ZyXEL Communications ZyWall USG 50-H Series, User Manual

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Chapter 19 IPSec VPN
ZyWALL USG 50-H User’s Guide
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IPSec SA using Manual Keys
You might set up an IPSec SA using manual keys when you want to establish a VPN tunnel
quickly, for example, for troubleshooting. You should only do this as a temporary solution,
however, because it is not as secure as a regular IPSec SA.
In IPSec SAs using manual keys, the ZyWALL and remote IPSec router do not establish an
IKE SA. They only establish an IPSec SA. As a result, an IPSec SA using manual keys has
some characteristics of IKE SA and some characteristics of IPSec SA. There are also some
differences between IPSec SA using manual keys and other types of SA.
IPSec SA Proposal using Manual Keys
In an IPSec SA using manual keys, you can only specify one encryption algorithm and one
authentication algorithm. You cannot specify several proposals. There is no DH key exchange,
so you have to provide the encryption key and the authentication key the ZyWALL and remote
IPSec router use.
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The ZyWALL and remote IPSec router must use the same encryption key and
authentication key.
Authentication and the Security Parameter Index (SPI)
For authentication, the ZyWALL and remote IPSec router use the SPI, instead of pre-shared
keys, ID type and content. The SPI is an identification number.
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The ZyWALL and remote IPSec router must use the same SPI.
NAT for Inbound and Outbound Traffic
The ZyWALL can translate the following types of network addresses in IPSec SA.
• Source address in outbound packets - this translation is necessary if you want the
ZyWALL to route packets from computers outside the local network through the IPSec
SA.
• Source address in inbound packets - this translation hides the source address of computers
in the remote network.
• Destination address in inbound packets - this translation is used if you want to forward
packets (for example, mail) from the remote network to a specific computer (like the mail
server) in the local network.
Each kind of translation is explained below. The following example is used to help explain
each one.