A method and system for enabling secure IPsec tunnels within NAT without compromising security. A local network is configured with a gateway machine connected to the Internet and having an IPsec ID for interfacing with the Internet and a local IP/interface address for interfacing with the local network. Client machines are connected to the gateway machine and communicate with the Internet via the gateway and network address translation (NAT) techniques. Each client machine is configured with a local IP/interface address. The client machines are also provided with an alias of the IPsec ID for the gateway machine. When an IPsec request is received by the gateway machine to establish a tunnel (secure communication) with one of the clients, the gateway machine forwards the packet to the particular client using NAT. The client machine receives the request and since it has an alias of the gateway's IPsec ID, the client machine will confirm that it has one of the IPsec IDs in the packet. The client machine sends the reply packet back to the gateway machine, which then forwards it to the requesting machine over the Internet. The requesting machine receives the packet and a confirmation that it has reached its intended recipient and opens the secure IKE tunnel with the particular client via the gateway machine. In this manner authentication of the IKE tunnel and establishment of a secure IPsec session is completed with a client machine that is accessible only via a gateway implementing NAT.
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