Security Considerations; Triangle Route - ZyXEL Communications eircom D1000 User Manual

Chapter 13 Firewall
10 For local services that are enabled, protect against misuse. Protect by configuring the services to
communicate only with specific peers, and protect by configuring rules to block packets for the
services at specific interfaces.
11 Protect against IP spoofing by making sure the firewall is active.
12 Keep the firewall in a secured (locked) room.

13.6.3 Security Considerations

Note: Incorrectly configuring the firewall may block valid access or introduce security
risks to the 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:
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.

13.6.4 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 109 Ideal Firewall Setup
13.6.4.1 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),
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LAN
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WAN
eircom D1000 modem User's Guide