What is a Proxy Server 2.0 LAT?.

What is a Proxy Server LAT?

This is probably one of the most common questions I am asked as a security
professional. The LAT, or Local Address Table, is a series of IP address
pairs that define your internal network. Each pair defines a range of IP
addresses or a single pair.

That LAT is generated upon installation of Proxy Server. It defines the
internal IP addresses. Proxy Server uses the Windows NT Routing Table to
auto-generate the LAT. It is possible that the when the LAT is
auto-generated, that errors in the LATs construction will be found. You
should always manually comb through the LAT and check for errors. It is not
uncommon to find external IP addresses in the LAT, or entire subnets of your
internal IP addresses will not appear on the LAT. It is generally a good
idea to have all of your internal IP addresses in the LAT.

* NO EXTERNAL IP ADDRESSES SHOULD APPEAR IN YOUR LAT.

Upon installing the Proxy Server client software, it adds a file named
msplat.txt into the \Mspclnt directory. The msplat.txt file contains the
LAT. This file is regularly updated from the server to ensure that the LAT
the client is using is current.

What is the LAT used for?

Every time a client attempts to use a Winsock application to establish a
connection, the LAT is referenced to determine if the IP address the client
is attempting to reach is internal or external. If the IP address is
internal, Proxy Server is bypassed and the connection is made directly. If
the IP address the client is attempting to connect to DOES NOT appear in the
LAT, it is determined that the IP address is remote and the connection is
made through Proxy Server. By knowing this information, someone on your
internal network could easily edit his or her LAT table to bypass Proxy
Server.

Some Administrators may not see this as a problem because the LAT is
regularly updated from the server, so any changes the user made to his or
her LAT will be overwritten. However, if the user saves their LAT with the
filename Locallat.txt, the client machine will reference both the msplat.txt
and the locallat.txt to determine if an IP address is local or remote. So,
by using the locallat.txt method, a user can, in theory, permanently bypass
Proxy Server. The locallat.txt file is never overwritten unless the user
does so manually.

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