>>Could you use WCF instead of the old style asmx webservice ? If so look at wsDualHttpBinding. here's plenty of example code out there.....
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>The WCF does not support changing the URL once the reference is added to the project. At least, there is no property available for that. I spent some threads on that last year on this topic. I do not know if this has been improved in 2012.

You didn't mention a requirement for that but, AFAICS you can construct whatever endpoint you want.

>Putting aside the use of wsDualHttpBinding for the VPN support, wouldn't the SSLStream class be resulting in the same? We made a test yesterday with a valid X509 certificate and processing SSL socket packets across the wire from a server to a client having a listener waiting for the response and sending one back. Thus, once the listener was started at the client site, the server was able to submit requests to it and receive immediate responses.

No experience there I'm afraid. Are you using TcpListener ?