Understanding Site-to-Site VPNs on FortiGate: A Primer

When it comes to securing site-to-site connections, FortiGate firewalls offer robust support for Internet Protocol Security (IPSec) Virtual Private Networks (VPNs). In this primer, we'll explore the fundamental concepts of IPSec and how they are implemented in FortiGate site-to-site VPN configurations.

Tunnel Mode vs. Transport Mode

In IPSec, there are two primary modes of operation: Tunnel Mode and Transport Mode. Tunnel Mode encapsulates the entire IP packet, providing a virtual 'secure hop' between two gateways. This mode is often used for site-to-site VPNs, where data needs to be encrypted and authenticated as it traverses public networks. Transport Mode, on the other hand, provides a secure connection between two endpoints by encapsulating the IP payload only.

Authentication Header (AH) vs. Encapsulating Security Payload (ESP)

IPSec authentication is provided through two protocols: Authentication Header (AH) and Encapsulating Security Payload (ESP). AH authenticates the entire IP packet, including the outer IP header, while ESP authenticates only the IP datagram portion of the packet. Both protocols can be used in combination with each other or separately.

ESP Encryption Options

When using ESP, there are several encryption options available, including confidentiality-only encryption, authentication-only encryption, and both confidentiality and authentication. This allows administrators to tailor their IPSec configurations to meet specific security requirements.

AH Authentication

The AH protocol provides authentication services only, ensuring that data is not tampered with during transmission. It does this by authenticating IP headers and payloads, excluding certain header fields that can be legitimately changed in transit, such as the Time To Live (TTL) field.

In conclusion, a solid understanding of IPSec fundamentals is essential for configuring and troubleshooting site-to-site VPNs on FortiGate firewalls. By grasping the differences between Tunnel Mode and Transport Mode, AH and ESP protocols, and encryption options, administrators can ensure their site-to-site connections are secure and reliable.

Site to site vpn fortigate