Triangle Route - ZyXEL Communications VMG1312-T10C User Manual

Does this rule stop LAN users from accessing critical resources on the Internet? For example, if IRC
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is blocked, are there users that require this service?
Is it possible to modify the rule to be more specific? For example, if IRC is blocked for all users, will
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a rule that blocks just certain users be more effective?
Does a rule that allows Internet users access to resources on the LAN create a security
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vulnerability? For example, if FTP ports (TCP 20, 21) are allowed from the Internet to the LAN,
Internet users may be able to connect to computers with running FTP servers.
Does this rule conflict with any existing rules?
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Once these questions have been answered, adding rules is simply a matter of entering the
information into the correct fields in the web configurator screens.

14.6.4 Triangle Route

When the firewall is on, your Device acts as a secure gateway between your LAN and the Internet.
In an ideal network topology, all incoming and outgoing network traffic passes through the Device
to protect your LAN against attacks.
Figure 113 Ideal Firewall Setup
14.6.4.1 The "Triangle Route" Problem
A traffic route is a path for sending or receiving data packets between two Ethernet devices. You
may have more than one connection to the Internet (through one or more ISPs). If an alternate
gateway is on the LAN (and its IP address is in the same subnet as the Device's LAN IP address),
the "triangle route" (also called asymmetrical route) problem may occur. The steps below describe
the "triangle route" problem.
A computer on the LAN initiates a connection by sending out a SYN packet to a receiving server on
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the WAN.
The Device reroutes the SYN packet through Gateway A on the LAN to the WAN.
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The reply from the WAN goes directly to the computer on the LAN without going through the
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Device.
As a result, the Device resets the connection, as the connection has not been acknowledged.
VMG1312-T10C User's Guide
LAN
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WAN
Chapter 14 Firewall
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