Ethereal-dev: Re: [Ethereal-dev] 'Add Expression' button in 'Edit Capture Filte r' window ?
On Fri, Dec 13, 2002 at 08:31:21AM -0000, Richard Urwin wrote:
> What investment do we have in the libpcap parser?
What do you mean by "investment"?
There *is* a fair bit invested in the libpcap parser and code generator
by libpcap developers (of which I am one); I am unconvinced that it
would be a worthwhile investment of our time to attempt to duplicate -
and match the further investment in - the code generator.
The parser, itself (in the sense of "grammar.y" and "scanner.l"), is
another matter. That's the easy part.
> One option would be to
> build our own capture filter parser somewhat closer to the display
> filter syntax.
As noted, there's more involved than just a parser.
It wouldn't be hard to build the parser. The question then is what it
would produce.
There are two options I see, at present:
1) producing BPF code;
2) producing a libpcap-format capture filter string.
1) involves duplicating the effort that's gone into libpcap - and
continuing to duplicate that effort. I'm not sure that's worthwhile.
2) doesn't involve duplicating that effort; the major problem is that we
might have to use the
expr relop expr
True if the relation holds, where relop is one of
>, <, >=, <=, =, !=, and expr is an arithmetic
expression composed of integer constants
(expressed in standard C syntax), the normal
binary operators [+, -, *, /, &, |], a length
operator, and special packet data accessors. To
access data inside the packet, use the following
syntax:
proto [ expr : size ]
Proto is one of ether, fddi, ip, arp, rarp, tcp,
udp, or icmp, and indicates the protocol layer for
the index operation. The byte offset, relative to
the indicated protocol layer, is given by expr.
Size is optional and indicates the number of bytes
in the field of interest; it can be either one,
two, or four, and defaults to one. The length
operator, indicated by the keyword len, gives the
length of the packet.
feature in the generated string, but, in current versions of libpcap,
there's a leak in the code generator that can cause it, in some cases,
to forget that BPF "registers" are no longer being used, so sufficiently
complicated expressions might produce errors *and* all subsequent
compiles in the same program would also prodce errors even if the
expression isn't too complicated.
However:
1) I fixed that memory leak in the current CVS version of
libpcap (and 0.8 might be released early next year; it would
include that fix);
2) that might not be *too* severe a problem, especially for
expressions that could be compiled into expressions not using
that feature (e.g., "ip.addr == XXX" would compile into "ip
host XXX").
We would, of course, want to take any string that fails to parse in our
syntax and hand it directly to "pcap_compile()", so that libpcap-format
strings continue to work.
