Filters; General; Creating A Filter; Syntax For Filter Stanzas - Digi PortServer II User Manual

Filters

General

Filters can be used in PPP, SLIP, and CSLIP connections to bringup a connection, to
keep the connection up, to pass or block IP packets, and to log IP packets.

Creating a filter

To configure a filter, enter the following command:
set filter name=<filtername> [stanza#]
Each filter comprises up to 32 stanzas. When a filter is active, stanza s1 is processed
first, s2 second, and so on. As soon as a stanza is found that matches the packet being
processed, the action specified in that stanza is taken, and the remaining stanzas are
ignored. Each stanza may contain several tokens or conditions; all conditions must be
true for the stanza to be true.
Important: It is not possible to add a stanza into the middle of a filter that has been
completed. You can only add a line to the end, or replace an existing
line. Otherwise, you should create a new filter that includes the addi-
tional stanza.

Syntax for filter stanzas

Configuring actions that will not be taken

The exclamation mark (" ! ") changes the sense of the filter action. If the packet
matches a stanza and the stanza is preceded by " ! ", then the action is not taken, and no
other stanzas are processed. Also, if the last stanza is preceded by " ! ", the action taken
on exiting the filter differs to that taken if the " ! " is omitted (see the example under A
filter that will bring up a connection when it detects any IP packet except DNS later in
this chapter).
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