How to Configure Windows Sandbox Local Traefik PassTLSClientCert Middleware via XML in Windows 11

Forwarding client mutual TLS (mTLS) certificate details to backend application microservices via HTTP request headers requires careful testing to ensure certificate subjects and SHA-256 fingerprints are decoded correctly. Traefik’s PassTLSClientCert middleware parses client x509 certificates and injects sanitised pem-encoded headers. Windows Sandbox on Windows 11 provides an isolated container for validating mTLS header injection safely.

Why Test PassTLSClientCert in Windows Sandbox

Testing mutual TLS setups directly on developer machines often pollutes local certificate trust stores. Launching Traefik inside an isolated Windows Sandbox enables engineers to generate self-signed test root CAs, issue ephemeral client certificates, and verify proxy header forwarding without touching host certificate authorities.

Authoring the Windows Sandbox WSB XML

Save the following XML document as TraefikMTLSSandbox.wsb on your Windows 11 machine:

<Configuration>
  <VGpu>Default</VGpu>
  <Networking>Default</Networking>
  <MappedFolders>
    <MappedFolder>
      <HostFolder>C:\TraefikMTLSLab</HostFolder>
      <SandboxFolder>C:\TraefikMTLSLab</SandboxFolder>
      <ReadOnly>true</ReadOnly>
    </MappedFolder>
  </MappedFolders>
  <LogonCommand>
    <Command>powershell.exe -ExecutionPolicy Bypass -NoExit -File C:\TraefikMTLSLab\setup-mtls.ps1</Command>
  </LogonCommand>
</Configuration>

PowerShell Deployment Bootstrapper

Save the bootstrapping script as C:TraefikMTLSLabsetup-mtls.ps1:

Write-Host "Setting up Traefik PassTLSClientCert Lab..." -ForegroundColor Cyan

$Dir = "C:Traefik"
New-Item -ItemType Directory -Force -Path $Dir | Out-Null

$ZipUrl = "https://github.com/traefik/traefik/releases/download/v3.0.0/traefik_v3.0.0_windows_amd64.zip"
Invoke-WebRequest -Uri $ZipUrl -OutFile "$Dir	raefik.zip"
Expand-Archive -Path "$Dir	raefik.zip" -DestinationPath $Dir -Force

$Config = @"
http:
  middlewares:
    mtls-header-pass:
      passTLSClientCert:
        pem: true
        info:
          notAfter: true
          notBefore: true
          sans: true
          subject:
            country: true
            province: true
            locality: true
            organization: true
            commonName: true
  routers:
    secure-mtls:
      rule: "PathPrefix(`/mtls`)"
      middlewares:
        - "mtls-header-pass"
      service: "backend-service"
  services:
    backend-service:
      loadBalancer:
        servers:
          - url: "http://127.0.0.1:8080"
"@

Set-Content -Path "$Dirdynamic_conf.yml" -Value $Config
Write-Host "Traefik PassTLSClientCert Configured for mTLS Header Forwarding." -ForegroundColor Green

Testing Header Forwarding Workflows

Launch the sandbox configuration and run curl requests with client certificate arguments against https://127.0.0.1/mtls. Verify that backend services receive X-Forwarded-Tls-Client-Cert-Info and X-Forwarded-Tls-Client-Cert headers cleanly.