Security 8-87

About generic filters

One of the more difficult aspects of writing Generic filter sets is determining how many bytes into the packet (the Offset) the value you want to test is. In order to find the Offset value a good understanding of packet formats is helpful. The following figure depicts a typical packet format structure for purposes of this example.

Destination

Source

 

Frame Type

 

Frame Data

 

 

 

CRC

Address

Address

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

6 octets

6 octets

 

2 octets

 

64 - 1500 octets

 

4 octets

 

 

 

 

 

 

 

 

 

 

 

0

 

1

 

 

 

2

 

 

 

3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Version

IHL Type of Service

Total Length

 

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Identification

Flags

Fragment Offset

 

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Time to Live

Protocol

 

Header Checksum

 

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 

 

Source Address

 

 

 

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 

 

Destination Address

 

 

 

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 

Options

 

 

Padding

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 

 

 

data

 

 

 

 

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

IP Header Format

Note that one tick mark represents one bit position.

0123

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Source Port Destination Port +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Sequence Number +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Acknowledgment Number +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Data

UAPRSF

 

Offset Reserved

RCSSYI

Window

GKHTNN

 

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Checksum Urgent Pointer +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Options Padding +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

data +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

TCP Header Format

Note that one tick mark represents one bit position.

Observe that the first six bytes of the packet (frame) are used to indicate the destination MAC address and the next six are used define the source MAC address. So, for example, to write a Generic filter set that would block all incoming traffic to Mac address 00-00-c8-e3-95 you could create a filter rule like this:

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Netopia D3232 IDSL, D7171 SDSL, D7100 SDSL, D3100-I IDSL manual About generic filters, Crc

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