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 From:  Jeff Buehler <jeff at buehlertech dot com>
 To:  Andreas Ferrari <aferrari at stasoft dot ch>
 Cc:  "Neil A. Hillard" <m0n0 at dana dot org dot uk>, m0n0wall at lists dot m0n0 dot ch
 Subject:  Re: [m0n0wall] bad tcp cksum
 Date:  Wed, 04 Oct 2006 08:13:29 -0700
I think Neil is just saying it may be worth exploring other possible 
reasons for the email to be failing rather than focusing on the bad TCP 
checksums, which may or may not be related (is that right, Neil?).  It 
seems to me that that woulds be a indication of part of the problem, 
though, but it may confuse the issue.  For example, are you certain that 
the Postfix server doesn't have problems with large emails in general?  
I was having an issue with a mail server that had memory corruption due 
to memory timings in the bios that ONLY failed on very large email 
transfers (over 1.5 meg) and it took me quite a while to discover the 
cause, or even the problem, since none of my users reported it for a 
very long time.

If it were me, I would try sending large emails from other locations to 
verify the server is functioning properly (just to discount that as a 
possibility), then I would try sending large files from the M0n0 VPN via 
FTP to see if the failure is specific to SMTP.  I would suspect memory 
on the M0n0, or the network card.  I have found with M0n0wall that using 
IPSEC changing the type of compression makes a difference with MTU 
issues (PPTP doesn't have that option). Finally messing with MTU 
settings is the only thing left, and I have had very significant 
problems that were hard to correct with MTU (just changing this and that 
didn't work - it required very deliberate changes on a number of systems 
along with reboots in some cases) running M0n0wall to Windows systems 

I hope it resolves easily!


Andreas Ferrari wrote:
> > This is probably a red herring.
> Thanks for your answer....
> Dou you think i will report something that i have dreamd?
> The fact is that it still not working, I tried lowering the mtu 
> without sucess...
> So if you or someone have any ideas what there could be wrong please 
> give me a hint.
> Neil A. Hillard schrieb:
>> Hi,
>>         I've been away for a while and haven't seen a response to this
>> but apologies if I've missed it.
>> In message <451A397A dot 8010006 at stasoft dot ch>, Andreas Ferrari
>> <aferrari at stasoft dot ch> writes
>>> One of our customer has a VPN from his home office (wrap+m0n0 ver 1.22)
>>> to his office (wrap+m0n0 ver 1.22), that works fine. When he tries to
>>> send a mail from home over the VPN to the postfix server then the mail
>>> is never delivered to the mailserver.
>>> I noticed that small plaintext messages can be delivered to the postfix
>>> server but if the mail gets bigger then only the first part of the data
>>> are sent.
>>> On the mailserver i started tcpdump and could see that the mail could
>>> not be sent because there are a lot of bad tcp cksum messages.
>>> So the mailserver gets the first part right and the rest of the data
>>> are bad. How could that happend?
>> This is probably a red herring.  I suspect that you'll find that this is
>> due to TCP Checksum Offloading.  I spent a couple of days chasing what I
>> thought were problems cause by this - turns out it was something else.
>> HTH,
>>                                 Neil.