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Date: Mon, 10 May 2010 22:49:17 +0200
From: Andrea Venturoli <ml@netfence.it>
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Subject: Warnings with TSO on em
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Hello.
For quite a while, I've been seeing in the logs a lot of messages like 
the following:

snort: (snort_decoder) WARNING: IP dgm len < IP Hdr len!

I'm not sure about this, but I suspect they started when I upgraded from 
6.3 to 7.2.
Today, while investigating another problem, I decided I had to move this 
out of the way; tcpdump showed zero-length IP packets and Google told me 
they might be the result of TSO on the em cards.
So I checked and, yes, I had TSO enabled (by default).
"ifconfig em0 -tso" made this noise go away.

Now I'm looking for some more insight: if it's only a performance 
problem, I don't think I'll be hit, but could there be other side effects?



# uname -a
FreeBSD xxxxx.xxxxxxxx.xx 7.2-RELEASE-p7 FreeBSD 7.2-RELEASE-p7 #9: ...
# pciconf -l|grep em
em0@pci0:6:0:0: class=0x020000 card=0x34768086 chip=0x10968086 rev=0x01 
hdr=0x00
em1@pci0:6:0:1: class=0x020000 card=0x34768086 chip=0x10968086 rev=0x01 
hdr=0x00


I found some thread suggesting this might possibly be a bug with the em 
driver, but there was no follow up for more than a year.


  bye & Thanks
	av.

P.S. I had another problem on this machine: when I bonded the the two 
"em" interfaces into a "lagg" one, carp stopped working properly. Now I 
wonder if the two issues might be related...