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Configure 1-to-1 NAT Through a Branch Office VPN Tunnel

When you create a Branch Office VPN (BOVPN) tunnel between two networks that use the same private IP address range, an IP address conflict occurs. To create a tunnel without this conflict, both networks must apply 1-to-1 NAT to the VPN. 1-to-1 NAT makes the IP addresses on your computers appear to be different from their true IP addresses when traffic goes through the VPN.

1-to-1 NAT creates a map from one or more IP addresses in one range to a second IP address range of the same size. Each IP address in the first range corresponds to an IP address in the second range. In this topic, we refer to the first range as the real IP addresses and to the second range as the masqueraded IP addresses. For more information on 1-to-1 NAT, see About 1-to-1 NAT.

1-to-1 NAT and VPNs

When you use 1-to-1 NAT through a BOVPN tunnel:

  • When a computer in your network sends traffic to a computer at the remote network, the Firebox changes the source IP address of the traffic to an IP address in the masqueraded IP address range. The remote network sees the masqueraded IP addresses as the source of the traffic.
  • When a computer at the remote network sends traffic to a computer at your network through the VPN, the remote office sends the traffic to the masqueraded IP address range. The Firebox changes the destination IP address to the correct address in the real IP address range and then sends the traffic to the correct destination.

1-to-1 NAT through a VPN affects only the traffic through that VPN. The rules you see when you select Network > NAT do not affect traffic through a VPN.

Other Reasons to Use 1-to-1 NAT Through a VPN

You can also use 1-to-1 NAT through a VPN if the network you want to make a VPN connection to already has a VPN to a network that has the same private IP addresses you use in your network. An IPSec device cannot send traffic to two different remote networks when the two networks have the same private IP addresses. You use 1-to-1 NAT through the VPN to enable the computers in your network to appear to have different (masqueraded) IP addresses. However, unlike the situation described at the start of this topic, you have to use NAT only on your end of the VPN, instead of on both ends.

A similar situation exists when two remote offices have the same private IP addresses, and both remote offices want to make a VPN to your Firebox. In this case, one of the remote offices must use NAT through the VPN to your Firebox to resolve the IP address conflict.

Alternative to NAT

If your office uses a common private IP address range (for example, 192.168.0.x or 192.168.1.x), it is very likely that you will have a problem with IP address conflicts in the future. These IP address ranges are often used by broadband routers or other electronic devices in homes and small offices. We recommend that you change to a less common private IP address range (for example, 10.x.x.x or 172.16.x.x).

How to Set Up the VPN

These steps and the subsequent example apply to a branch office VPN that is not configured as a BOVPN virtual interface. For a BOVPN virtual interface, you configure 1-to-1 NAT as you would for a physical interface. For more information, see Configure Firewall 1-to-1 NAT.

  1. Select a range of IP addresses that your computers show as the source IP addresses when traffic comes from your network and goes to the remote network through the BOVPN. Make sure to consult with the network administrator for the other network to select a range of IP addresses that are not in use.
    Do not use any of the IP addresses from: 
    • The trusted, optional, or external network connected to your Firebox
    • A secondary network connected to a trusted, optional, or external interface of your Firebox
    • A routed network configured in your Firebox policy (Network > Routes)
    • Networks to which you already have a BOVPN tunnel
    • Mobile VPN virtual IP address pools
    • Networks that the remote IPSec device can reach through its interfaces, network routes, or VPN routes
  2. Configure Manual BOVPN Gateways for the local and remote Fireboxes.
  3. Configure Manual BOVPN Tunnels.
    In the Tunnel Route Settings dialog box for each Firebox, select the 1:1 NAT check box and type its masqueraded IP address range in the adjacent text box.

The number of IP addresses in this text box must be exactly the same as the number of IP addresses in the Local text box at the top of the dialog box. For example, if you use slash notation to specify a subnet, the value after the slash must be the same in both text boxes. For more information, see About Slash Notation.

You do not have to define any parameters in the Network > NAT settings. These settings do not affect VPN traffic.

Example

Suppose two companies, Site A and Site B, want to set up a Branch Office VPN between their trusted networks. Both companies use a WatchGuard Firebox with Fireware. Both companies use the same IP addresses for their trusted networks, 192.168.1.0/24. Both Fireboxes use 1-to-1 NAT through the VPN. Site A sends traffic to the masqueraded range at Site B and the traffic goes outside the local subnet of Site A. Also, Site B sends traffic to the masqueraded range that Site A uses. This solution solves the IP address conflict at both networks. The two companies agree that:

  • The Site A trusted network is configured to appear to come from the 192.168.100.0/24 range when traffic goes through the VPN. This is the masqueraded IP address range of Site A for this VPN.
  • The Site B trusted network is configured to appear to come from the 192.168.200.0/24 range when traffic goes through the VPN. This is the masqueraded IP address range of Site B for this VPN.

Make sure to configure your internal DNS servers to correctly resolve host names for network resources located at the remote site. For example, you might have an intranet.example.com web server located at Site A. With the IP addresses in our example, if a user at Site A goes to http://intranet.example.com, your DNS server resolves the domain name to 192.168.1.80. If a user at Site B goes to http://intranet.example.com, your DNS server must resolve the domain name to 192.168.200.80, which is the masqueraded IP address given by NAT.

Define a Branch Office Gateway on Each Device

First, you must add a gateway that identifies the remote IPSec device. When you add the gateway, it appears in the list of gateways.

To see the list of gateways from Policy Manager, select VPN > Branch Office Gateways.

Screen shot of Gateways dialog box, empty

To see the list of gateways, from Fireware Web UI, select VPN > Branch Office VPN.

Screen shot of Gateways list on BOVPN page

Configure the Tunnel at Site A

Configure the local tunnel on the Site A Firebox to use 1-to-1NAT so that traffic from the Site A trusted network appears to come from the 192.168.100.0/24 range when it goes through the VPN to Site B.

If 1-to-1 NAT must only be configured on one side of the VPN, you do not have to complete the subsequent procedures. The VPN on the Firebox at the other end of the tunnel must be configured to accept traffic from your masqueraded IP address range.

Configure the Tunnel at Site B

To set up 1-to-1 NAT from Site B to Site A, configure the tunnel route on the Site B device to use 1-to-1 NAT. With this configuration, traffic from the Site B trusted network appears to come from the 192.168.200.0/24 address range when it goes through the VPN to Site A.

See Also

About 1-to-1 NAT

About Manual Branch Office VPNs

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