[clug-talk] nasty problem

Mark Carlson carlsonmark at gmail.com
Wed May 17 19:42:33 PDT 2006


On 5/17/06, Robert Lewko <lewkor at gmail.com> wrote:
> Hi guys:
>
>  I have a program that sends UDP messages to its server.  Its been working
> just fine for about a week since I delivered it.  At about 2:00pm. my client
> phoned me to say that the server wasn't working.  When I looked at the
> software (putting fprintf''s into it) I could see that it was processing the
> packets that it receives and sending the correct data back.  I can ssh into
> the server and on both the server and the firewall I can use tcpdump to see
> the packet coming in and the packet leaving the server then making its way
> out of the firewall.  However, ethereal does not see the packet return to
> the client.  This affects me at home (shaw) and my client at his work site
> (telus).
>
>  One other thing.  For brief periods this afternoon I was getting packets
> through without making any changes either to the code, the server or the
> client machines.
>
>  I am using UDP because the client is using a satellite modem.  The server
> uses UDP port 9608.  When the server gets a packet it just gets the source
> address and returns the response packet to the sender.  Like I said I have
> had no problem with that port both while I have been developing the code and
> for the last week until this afternoon.
>
>  All the TCP stuff works - ssh, http, https, etc.  Does anyone know how to
> diagnose this?

You could try using a UDP "ping" program to see if your packets are
all getting lost.  Thats about the only thing I can think of that you
haven't tried yet.  Good luck, UDP can get nasty.

HTH

-Mark

P.S. IIRC UDP is highly sensitive to packet loss, since you are going
over satellite, you may want to consider "solar activity"
http://www.spaceweather.gc.ca/forecastmap_e.php
If you check the past 24 hour conditions right now, you'll see that in
the aural region (north of here) there was some "active intervals."  I
don't know if this would have had an effect... I just think its funny
that the government has a space weather agency.



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