ZyXEL Communications P-661H Series, User Manual, Page: 153 / 383

P-661H/HW Series User’s Guide
Chapter 8 Firewalls 153
• 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.
Note: 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.
If an initiation packet originates on the LAN, this means that someone is trying to make a
connection from the LAN to the Internet. Assuming that this is an acceptable part of the
security policy (as is the case with the default policy), the connection will be allowed. A cache
entry is added which includes connection information such as IP addresses, TCP ports,
sequence numbers, etc.
When the ZyXEL Device receives any subsequent packet (from the Internet or from the LAN),
its connection information is extracted and checked against the cache. A packet is only
allowed to pass through if it corresponds to a valid connection (that is, if it is a response to a
connection which originated on the LAN).
8.5.4 UDP/ICMP Security
UDP and ICMP do not themselves contain any connection information (such as sequence
numbers). However, at the very minimum, they contain an IP address pair (source and
destination). UDP also contains port pairs, and ICMP has type and code information. All of
this data can be analyzed in order to build "virtual connections" in the cache.
For instance, any UDP packet that originates on the LAN will create a cache entry. Its IP
address and port pairs will be stored. For a short period of time, UDP packets from the WAN
that have matching IP and UDP information will be allowed back in through the firewall.