Proxy Configuration

A proxy server handles client requests for resources. A proxy can return a requested resource from its cache or forward the request to the server where the resource resides. Proxies can improve network performance by reducing the number of requests sent to remote servers. Proxies can also be used to restrict access to resources.

Adaptive Proxies

In the .NET Framework, proxies come in two varieties: adaptive and static. Adaptive proxies adjust their settings when the network configuration changes. For example, if a laptop user starts a dialup network connection, an adaptive proxy would recognize this change, discover and run its new configuration script, and adjust its settings appropriately.

Adaptive proxies are configured by a configuration script (see Automatic Proxy Detection). The script generates a set of application protocols and a proxy for each protocol.

Changes in the network environment may require that the system use a new set of proxies. If a network connection goes down or a new network connection is initialized, the system must discover the appropriate source of the configuration script in the new environment and run the new script.

You can use the usesystemdefault attribute of the <proxy> element in your configuration file. The usesystemdefault attribute controls whether the static proxy settings (proxy address, bypass list, and bypass on local) should be read from the Internet Explorer proxy settings for the user. If this value is set to true, the static proxy settings from Internet Explorer will be used. If this value is false or not set, the static proxy settings can be specified in the configuration and will override the Internet Explorer proxy settings. This value must also be set to false or not set for adaptive proxies to be enabled.

The following example shows a typical adaptive proxy configuration.

<system.net>  
    <defaultProxy>  
      <proxy usesystemdefault="false" />
    </defaultProxy>  
</system.net>  

Static Proxies

Static proxies are usually configured explicitly by an application, or when a configuration file is invoked by an application or the system. Static proxies are useful in networks in which the topology changes infrequently, such as a desktop computer connected to a corporate network.

Several options control how a static proxy operates. You can specify the following:

  • The address of the proxy.

  • Whether the proxy should be bypassed for local addresses.

  • Whether the proxy should be bypassed for a set of addresses.

The following table shows the configuration options for a static proxy.

Attribute, property, or configuration file setting Description
proxyaddress or Address The address of the proxy to use.
bypassonlocal or BypassProxyOnLocal Controls whether the proxy is bypassed for local addresses.
bypasslist or BypassArrayList Describes, with regular expressions, a set of addresses that bypass the proxy.
usesystemdefault Controls whether the static proxy settings (proxy address, bypass list, and bypass on local) should be read from the Internet Explorer proxy settings for the user. If this value is set to true, then the static proxy settings from Internet Explorer will be used. On .NET Framework 2.0 when this value is set to true, the Internet Explorer proxy settings are not overridden by other proxy settings in the configuration file. On .NET Framework 1.1, the Internet Explorer proxy settings can be overridden by other proxy settings in the configuration file.

If this value is false or not set, then the static proxy settings can be specified in the configuration and will override the Internet Explorer proxy settings. This value must also be set to false or not set for adaptive proxies to be enabled.

The following example shows a typical static proxy configuration.

<system.net>  
    <defaultProxy>  
        <proxy  proxyaddress="http://proxy.contoso.com:3128"  
                bypassonlocal="true"  
        />  
        <bypasslist>  
            <add address="[a-z]+.blueyonderairlines.com$" />  
        </bypasslist>  
    </defaultProxy>  
</system.net>  

See Also

WebProxy
GlobalProxySelection
Automatic Proxy Detection