ZyXEL Communications P-2802H(W)(L)-I Series - Manual (Page 158)

Chapter 11 Firewalls
P-2802H(W)(L)-I Series User’s Guide
158
11.3 Security Considerations
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Incorrectly configuring the firewall may block valid access or introduce security
risks to the ZyXEL Device and your protected network. Use caution when
creating or deleting firewall rules and test your rules after you configure them.
Consider these security ramifications before creating a rule:
1 Does this rule stop LAN users from accessing critical resources on the Internet? For
example, if IRC is blocked, are there users that require this service?
2 Is it possible to modify the rule to be more specific? For example, if IRC is blocked for
all users, will a rule that blocks just certain users be more effective?
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..
11.4 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 85 Ideal Firewall Setup
11.4.1 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.