How to get/add/changed allowed server variables of IIS Web Site with PowerShell?

FYI adding server variable on server level by powershell:

Add-WebConfiguration /system.webServer/rewrite/allowedServerVariables -atIndex 0 -value @{name="RESPONSE_SERVER"}

this will add the following configuration in applicationhost.config

<system.webServer>
...
    <rewrite>
    ...
        <allowedServerVariables>
            <add name="RESPONSE_SERVER" />
        </allowedServerVariables>
    ...
    </rewrite>
</system.webServer>

Updated 2024/09/24

this answer works with PowerShell 7+

1. import the iis module

Import-Module IISAdministration

2. create the following two functions:

function Enable-IisServerVariable($variable) {
    $section = Get-IISConfigSection -SectionPath "system.webServer/rewrite/allowedServerVariables"
    $collection = Get-IISConfigCollection -ConfigElement $section
    $existingVariables = $collection | ForEach-Object { $_.Attributes["name"].Value }

    if (-not ($existingVariables -contains $variable)) {
        New-IISConfigCollectionElement -ConfigCollection $collection -ConfigAttribute @{"Name" = $variable}
        Write-Host "Server variable '$variable' has been added."
    } else {
        Write-Host "Server variable '$variable' is already enabled."
    }
}

function Enable-ReverseProxyHeaderForwarding() {
    foreach ($variable in @("HTTP_X_FORWARDED_PROTO", "HTTP_X_FORWARDED_HOST", "HTTP_X_FORWARDED_FOR")) {
        Enable-IisServerVariable $variable
    }
}

3. execute the function:

Enable-ReverseProxyHeaderForwarding

Original Answer

as noted in @matija's answer, you will need to ensure you have the web admin module loaded:

Import-Module WebAdministration

then, building on @lia's answer, you can define a function to add multiple variables in an idempotent fashion like this:

function Enable-ReverseProxyHeaderForwarding() {
    $requiredVariables = @("HTTP_X_FORWARDED_PROTO", "HTTP_X_FORWARDED_HOST", "HTTP_X_FORWARDED_FOR")
    $existingVariables = Get-WebConfiguration -Filter "/system.webServer/rewrite/allowedServerVariables/add" -PSPath "IIS:\Sites\" | ForEach-Object { $_.name }

    foreach ($variable in $requiredVariables) {
        if (-not ($existingVariables -contains $variable)) {
            Add-WebConfiguration "/system.webServer/rewrite/allowedServerVariables" -value @{name = $variable }
        }
    }
}

and then execute the function:

Enable-ReverseProxyHeaderForwarding

UPDATE: I have solved this problem

Step 1. Unlock server configuration section

%windir%\system32\inetsrv\appcmd.exe unlock config -section:system.webServer/rewrite/allowedServerVariables

Now it's posible to get allowed server variables with

import-module WebAdministration

$webSiteName = "your-site-name"
$allowedServerVariables = Get-WebConfigurationProperty -PSPath "MACHINE/WEBROOT/APPHOST/${webSiteName}" -filter "system.webServer/rewrite/allowedServerVariables/add" -Name name

Write-Host $allowedServerVariables.Value

Example for adding server variables

import-module WebAdministration

$webSiteName = "your-web-site-name"
add-webconfigurationproperty -pspath "iis:\sites\${webSiteName}"  -filter "system.webserver/rewrite/allowedservervariables" -name "." -value @{name='HTTP_X_ORIGINAL_ACCEPT_ENCODING'}

UPDATE 2 Better solution without PowerShell is to add allowed server variables globally on IIS web server. So each variable will be automatically inherited from new web site.