CentOS - Oct 2007 - File retrieval from outside hangs, internally is okay, only Centos5 affected

Hi!

I have a really weird problem with some of my servers, namely all the
ones running Centos5

When I try to download a file from the server to a machine outside our
Cisco 6500 router/firewall, the download hangs about half the times
(15 out of 40) when less than half a megabyte into the transfer
(varied from 76 kb to 496 kb).

One server has a portchannel (Cisco speak for ethernet bundle), others
do not, some use e1000, some use broadcom, I've tried httpd and scp
transfers, I've tried from three different clients,

Here's the really annoying part: the problem only occurs when
downloading from outside the firewall, not when transferring files
internally in the serverroom! And the switch is the firewall is a
modular chassis, so the data comes over the same backplane regardless.

And there's 42 files in /proc/net/sys/ipv4 which differ between EL4
and Centos5, so I'm a little lost here

Anybody got some ideas?

with kind regards,
Bent

PS Crossposted from Centos Forum, in case sombody on the list, who
doesn't read the forum, can help me

> I have a really weird problem with some of my servers, namely all the
> ones running Centos5
>
> When I try to download a file from the server to a machine outside our
> Cisco 6500 router/firewall, the download hangs about half the times
> (15 out of 40) when less than half a megabyte into the transfer
> (varied from 76 kb to 496 kb).
>
> One server has a portchannel (Cisco speak for ethernet bundle), others
> do not, some use e1000, some use broadcom, I've tried httpd and scp
> transfers, I've tried from three different clients,
>
> Here's the really annoying part: the problem only occurs when
> downloading from outside the firewall, not when transferring files
> internally in the serverroom! And the switch is the firewall is a
> modular chassis, so the data comes over the same backplane regardless.
>
> And there's 42 files in /proc/net/sys/ipv4 which differ between EL4
> and Centos5, so I'm a little lost here
I don't know if this would help, but I remember reading a post about a 
problem with tcp window scaling.  To disable tcp window scaling add the 
line

net.ipv4.tcp_window_scaling = 0

to /etc/sysctl.conf and then run sysctl -p to activate your change.

Barry

> I have a really weird problem with some of my servers, namely all the
> ones running Centos5
> 
> When I try to download a file from the server to a machine outside our
> Cisco 6500 router/firewall, the download hangs about half the times
> (15 out of 40) when less than half a megabyte into the transfer
> (varied from 76 kb to 496 kb).
> 
> One server has a portchannel (Cisco speak for ethernet bundle), others
> do not, some use e1000, some use broadcom, I've tried httpd and scp
> transfers, I've tried from three different clients,
> 
> Here's the really annoying part: the problem only occurs when
> downloading from outside the firewall, not when transferring files
> internally in the serverroom! And the switch is the firewall is a
> modular chassis, so the data comes over the same backplane regardless.
> 
> And there's 42 files in /proc/net/sys/ipv4 which differ between EL4
> and Centos5, so I'm a little lost here
> 
> Anybody got some ideas?
Just to state the obvious (well, obvious to me), which you don't seem to
have mentioned above: The filtering part of the 6500 is dropping the
traffic, and is dropping it because of something that Centos 5 is doing
differently from EL4.  I think there was a post to this list just last
week about something similar; I don't have time to search the archive,
but it is something to do with a TCP option/extension which is on in
Centos 5, but can be turned off via a setting in /proc somewhere; the
extension should be acceptable to all firewalls/routers (uses a
previously unused few bits in the TCP header), but some decide it's not
valid and drop packets/connections.

Craig Miskell
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