OpenVPN Setup (Server & Client)

Environment

VPN Server

Field	Spec or Version
CPU	QEMU Virtual CPU version 2.5+ (2 Cores)
RAM	2G
OS	Ubuntu 24.04.1 LTS (Noble Numbat)
Network	interface ~~ens18~~enp6s18 with static IP 192.168.3.100/24
DNS	192.168.3.1 (Private DNS Server)

VPN Client

Field	Spec or Version
CPU	QEMU Virtual CPU version 2.5+ (2 Cores)
RAM	2G
OS	Ubuntu 24.04.1 LTS (Noble Numbat)
Network	interface ~~ens18~~enp6s18 with static IP 192.168.5.100/24
DNS	192.168.5.1 (Private DNS Server)

Server - Package Installation

apt install -y openvpn

Server - Configuration

Copy from sample configuration file

cp /usr/share/doc/openvpn/examples/sample/config-files/server.conf /etc/openvpn/server/server.conf

Edit the copied file as following content

port 1194
proto tcp

# Use routing mode instead of bridge mode
dev tun

ca ca.crt
cert example.fake.crt
key example.fake.key
dh dh.pem

topology subnet
server 192.168.7.0 255.255.255.0

# Add route for subnet on server to connected VPN clients
# Notice: firewall rule adjustment is required
push "route 192.168.3.0 255.255.255.0"

Server - Firewall rule

Allow incoming connections to port 1194 if default policy of chain INPUT is NOT ACCEPT

Allow VPN interface and subnet to access other devices inside the server subnet (192.168.3.0/24)

iptables -A FORWARD -i tun0 -o enp6s18 -s 192.168.7.0/24 -d 192.168.3.0/24 -j ACCEPT
iptables -A FORWARD -i enp6s18 -o tun0 -m state --state RELATED,ESTABLISHED -j ACCEPT
iptables -t nat -A POSTROUTING -s 192.168.7.0/24 -o enp6s18 -j MASQUERADE

Don't forget to save the changes to both firewall configuration file and runtime configuration

# Save iptables runtime to file
iptables-save > /etc/iptables/rules.v4