What key in windows registry disables IE connection parameter "Automatically Detect Settings"?

I found the solution : it's the 9th byte of this key :

[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Internet Settings\Connections] "DefaultConnectionSettings"=hex:3c,00,00,00,1f,00,00,00,05,00,00,00,00,00,00, 00,00,00,00,00,00,00,00,00,01,00,00,00,1f,00,00,00,68,74,74,70,3a,2f,2f,31, 34,34,2e,31,33,31,2e,32,32,32,2e,31,36,37,2f,77,70,61,64,2e,64,61,74,90,0e, 1e,66,d3,88,c5,01,01,00,00,00,8d,a8,4e,9e,00,00,00,00,00,00,00,00

It's a bitfield:

• 0x1: (Always 1)
• 0x2: Proxy enabled
• 0x4: "Use automatic configuration script" checked
• 0x8: "Automatically detect settings" checked

Mask 0x8 to turn it off, i.e., subtract 8 if it's higher than 8.

Thanks to Jamie on google groups.

Update

Based on the VBScript by WhoIsRich combined with details in this answer, here's a PowerShell script to amend these & related settings:

function Set-ProxySettings {
    [CmdletBinding()]
    param ( #could improve with parameter sets 
        [Parameter(Mandatory = $false)]
        [bool]$AutomaticDetect = $true
        ,
        [Parameter(Mandatory = $false)]
        [bool]$UseProxyForLAN = $false
        ,
        [Parameter(Mandatory = $false)]
        [AllowNull()][AllowEmptyString()]
        [string]$ProxyAddress = $null
        ,
        [Parameter(Mandatory = $false)]
        [int]$ProxyPort = 8080 #closest we have to a default port for proxies
        ,
        [AllowNull()][AllowEmptyString()]
        [bool]$UseAutomaticConfigurationScript = $false
    )
    begin {
        [string]$ProxyRegRoot = 'HKCU:\Software\Microsoft\Windows\CurrentVersion\Internet Settings'
        [string]$DefaultConnectionSettingsPath = (Join-Path $ProxyRegRoot 'Connections')
        [byte]$MaskProxyEnabled = 2
        [byte]$MaskUseAutomaticConfigurationScript = 4
        [byte]$MaskAutomaticDetect = 8
        [int]$ProxyConnectionSettingIndex = 8
    }
    process {
    #this setting is affected by multiple options, so fetch once here 
    [byte[]]$DefaultConnectionSettings = Get-ItemProperty -Path $DefaultConnectionSettingsPath -Name 'DefaultConnectionSettings' | Select-Object -ExpandProperty 'DefaultConnectionSettings'

    #region auto detect
    if($AutomaticDetect) { 
        Set-ItemProperty -Path $ProxyRegRoot -Name AutoDetect -Value 1
        $DefaultConnectionSettings[$ProxyConnectionSettingIndex] = $DefaultConnectionSettings[$ProxyConnectionSettingIndex] -bor $MaskAutomaticDetect
    } else {
        Set-ItemProperty -Path $ProxyRegRoot -Name AutoDetect -Value 0
        $DefaultConnectionSettings[$ProxyConnectionSettingIndex] = $DefaultConnectionSettings[$ProxyConnectionSettingIndex] -band (-bnot $MaskAutomaticDetect)
    }
    #endregion

    #region defined proxy
    if($UseProxyForLAN) {
        if(-not ([string]::IsNullOrWhiteSpace($ProxyAddress))) {
            Set-ItemProperty -Path $ProxyRegRoot -Name ProxyServer -Value ("{0}:{1}" -f $ProxyAddress,$ProxyPort)
        }
        Set-ItemProperty -Path $ProxyRegRoot -Name ProxyEnable -Value 1
        $DefaultConnectionSettings[$ProxyConnectionSettingIndex] = $DefaultConnectionSettings[$ProxyConnectionSettingIndex] -bor $MaskProxyEnabled
    } else {
        Set-ItemProperty -Path $ProxyRegRoot -Name ProxyEnable -Value 0        
        $DefaultConnectionSettings[$ProxyConnectionSettingIndex] = $DefaultConnectionSettings[$ProxyConnectionSettingIndex] -band (-bnot $MaskProxyEnabled)
    }
    #endregion

    #region config script
    if($UseAutomaticConfigurationScript){
        $DefaultConnectionSettings[$ProxyConnectionSettingIndex] = $DefaultConnectionSettings[$ProxyConnectionSettingIndex] -bor $MaskUseAutomaticConfigurationScript
    }else{
        $DefaultConnectionSettings[$ProxyConnectionSettingIndex] = $DefaultConnectionSettings[$ProxyConnectionSettingIndex] -band (-bnot $MaskUseAutomaticConfigurationScript) 
    }
    #endregion

    #persist the updates made above
    Set-ItemProperty -Path $DefaultConnectionSettingsPath -Name 'DefaultConnectionSettings' -Value $DefaultConnectionSettings
    }
}

Another way to control this setting is by using an undocumented registry key AutoDetect=0 mentioned on this blog post:

Registry Key : HKCU\Software\Microsoft\Windows\CurrentVersion\Internet Settings\

DWORD AutoDetect = 0 or 1

So the .reg file to turn it off would look like:

Windows Registry Editor Version 5.00
[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Internet Settings]
"AutoDetect"=dword:00000000

For anyone looking to untick the 'Automatically Detect Settings' box without overwriting the other settings contained in the registry entry, you can use vbscript at logon.

On Error Resume Next

Set oReg   = GetObject("winmgmts:{impersonationLevel=impersonate}!\\.\root\default:StdRegProv")
sKeyPath   = "Software\Microsoft\Windows\CurrentVersion\Internet Settings\Connections"
sValueName = "DefaultConnectionSettings"

' Get registry value where each byte is a different setting.
oReg.GetBinaryValue &H80000001, sKeyPath, sValueName, bValue

' Check byte to see if detect is currently on.
If (bValue(8) And 8) = 8 Then

  ' Turn off detect and write back settings value.
  bValue(8) = bValue(8) And Not 8
  oReg.SetBinaryValue &H80000001, sKeyPath, sValueName, bValue

End If

Set oReg = Nothing