ZyXEL P-661H Series Bedienungsanleitung PDF herunterladen (Seite 185)

Chapter 12 VPN Screens

P-661H/HW Series User’s Guide

185

12.6 Keep Alive

When you initiate an IPSec tunnel with keep alive enabled, the ZyXEL Device automatically

renegotiates the tunnel when the IPSec SA lifetime period expires (see Section 12.12 on page

193 for more on the IPSec SA lifetime). In effect, the IPSec tunnel becomes an “always on”

connection after you initiate it. Both IPSec routers must have a ZyXEL Device-compatible

keep alive feature enabled in order for this feature to work.

If the ZyXEL Device has its maximum number of simultaneous IPSec tunnels connected to it

and they all have keep alive enabled, then no other tunnels can take a turn connecting to the

ZyXEL Device because the ZyXEL Device never drops the tunnels that are already connected.

When there is outbound traffic with no inbound traffic, the ZyXEL Device automatically

drops the tunnel after two minutes.

12.7 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 ZyXEL 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.

Figure 106 NAT Router Between IPSec Routers

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 Figure 106 on page 185, 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.

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