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 From:  Andreas Ferrari <aferrari at stasoft dot ch>
 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:  Fri, 06 Oct 2006 12:08:39 +0200
Hi

After hours of searching and lots of calls with the ISP
someone from the ISP gave me the hint.
It is not a m0n0wall problem, it is a problem with a router which does
not properly handle MTU/MSS.
The router is between the customers home network and his office...
its a Draytek Vigor!!
So dont use this routers!!!

thanks a lot for all the help

regards

Andreas

Andreas Ferrari schrieb:
> Morning
> 
> Ok i missunderstod you and yestarday was not really my day.
> I hope you accept my excuse.
> Yes the postfix server works, I tested it with other VPN connections and 
> large Mails so it cannot be the a problem of the postfix server.
> I also checked the transfer of files to the fileserver.
> Maybe its really a problem with the Memory of the M0n0 or the FlashCard,
> hope it works with a new one. And the last option is the MTU story..
> I will keep you up-to-date....
> 
> Neil A. Hillard schrieb:
> 
>> Jeff,
>>
>> In message <4523CF99 dot 1050406 at buehlertech dot com>, Jeff Buehler
>> <jeff at buehlertech dot com> writes
>>
>>> 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 specifically.
>>
>>
>>
>> Yep.  My point was that the bad checksums may just lead you down the
>> wrong path and waste your time.
>>
>> To confirm whether this is a problem or is just TCP offloading, perform
>> a packet capture using a separate machine, either in a tap, a spanned
>> port on a manageable switch or a hub.
>>
>> At least that way you'll know if this really is a problem.
>>
>> HTH,
>>
>>
>>                                 Neil.
>>
>>
>>> 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.
>>>>>
>>>>
>>>>
>>>
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>>
>>
> 
> 


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