kern/119635: Bad UDP packet checksum with em(4) and rxcsum/txcsum enabled

[Bernd Heller]

The following reply was made to PR kern/119635; it has been noted by GNATS.

From: Bernd Heller <bdheller at users.sourceforge.net>
To: Kris Kennaway <kris at FreeBSD.org>
Cc: freebsd-gnats-submit at FreeBSD.org
Subject: Re: kern/119635: Bad UDP packet checksum with em(4) and rxcsum/txcsum enabled
Date: Sun, 13 Jan 2008 22:59:52 +0100

 Hi Kris,
 
 yes, I ran tcpdump from another system on the same subnet, connected  
 by a hub.
 
 Bernd
 
 On 13.01.2008, at 20:04, Kris Kennaway wrote:
 
 > Bernd Heller wrote:
 >
 >>> Description:
 >> I have an Intel Pro/1000 PT Desktop Adapter network card in that  
 >> machine, and by default the em(4) driver has rxcsum/txcsum enabled.
 >> But the checksums for some UDP packets generated this way are  
 >> wrong. I noticed this when trying to run avahi-daemon. It sends UDP  
 >> multicast packets, but they were never recognized on the network.  
 >> Only when using tcpdump did I notice the bad checksums. After I  
 >> disabled rxcsum/txcsum on the interface all was working perfectly.
 >> I could reproduce the same issue in a VMware machine using FreeBSD  
 >> 7.0-RC1 i386 and amd64. 6.2 seems to be unaffected.
 >>> How-To-Repeat:
 >> Run avahi-daemon to publish any services (it will do so by default)  
 >> and analyze the UDP packages sent out.
 >>> Fix:
 >> Workaround: disable rxcsum/txcsum on the interface.
 >
 > Please confirm that you ran tcpdump from ANOTHER system, not the one  
 > generating the packets.  See the tcpdump manpage for more discussion  
 > of hardware checksumming and tcpdump.
 >
 > Kris