Connection problem from Windows Client to Linux Server

Hi, I have a problem to get data from a linux server over UDP.
I have two applications (self written), the first runs under Windows
(Client), the other on Slackware 10.2, Kernel 2.4.32. The Windws
Client asks the Linux Server for an UDP data packet, and Linux answers
with the requested data.
So far so good.
Now I tried to get data with size over 1500 bytes, and I get ...
nothing.
To get more information, I've started Wireshark, and got following
line at the client side from the server: "Flags: 0x06 (Don't Fragment)
(More Fragments)". My MTU settings are 1500 on linux, 1492 on windows,
both are default values I think. The send data has none of this Bits
set (Flags: 0x0).
When I start the linux application on an Kernel 2.6 system, everything
is ok, there is no 'Don't Fragment' Bit set.
The problem is, I can't use an newer kernel at the moment, because
some other software only runs on 2.4 Kernels.
Does anyone have an idea?

mfg <(E-Mail Removed)> wrote:
> Now I tried to get data with size over 1500 bytes, and I get ...
> nothing.

That's because your Don't Fragment flag is set. The datagram is over the 1492
bytes limit, and should be discarded.

> When I start the linux application on an Kernel 2.6 system, everything
> is ok, there is no 'Don't Fragment' Bit set.

Ok.

> Does anyone have an idea?

What are you trying to do? What is the nature of the data content?

You could chunk your data into 1k blocks and label them 1 of 3, 2 of 3, 3 of
3, or whatever number of blocks you have. The receiver discards all blocks,
if a complete set is not received. (Add a rolling set number, so that it does
not get confused with later transmissions, eg receiving 1 and 2 from first
set, then 3 from a different data set.)

Alternatively you may want to use TCP, if reliable (signed-for) delivery is
required. UDP is for real time (pigeon) delivery, that may get lost.

Mark.

--
Mark Hobley
393 Quinton Road West
QUINTON
Birmingham
B32 1QE

Email: markhobley at hotpop dot donottypethisbit com

http://markhobley.yi.org/

Now I got the solution!
The problem was on both sides (Linux and Windows):
The first problem was, that the Linux Kernel 2.4.32 send our UDP
packets in an non standard network protocol (Kernel 2.6.19 does it
right...),
and the second problem, that the firewall we used don't informed about
blocking those non standard protocol.
So the windows firewall blocks it and set the "Don't Fragment" bit.
By the way, I use ZoneAlarm since years, but this bug costs me many
time, so i will look out for another firewall :-(.
I think comodo will have an option to solve this problem.

Thanks,
Manfred Gschweitl

I have forgotten one thing:
The solution is to uninstall the firewall. Only to disable it will not
help (Do'h)!

Manfred Gschweitl