symfony/var-dumper · error · RuntimeException

Server start failed on

Error message

Server start failed on "%s": 

What it means

DumpServer::start() opens a stream_socket_server on the configured host (e.g. tcp://127.0.0.1:9912). If the socket cannot be created — port already in use, permission denied, bad address — a RuntimeException is thrown appending the OS error string and errno.

Solutions

1. Kill the process already bound to the port (lsof -i :9912 / netstat) or stop the duplicate server:dump run
2. Choose a free port (e.g. --port=9913) and update your client VAR_DUMPER_SERVER accordingly
3. Verify the host string format is tcp://host:port with a port in 1-65535

Example fix

// before
$server->start('tcp://127.0.0.1:9912'); // in use
// after
$server->start('tcp://127.0.0.1:9913');

$parts = parse_url($host);
if (!$parts || !isset($parts['port']) || $parts['port'] < 1 || $parts['port'] > 65535) {
    throw new \InvalidArgumentException('Dump server host must be tcp://host:port with a valid port');
}
$conn = @fsockopen($parts['host'] ?? '127.0.0.1', $parts['port'], $errno, $errstr, 1);
if (is_resource($conn)) {
    fclose($conn);
    throw new \RuntimeException("Port {$parts['port']} already in use");
}

function isBindableAddress(string $host): bool {
    return (bool) preg_match('#^tcp:\/\/[\w.\-]+:\d{1,5}$#', $host);
}

try {
    $server->start();
} catch (\RuntimeException $e) {
    if (str_contains($e->getMessage(), 'Server start failed')) {
        // check port in use / pick alternate port, or surface the OS errno
    } else {
        throw $e;
    }
}

When it happens

Trigger: Another process (including a second server:dump instance) already listens on the port; port below 1024 without root; invalid host string like tcp://localhost:99999; firewall blocking bind.

Common situations: Leftover dump server from a previous run still holding the port in dev; Docker/WSL port conflicts; typo'd host/port in dump config or VAR_DUMPER_SERVER env vars.

AI-assisted analysis of symfony/var-dumper@e9d9cf5dcd (2026-09-14). Data as JSON: /api/errors/1246ab6569078d29. Report an issue: GitHub.

Appendix: source

Thrown at Server/DumpServer.php:49

     * @var resource|null
     */
    private $socket;

    public function __construct(
        string $host,
        private ?LoggerInterface $logger = null,
    ) {
        if (!str_contains($host, '://')) {
            $host = 'tcp://'.$host;
        }

        $this->host = $host;
    }

    public function start(): void
    {
        if (!$this->socket = stream_socket_server($this->host, $errno, $errstr)) {
            throw new \RuntimeException(\sprintf('Server start failed on "%s": ', $this->host).$errstr.' '.$errno);
        }
    }

    /**
     * @param-immediately-invoked-callable $callback
     */
    public function listen(callable $callback): void
    {
        if (null === $this->socket) {
            $this->start();
        }

        foreach ($this->getMessages() as $clientId => $message) {
            $this->logger?->info('Received a payload from client {clientId}', ['clientId' => $clientId]);

            $payload = @unserialize(base64_decode($message), ['allowed_classes' => [Data::class, Stub::class, ClassDumpStub::class]]);

            // Impossible to decode the message, give up.

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