Operation Manual

Filters and Rules
55 NT/MPRI – 4 Special NT/MPRI Settings
– Reject: the packet is not sent on. The error message “Destina-
tion not reachable” is sent back to the source address.
Each packet is compared with the criteria of each rule in succession un-
til the first rule is found which applies to it. If the applicable action is
“Deny” or “Reject” the filtering process for this packet is concluded.
If no rule applying to this packet is found, or if the default action is “Ac-
cept”, the packet is sent on to the next filter instance.
Bear in mind the two following aspects when creating a filter:
l A filter profile always handles all packets (the rules apply to cer-
tain packets and the default action to all others).
l The sequence of the rules is extermely important! It is essential
that more general rules be placed toward the bottom of the pro-
file. Otherwise, more specific rules that are located lower down in
the hierarchy would never be applied.
The basic principle should be applied when designing a filter profile:
handle minority cases first.
A simple example: Suppose only call destination A is to be granted ac-
cess to computer B in the LAN. To set this condition, define the follow-
ing two rules in the global input filter:
1. A is granted access to computer B. Thus the first rule states: ac-
cept packets for all services which have the IP range of A as the
source and B’s IP address as the destination.
2. No one (i.e., no one else) is permitted access to the computer.
Thus the second rule states: deny packets for all services which
have any IP address and are addressed to the IP address of B.
The illustrations below show the order in which the filter instances are
applied for incoming and outgoing packets and those forwarded over
ISDN to another network. It is assumed that all filter instances are en-
abled and that the applicable or default rule for the packet in the exam-
ple is “Accept” so that the longest possible packet path is illustrated:
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