Setting up a secure VPN (Virtual Private Network) between a FortiGate firewall and Cisco VPN clients is a common requirement for many organizations. This allows remote users to securely access internal network resources. This comprehensive guide walks you through the process of configuring an IPsec VPN on a FortiGate device to be compatible with Cisco VPN clients. You'll find detailed steps, configuration snippets, and troubleshooting tips to ensure a smooth and successful deployment. Let's dive in!

Understanding the Basics

Before we jump into the configuration, let's cover some fundamental concepts. IPsec (Internet Protocol Security) is a suite of protocols that provides secure communication over IP networks. It achieves this by authenticating and encrypting each IP packet in a data stream. Think of it as creating a secure tunnel through the internet.

• IKE (Internet Key Exchange): This protocol is used to establish a secure channel between the FortiGate and the Cisco VPN client. It handles authentication and key exchange, setting the stage for the IPsec tunnel.
• IPsec Phase 1 (Main Mode or Aggressive Mode): This is the first phase of the IPsec tunnel negotiation. It establishes a secure channel for further negotiation.
• IPsec Phase 2 (Quick Mode): This phase negotiates the specific security parameters for the data transfer, such as encryption algorithms and hashing methods.
• Encryption Algorithms: These algorithms are used to encrypt the data transmitted through the VPN tunnel. Common examples include AES (Advanced Encryption Standard) and 3DES (Triple DES).
• Hashing Algorithms: Hashing algorithms create a one-way hash of the data to ensure its integrity. Common examples include SHA1 (Secure Hash Algorithm 1) and SHA256 (Secure Hash Algorithm 256).
• Perfect Forward Secrecy (PFS): PFS ensures that even if the encryption key for a particular session is compromised, past sessions remain secure. This is achieved by generating a new key for each session.

Understanding these concepts is crucial for troubleshooting and fine-tuning your IPsec VPN configuration. Make sure you grasp the basic principles of cryptography and network security.

FortiGate Configuration

Now, let's move on to the FortiGate configuration. We'll be using the FortiGate GUI (Graphical User Interface) for this guide, but you can also achieve the same results using the CLI (Command Line Interface).

Step 1: Create a New VPN Tunnel

1. Log in to your FortiGate GUI.
2. Go to VPN > IPsec Wizard.
3. Give your VPN tunnel a descriptive name (e.g., "CiscoVPN").
4. Select "Remote Access" as the template type. This configures the FortiGate for remote users connecting with VPN clients.
5. Click Next.

Step 2: Configure Authentication

1. Select "Pre-shared Key" as the authentication method. This is the most common and straightforward method for VPN authentication. Guys, make sure to use a strong and complex pre-shared key!
2. Enter your pre-shared key in the "Pre-shared Key" field. Store this key securely, as it will be needed on the Cisco VPN client.
3. In the "User Group" field, select an existing user group or create a new one to manage VPN access. You can define which users are allowed to connect to the VPN.
4. Click Next.

Step 3: Configure IPsec Settings

1. Remote Gateway: Enter the IP address or FQDN (Fully Qualified Domain Name) of the Cisco VPN client's public IP address. Since we're dealing with remote access, you can typically set this to 0.0.0.0 to allow connections from any IP address. However, for enhanced security, consider restricting it to specific IP addresses or ranges if possible.
2. Interface: Choose the FortiGate interface that will be used for the VPN connection (usually the interface connected to the internet).
3. Local Address: Specify the internal network subnet that the VPN clients will be able to access. This defines which resources are available to remote users.
4. Client Address Range: Define an IP address range that will be assigned to the VPN clients when they connect. This range should be different from your existing internal network subnets to avoid conflicts.
5. Click Next.

Step 4: Advanced Settings (Phase 1)

1. Click on "Advanced Settings" to fine-tune the Phase 1 parameters. These settings define how the initial secure channel is established.
2. IKE Version: Select "IKEv1" for compatibility with older Cisco VPN clients. While IKEv2 is more modern and offers better security, IKEv1 is often necessary for backward compatibility.
3. Mode: Choose "Main" mode for a more secure but slower initial negotiation, or "Aggressive" mode for a faster but less secure negotiation. Main mode is generally recommended for production environments.
4. Encryption: Select an encryption algorithm such as "AES256" or "AES128." AES256 provides stronger encryption, but AES128 may offer better performance on older devices. The choice depends on your security requirements and the capabilities of the Cisco VPN clients.
5. Authentication: Choose a hashing algorithm such as "SHA256" or "SHA1." SHA256 is generally preferred for its stronger security.
6. DH Group: Select a Diffie-Hellman group such as "14 (2048 bit)" or "5 (1536 bit)." This group determines the strength of the key exchange. Higher numbers offer better security but may require more processing power.
7. Key Lifetime: Set the lifetime of the Phase 1 key in seconds. A shorter lifetime improves security but requires more frequent re-negotiation. 86400 seconds (24 hours) is a common value.

