Bypass proxy server for local Addresses in Internet Explorer explained

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If you are a Proxy Administrator in an organisation, one of the most common problems with users and their Internet Explorer browsers is the “Bypass proxy” settings and how they work. This article should bring some light on proxy bypass.

When click Tools – Internet Options – Connections – LAN Settings on your Internet Explorer, you can see “Proxy Server” settings where you enter the IP/Hostname of the proxy server and the port number on which it listens on. This is the host to which all browser requests will be forwarded to. This helps improve performance from caching the pages or objects where necessary and these days proxies are evolving from just being a caching appliance to a more Internet Access Control devices.

There are instances where you would prefer to bypass the proxy and access the website or resource directly. A classic example is the locally available resources like Intranet or an extranet service which is available through a direct network access or is a service which doesn’t work very well with proxies.

You can choose to enable the option “Bypass Proxy Server for local addresses” to bypass any local addresses. But, what here means as local ip addresses? Is it the local subnet? local pcs or servers? how does the browser understand this?

In simple terms, Internet Explorer is designed to assume only a hostname as a local address and everything else as either an external address.

With the above, the following is true when it comes to handling bypass for local addresses where:

A URL with a hostname is referred as a local address. For example, http://intranet is a local address and hence will bypass proxy.

A URL with a domain name (with “.”s in it) is considered as a Fully Qualified Domain Name and will be passed through the proxy even if it is a local address. For instance, the same resource above if you refer it with its hostname.domainname like http://intranet.mynetwork.local where it is a part of the domain mynetwork.local and the traffic will still pass through the proxy even if “Bypass proxy server for local addresses” is selected.

A URL with an IP Address is considered as an external resource or server even if it is in the local subnet and will be passed through the proxy even if “bypas proxy” is selected. For example. http://192.168.0.100 which is same as http://intranet is passed through the proxy as it is considered as an external resource.

So, in the case of a FQDN or an IP Address, the only way to bypass proxy would be click the Advanced button and add these to the Exception List. You can use wildcards in these. For instance, intranet*, 10.1.1.1,www.mygoogle.com, 172* are all valid.

For more detailed information, check this Microsoft Technet