react/create-react-app · error · Error

Could not find an open port at

Error message

Could not find an open port at ${host}.\nNetwork error message: ${err.message}\n

What it means

WebpackDevServerUtils.choosePort first probes the host with a network check before scanning for a free port. If that probe itself rejects (the host is unreachable / DNS fails), the error callback throws this message combining the host and the underlying network error. It is distinct from 'port in use', which is handled interactively above.

Solutions

1. Set HOST to a valid local interface: `HOST=127.0.0.1 npm start` (or localhost).
2. If you need LAN access, confirm the chosen HOST is assigned to a local interface (run ifconfig/ipconfig) before starting.
3. Inside Docker, bind to 0.0.0.0 only if the container networking permits and you intend external access; otherwise use 127.0.0.1.
4. Disable network adapters that conflict, or reconnect VPN, then retry.

Example fix

// before
// .env
HOST=dev.local.example
// after
// .env
HOST=127.0.0.1

const net = require('net');
const dns = require('dns').promises;
async function canReachHost(host) {
  // loopback/ips are bindable; hostnames must resolve
  if (net.isIP(host)) return true;
  try { await dns.lookup(host); return true; } catch { return false; }
}
// before choosePort:
const host = process.env.HOST || '0.0.0.0';
if (!(await canReachHost(host))) {
  throw new Error(`HOST '${host}' is not reachable/bindable on this machine`);
}

const isBindableHost = (h) =>
  h === 'localhost' || h === '0.0.0.0' || net.isIP(h) !== 0;

try {
  const port = await choosePort(process.env.HOST || '0.0.0.0', 3000);
  if (!port) process.exit(0);
} catch (e) {
  if (/Could not find an open port|Network error/.test(e.message)) {
    console.error('HOST not reachable. Try HOST=127.0.0.1');
    process.exit(1);
  }
  throw e;
}

When it happens

Trigger: choosePort(host, defaultPort) is called and the internal is-port-reachable/network probe for `host` rejects. The rejection handler runs and throws. Common when HOST is set to a hostname the machine cannot bind or resolve.

Common situations: Setting HOST to a non-loopback address not assigned to any interface. Docker/container setups where the configured HOST isn't available inside the namespace. VPN or network changes making a previously-valid HOST unreachable. Misconfigured DNS for a custom hostname.

AI-assisted analysis of react/create-react-app@6254386531 (2026-08-12). Data as JSON: /api/errors/9357170dec961da8. Report an issue: GitHub.

Appendix: source

Thrown at packages/react-dev-utils/WebpackDevServerUtils.js:428

                message +
                  `${existingProcess ? ` Probably:\n  ${existingProcess}` : ''}`
              ) + '\n\nWould you like to run the app on another port instead?',
            initial: true,
          };
          prompts(question).then(answer => {
            if (answer.shouldChangePort) {
              resolve(port);
            } else {
              resolve(null);
            }
          });
        } else {
          console.log(chalk.red(message));
          resolve(null);
        }
      }),
    err => {
      throw new Error(
        chalk.red(`Could not find an open port at ${chalk.bold(host)}.`) +
          '\n' +
          ('Network error message: ' + err.message || err) +
          '\n'
      );
    }
  );
}

module.exports = {
  choosePort,
  createCompiler,
  prepareProxy,
  prepareUrls,
};

View on GitHub (pinned to 6254386531)