Getting through firewalls using HttpWebRequest

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Bruce Wolfe - 20 Jul 2004 15:50 GMT

We are having a problem with firewalls. We can't pinpoint the exact
problem but the working theory at this point is the request from the
client software is reaching the server through port 80. The response
from the server is not making it back because the return port back to
the client software being blocked by the firewall. This seems to be an
intermittent problem because some of the ports are open through the
firewall and some are not. Outbound from the client to the server
(port 80) is obviously not blocked. The firewall and the settings are
at the client site and out of our control. First question: Is this a
good theory? 2nd: What is a good , general strategy for dealing with
these issues? 3rd: Can we control the inbound port - one that is not
blocked? 4th: Can we use a persistent connection so the inbound port
will not change? Would the inbound port change anyway even of we had a
persistent connection?

Thanks. Any help with this would be greatly appreciated.

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Dale - 21 Jul 2004 02:46 GMT

Generally, if the request is initiated by the client, then the response can
made on any port and the firewall will allow it. The firewall should
associate the return data with the request and allow the packet. What is it
that makes you come to this conclusion?

Dale

> We are having a problem with firewalls. We can't pinpoint the exact
> problem but the working theory at this point is the request from the
[quoted text clipped - 12 lines]
>
> Thanks. Any help with this would be greatly appreciated.

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Bruce Wolfe - 21 Jul 2004 13:56 GMT

Dale, Thanks for the reply. I did some more tests and have arrived at
the same conclusion. At this point, we are not sure what the problem
is. We think it might be related to hitting ports that are dedicated
to another service. I'll post again when I have better information.
Thanks again.
--Bruce

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KumarForG - 03 Aug 2004 09:29 GMT

Hi Bruce and Dale,

    i have exactly the same scenario with the client environment remote
to us and us having a dotnet webservice to be called from a dotnet
winform client.

    The thing is, we have not yet discarded the theory that the firewall
is disallowing response from the webservice.

    Bruce, can you please tell me as to how you concluded the theory to
be incorrect.
    I see in Ethereal traces that there are only TCP ack=0 packets to the
webserver, but there are no responses.

    the firewall's ( a Cisco Checkpoint Firewall ) behaviour is unknown
to me.

any info in this regard is greatly appreciated as it looks like a dead
end with my not having any documentation or support from cisco

regards
-kumarforg

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