How to get notified if packets are dropped because of checksum mismatch error

How to get notified if packets are dropped because of checksum mismatch error

[Arif Hossain]

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Hi all,

I have a netfilter_queue app which de-obfuscates a already obfuscated
udp packets. de-obfuscation process ends successfully but somehow packet
is not reaching to the udp daemon. i'm suspecting this is happening
because of a udp checksum mismatch. so i'm wondering how can i get logs
of packets which are dropped because of udp checksum mismatch?

I'm receiving the packets ok in my netfilter_queue app. and when i issue
set_nfq_verdict() it also returns a positive value. problem is packet is
not delivered to the udp daemon i'm expecting. i've straced my
netfilter_queue app. it shows that it issues a sendmsg() to the udp
socket which is listening for this. I've also straced my  udp daemon.
its stuck in poll() for receiving udp packet. but it does not return
from this poll meaning that something is wrong. packet is lost in
oblivion after set_nfq_verdict(). how can i debug it?

i've heard linux by default dropps a packet if it does not have correct
checksum if checksum is present, this behavior can be modified by
recompiling the kernel. using a custom kernel is not in our agenda.

my platform :
Linux <hostname> 2.6.32-220.7.1.el6.x86_64 #1 SMP Wed Mar 7 00:52:02 GMT
2012 x86_64 x86_64 x86_64 GNU/Linux

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