Creating Packet Filters
12-7
The Ethernet and FDDI packet fields in Figure 12-1 are used as
operands
in
the packet filter. The two simplest operands are described in Table 12-3.
The
operators
that you specify in the packet filter allow the filter to make
a logical decision about whether the packet should be forwarded or
discarded. These operators are described in Table 12-4.
The operators
and
,
or
,
and
exclusive or
are bit-wise operators. Each bit of
the operands is logically compared to produce the resulting bit.
Table 12-3
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 12-4
Packet Filter Operators
Operator
Result
Opcode
equal
true if operand 1 = operand 2
eq
not equal
true if operand 1
≠
operand 2
ne
less than
true if operand 1 < operand 2
lt
less than or equal
true if operand 1
≤
operand 2
le
greater than
true if operand 1 > operand 2
gt
greater than or equal
true if operand 1
≥
operand 2
ge
and
operand 1 bit-wise AND operand 2
and
or
operand 1 bit-wise OR operand 2
or
exclusive or
operand 1 bit-wise XOR operand 2
xor
not
true if operand 1 = false
not
shift left
operand 1 SHIFT LEFT operand 2
shiftl
shift right
operand 1 SHIFT RIGHT operand 2
shiftr
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Страница 174: ...V Appendix A Packet Filter Opcodes Examples and Sytax Errors Appendix B Technical Support APPENDIXES ...