kern/91032: invalid IP checksum under if_bridge(4)+em(4) combination

Ralf S. Engelschall rse at
Wed Dec 28 13:10:08 PST 2005

The following reply was made to PR kern/91032; it has been noted by GNATS.

From: "Ralf S. Engelschall" <rse at>
To: Gleb Smirnoff <glebius at>
Cc: FreeBSD-gnats-submit at
Subject: Re: kern/91032: invalid IP checksum under if_bridge(4)+em(4) combination
Date: Wed, 28 Dec 2005 22:05:24 +0100

 On Wed, Dec 28, 2005, Gleb Smirnoff wrote:
 > On Wed, Dec 28, 2005 at 09:45:38PM +0100, Ralf S. Engelschall wrote:
 > R> Another workaround would have been to put into the box a 100baseTX NIC
 > R> driven by fxp(4) instead of the 1000baseTX NIC driven by em(4). Because
 > R> the combination of if_bridge(4) and fxp(4) I've running fine with mostly
 > R> the same configuration on another server.
 > Not all fxp(4) cards can do checksum offloading. If you put the one
 > that can't, then the seconds workaround doesn't differ from the first
 > one. An problem ins't em specific in this case probably.
 Yes, of course. I think we actually have two problems here: first
 if_bridge(4) unconditionally clears the checksum, although I see no
 reason for doing this in case of non-generated and just forwarded
 packets. And the second problem (that the checksum is not re-calculated)
 is more em(4) related or at least related to the packet flow through the
 system in case a network device is attached to if_bridge(4).
 > So important questions is: does the fxp card you used can do offloading?
 According to the ifconfig(8) output on that box and the fact that I do
 not see any "TXCSUM" in the "options", I would say this particular card
 is not capable of doing "checksum offloading":
 |         options=8<VLAN_MTU>
 |         inet netmask 0xffff0000 broadcast
 |         ether 00:07:e9:d9:4d:43
 |         media: Ethernet autoselect (100baseTX <full-duplex>)
 |         status: active
 Ok, then it is clear why the bridging works fine on this FreeBSD
 6.0-STABLE box while it was broken (and required the "ifconfig em0
 -txcsum") on the other box.
 rse at                        Ralf S. Engelschall                       rse at
 FreeBSD committer            

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