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Disabling NAT-T on MX Series Routers for Handling NAT with IPsec-Protected Packets


Before Junos OS Release 17.4R1, Network Address Translation-Traversal (NAT-T) is not supported for the Junos VPN Site Secure suite of IPsec features on the MX Series routers. By default, Junos OS detects whether either one of the IPsec tunnels is behind a NAT device and automatically switches to using NAT-T for the protected traffic. To avoid running unsupported NAT-T in Junos OS releases before 17.4R1, you must disable NAT-T by including the disable-natt statement at the [edit services ipsec-vpn] hierarchy level. When you disable NAT-T, the NAT-T functionality is globally switched off. When you disable NAT-T and a NAT device is present between the two IPsec gateways, ISAKMP messages are negotiated using UDP port 500 and data packets are encapsulated with Encapsulating Security Payload (ESP).

Network Address Translation-Traversal (NAT-T) is a method for getting around IP address translation issues encountered when data protected by IPsec passes through a NAT device for address translation. Any changes to the IP addressing, which is the function of NAT, causes IKE to discard packets. After detecting one or more NAT devices along the data path during Phase 1 exchanges, NAT-T adds a layer of User Datagram Protocol (UDP) encapsulation to IPsec packets so they are not discarded after address translation. NAT-T encapsulates both IKE and ESP traffic within UDP with port 4500 used as both the source and destination port. Because NAT devices age out stale UDP translations, keepalive messages are required between the peers.

The location of a NAT device can be such that:

  • Only the IKEv1 or IKEv2 initiator is behind a NAT device. Multiple initiators can be behind separate NAT devices. Initiators can also connect to the responder through multiple NAT devices.

  • Only the IKEv1 or IKEv2 responder is behind a NAT device.

  • Both the IKEv1 or IKEv2 initiator and the responder are behind a NAT device.

Dynamic endpoint VPN covers the situation where the initiator's IKE external address is not fixed and is therefore not known by the responder. This can occur when the initiator's address is dynamically assigned by an ISP or when the initiator's connection crosses a dynamic NAT device that allocates addresses from a dynamic address pool.

Configuration examples for NAT-T are provided for the topology in which only the responder is behind a NAT device and the topology in which both the initiator and responder are behind a NAT device. Site-to-site IKE gateway configuration for NAT-T is supported on both the initiator and responder. A remote IKE ID is used to validate a peer’s local IKE ID during Phase 1 of IKE tunnel negotiation. Both the initiator and responder require a local identify and remote identity string.

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