How does Windows actually detect LAN (proxy) settings when using Automatic Configuration

Question

When Windows Internet Properties -> Connections -> LAN Settings -> Automatic Configuration is set to "Automatically detect settings" how does Windows actually determine/discover what the settings are? Is it a network broadcast or some kind of targeted query to a server configured somewhere in the registry, or something else?

This is because in your organization proxy settings are probably distributed through DHCP (or maybe group policy), whereas Chrome only supports the discovery through DNS described in my answer. — Tomalak, Aug 08 '13 at 18:27

Tomalak · Accepted Answer · 2008-10-10T12:37:30.513

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Its simple: Browsers (Firefox works the same) query GET http://wpad/wpad.dat.

If a web server named wpad is resolveable, it should serve wpad.dat, a script file analog to netscape PAC files. MIME type must also be "application/x-ns-proxy-autoconfig".

edited Oct 10 '08 at 12:37

answered Oct 10 '08 at 12:31

Tomalak

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-1 because the mechanism is more complicated. Mozilla's implementation fits the simple description, IE is more conformant to WPAD. – benc Oct 30 '09 at 06:35
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Sorry, but I have to disagree. WPAD is as simple as that. IE behaves exactly as described, and so does Firefox - not sure what you refer to. Can you elaborate? – Tomalak Oct 30 '09 at 09:44
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Interested to know what the timeout is for this, I found this question and answer after disabling the "Automatically detect settings" option to stop Chrome from hanging on "Downloading proxy script", when Chrome tries to load it's first web page. – StackEng2010 Mar 25 '18 at 22:37

score 9 · Answer 2 · edited Nov 24 '17 at 13:36

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This info about WPAD (Web Proxy Auto Discovery) seems to describe the process in detail, though I have confirmed that what Tomalak says is also actually occurring.

edited Nov 24 '17 at 13:36

Glenn

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answered Oct 10 '08 at 12:57

Craig Day

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score 8 · Answer 3 · answered Oct 30 '09 at 06:39

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The IE configuration described enables a WPAD implementation. Here's the Microsoft explanation of the entire mechanism (probably too much detail for a single post).

answered Oct 30 '09 at 06:39

benc

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score 7 · Answer 4 · answered Oct 10 '08 at 12:25

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It's a network broadcast, usually using DHCP.

That there wikipedia page should tell you all you need to know.

answered Oct 10 '08 at 12:25

Ryan

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I suppose the people who modded this down didn't know that this broken auto-configuration protocol first queries the DHCP server, and only then goes on to try the usual list of default wpad-like URLs. – Alexander Oct 11 '08 at 22:43
I believe in MS DHCP config you can specify the proxy server. – Robert Wagner Nov 18 '08 at 09:42
Yes, DHCP is/was used by Microsoft. Other implementations vary. – Ricardo Pardini Jun 13 '13 at 07:02
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Help! How can I see what the DHCP is broadcasting about proxies? – Colonel Panic Jul 01 '13 at 12:46

score -5 · Answer 5 · answered Apr 25 '14 at 10:44

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•Go to Tools > Options > General > Connection Settings > •Set to “Manual Proxy Configuration”

answered Apr 25 '14 at 10:44

sautrav adhikary

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How does Windows actually detect LAN (proxy) settings when using Automatic Configuration

5 Answers5

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