Packet Filter Examples

A-9

Packet Filter

The following examples of using the packet filter language start with basic

Examples

packet filter concepts.

Destination This filter operates on the destination address field of a frame. It Address Filter allows packets to be forwarded that are destined for stations with an

Organizationally Unique Identifier (OUI) of 08-00-02. To customize this filter to another OUI value, change the literal value loaded in the last pushLiteral.l instruction. Note that the OUI must be padded with an additional 00 to fill out the literal to 4 bytes.

name

“Forward to 08-00-02”

 

 

 

pushField.l

0

#

Get first

4

bytes of

 

 

#

destination

address

pushLiteral.l

0xffffff00

#

Set up mask

to isolate first

 

 

#

3 bytes

 

 

and

 

#

Top of stack now has OUI

pushLiteral.l

0x08000200

#

Load

OUI

value

eq

 

#

Check

for

match

Source Address This filter operates on the source address field of a frame. It allows packets Filter to be forwarded that are from stations with an OUI of 08-00-02. To

customize this filter to another OUI value, change the literal value loaded in the last pushLiteral.l instruction. Note that the OUI must be padded with an additional 00 to fill out the literal to 4 bytes.

name

“Forward from 08-00-02”

 

 

pushField.l

6

#

Get first

4

bytes of source

 

 

#

address

 

 

pushLiteral.l

0xffffff00

#

Set up mask

to isolate first

 

 

#

3 bytes

 

 

and

 

#

Top of stack now has OUI

pushLiteral.l

0x08000200

#

Load

OUI

value

eq

 

#

Check

for

match

Length Filter This filter operates on the length field of a frame. It allows packets to be forwarded that are less than 400 bytes in length. To customize this filter to another length value, change the literal value loaded in the pushLiteral.w instruction.

name

“Forward

< 400”

 

 

 

pushField.w

12

#

Get length

field

pushLiteral.w

400

#

Load

length

limit

lt

 

#

Check

for frame length < limit

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3Com 2200 manual Packet Filter, Examples, Packet filter concepts