Step 5: Advanced Settings (Phase 2)

1. Configure the Phase 2 parameters. These settings define the security parameters for the actual data transfer.
2. Protocol: Select "ESP" (Encapsulating Security Payload) as the protocol. ESP provides both encryption and authentication.
3. Encryption: Choose an encryption algorithm that matches the Phase 1 setting (e.g., "AES256" or "AES128").
4. Authentication: Choose a hashing algorithm that matches the Phase 1 setting (e.g., "SHA256" or "SHA1").
5. PFS: Enable Perfect Forward Secrecy (PFS) by selecting a Diffie-Hellman group (e.g., "14 (2048 bit)"). This enhances security by generating a new key for each session.
6. Key Lifetime: Set the lifetime of the Phase 2 key in seconds. A common value is 3600 seconds (1 hour).
7. Click Next and then Create to finish the wizard.

Step 6: Create Firewall Policies

You need to create firewall policies to allow traffic to flow through the VPN tunnel. Create two policies:

1. VPN to Internal: Allow traffic from the VPN client address range to the internal network subnet. This allows remote users to access internal resources.
2. Internal to VPN: Allow traffic from the internal network subnet to the VPN client address range. This allows internal users to respond to requests from remote users.

Make sure these policies are placed in the correct order in your firewall policy list.

Cisco VPN Client Configuration

Now, let's configure the Cisco VPN client to connect to the FortiGate VPN.

Step 1: Download and Install the Cisco VPN Client

Download the Cisco VPN client software from the Cisco website or your organization's IT department. Install the client on the remote user's computer.

Step 2: Create a New VPN Connection

1. Open the Cisco VPN client.
2. Click "New" to create a new VPN connection profile.
3. Enter a descriptive name for the connection (e.g., "FortiGate VPN").
4. Connection Type: Select "IPsec" or "IKEv1 IPsec."
5. Server Address: Enter the public IP address or FQDN of your FortiGate device.
6. Authentication Method: Select "Pre-shared Key."
7. Pre-shared Key: Enter the same pre-shared key that you configured on the FortiGate.
8. Group Name (Optional): Some Cisco VPN clients require a group name. If so, you can enter a name (it doesn't necessarily need to match anything on the FortiGate side).

Step 3: Configure Advanced Settings

1. Go to the "Advanced" settings tab.
2. IKE Policy: Ensure that the IKE policy matches the Phase 1 settings on the FortiGate (e.g., encryption algorithm, hashing algorithm, DH group).
3. IPsec Policy: Ensure that the IPsec policy matches the Phase 2 settings on the FortiGate (e.g., encryption algorithm, hashing algorithm, PFS).
4. NAT Traversal: Enable NAT Traversal if the Cisco VPN client is behind a NAT device.

Step 4: Save and Connect

1. Save the VPN connection profile.
2. Click "Connect" to initiate the VPN connection.
3. Enter the username and password for the user account that is allowed to connect to the VPN (as defined in the FortiGate user group).

Troubleshooting

If you encounter issues connecting to the VPN, here are some troubleshooting tips:

• Check the FortiGate logs: The FortiGate logs provide valuable information about the VPN connection process. Look for error messages or warnings that can help identify the problem.
• Verify the pre-shared key: Ensure that the pre-shared key is identical on both the FortiGate and the Cisco VPN client. Even a small typo can prevent the connection from establishing.
• Check the firewall policies: Make sure that the firewall policies are correctly configured to allow traffic to flow through the VPN tunnel.
• Verify the IPsec and IKE settings: Double-check that the IPsec and IKE settings on the FortiGate and the Cisco VPN client match. Any discrepancies can cause connection problems.
• MTU Issues: Sometimes, the Maximum Transmission Unit (MTU) size can cause issues. Try reducing the MTU size on the FortiGate interface.
• Cisco VPN Client Compatibility: Ensure you're using a compatible version of the Cisco VPN client. Older clients might not support newer encryption algorithms or protocols.

Conclusion

Configuring a FortiGate IPsec VPN to work with Cisco VPN clients requires careful attention to detail. By following these steps and understanding the underlying concepts, you can create a secure and reliable VPN connection for your remote users. Remember to always prioritize security best practices and regularly review your VPN configuration to ensure its effectiveness. With the proper configuration, remote users can securely access internal network resources, enhancing productivity and collaboration. Good luck, and happy networking! If you have any specific scenarios or error messages, provide them, and I can offer more tailored guidance.