Chapter 8 Firewalls
P-793H User’s Guide
124
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Later, an inbound packet reaches the interface. This packet is part of the connection
previously established with the outbound packet. The inbound packet is evaluated
against the inbound access list, and is permitted because of the temporary access list
entry previously created.
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The packet is inspected by a firewall rule, and the connection's state table entry is
updated as necessary. Based on the updated state information, the inbound extended
access list temporary entries might be modified, in order to permit only packets that are
valid for the current state of the connection.
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Any additional inbound or outbound packets that belong to the connection are inspected
to update the state table entry and to modify the temporary inbound access list entries as
required, and are forwarded through the interface.
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When the connection terminates or times out, the connection's state table entry is deleted
and the connection's temporary inbound access list entries are deleted.
8.5.2 Stateful Inspection and the ZyXEL Device
Additional rules may be defined to extend or override the default rules. For example, a rule
may be created which will:
• Block all traffic of a certain type, such as IRC (Internet Relay Chat), from the LAN to the
Internet.
• Allow certain types of traffic from the Internet to specific hosts on the LAN.
• Allow access to a Web server to everyone but competitors.
• Restrict use of certain protocols, such as Telnet, to authorized users on the LAN.
These custom rules work by evaluating the network traffic’s Source IP address, Destination IP
address, IP protocol type, and comparing these to rules set by the administrator.
"
The ability to define firewall rules is a very powerful tool. Using custom rules, it
is possible to disable all firewall protection or block all access to the Internet.
Use extreme caution when creating or deleting firewall rules. Test changes
after creating them to make sure they work correctly.
Below is a brief technical description of how these connections are tracked. Connections may
either be defined by the upper protocols (for instance, TCP), or by the ZyXEL Device itself (as
with the "virtual connections" created for UDP and ICMP).
8.5.3 TCP Security
The ZyXEL Device uses state information embedded in TCP packets. The first packet of any
new connection has its SYN flag set and its ACK flag cleared; these are "initiation" packets.
All packets that do not have this flag structure are called "subsequent" packets, since they
represent data that occurs later in the TCP stream.
If an initiation packet originates on the WAN, this means that someone is trying to make a
connection from the Internet into the LAN. Except in a few special cases (see "Upper Layer
Protocols" shown next), these packets are dropped and logged.
Summary of Contents for G.SHDSL.bis 4-port Security Gateway P-793H
Page 2: ......
Page 7: ...Safety Warnings P 793H User s Guide 7 This product is recyclable Dispose of it properly ...
Page 8: ...Safety Warnings P 793H User s Guide 8 ...
Page 30: ...List of Figures P 793H User s Guide 30 ...
Page 36: ...List of Tables P 793H User s Guide 36 ...
Page 38: ...38 ...
Page 68: ...Chapter 4 Point to 2 point Configuration P 793H User s Guide 68 ...
Page 70: ...70 ...
Page 114: ...Chapter 7 Network Address Translation NAT Screens P 793H User s Guide 114 ...
Page 116: ...116 ...
Page 152: ...Chapter 10 Content Filtering P 793H User s Guide 152 ...
Page 180: ...Chapter 12 Static Route P 793H User s Guide 180 ...
Page 194: ...Chapter 14 Dynamic DNS Setup P 793H User s Guide 194 ...
Page 216: ...Chapter 16 Universal Plug and Play UPnP P 793H User s Guide 216 ...
Page 217: ...217 PART IV Maintenance System 219 Logs 225 Tools 229 Diagnostic 235 ...
Page 218: ...218 ...
Page 224: ...Chapter 17 System P 793H User s Guide 224 ...
Page 234: ...Chapter 19 Tools P 793H User s Guide 234 ...
Page 238: ...238 ...
Page 244: ...Chapter 21 Introducing the SMT P 793H User s Guide 244 ...
Page 248: ...Chapter 22 General Setup P 793H User s Guide 248 ...
Page 256: ...Chapter 23 WAN Setup P 793H User s Guide 256 ...
Page 262: ...Chapter 24 LAN Setup P 793H User s Guide 262 ...
Page 278: ...Chapter 27 Static Route Setup P 793H User s Guide 278 ...
Page 308: ...Chapter 30 Filter Configuration P 793H User s Guide 308 ...
Page 310: ...Chapter 31 SNMP Configuration P 793H User s Guide 310 ...
Page 312: ...Chapter 32 System Password P 793H User s Guide 312 ...
Page 322: ...Chapter 33 System Information Diagnosis P 793H User s Guide 322 ...
Page 352: ...Chapter 37 Schedule Setup P 793H User s Guide 352 ...
Page 360: ...360 ...
Page 366: ...Appendix B Wall mounting Instructions P 793H User s Guide 366 ...
Page 400: ...Appendix F IP Address Assignment Conflicts P 793H User s Guide 400 ...
Page 404: ...Appendix G Common Services P 793H User s Guide 404 ...
Page 432: ...Appendix K Legal Information P 793H User s Guide 432 ...
Page 443: ...Index P 793H User s Guide 443 ...
Page 444: ...Index P 793H User s Guide 444 ...