Farallon Communications R9100 Filter basics, Established connections, Example IP filter set screen

13-32 User’s Reference Guide

Established connections

The TCP header contains one bit called the ACK bit (or TCP Ack bit). This ACK bit appears only with TCP, not UDP. The ACK bit is part of the TCP mechanism that guaranteed the delivery of data. The ACK bit is set whenever one side of a connection has received data from the other side. Only the first TCP packet will not have the ACK bit set; once the TCP connection is in place, the remainder of the TCP packets with have the ACK bit set.

The ACK bit is helpful for firewall design and reduces the number of potential filter rules. A filter rule could be created just allowing incoming TCP packets with the ACK bit set, since these packets had to be originated from the local network.

Example IP filter set screen

This is an example of the Netopia IP filter set screen:

Change Filter
Enabled:	Yes
Forward:	No
Source IP Address:	0.0.0.0
Source IP Address Mask:	0.0.0.0
Dest. IP Address:	0.0.0.0
Dest. IP Address Mask:	0.0.0.0
Protocol Type:	TCP
Source Port Compare...	No Compare
Source Port ID:	0
Dest. Port Compare...	Equal
Dest. Port ID:	2000
Established TCP Conns. Only:	No

Return/Enter accepts * Tab toggles * ESC cancels.

Enter the IP specific information for this filter.

Filter basics

In the source or destination IP address fields, the IP address that is entered must be the network address of the subnet. A host address can be entered, but the applied subnet mask must be 32 bits (255.255.255.255).

The Netopia R9100 has the ability to compare source and destination TCP or UDP ports. These options are as follows:

Item	What it means
No Compare	Does not compare TCP or UDP port
Not Equal To	Matches any port other than what is defined
Less Than	Anything less than the port defined