USRobotics NETServer/16 Bedienungsanleitung PDF herunterladen (Seite 122)

8-8 Packet Filters

TCP/IP packet filtering

After the filter name, rule number and permit/deny, IP rules start

with the following parameters:

<source address/mask> <destination address/mask> <

tcp

udp

icmp

Depending on the protocol, there can be more options following

these parameters. See TCP and UDP parameters and Filtering

ICMP packets (below) for more information.

Source Address

The address given here is compared to the source address of the

packet. Note that only the part of the address specified by the

mask field is used in the comparison. If a match is found, the

packet is forwarded (rules containing permit) or discarded

(rules containing deny).

The following rule example permits source addresses that match

the first 16 bits of the given IP address (that is, addresses begin-

ning with 192.77):

permit 192.77.200.203/16

Note: The source address and destination address fields gener-

ally are used to limit permitted access to trusted hosts and

networks only, to explicitly deny access to hosts and networks

that are not trusted, or to limit external access to a given host

(for example, a web server or a firewall). For example, the

following rule permits (SMTP) E-mail packets only if they are

from the host 192.77.203.24.

permit 192.77.203.24/32 0.0.0.0/0 tcp dst eq 25

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USRobotics NETServer/16 Bedienungsanleitung Seite 122