With businesses increasingly relying on secure remote connectivity, IPSec VPNs have become essential for organizations that need encrypted communication between remote offices, mobile users, or cloud environments. FortiGate firewalls provide a robust and scalable solution for implementing IPSec VPNs. This guide will walk you through the step-by-step process of configuring an IPSec VPN on a FortiGate firewall in 2025.
Table of Contents
What is IPSec VPN?
An IPSec VPN (Internet Protocol Security Virtual Private Network) is a secure method of connecting remote networks or users over the internet using encryption and authentication techniques. IPSec ensures data confidentiality, integrity, and authenticity, making it a preferred choice for enterprise security.
Types of IPSec VPNs in FortiGate
FortiGate supports multiple IPSec VPN configurations:
- Site-to-Site VPN – Securely connects two remote offices over the internet.
- Remote Access VPN – Allows individual users to connect securely to the corporate network.
- Dial-Up VPN – Users connect from different locations with dynamic IPs.
- Hub-and-Spoke VPN – Centralized VPN topology connecting multiple branch offices to a central hub.
Pre-Configuration Requirements
Before setting up an IPSec VPN on FortiGate, ensure you have:
- FortiGate firewall with firmware updated to the latest stable release.
- Static Public IPs (recommended for Site-to-Site VPNs).
- User authentication details for Remote Access VPNs.
- Proper network topology and routing plan.
- Required ports open: IPSec uses UDP 500 and 4500 (NAT-T) for IKE.
Step-by-Step Configuration for Site-to-Site IPSec VPN
Step 1: Define Phase 1 Settings
- Log in to the FortiGate Web GUI.
- Navigate to VPN > IPSec Tunnels > Create New.
- Select Site-to-Site as the VPN type.
- Configure the following under Phase 1:
- Name:
BranchVPN
- Remote Gateway: Public IP of the remote FortiGate device.
- Interface: WAN interface
- Authentication Method: Pre-shared Key (PSK)
- IKE Version: IKEv2 (recommended for security and performance)
- Encryption: AES-256
- Authentication: SHA-256
- DH Group: 14
- Key Lifetime: 86400 seconds (default)
- Name:
Step 2: Configure Phase 2 Settings
- In the same VPN settings, navigate to Phase 2.
- Configure the following:
- Phase 2 Name:
BranchVPN_Ph2
- Local Address: Local subnet of FortiGate (
192.168.1.0/24
) - Remote Address: Remote subnet (
192.168.2.0/24
) - Encryption: AES-256
- Authentication: SHA-256
- Enable Perfect Forward Secrecy (PFS): Group 14
- Key Lifetime: 3600 seconds
- Phase 2 Name:
Step 3: Configure Firewall Policies
- Navigate to Policy & Objects > Firewall Policy.
- Create a new policy with the following settings:
- Incoming Interface:
BranchVPN
- Outgoing Interface:
LAN
- Source:
192.168.2.0/24
- Destination:
192.168.1.0/24
- Service: ALL
- Action: ACCEPT
- Incoming Interface:
- Enable NAT Traversal if required.
Step 4: Configure Static Routes
- Go to Network > Static Routes.
- Add a new route:
- Destination:
192.168.2.0/24
- Gateway:
BranchVPN
- Distance:
10
- Destination:
Step 5: Test and Verify the VPN Connection
- Go to VPN > Monitor > IPSec Monitor.
- Check the Status column to ensure the VPN is up.
- Run the following CLI commands to verify tunnel status:
diagnose vpn tunnel list diagnose debug application ike -1
- Perform a ping test from one site to another.
Step-by-Step Configuration for Remote Access IPSec VPN
Step 1: Configure the IPSec VPN Server on FortiGate
- Navigate to VPN > IPSec Wizard.
- Select Remote Access and choose the FortiClient VPN option.
- Configure:
- Authentication Type: Pre-shared Key
- User Group: Create a user group under User & Device > User Groups.
- Tunnel Type: Full Tunnel or Split Tunnel
- Local Subnet: Define allowed network access
Step 2: Configure User Authentication
- Go to User & Device > User Definition.
- Create a new user and assign it to the VPN group.
Step 3: Configure Firewall Policy
- Create an inbound firewall rule allowing VPN users access to LAN.
- Ensure DNS and DHCP settings are correctly assigned to VPN clients.
Step 4: Configure FortiClient VPN on End-Users’ Devices
- Download and install FortiClient VPN.
- Configure a new VPN connection:
- Remote Gateway: Public IP of FortiGate
- Authentication: Pre-shared Key
- User Credentials: Username & Password
- Connect and verify network access.
Troubleshooting Common IPSec VPN Issues
1. VPN Tunnel Not Establishing
- Ensure correct PSK and encryption settings.
- Check firewall policies for allowed IPSec traffic.
- Verify logs under Log & Report > VPN Event Log.
2. No Internet Access After VPN Connection
- Check split-tunneling configuration.
- Ensure correct DNS settings are assigned.
- Verify NAT and firewall rules.
3. Intermittent VPN Disconnects
- Adjust DPD (Dead Peer Detection) settings.
- Optimize IKE keepalive timers.
- Ensure stable internet connectivity.
Conclusion
Setting up an IPSec VPN on a FortiGate firewall ensures secure remote access and site-to-site connectivity. By following this guide, you can establish a stable and encrypted connection in compliance with 2025 best practices. Whether you’re securing remote employees or interconnecting branch offices, IPSec VPN remains a fundamental part of network security.
Start configuring your FortiGate IPSec VPN today and enhance your network security!
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