done

/etc/openvpn/server.conf

• Edit the configuration file /etc/openvpn/server.conf:

port 1194
proto udp
dev tun

ca /etc/openvpn/pki/ca.crt
cert /etc/openvpn/pki/issued/server.crt
key /etc/openvpn/pki/private/server.key # the server.key private key must be kept secret
dh /etc/openvpn/pki/dh.pem

# internal tun0 connection IP
server 10.50.8.0 255.255.255.0

ifconfig-pool-persist ipp.txt

keepalive 10 120

# Compression - must be turned on at both end
#comp-lzo

persist-key
persist-tun

# parameters to be adjusted according to your network configuration
push "dhcp-option DNS 192.168.0.200"
push "dhcp-option DOMAIN std.local"
push "route 192.168.0.0 255.255.255.0"

status /var/log/openvpn-status.log

# verbose mode
verb 3

Systemd

• Enable the OpenVPN Server service:

root@host:~# systemctl enable openvpn@server.service

• Start the OpenVPN Server service:

root@host:~# systemctl start openvpn@server.service

Gateway mode

Gateway mode allows us to make the 192.168.0.0/24 network accessible from the client side.

nftables

A few netfilter rules will allow vpn clients to access the entire network.

Identify network interfaces

• List network interfaces:

root@host:~# ip addr sh
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host 
       valid_lft forever preferred_lft forever
2: ens192: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP group default qlen 1000
    link/ether 01:02:a0:21:fd:54 brd ff:ff:ff:ff:ff:ff
    inet OPENVPN_IP brd X.X.X.X scope global wan
       valid_lft forever preferred_lft forever
    inet6 fe80::ff:fe5d:f333/64 scope link 
       valid_lft forever preferred_lft forever
3: enp2s0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP group default qlen 1000
    link/ether 11:a2:a9:21:fd:54 brd ff:ff:ff:ff:ff:ff
    inet 192.168.0.254 brd X.X.X.X scope global wan
       valid_lft forever preferred_lft forever
    inet6 fe80::6a05:caff:fe39:c153/64 scope link 
       valid_lft forever preferred_lft forever
4: tun0: <POINTOPOINT,MULTICAST,NOARP,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UNKNOWN group default qlen 500
    link/none 
    inet 10.50.8.1 peer 10.50.8.2/32 scope global tun0
       valid_lft forever preferred_lft forever
    inet6 fe80::7ea2:577f:e834:7a20/64 scope link stable-privacy 
       valid_lft forever preferred_lft forever

Temporary rules

• Enter masquerade rules to make your internal network accessible to Windows VPN clients:

root@host:~# nft add table ip NAT

root@host:~# nft add chain ip NAT my_masquerade '{ type nat hook postrouting priority 100; }'

root@host:~# nft add rule NAT my_masquerade ip saddr { 10.50.8.0/24 } oifname enp2s0 masquerade

Persistant rules

• To make our nat persistent, edit the /etc/nftables.conf file:

#!/usr/sbin/nft -f

flush ruleset

table inet filter {
        chain input {
                type filter hook input priority 0;
        }
        chain forward {
                type filter hook forward priority 0;
        }
        chain output {
                type filter hook output priority 0;
        }
}
table ip NAT {
        chain my_masquerade {
                type nat hook postrouting priority 100; policy accept;
                ip saddr { 10.50.8.0/24 } oifname "enp2s0" masquerade comment "outgoing NAT"
        }
}

• Enable the nftables service with the systemctl command:

root@host:~# systemctl enable nftables.service

Gateway mode

• Edit /etc/sysctl.conf and add:

net.ipv4.ip_forward=1

• And run this command to take this parameter into account:

root@host:~# sysctl -p /etc/sysctl.conf

What do I do if my OpenVPN only has one network interface and it's not my default gateway?

Of course, we may find ourselves in a situation where we can't install OpenVPN on our router. In this case, we could for example, install our OpenVPN server as a virtual machine.

We obviously need to create a port forwarder on our wan router to redirect OpenVPN traffic (step 1 on the diagram) to our OpenVPN server (step 2).

And that's it, it won't change much in terms of configuration because we still need to enable gateway mode and create a NAT rule.

In fact, all we need to do is apply exactly the same rules as before and we're done.

Windows client configuration

• Files to be taken from the server and copied to the client:
  • ca.crt: /etc/openvpn/pki/ca.crt
  • client01.crt: /etc/openvpn/pki/issued/client01.crt
  • client01.key: /etc/openvpn/pki/private/client01.key

• You should see the following client files:

• Edit the file C:\Program Files\OpenVPN\config\client.ovpn:

client

dev tun

proto udp

remote OPENVPN_IP 1194

resolv-retry infinite
nobind
persist-key
persist-tun

ca ca.crt
cert client01.crt
key client01.key

#comp-lzo

verb 3

• Open OpenVPN with administrator rights (otherwise the routing rules will not work) and connect:

Note: Running in administrator mode no longer seems to be necessary in the latest versions.