chadtmiller.com/pds.js

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// @pdsjs/sites - hostname → site-name resolution
//
// A site's record key is a DNS label, and the label is the subdomain it
// serves on: `blog` serves at `blog.<siteDomain>`. The apex serves the site
// named by `apexName`. Hostnames outside `siteDomain` are not ours at all —
// the caller falls through to normal PDS dispatch for them.

/**
 * One DNS label: 1-63 chars, lowercase alphanumeric with interior hyphens.
 * Site rkeys must match this because they become subdomains.
 */
const LABEL = /^[a-z0-9]([a-z0-9-]{0,61}[a-z0-9])?$/;

/**
 * Resolve a request hostname to the site name it serves.
 *
 * @param {string} hostname - request hostname, may carry mixed case
 * @param {string} siteDomain - apex domain sites serve under
 * @param {string} apexName - site name bound to the apex itself
 * @returns {{site: string}|{notFound: true}|null} The site to serve; a
 *   claimed-but-unservable hostname (deeper subdomains, malformed labels)
 *   that should get the sites 404 page; or null when the hostname is not
 *   under the site domain and the request is not ours.
 */
export function hostnameToSite(hostname, siteDomain, apexName) {
  const host = hostname.toLowerCase().replace(/\.$/, '');
  const domain = siteDomain.toLowerCase();

  if (host === domain) return { site: apexName };
  if (!host.endsWith(`.${domain}`)) return null;

  const label = host.slice(0, -(domain.length + 1));
  // Multi-label subdomains are claimed by the wildcard route but carry no
  // site (and sit outside the one-level Universal SSL wildcard anyway).
  if (label.includes('.') || !LABEL.test(label)) return { notFound: true };
  return { site: label };
}

/**
 * Whether a site name is usable as a subdomain label.
 * @param {string} name
 * @returns {boolean}
 */
export function isValidSiteName(name) {
  return LABEL.test(name);
}