Setting A Web Proxy Through Configuration In .NET Applications

Specially in enterprise environments, proxy servers are used to access the Internet.

In a Windows / Internet Explorer environments there is a proxy server configuration in Internet Properties > Connections > LAN settings > Proxy server.

Although these configurations are tightly connected to Internet Explorer, any well behaved Windows application should, at least, allow the user to choose to use them.

In applications targeting the .NET framework, these the proxy server can be set on a per call basis. Several networking classes have a Proxy property that receives a value implementing the IWebProxy interface.

In order for the Windows/ Internet Explorer configuration to be used, the application must be configured to use the default proxy settings.

This configuration is done in the machine or application file in the proxy element of the defaultProxy configuration section in the system.net section group:

<configuration>
  <system.net>
    <defaultProxy enabled="true">
      <proxy usesystemdefault="True"/>
    </defaultProxy>
  </system.net>
</configuration>

You can use this configuration also to set a specific proxy to be used by your application. In the following example, a proxy setting for using the Fiddler Tool is used:

<configuration>
  <system.net>
    <defaultProxy enabled="true">
      <proxy proxyaddress="http://ipv4.fiddler"/>
      <proxy proxyaddress="http://127.0.0.1:8888"/>
    </defaultProxy>
  </system.net>
</configuration>

Unfortunately, this is a MachineToApplication setting and, for that reason, is not allowed in the user settings configuration file, when in a shared installation. In these type of installations, the default Windows / Internet Explorer settings should be used as a default. If a user needs or wants to specify proxy server settings, application specific proxy server settings must be used but, assigning the user defined proxy server configuration to the GlobalProxySelection.Select property will allow its use for the entire application.

Updated: Corrected proxy address when using Fiddler following Eric Lawrence’s comment:

> <proxy proxyaddress="http://ipv4.fiddler/" />

That line should not work. Fiddler doesn't register anything in DNS, so for "ipv4.fiddler" to have any meaning, Fiddler must already be being used as the proxy.

The proper setting for Fiddler use should be:

<proxy proxyaddress="http://127.0.0.1:8888" />