Modify routes and url when using Varnish

I have a project where Varnish is used in front of a Slim 4 project. Due to some project specialities, the original url should be hidden and the incoming request is rewritten to some other headers.

Example

Original call to demo.xyz is coming to Varnish
Varnish is transforming the request to http://slimapp.dev/url/demo.xyz
The original host is saved in the header with X-Orig-Host: demo.xyz

The Slim App needs to detect if it is a Varnish request (I already got that) and needs to transform the routes.

Routes

Without Varnish:

$app->group('/url/{url}', function (RouteCollectorProxy $group) {
    $group->get('[/city/{city}]', IndexAction::class)->setName('home');
    $group->get('/search[/{query}]', SearchQueryAction::class)->setName('search-query');
});

With Varnish:

$app->get('[/city/{city}]', IndexAction::class)->setName('home');
$app->get('/search[/{query}]', SearchQueryAction::class)->setName('search-query');

So when creating a route with url_for (Twig-View package) the urls should be generated like that:

Route w/o Varnish: http://slimapp.dev/url/demo.xyz/article/123
Route w/ Varnish: http://demo.xyz/article/123

Ideas:

Modify the original UriInterface object but I have no idea how.
Write a TwigExtension for Twig-View to modify the methods for url_for and so on.

The first ideas feels kind of more reliable and the right place to do. The other one would mean I would need to overwrite the helper methods to build the url based on the X-Orig-Host.

Any idea how I can modify the Uri / probably overwriting the create method?

Thanks in advance!

P. S.: The same questions I placed in Slim Discourse.

UPDATE 1

routes.php

return function (App $app) {

    $app->get('/', function () {
        die('No URL defined.');
    })->setName('root');

    $app->group('/url/{url}', function (RouteCollectorProxy $group) {
        $group->get('[/city/{city}]', IndexAction::class)->setName('home');

        # Internal page search
        $group->get('/search[/{query}]', SearchQueryAction::class)->setName('search-query');
    });

};

middleware.php

return function(App $app) {
    $c = $app->getContainer();

    $app->addBodyParsingMiddleware();
    $app->add(TwigMiddleware::class);
    $app->addRoutingMiddleware();
    $app->add(VarnishBasePathMiddleware::class);

    $displayErrorDetails = $c->get(Configuration::class)->getBool('displayErrorDetails');
    $errorMiddleware = $app->addErrorMiddleware($displayErrorDetails, false, false);
    $errorHandler = $errorMiddleware->getDefaultErrorHandler();
    $errorHandler->registerErrorRenderer('text/html', HtmlErrorRenderer::class);
    $errorHandler->registerErrorRenderer('application/json', JsonErrorRenderer::class);
};

Error message

[404] Not found. in /usr/share/nginx/html/vendor/slim/slim/Slim/Middleware/RoutingMiddleware.php on line 91.

Backtrace: #0 /usr/share/nginx/html/vendor/slim/slim/Slim/Middleware/RoutingMiddleware.php(57): Slim\Middleware\RoutingMiddleware->performRouting(Object(Slim\Psr7\Request)) #1 /usr/share/nginx/html/vendor/slim/slim/Slim/MiddlewareDispatcher.php(132): Slim\Middleware\RoutingMiddleware->process(Object(Slim\Psr7\Request), Object(class@anonymous)) #2 /usr/share/nginx/html/src/Middleware/VarnishBasePathMiddleware.php(28): class@anonymous->handle(Object(Slim\Psr7\Request)) #3 /usr/share/nginx/html/vendor/slim/slim/Slim/MiddlewareDispatcher.php(180): App\Middleware\VarnishBasePathMiddleware->process(Object(Slim\Psr7\Request), Object(class@anonymous)) #4 /usr/share/nginx/html/vendor/slim/twig-view/src/TwigMiddleware.php(125): class@anonymous->handle(Object(Slim\Psr7\Request)) #5 /usr/share/nginx/html/vendor/slim/slim/Slim/MiddlewareDispatcher.php(180): Slim\Views\TwigMiddleware->process(Object(Slim\Psr7\Request), Object(class@anonymous)) #6 /usr/share/nginx/html/vendor/slim/slim/Slim/Middleware/ErrorMiddleware.php(89): class@anonymous->handle(Object(Slim\Psr7\Request)) #7 /usr/share/nginx/html/vendor/slim/slim/Slim/MiddlewareDispatcher.php(132): Slim\Middleware\ErrorMiddleware->process(Object(Slim\Psr7\Request), Object(class@anonymous)) #8 /usr/share/nginx/html/vendor/slim/slim/Slim/MiddlewareDispatcher.php(73): class@anonymous->handle(Object(Slim\Psr7\Request)) #9 /usr/share/nginx/html/vendor/slim/slim/Slim/App.php(208): Slim\MiddlewareDispatcher->handle(Object(Slim\Psr7\Request)) #10 /usr/share/nginx/html/public/index.php(57): Slim\App->handle(Object(Slim\Psr7\Request)) #11 {main}

Solution

You can implement you own ServerRequestCreatorInterface class like this:

use Psr\Http\Message\ServerRequestInterface;
use Slim\Factory\ServerRequestCreatorFactory;
use Slim\Interfaces\ServerRequestCreatorInterface;
use Slim\Psr7\Factory\ServerRequestFactory;

class VarnishServerRequestCreator implements ServerRequestCreatorInterface
{
    public function createServerRequestFromGlobals(): ServerRequestInterface
    {
        $request = ServerRequestFactory::createFromGlobals();
        $serverParams = $request->getServerParams();

        if (isset($serverParams['X-Orig-Host'])) {
            // Replace the hostname
            $uri = $request->getUri()->withHost($serverParams['X-Orig-Host']);
            $request = $request->withUri($uri);
        }

        return $request;
    }
}

ServerRequestCreatorFactory::setServerRequestCreator(new VarnishServerRequestCreator());

To output the full url in your Twig templates use the full_url_for function. Example:

{{ full_url_for('article') }}

Output:

https://demo.xyz/article

To change the base path you could add this special middleware:

<?php

namespace App\Middleware;

use Psr\Http\Message\ResponseInterface;
use Psr\Http\Message\ServerRequestInterface;
use Psr\Http\Server\MiddlewareInterface;
use Psr\Http\Server\RequestHandlerInterface;
use Slim\App;

final class VarnishBasePathMiddleware implements MiddlewareInterface
{
    /**
     * @var App The slim app
     */
    private $app;

    public function __construct(App $app)
    {
        $this->app = $app;
    }

    public function process(ServerRequestInterface $request, RequestHandlerInterface $handler): ResponseInterface
    {
        $serverParams = $request->getServerParams();

        // Detect Varnish
        if (isset($serverParams['X-Orig-Host'])) {
            // Change the base path
            $this->app->setBasePath('/url/demo.xyz');
        }

        return $handler->handle($request);
    }
}

The VarnishBasePathMiddleware must be add after the RoutingMiddleware

$app->add(TwigMiddleware::class);

$app->addRoutingMiddleware();

$app->add(\App\Middleware\VarnishBasePathMiddleware::class); // <--- here

$app->addErrorMiddleware(...)