IPsec overheads

The FortiGate sets an IPsec tunnel Maximum Transmission Unit (MTU) of 1436 for 3DES/SHA1 and an MTU of 1412 for AES128/SHA1, as seen with diag vpn tunnel list. This indicates that the FortiGate allocates 64 bytes of overhead for 3DES/SHA1 and 88 bytes for AES128/SHA1, which is the difference if you subtract this MTU from a typical ethernet MTU of 1500 bytes.

During the encryption process, AES/DES operates using a specific size of data which is block size. If data is smaller than that, it will be padded for the operation. MD5/SHA-1 HMAC also operates using a specific block size.

The following table describes the potential maximum overhead for each IPsec encryption:

Authentication

To protect data via encryption, a VPN must ensure that only authorized users can access the private network. You must use either a preshared key on both VPN gateways or RSA X.509 security certificates. The examples in this guide use only preshared key authentication. Refer to the Fortinet Knowledge Base for articles on RSA X.509 security certificates.

Phase 1 and Phase 2 settings

Preshared keys

A preshared key contains at least six random alphanumeric characters. Users of the VPN must obtain the preshared key from the person who manages the VPN server and add the preshared key to their VPN client configuration.

Although it looks like a password, the preshared key, also known as a shared secret, is never sent by either gateway. The preshared key is used in the calculations at each end that generate the encryption keys. As soon as the VPN peers attempt to exchange encrypted data, preshared keys that do not match will cause the process to fail.

Additional authentication

To increase security, you can require additional means of authentication from users, such as:

l An identifier, called a peer ID or a local ID. l Extended authentication (XAUTH) which imposes an additional user name/password requirement.

A Local ID is an alphanumeric value assigned in the Phase 1 configuration. The Local ID of a peer is called a Peer ID.

In FortiOS 5.2, new authentication methods have been implemented for IKE: ECDSA-256, ECDSA-384, and ECDSA-521. However, AES-XCBC is not supported.

Full CA chain checking

Added a new option (enabled by default) to fail certificate verification if any of the CAs in the trust chain are not found in the CA store. When disabled, a sub-CA is sufficient to pass certificate verification.

Syntax

config vpn certificate setting set check-ca-chain {enable | disable}

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