19216811.liveChapter 7 Firewalls
P-660R-F1 Series User’s Guide
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3
Does a rule that allows Internet users access to resources on the LAN create a security
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.
4
Does this rule conflict with any existing rules?
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.
7.5.4
Triangle Route
When the firewall is on, your ZyXEL 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 ZyXEL Device to protect your LAN against attacks.
Figure 61
Ideal Firewall Setup
7.5.4.1
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 ZyXEL Device’s LAN IP
address), the “triangle route” (also called asymmetrical route) problem may occur. The steps below
describe the “triangle route” problem.
1
A computer on the LAN initiates a connection by sending out a SYN packet to a receiving server on
the WAN.
2
The ZyXEL Device reroutes the SYN packet through Gateway
A
on the LAN to the WAN.
3
The reply from the WAN goes directly to the computer on the LAN without going through the ZyXEL
Device.
1
2
WAN
LAN