Phase 1 and Phase 2 settings

A VPN tunnel is established in two phases: Phase 1 and Phase 2. Several parameters determine how this is done. Except for IP addresses, the settings simply need to match at both VPN gateways. There are defaults that are appropriate for most cases.

FortiClient distinguishes between Phase 1 and Phase 2 only in the VPN Advanced settings and uses different terms. Phase 1 is called the IKE Policy. Phase 2 is called the IPsec Policy.

Phase 1

In Phase 1, the two VPN gateways exchange information about the encryption algorithms that they support and then establish a temporary secure connection to exchange authentication information.

Phase 1 and Phase 2 settings

When you configure your FortiGate unit or FortiClient application, you must specify the following settings for Phase 1:

Remote gateway	The remote VPN gateway’s address. FortiGate units also have the option of operating only as a server by selecting the “Dialup User” option.
Preshared key	This must be the same at both ends. It is used to encrypt Phase 1 authentication information.
Local interface	The network interface that connects to the other VPN gateway. This applies on a FortiGate unit only.

All other Phase 1 settings have default values. These settings mainly configure the types of encryption to be used. The default settings on FortiGate units and in the FortiClient application are compatible. The examples in this guide use these defaults.

For more detailed information about Phase 1 settings, see Phase 1 parameters on page 46.

Phase 2

Similar to the Phase 1 process, the two VPN gateways exchange information about the encryption algorithms that they support for Phase 2. You may choose different encryption for Phase 1 and Phase 2. If both gateways have at least one encryption algorithm in common, a VPN tunnel can be established. Keep in mind that more algorithms each phase does not share with the other gateway, the longer negotiations will take. In extreme cases this may cause timeouts during negotiations.

To configure default Phase 2 settings on a FortiGate unit, you need only select the name of the corresponding Phase 1 configuration. In FortiClient, no action is required to enable default Phase 2 settings.

For more detailed information about Phase 2 settings, see Phase 2 parameters on page 66.

Security Association

The establishment of a Security Association (SA) is the successful outcome of Phase 1 negotiations. Each peer maintains a database of information about VPN connections. The information in each SA can include cryptographic algorithms and keys, keylife, and the current packet sequence number. This information is kept synchronized as the VPN operates. Each SA has a Security Parameter Index (SPI) that is provided to the remote peer at the time the SA is established. Subsequent IPsec packets from the peer always reference the relevant SPI. It is possible for peers to have multiple VPNs active simultaneously, and correspondingly multiple SPIs.

The IPsec SA connect message generated is used to install dynamic selectors. These selectors can be installed via the auto-negotiate mechanism. When phase 2 has auto-negotiate enabled, and phase 1 has mesh selectortype set to subnet, a new dynamic selector will be installed for each combination of source and destination subnets. Each dynamic selector will inherit the auto-negotiate option from the template selector and begin SA negotiation. Phase 2 selector sources from dial-up clients will all establish SAs without traffic being initiated from the client subnets to the hub.

 

Remote IP address change detection

SAs are stored in a hash table when keyed off the IPsec SA SPI value. This enables the FortiGate, for each inbound ESP packet received, to immediately look up the SA and compare the stored IP address against the one in the incoming packet. If the incoming and stored IP addresses differ, an IP address change can be made in the kernel SA, and an update event can be triggered for IKE.

IKE and IPsec packet processing

Internet Key Exchange (IKE) is the protocol used to set up SAs in IPsec negotiation. As described in Phase 1 parameters on page 46, you can optionally choose IKEv2 over IKEv1 if you configure a route-based IPsec VPN.

IKEv2 simplifies the negotiation process, in that it provides no choice of Aggressive or Main mode in Phase 1. IKEv2 also uses less bandwidth.

The following sections identify how IKE versions 1 and 2 operate and differentiate.

IKEv1

Phase 1

A peer, identified in the IPsec policy configuration, begins the IKE negotiation process. This IKE Security Association (SA) agreement is known as Phase 1. The Phase 1 parameters identify the remote peer or clients and supports authentication through pre-shared key (PSK) or digital certificate. You can increase access security further using peer identifiers, certificate distinguished names, group names, or the FortiGate extended authentication (XAuth) option for authentication purposes. Basically, Phase 1 authenticates a remote peer and sets up a secure communication channel for establishing Phase 2, which negotiates the IPsec SA.

IKE Phase 1 can occur in either Main mode or Aggressive mode. For more information, see Phase 1 parameters on page 46.

IKE Phase 1 is successful only when the following are true:

l Each peer negotiates a matching IKE SA policy. l Each peer is authenticated and their identities protected. l The Diffie-Hellman exchange is authenticated (the pre-shared secret keys match).

For more information on Phase 1, see Phase 1 parameters on page 46.

Phase 2

Phase 2 parameters define the algorithms that the FortiGate unit can use to encrypt and transfer data for the remainder of the session in an IPsec SA. The basic Phase 2 settings associate IPsec Phase 2 parameters with a Phase 1 configuration.

In Phase 2, the VPN peer or client and the FortiGate unit exchange keys again to establish a more secure communication channel. The Phase 2 Proposal parameters select the encryption and authentication algorithms needed to generate keys for protecting the implementation details of the SA. The keys are generated automatically using a Diffie-Hellman algorithm.

IKE and IPsec packet processing

In Phase 2, Quick mode selectors determine which IP addresses can perform IKE negotiations to establish a tunnel. By only allowing authorized IP addresses access to the VPN tunnel, the network is more secure. For more information, see Phase 2 parameters on page 66. IKE Phase 2 is successful only when the following are true:

• The IPsec SA is established and protected by the IKE SA.
• The IPsec SA is configured to renegotiate after set durations (see Phase 2 parameters on page 66 and Phase 2 parameters on page 66).
• Optional: Replay Detection is enabled. Replay attacks occur when an unauthorized party intercepts a series of IPsec packets and replays them back into the tunnel. See Phase 2 parameters on page 66. l Optional: Perfect Forward Secrecy (PFS) is enabled. PFS improves security by forcing a new Diffie-Hellman exchange whenever keylife expires. See Phase 2 parameters on page 66.

For more information on Phase 2, see Phase 2 parameters on page 66.

With Phase 2 established, the IPsec tunnel is fully negotiated and traffic between the peers is allowed until the SA terminates (for any number of reasons; time-out, interruption, disconnection, etc).

The entire IKEv1 process is demonstrated in the following diagram:

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