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[i] i = start_offset, length = 1 [:j] start_offset = 0, length = j [i:] start_offset = i, end_offset = end_of_field Offsets can be negative, in which case they indicate the offset from the end of the field.The last byte of the field is at offset -1, the last but one byte is at offset -2, and so on.The third filter expression includes the constraint that offset 199 in the frame exists, in other words the length of the frame is at least 200.A special caveat must be given regarding fields that occur more than once per packet.Note: all protocol and field names that are available in Wireshark and TShark filters are listed in the comprehensive Boolean values are either true or false.In a display filter expression testing the value of a Boolean field, ``true'' is expressed as 1 or any other non-zero value, and ``false'' is expressed as zero.a11auth.subtype Gen Auth Ext Sub Type Unsigned 8-bit integer Mobile Auth Extension Sub Type.a11canid CANID Byte array CANID a11code Reply Code Unsigned 8-bit integer PDSN Code.
Think of a protocol or field in a filter as implicitly having the ``exists'' operator.
Every one of these fields can be used in a display filter. a11.ackstat Reply Status Unsigned 8-bit integer A11 Registration Ack Status. a11spi SPI Unsigned 32-bit integer Authentication Header Security Parameter Index.
a11.b Broadcast Datagrams Boolean Broadcast Datagrams requested a11Care of Address address Care of Address.
So, for instance, the following filters are equivalent: Remember that whenever a protocol or field name occurs in an expression, the ``exists'' operator is implicitly called. This means that the first filter expression must be read as ``show me the packets for which exists and equals 80, and exists and equals 192.168.2.1''.
The second filter expression means ``show me the packets where not (llc exists)'', or in other words ``where llc does not exist'' and hence will match all packets that do not contain the llc protocol.