header-middleware-next/src/utils/checkIP.js

A lightweight and flexible middleware utility for managing HTTP headers in Next.js applications. Supports header extraction, transformation, masking, and safe injection for Edge and API routes.

/**
 * Function to extract and verify the client's IP address from an incoming HTTP request.
 * It attempts to determine the IP by checking multiple standard headers and connection properties,
 * accounting for common proxy forwarding and network configurations.
 *
 * @param {Object} req - The HTTP request object containing headers and connection information.
 * @returns {Object} - An object representing the detection result, including status type, name, message, and meta information with the IP.
 */
export default async function checkIP(req) {
  /**
   * Attempt to retrieve the client's IP address using multiple fallbacks:
   * 1. 'x-forwarded-for' header - may contain a list of IPs when behind proxies; use the first IP.
   * 2. req.ip - commonly set by frameworks like Express.
   * 3. req.connection.remoteAddress - legacy Node.js property for remote IP.
   * 4. req.socket.remoteAddress - socket-level IP address.
   * 5. req.connection.socket.remoteAddress - additional legacy fallback.
   * If none are available, default to an empty string.
   */
  let ip =
    req.headers['x-forwarded-for']?.split(',')[0]?.trim() ||
    req.ip ||
    req.connection?.remoteAddress ||
    req.socket?.remoteAddress ||
    req.connection?.socket?.remoteAddress ||
    '';

  // If no IP address could be resolved, return a warning with empty meta IP.
  if (!ip) {
    return {
      type: 'warning',
      name: 'IP',
      message: 'IP address could not be determined',
      meta: { ip: '' }
    };
  }

  // On successful IP detection, return a success type with the resolved IP in the message and meta.
  return {
    type: 'success',
    name: 'IP',
    message: `IP detected: ${ip}`,
    meta: { ip }
  };
}