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10-40 Firmware User 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 guarantees 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 filter set screen
This is an example of the Netopia filter set screen:
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 Firmware Version 5.4 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
Change Input Filter 1
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