Help! Strange download problem

I recently switched over from a dedicated ADSL router box to
using my Linux box as an ADSL/PPPoE client, firewall, NAT, etc.,
box. The Linux box is running RedHat 7.2. Firewall is using iptables,
and is set-up to do masquerading.

It seems that ever since the switchover, I have been running into
data corruption problems when transferring data to a Win2K box on
the LAN behind the Linux box. I see this principally when doing
HTTP file transfers, but also in SMTP and NNTP transfer from my
ISP's POP and news servers.

Here is a typical scenario:

If I download a file to the Linux box, I get 100% success.

If I download the same file to the Win2K box, I get only about 10%
success. In the failing cases, the transfer completes but the data
received is corrupt, as indicated when I try to unzip the file
using WinZip.

In both cases I am downloading via HTTP using wget.

Using ethereal, I captured the message flow through the ethernet
adapter that's connected to the ADSL modem. I compared the TCP
stream contents between a successful Linux download, a successful
Win2K download, and a failed Win2K download.

Except for an HTML date/time header, the data is the same for both
successful downloads, but differs from the other two for the
failed Win2K download.

The CLAMPMSS option is set to 1412 in /etc/sysconfig/network-scripts/
ifcfg-ppp0. This adds the "-m 1412" option to the pppoe commmand
line.

The MTU for the ppp0 interface is set to 1492. The MTU for the LAN
side of the box (eth0) is set to 1500. I tried changing it to 1492
but that did not help.

On the Win2K box, I tried setting the MTU for the LAN interface to
1492 without effect.

I can't think of anything else to try, and I'm looking for any
suggestions.

MV

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"Martin Vuille" <(E-Mail Removed)> wrote in message
news:Xns93C0F3E88DBEjpmvrealtime@206.123.6.7

> Except for an HTML date/time header, the data is the same for both
> successful downloads, but differs from the other two for the
> failed Win2K download.
>
> The CLAMPMSS option is set to 1412 in /etc/sysconfig/network-scripts/
> ifcfg-ppp0. This adds the "-m 1412" option to the pppoe commmand
> line.
>
> The MTU for the ppp0 interface is set to 1492. The MTU for the LAN
> side of the box (eth0) is set to 1500. I tried changing it to 1492
> but that did not help.
>
> On the Win2K box, I tried setting the MTU for the LAN interface to
> 1492 without effect.
>
> I can't think of anything else to try, and I'm looking for any
> suggestions.

Have you explicitly disabled ECN via "echo 0 > /proc/sys/net/ipv4/tcp_ecn"

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