Fix: client IP resolution for Flarum(v1.x) behind proxy

extend.php

<?php

/*
 * This file is part of Flarum.
 *
 * For detailed copyright and license information, please view the
 * LICENSE file that was distributed with this source code.
 */

use Flarum\Extend;
use Psr\Http\Message\ResponseInterface;
use Psr\Http\Message\ServerRequestInterface;
use Psr\Http\Server\MiddlewareInterface;
use Psr\Http\Server\RequestHandlerInterface;

class ForwardedIpMiddleware implements MiddlewareInterface
{
    public function process(
        ServerRequestInterface $request,
        RequestHandlerInterface $handler): ResponseInterface
    {
        $server = $request->getServerParams();

        $ip = match (true) {
            isset($server['HTTP_X_FORWARDED_FOR']) => trim(explode(
                ',', $server['HTTP_X_FORWARDED_FOR'])[0] ?? ''),
            isset($server['HTTP_X_REAL_IP']) => trim($server['HTTP_X_REAL_IP']),
            $server['REMOTE_ADDR'] !== 'unix:' => $server['REMOTE_ADDR'],
            default => '127.0.0.1',
        };

        return $handler->handle($request->withAttribute('ipAddress',
            $ip ?: '127.0.0.1'
        ));
    }
}

return [
    // Register extenders here to customize your forum!
    (new Extend\Middleware('api'))
        ->add(ForwardedIpMiddleware::class),
];