![\"\"](\"https:\/\/nuxx.net\/blog\/wp-content\/uploads\/2026\/06\/internet_options_proxy_when_fiddler_running.png\" "\\"Proxy")
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Beware: The use of Fiddler<\/a> to troubleshoot Internet Explorer<\/a> issues can complicate the use of the Local intranet<\/em> zone by effectively disabling\u00a0the intended behavior of the Include all sites that bypass the proxy server<\/em> setting. KB174360<\/a>\u00a0describes the\u00a0Local intranet<\/em> zone as follows:<\/p>\n By default, the Local Intranet zone contains all network connections that were established by using a Universal Naming Convention (UNC) path, and Web sites that bypass the proxy server or have names that do not include periods (for example, http:\/\/local), as long as they are not assigned to either the Restricted Sites or Trusted Sites zone.<\/em><\/p><\/blockquote>\n When Fiddler is enabled, proxy settings on a client machine are changed to direct all traffic to 127.0.0.1 as seen above. This results in no websites automatically matching the\u00a0Local intranet<\/em> zone because none bypass the proxy server.<\/p>\n While troubleshooting a suspected issue with custom user agent<\/a>\u00a0entries I had Fiddler running, as is my normal practice. \u00a0The reported issues was\u00a0custom user agent strings not being sent, something that won’t happen on IE9 unless\u00a0Compatibility View<\/em> is enabled. I’d first thought there was a problem with\u00a0the\u00a0Display intranet sites in Compatibility View<\/em> option not working due to the Local intranet<\/em> zone assignment not working, but\u00a0my issue actually turned out to be Fiddler getting in the way by causing no sites to match the Local intranet<\/em> zone. When Fiddler\u00a0was disabled\u00a0and I switched to using an external tap<\/a> for monitoring, behavior returned to normal.<\/p>\n