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How to Configure IPSec VPN on FortiGate: A Complete Guide for 2025
How to Configure IPSec VPN on FortiGate: A Complete Guide for 2025

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.

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!

Disclosure: My content is reader-supported. This means if you click on some of my links, then i may earn a commission. See how my blog is funded, why it matters, and how you can support me. Here’s my editorial process.

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