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Dynamic DNS configuration

This section describes how to configure a site-to-site VPN, in which one FortiGate unit has a static IP address and the other FortiGate unit has a domain name and a dynamic IP address.

The following topics are included in this section:

Dynamic DNS over VPN concepts

A typical computer has a static IP address and one or more DNS servers to resolve fully qualified domain names (FQDN) into IP addresses. A domain name assigned to this computer is resolved by any DNS server having an entry for the domain name and its static IP address. The IP address never changes or changes only rarely so the DNS server can reliably say it has the correct address for that domain all the time.

Dynamic DNS (DDNS)

It is different when a computer has a dynamic IP address, such as an IP address assigned dynamically by a DHCP server, and a domain name. Computers that want to contact this computer do not know what its current IP address is. To solve this problem there are dynamic DNS (DDNS) servers. These are public servers that store a DNS entry for your computer that includes its current IP address and associated domain name. These entries are kept up to date by your computer sending its current IP address to the DDNS server to ensure its entry is always up to date. When other computers want to contact your domain, their DNS gets your IP address from your DDNS server. To use DDNS servers, you must subscribe to them and usually pay for their services.

When configuring DDNS on your FortiGate unit, go to Network > DNS and enable Enable FortiGuard DDNS. Then select the interface with the dynamic connection, which DDNS server you have an account with, your domain name, and account information. If your DDNS server is not on the list, there is a generic option where you can provide your DDNS server information.

Routing

When an interface has some form of changing IP address (DDNS, PPPoE, or DHCP assigned address), routing needs special attention. The standard static route cannot handle the changing IP address. The solution is to use the dynamic-gateway command in the CLI. Say for example you already have four static routes, and you have a PPPoE connection over the wan2 interface and you want to use that as your default route.

The route is configured on the dynamic address VPN peer trying to access the static address FortiGate unit.

Configuring dynamic gateway routing - CLI

config router static

edit 5

set dst 0.0.0.0 0.0.0.0

set dynamic-gateway enable

set device wan2

next

end

 

For more information on DDNS, see the System Administration handbook chapter.

DDNS over VPN

IPsec VPN expects an IP address for each end of the VPN tunnel. All configuration and communication with that tunnel depends on the IP addresses as reference points. However, when the interface the tunnel is on has DDNS enabled there is no set IP address. The remote end of the VPN tunnel now needs another way to reference your end of the VPN tunnel. This is accomplished using Local ID.

A FortiGate unit that has a domain name and a dynamic IP address can initiate VPN connections anytime. The remote peer can reply to the local FortiGate unit using the source IP address that was sent in the packet header because it is current. Without doing a DNS lookup first, the remote peer runs the risk of the dynamic IP changing before it attempts to connect. To avoid this, the remote peer must perform a DNS lookup for the domain name of to be sure of the dynamic IP address before initiating the connection.

Remote Gateway

When configuring the Phase 1 entry for a VPN tunnel, the Remote Gateway determines the addressing method the remote end of the tunnel uses as one of Static IP Address, Dialup User, or Dynamic DNS. There are different fields for each option.

When you select the Dynamic DNS VPN type there is a related field called Dynamic DNS. The Dynamic DNS field is asking for the FQDN of the remote end of the tunnel. It uses this information to look up the IP address of the remote end of the tunnel through the DDNS server associated with that domain name.

Local ID (peer ID)

The Local ID or peer ID can be used to uniquely identify one end of a VPN tunnel. This enables a more secure connection. Also if you have multiple VPN tunnels negotiating, this ensures the proper remote and local ends connect. When you configure it on your end, it is your Local ID. When the remote end connects to you, they see it as your peer ID.

If you are debugging a VPN connection, the Local ID is part of the VPN negotiations. You can use it to help troubleshoot connection problems.

note icon In circumstances where multiple remote dialup VPN tunnels exist, each tunnel must have a peer ID set.
Configuring your Local ID
  1. Go to VPN > IPsec Wizard and create the new custom tunnel or go to VPN > IPsec Tunnels and edit an existing tunnel.
  2. Edit the Phase 1 Proposal (if it is not available, you may need to click the Convert To Custom Tunnel button).
  3. In the Phase 1 Proposal section, enter your Local ID.
  4. Select OK.

The default configuration is to accept all local IDs (peer IDs). If you have Local ID set, the remote end of the tunnel must be configured to accept your local ID.

Accepting a specific Peer ID
  1. Go to VPN > IPsec Tunnels and create the new custom tunnel or edit an existing tunnel.
  2. Edit Authentication (if it is not available, you may need to click the Convert To Custom Tunnel button).
  3. Set Mode to Aggressive.
  4. For Peer Options, select This peer ID. This option becomes visible only when Aggressive mode is selected.
  5. In the Peer ID field, enter the string the other end of the tunnel used for its local ID.
  6. Configure the rest of the Phase 1 entry as required.
  7. Select OK.

Route-based or policy-based VPN

VPN over dynamic DNS can be configured with either route-based or policy-based VPN settings. Both are valid, but have differences in configuration. Choose the best method based on your requirements. For more information on route-based and policy-based, see IPsec VPN overview.

Route-based VPN configuration requires two security policies to be configured (one for each direction of traffic) to permit traffic over the VPN virtual interface, and you must also add a static route entry for that VPN interface or the VPN traffic will not reach its destination. See Dynamic DNS configuration and Dynamic DNS configuration.

Policy-based VPN configuration uses more complex and often more IPsec security policies, but does not require a static route entry. It has the benefit of being able to configure multiple policies for handling multiple protocols in different ways, such as more scanning of less secure protocols or guaranteeing a minimum bandwidth for protocols such as VoIP. See Dynamic DNS configuration and Dynamic DNS configuration.

DDNS topology

In this scenario, two branch offices each have a FortiGate unit and are connected in a gateway-to-gateway VPN configuration. One FortiGate unit has a domain name (example.com) with a dynamic IP address. See branch_2 in the figure below.

Whenever the branch_2 unit connects to the Internet (and possibly also at predefined intervals set by the ISP), the ISP may assign a different IP address to the FortiGate unit. The unit has its domain name registered with a dynamic DNS service. The branch_2 unit checks in with the DDNS server on a regular basis, and that server provides the DNS information for the domain name, updating the IP address from time to time. Remote peers have to locate the branch_2 FortiGate unit through a DNS lookup each time to ensure the address they get is current and correct.

Example dynamic DNS configuration

When a remote peer (such as the branch_1 FortiGate unit above) initiates a connection to example.com, the local DNS server looks up and returns the IP address that matches the domain name example.com. The remote peer uses the retrieved IP address to establish a VPN connection with the branch_2 FortiGate unit.

Assumptions

  • You have administrator access to both FortiGate units.
  • Both FortiGate units have interfaces named wan1 and internal. (If not, you can use the alias feature to assign these labels as “nicknames” to other interfaces to follow this example.)
  • Both FortiGate units have the most recent firmware installed, have been configured for their networks, and are currently passing normal network traffic.
  • The branch_2 FortiGate unit has its wan1 interface defined as a dynamic DNS interface with the domain name of example.com.
  • A basic gateway-to-gateway configuration is in place (see Gateway-to-gateway configurations on page 1) except one of the FortiGate units has a static domain name and a dynamic IP address instead of a static IP address.
  • The FortiGate unit with the domain name is subscribed to one of the supported dynamic DNS services. Contact one of the services to set up an account. For more information and instructions about how to configure the FortiGate unit to push its dynamic IP address to a dynamic DNS server, see the System Administration  handbook chapter.