Asus SL1000 - Manual (Page 61)

Internet Security Router User ’s Manual Chapter 9. Configuring Firewall/NAT Settings
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Configuring Firewall/NAT Settings
The Internet Security Router provides built-in firewall/NAT functions, enabling you to protect the system
against denial of service (DoS) attacks and other types of malicious accesses to your LAN while providing
Internet access sharing at the same time. You can also specify how to monitor attempted attacks, and who
should be automatically notified.
This chapter describes how to create/modify/delete ACL (Access Control List) rules to control the data passing
through your network. You will use firewall configuration pages to:
„ Create, modify, delete and view inbound/outbound ACL rules.
„ Create, modify and delete pre-defined services, IP pools, NAT pools, application filters and time
ranges to be used in inbound/outbound ACL configurations.
„ View firewall statistics.
Note: When you define an ACL rule, you instruct the Internet Security Router to examine each data packet it
receives to determine whether it meets criteria set forth in the rule. The criteria can include the network or
internet protocol it is carrying, the direction in which it is traveling (for example, from the LAN to the Internet or
vice versa), the IP address of the sending computer, the destination IP address, and other characteristics of
the packet data.
If the packet matches the criteria established in a rule, the packet can either be accepted (forwarded towards
its destination), or denied (discarded), depending on the action specified in the rule.
9.1 Firewall Overview
9.1.1 Stateful Packet Inspection
The stateful packet inspection engine in the Internet Security Router maintains a state table that is used to
keep track of connection states of all the packets passing through the firewall. The firewall will open a
“hole” to
allow the packet to pass through if the state of the packet that belongs to an already established connection
matches the state maintained by the stateful packet inspection engine. Otherwise, the packet will be dropped.
This
“hole” will be closed when the connection session terminates. No configuration is required for stateful
packet inspection; it is enabled by default when the firewall is enabled. Please refer to section 12.1 Configure
System Services to enable or disable firewall service on the Internet Security Router.
9.1.2 DoS (Denial of Service) Protection
Both DoS protection and stateful packet inspection provide first line of defense for your network. No
configuration is required for both protections on your network as long as firewall is enabled for the Internet
Security Router. By default, the firewall is enabled at the factory. Please refer to section 12.1 Configure System
Services to enable or disable firewall service on the Internet Security Router.
9.1.3 Firewall and Access Control List (ACL)
9.1.3.1 Priority Order of ACL Rule
All ACL rules have a rule ID assigned – the smaller the rule ID, the higher the priority. Firewall monitors the
traffic by extracting header information from the packet and then either drops or forwards the packet by looking
for a match in the ACL rule table based on the header information. Note that the ACL rule checking starts from
the rule with the smallest rule ID until a match is found or all the ACL rules are examined. If no match is found,
the packet is dropped; otherwise, the packet is either dropped or forwarded based on the action defined in the
matched ACL rule.