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[lwip-devel] Bypassing checksums

From: Gisle Vanem
Subject: [lwip-devel] Bypassing checksums
Date: Wed, 18 Jan 2017 17:29:20 +0100
User-agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:45.0) Gecko/20100101 Thunderbird/45.6.0

While building lwIP on Windows and testing using e.g. Google Chrome
on the same Windows machine I build lwIP on, the lwIP application sense-fully
ignores incoming packets with wrong checksums.

AFAICS, since IP+UDP+TCP checksum routines are most often implemented in the
NIC driver, packets coming from 'WinPap' will have wrong (no?) checksums. Or
is there another reason?

Hence in my lwipopts.h I set these to 0 to ignore wrong checksums:
  #if (LWIP_DEBUG > 0)
    #define CHECKSUM_CHECK_IP       0
    #define CHECKSUM_CHECK_TCP      0
    #define CHECKSUM_CHECK_UDP      0
    #define CHECKSUM_CHECK_ICMP     0
    #define CHECKSUM_CHECK_ICMP6    0

Without modifying the NIC-driver to NOT use this checksum
off-loading feature, is there a better way for my lwip test-app
to accept packets from my local machine?

Is there was a way to accept wrong checksums from a certain
source IP only? That would be perfect.


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