Re: [PATCH] --bad CRC

[Martin Mačok <martin.macok () underground cz>]

On Wed, Dec 21, 2005 at 08:54:33PM +0100, Ed3f wrote:

> I have written a very simple patch for 3.95 that adds a "--bad" option.

As you probably know, some chipsets (usualy fast ethernet) adds CRC to
outgoing packets automatically. This means that OS (drivers) does not
need to compute it and simply push packets to the hardware with no
(ie. bad) checksum. It gets corrected automatically by the hardware
before pushing the packets on the wire. Correct me if I'm wrong. (One
sideefect of this are occasinal questions on tcpdump/ethereal/etc.
mailing lists for "why my machine generates packets with bad
checksum?" -- because libpcap gets the packets before the checksum is
added by hardware...)

Are you aware of this and have you tried --bad from machine with such
hardware and checked the packets on the other side?

Martin Mačok
ICT Security Consultant


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