D-Link DFL-1660 User Manual page 438

9.3.5. NAT Traversal
evolved. NAT traversal is an add-on to the IKE and IPsec protocols that allows them to function
when being NATed. NetDefendOS supports the RFC3947 standard for NAT-Traversal with IKE.
NAT traversal is divided into two parts:
• Additions to IKE that lets IPsec peers tell each other that they support NAT traversal, and the
specific versions supported. NetDefendOS supports the RFC3947 standard for NAT-Traversal
with IKE.
• Changes to the ESP encapsulation. If NAT traversal is used, ESP is encapsulated in UDP, which
allows for more flexible NATing.
Below is a more detailed description of the changes made to the IKE and IPsec protocols.
NAT traversal is only used if both ends have support for it. For this purpose, NAT traversal aware
VPNs send out a special "vendor ID" to tell the other end of the tunnel that it understands NAT
traversal, and which specific versions of the draft it supports.
Achieving NAT Detection
To achieve NAT detection both IPsec peers send hashes of their own IP addresses along with the
source UDP port used in the IKE negotiations. This information is used to see whether the IP
address and source port each peer uses is the same as what the other peer sees. If the source address
and port have not changed, then the traffic has not been NATed along the way, and NAT traversal is
not necessary. If the source address and/or port has changed, then the traffic has been NATed, and
NAT traversal is used.
Changing Ports
Once the IPsec peers have decided that NAT traversal is necessary, the IKE negotiation is moved
away from UDP port 500 to port 4500. This is necessary since certain NAT devices treat UDP
packet on port 500 differently from other UDP packets in an effort to work around the NAT
problems with IKE. The problem is that this special handling of IKE packets may in fact break the
IKE negotiations, which is why the UDP port used by IKE has changed.
UDP Encapsulation
Another problem that NAT traversal resolves is that the ESP protocol is an IP protocol. There is no
port information as we have in TCP and UDP, which makes it impossible to have more than one
NATed client connected to the same remote gateway and at the same time. Because of this, ESP
packets are encapsulated in UDP. ESP-UDP traffic is sent on port 4500, the same port as IKE when
NAT traversal is used. Once the port has been changed, all following IKE communication is done
over port 4500. Keep-alive packets are also sent periodically to keep the NAT mapping alive.
NAT Traversal Configuration
Most NAT traversal functionality is completely automatic and in the initiating firewall no special
configuration is needed. However, for responding firewalls two points should be noted:
• On responding firewalls, the Remote Endpoint field is used as a filter on the source IP of
received IKE packets. This should be set to allow the NATed IP address of the initiator.
• When individual pre-shared keys are used with multiple tunnels connecting to one remote
firewall which are then NATed out through the same address, it is important to make sure the
Local ID is unique for every tunnel. The Local ID can be one of
• Auto - The local ID is taken as the IP address of the outgoing interface. This is the
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Chapter 9. VPN