Security 10-33
Basic Firewall blocks undesirable traffic originating from the WAN (in most cases, the Internet), but for wards all
traffic originating from the LAN. It follows the conser vative “that which is not expressly permitted is prohibited”
approach: unless an incoming packet expressly matches one of the constituent input filters, it will not be
for warded to the LAN.
The five input filters and one output filter that make up Basic Firewall are shown in the table below.
Basic Firewall’s filters play the following roles.
Input filters 1 and 2:
These block WAN-originated OpenWindows and X-Windows sessions. Ser vice origination
requests for these protocols use por ts 2000 and 6000, respectively. Since these are greater than 1023,
OpenWindows and X-Windows traffic would other wise be allowed by input filter 4. Input filters 1 and 2 must
precede input filter 4; other wise they would have no effect since filter 4 would have already for warded
OpenWindows and X-Windows traffic.
Input filter 3:
This filter explicitly for wards all WAN-originated ICMP traffic to permit devices on the WAN to ping
devices on the LAN. Ping is an Internet ser vice that is useful for diagnostic purposes.
Input filters 4 and 5:
These filters for ward all TCP and UDP traffic, respectively, when the destination por t is
greater than 1023. This type of traffic generally does not allow a remote host to connect to the LAN using one
of the potentially intrusive Internet ser vices, such as Telnet, FTP, and WWW.
Setting
Input filter
1
Input filter
2
Input filter
3
Input filter
4
Input filter
5
Output
filter 1
Enabled
Yes
Yes
Yes
Yes
Yes
Yes
For ward
No
No
Yes
Yes
Yes
Yes
Source IP
address
0.0.0.0
0.0.0.0
0.0.0.0
0.0.0.0
0.0.0.0
0.0.0.0
Source IP
address mask
0.0.0.0
0.0.0.0
0.0.0.0
0.0.0.0
0.0.0.0
0.0.0.0
Dest. IP
address
0.0.0.0
0.0.0.0
0.0.0.0
0.0.0.0
0.0.0.0
0.0.0.0
Dest. IP
address mask
0.0.0.0
0.0.0.0
0.0.0.0
0.0.0.0
0.0.0.0
0.0.0.0
Protocol type
TCP
TCP
ICMP
TCP
UDP
0
Source por t
comparison
No
Compare
No
Compare
N/A
No
Compare
No
Compare
N/A
Source por t ID
0
0
N/A
0
0
N/A
Dest. por t
comparison
Equal
Equal
N/A
Greater
Than
Greater
Than
N/A
Dest. por t ID
2000
6000
N/A
1023
1023
N/A
Summary of Contents for Netopia Embedded Software
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