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Can I make an eBay web services request from behind a proxy server?
Yes, you can make SOAP calls from behind a proxy server, by configuring the proxy settings.
For .NET applications, you need to configure the proxy settings programmatically or in the machine.config / web.config file; as per this Microsoft Article:
PRB: Error on .NET client that consumes a Web service through an HTTP proxy server
For Java applications, you need to set the Java environment variables, "https.proxyHost" and "https.proxyPort" to allow HTTPS connections to work properly through your proxy server.
Java Networking and Proxies If you are using Axis, you need to set the java properties in Axis.
Web Services - Axis