Openvpn

OpenVPN

Overview

Deploy and manage OpenVPN — the industry-standard open-source VPN. This skill covers full server setup with PKI (EasyRSA), client certificate management, routing modes (TUN/TAP), split tunneling, site-to-site links, MFA integration, performance tuning, and monitoring. Suitable for remote access VPN, connecting offices, and securing traffic on untrusted networks.

Instructions

Step 1: Server Installation & PKI Setup

# Ubuntu/Debian
apt update && apt install -y openvpn easy-rsa

# Initialize PKI
make-cadir ~/openvpn-ca && cd ~/openvpn-ca
cat > vars <<'EOF'
set_var EASYRSA_REQ_COUNTRY    "US"
set_var EASYRSA_REQ_PROVINCE   "California"
set_var EASYRSA_REQ_CITY       "San Francisco"
set_var EASYRSA_REQ_ORG        "MyCompany"
set_var EASYRSA_REQ_EMAIL      "admin@company.com"
set_var EASYRSA_ALGO           ec
set_var EASYRSA_CURVE          secp384r1
set_var EASYRSA_CA_EXPIRE      3650
set_var EASYRSA_CERT_EXPIRE    825
EOF

./easyrsa init-pki
./easyrsa build-ca nopass

# Server certificate + DH + TLS auth
./easyrsa gen-req server nopass
./easyrsa sign-req server server
./easyrsa gen-dh
openvpn --genkey secret ta.key

# Copy to OpenVPN
cp pki/ca.crt pki/issued/server.crt pki/private/server.key pki/dh.pem ta.key /etc/openvpn/server/

Step 2: Server Configuration

/etc/openvpn/server/server.conf:

port 1194
proto udp
dev tun
ca ca.crt
cert server.crt
key server.key
dh dh.pem
tls-auth ta.key 0

server 10.8.0.0 255.255.255.0
topology subnet
push "redirect-gateway def1 bypass-dhcp"
push "dhcp-option DNS 1.1.1.1"
push "dhcp-option DNS 1.0.0.1"
keepalive 10 120

cipher AES-256-GCM
auth SHA384
tls-version-min 1.2
user nobody
group nogroup
persist-key
persist-tun

status /var/log/openvpn/status.log
log-append /var/log/openvpn/openvpn.log
verb 3
max-clients 100
crl-verify /etc/openvpn/server/crl.pem

Enable forwarding and NAT:

echo "net.ipv4.ip_forward = 1" >> /etc/sysctl.conf && sysctl -p
IFACE=$(ip route get 1.1.1.1 | awk '{print $5; exit}')
iptables -t nat -A POSTROUTING -s 10.8.0.0/24 -o "$IFACE" -j MASQUERADE
apt install -y iptables-persistent && netfilter-persistent save
systemctl enable --now openvpn-server@server

Step 3: Client Certificate & Profile Generation

cd ~/openvpn-ca
./easyrsa gen-req alice nopass
./easyrsa sign-req client alice

Generate a self-contained .ovpn file:

#!/bin/bash
# generate-client.sh
CLIENT=$1; CA_DIR=~/openvpn-ca; SERVER_IP="your.server.ip"
cat > ~/client-configs/$CLIENT.ovpn <<EOF
client
dev tun
proto udp
remote $SERVER_IP 1194
resolv-retry infinite
nobind
persist-key
persist-tun
remote-cert-tls server
cipher AES-256-GCM
auth SHA384
key-direction 1
verb 3
<ca>
$(cat "$CA_DIR/pki/ca.crt")
</ca>
<cert>
$(sed -n '/BEGIN CERTIFICATE/,/END CERTIFICATE/p' "$CA_DIR/pki/issued/$CLIENT.crt")
</cert>
<key>
$(cat "$CA_DIR/pki/private/$CLIENT.key")
</key>
<tls-auth>
$(cat "$CA_DIR/ta.key")
</tls-auth>
EOF
echo "Created: ~/client-configs/$CLIENT.ovpn"

