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.

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.

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Types of IPSec VPNs in FortiGate

FortiGate supports multiple IPSec VPN configurations:

1. Site-to-Site VPN – Securely connects two remote offices over the internet.
2. Remote Access VPN – Allows individual users to connect securely to the corporate network.
3. Dial-Up VPN – Users connect from different locations with dynamic IPs.
4. 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

1. Log in to the FortiGate Web GUI.
2. Navigate to VPN > IPSec Tunnels > Create New.
3. Select Site-to-Site as the VPN type.
4. 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)

Step 2: Configure Phase 2 Settings

1. In the same VPN settings, navigate to Phase 2.
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

Step 3: Configure Firewall Policies

1. Navigate to Policy & Objects > Firewall Policy.
2. 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
3. Enable NAT Traversal if required.

Step 4: Configure Static Routes

1. Go to Network > Static Routes.
2. Add a new route:
   • Destination: 192.168.2.0/24
   • Gateway: BranchVPN
   • Distance: 10

Step 5: Test and Verify the VPN Connection

1. Go to VPN > Monitor > IPSec Monitor.
2. Check the Status column to ensure the VPN is up.
3. Run the following CLI commands to verify tunnel status:diagnose vpn tunnel list diagnose debug application ike -1
4. 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

1. Navigate to VPN > IPSec Wizard.
2. Select Remote Access and choose the FortiClient VPN option.
3. 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

1. Go to User & Device > User Definition.
2. Create a new user and assign it to the VPN group.

Step 3: Configure Firewall Policy

1. Create an inbound firewall rule allowing VPN users access to LAN.
2. Ensure DNS and DHCP settings are correctly assigned to VPN clients.

Step 4: Configure FortiClient VPN on End-Users’ Devices

1. Download and install FortiClient VPN.
2. Configure a new VPN connection:
   • Remote Gateway: Public IP of FortiGate
   • Authentication: Pre-shared Key
   • User Credentials: Username & Password
3. 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!