Vpn, Nat, And Nat Traversal - ZyXEL Communications SBG3500-N000 User Manual

Transport mode ESP with authentication is not compatible with NAT.
Table 107 VPN and NAT
SECURITY PROTOCOL
AH
AH
ESP
ESP

22.7.6 VPN, NAT, and NAT Traversal

NAT is incompatible with the AH protocol in both transport and tunnel mode. An IPSec VPN using
the AH protocol digitally signs the outbound packet, both data payload and headers, with a hash
value appended to the packet, but a NAT device between the IPSec endpoints rewrites the source or
destination address. As a result, the VPN device at the receiving end finds a mismatch between the
hash value and the data and assumes that the data has been maliciously altered.
NAT is not normally compatible with ESP in transport mode either, but the SBG3500-N Series's NAT
Traversal feature provides a way to handle this. NAT traversal allows you to set up an IKE SA when
there are NAT routers between the two IPSec routers.
Figure 151 NAT Router Between IPSec Routers
A
Normally you cannot set up an IKE SA with a NAT router between the two IPSec routers because
the NAT router changes the header of the IPSec packet. NAT traversal solves the problem by adding
a UDP port 500 header to the IPSec packet. The NAT router forwards the IPSec packet with the UDP
port 500 header unchanged. In the above figure, when IPSec router A tries to establish an IKE SA,
IPSec router B checks the UDP port 500 header, and IPSec routers A and B build the IKE SA.
For NAT traversal to work, you must:
• Use ESP security protocol (in either transport or tunnel mode).
• Use IKE keying mode.
• Enable NAT traversal on both IPSec endpoints.
• Set the NAT router to forward UDP port 500 to IPSec router A.
Finally, NAT is compatible with ESP in tunnel mode because integrity checks are performed over the
combination of the "original header plus original payload," which is unchanged by a NAT device. The
compatibility of AH and ESP with NAT in tunnel and transport modes is summarized in the following
table.
Table 108 VPN and NAT
SECURITY PROTOCOL
AH
AH
Chapter 22 IPSec VPN
MODE NAT
Transport N
Tunnel N
Transport N
Tunnel Y
NAT Router
MODE NAT
Transport N
Tunnel N
SBG3500-N Series User's Guide
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B