Odd Network Problem with FC5+

[Tony Nelson]

At 10:44 PM -0600 11/14/07, blake at ispn.net wrote:
>I seem to be having trouble sending mail from an FC5 server to
>mail.jococourthouse.com.
>
>The problem manifests itself as extreme slowness, which eventually leads
>to a timeout before I can even receive a full welcome prompt from the
>remote side.
>
>I have tested on ~ 10 boxes (different hardware) around the US ranging
>from FC5 through FC8 and NONE can telnet to this mx while RH, FC3 RH4,
>windows, and cisco gear on the same networks can ALL telnet without a
>problem.
>
>wireshark on the FC5 boxes show that the TCP handshake occurs as normal,
>however from then on all I receive from the remote side are 60 byte frames
>labeled 'TCP segment of a reassembled PDU' until I eventually receive an
>RST from the remote. - A capture on the working boxes looks normal.
>
>I suspect the problem is related to an IP/TCP change in the kernel or
>FC5+, however I have no idea what change could have occurred. Any insight
>or recommendations for resolving this problem would be appreciated.

TCP Window Scaling, introduced about that time, and a bad router?


At 5:57 PM +0200 7/24/06, Alexander Dalloz wrote:
>Stefan Kuhlemann schrieb:
>
>> even more strange - today around 10:00 o'clock the sendmail started to
>> work normal until 16:00 o'clock. (with nothing changed since sunday
>> 22:00)
>>
>> My feelings tell me that the reason might be some strange 'ill-broken'
>> firewall network stack or some weird netwok communication issues (like
>> the 'fun' with the ECN-flag some years ago)
>> I'm tomorrow 'on-site'......
>>
>> Stefan
>
>Try following kernel parameter change
>
>sysctl -w net.ipv4.tcp_window_scaling=0
>
>and see if that helps. TCP window scaling behaviour changed with kernel
>2.6.17 to be again more aggressive[1]. Some other systems are just
>broken in this regard.
>
>Alexander
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