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322 C
HAPTER
15: P
ACKET
F
ILTERING
The Ethernet and FDDI packet fields in Figure 24 are used as operands in
the packet filter. The two simplest operands are described in Table 65.
Packet Filter Opcodes Opcodes are instructions used in packet filter definitions. The available
opcodes are described in Table 66.
Table 65 Two Packet Filter Operands
Operand Description Opcode
packet field A field in the packet that can reside at any offset.
The size of the field can be 1, 2, 4, or 6 bytes.
Typically, you only specify a 6-byte field when you
want the filter to examine a 48-bit address. pushField
constant A literal value to which you are comparing a packet
field. As with a field, a constant can be 1, 2, 4, or 6
bytes long. pushLiteral
Table 66 Packet Filtering Opcodes
Opcode
Memory
Requirements
Description
name “<name>” 2 + n bytes,
where n is the length
of the <name>
Assigns a user-defined <name> to the packet filter.
The name may be any sequence of ASCII characters
other than quotation marks. The name is limited to 32
characters. You can include only a single name
statement in each packet filter program.
pushField.size <offset> 3 bytes Pushes a field from the target packet onto the stack.
Packet data starting at <offset> is copied onto the
stack. The most significant byte of the field is the byte
at the specified offset. The size field of the instruction
determines the number of bytes pushed. The pushField
instruction provides direct access to any 1, 2, 4, or 6
byte (.b, .w, .l, or .a) field contained within the first 64
bytes of the target packet.
Specify the offset as an octal, decimal, or hexadecimal
number.
■
Precede an octal number by a “0”.
■
Precede a hexadecimal number by either “0x” or
“0X”.
■
Use either upper or lower case letters for the
hexadecimal digits “a” through “f”.