Step 4: Split Tunneling & Site-to-Site

Split tunnel — only route specific networks through VPN:

# Server: replace redirect-gateway with specific routes
push "route 10.0.0.0 255.0.0.0"
push "route 192.168.0.0 255.255.0.0"

Client-side override:

route-nopull
route 10.0.0.0 255.0.0.0
route 192.168.1.0 255.255.255.0

Site-to-site — connect two offices:

# Office A (server): route to Office B LAN
ifconfig 10.9.0.1 10.9.0.2
route 192.168.2.0 255.255.255.0

# Office B (client): route to Office A LAN
remote office-a.example.com 1195
ifconfig 10.9.0.2 10.9.0.1
route 192.168.1.0 255.255.255.0

Step 5: MFA & Client Revocation

Add TOTP (Google Authenticator):

apt install -y libpam-google-authenticator
su - vpnuser -c "google-authenticator -t -d -f -r 3 -R 30 -w 3"

Add to server.conf: plugin /usr/lib/openvpn/openvpn-plugin-auth-pam.so openvpn Client adds: auth-user-pass

Revoke a client:

cd ~/openvpn-ca
./easyrsa revoke alice
./easyrsa gen-crl
cp pki/crl.pem /etc/openvpn/server/crl.pem
systemctl restart openvpn-server@server

Step 6: Monitoring & Performance

# Connected clients
cat /var/log/openvpn/status.log

# Management interface (add to server.conf: management 127.0.0.1 7505)
echo "status" | nc 127.0.0.1 7505
echo "kill alice" | nc 127.0.0.1 7505

Performance tuning (add to server.conf):

sndbuf 0
rcvbuf 0
push "sndbuf 393216"
push "rcvbuf 393216"
fragment 1400
mssfix 1400

Examples

Example 1: Deploy a full OpenVPN server with client provisioning

User prompt: "Set up an OpenVPN server on our Ubuntu 22.04 VPS at 198.51.100.25. Create a full PKI with EasyRSA, configure UDP on port 1194 with AES-256-GCM, push DNS 1.1.1.1 to clients, and generate .ovpn profiles for alice, bob, and carol."

The agent will install openvpn and easy-rsa, initialize a PKI with EC keys (secp384r1), build a CA, generate server and three client certificates, create a server.conf with UDP/TUN/AES-256-GCM and NAT masquerading, write a generate-client.sh script, produce self-contained .ovpn files for alice, bob, and carol with embedded certificates, enable IP forwarding, persist iptables rules, and start the service.

Example 2: Add TOTP two-factor authentication to an existing OpenVPN server

User prompt: "Our OpenVPN server is running but we need to add Google Authenticator MFA. Set it up so each user needs their certificate plus a TOTP code to connect."

The agent will install libpam-google-authenticator, create a PAM config at /etc/pam.d/openvpn requiring the Google Authenticator module, add the PAM plugin directive to server.conf, run google-authenticator setup for each VPN system user, add auth-user-pass to the client .ovpn templates, and restart the OpenVPN service. It will explain that users enter their username and TOTP code when prompted.

Guidelines

• Always use tls-auth or tls-crypt to add an HMAC firewall that drops unauthenticated packets before they reach the TLS stack
• Use elliptic curve keys (EASYRSA_ALGO ec, EASYRSA_CURVE secp384r1) for faster handshakes and stronger security than RSA-2048
• Keep the CA private key offline or on a separate machine; if compromised, an attacker can issue valid client certificates
• Set crl-verify in server.conf and regenerate the CRL after every revocation, or revoked clients will still be able to connect
• Prefer UDP over TCP for the VPN tunnel to avoid TCP-over-TCP performance degradation; use TCP 443 only as a fallback for restrictive networks
• Test split tunnel configurations by checking client routing tables (ip route) after connecting to verify only intended traffic routes through the VPN