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Chapter 17 VPN
FMG3024-D10A / FMG3025-D10A Series User’s Guide
177
• Aggressive Mode is quicker than Main Mode because it eliminates several steps when the
communicating parties are negotiating authentication (phase 1). However the trade-off is that
faster speed limits its negotiating power and it also does not provide identity protection. It is
useful in remote access situations where the address of the initiator is not know by the responder
and both parties want to use pre-shared key authentication.
17.3.5 IPSec and NAT
Read this section if you are running IPSec on a host computer behind the Device.
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. When using AH protocol, packet contents (the data payload) are not
encrypted.
A NAT device in between the IPSec endpoints will rewrite either the source or destination address
with one of its own choosing. The VPN device at the receiving end will verify the integrity of the
incoming packet by computing its own hash value, and complain that the hash value appended to
the received packet doesn't match. The VPN device at the receiving end doesn't know about the
NAT in the middle, so it assumes that the data has been maliciously altered.
IPSec using ESP in Tunnel mode encapsulates the entire original packet (including headers) in a
new IP packet. The new IP packet's source address is the outbound address of the sending VPN
gateway, and its destination address is the inbound address of the VPN device at the receiving end.
When using ESP protocol with authentication, the packet contents (in this case, the entire original
packet) are encrypted. The encrypted contents, but not the new headers, are signed with a hash
value appended to the packet.
Tunnel mode ESP with authentication is compatible with NAT because integrity checks are
performed over the combination of the "original header plus original payload," which is unchanged
by a NAT device.
Transport mode ESP with authentication is not compatible with NAT.
17.3.6 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 Device’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.
Table 53 VPN and NAT
SECURITY PROTOCOL MODE NAT
AH Transport N
AH Tunnel N
ESP Transport N
ESP Tunnel Y