/**
 * Guards for server-side fetches of user-supplied URLs (link previews, the
 * image proxy, Bandcamp resolution): the server must not become a way to reach
 * addresses only it can see — loopback, the LAN, container networks, or the
 * cloud metadata endpoint at 169.254.169.254.
 *
 * @module
 */

function parseIPv4(s: string): number[] | null {
  const parts = s.split(".");
  if (parts.length !== 4) return null;
  const bytes = parts.map((p) => /^\d{1,3}$/.test(p) ? Number(p) : NaN);
  return bytes.every((b) => b >= 0 && b <= 255) ? bytes : null;
}

function isPrivateIPv4([a, b]: number[]): boolean {
  return a === 0 || // "this" network
    a === 10 ||
    a === 127 ||
    (a === 100 && b >= 64 && b <= 127) || // carrier-grade NAT
    (a === 169 && b === 254) || // link-local, cloud metadata
    (a === 172 && b >= 16 && b <= 31) ||
    (a === 192 && b === 0) || // IETF protocol assignments (192.0.0.0/24 …)
    (a === 192 && b === 168) ||
    (a === 198 && (b === 18 || b === 19)) || // benchmarking
    a >= 224; // multicast, reserved, broadcast
}

/** Parses `s` into its 16 bytes, or null when it isn't an IPv6 address. */
function parseIPv6(s: string): number[] | null {
  let addr = s.toLowerCase().replace(/%.*$/, ""); // drop a zone id
  // An embedded dotted quad (::ffff:10.0.0.1) becomes the last two groups.
  const lastColon = addr.lastIndexOf(":");
  if (lastColon === -1) return null;
  const tail = addr.slice(lastColon + 1);
  if (tail.includes(".")) {
    const v4 = parseIPv4(tail);
    if (!v4) return null;
    addr = `${addr.slice(0, lastColon + 1)}${
      ((v4[0] << 8) | v4[1]).toString(16)
    }:${((v4[2] << 8) | v4[3]).toString(16)}`;
  }
  const halves = addr.split("::");
  if (halves.length > 2) return null;
  const head = halves[0] ? halves[0].split(":") : [];
  const rest = halves.length === 2 && halves[1] ? halves[1].split(":") : [];
  const fill = 8 - head.length - rest.length;
  if (halves.length === 1 ? fill !== 0 : fill < 1) return null;
  const groups = [
    ...head,
    ...Array(halves.length === 2 ? fill : 0).fill("0"),
    ...rest,
  ];
  if (!groups.every((g) => /^[0-9a-f]{1,4}$/.test(g))) return null;
  return groups.flatMap((g) => {
    const n = parseInt(g, 16);
    return [n >> 8, n & 0xff];
  });
}

function isPrivateIPv6(b: number[]): boolean {
  const zeroPrefix = (n: number) => b.slice(0, n).every((x) => x === 0);
  // IPv4 carried inside IPv6 is judged as the IPv4 address it carries.
  if (zeroPrefix(10) && b[10] === 0xff && b[11] === 0xff) { // ::ffff:a.b.c.d
    return isPrivateIPv4(b.slice(12));
  }
  if (zeroPrefix(12)) { // ::, ::1 and the deprecated ::a.b.c.d
    return true;
  }
  if (
    // 64:ff9b::/96 NAT64
    b[0] === 0 && b[1] === 0x64 && b[2] === 0xff && b[3] === 0x9b &&
    b.slice(4, 12).every((x) => x === 0)
  ) return isPrivateIPv4(b.slice(12));
  if (b[0] === 0x20 && b[1] === 0x02) { // 2002::/16 6to4
    return isPrivateIPv4(b.slice(2, 6));
  }
  return (b[0] & 0xfe) === 0xfc || // fc00::/7 unique local
    (b[0] === 0xfe && (b[1] & 0xc0) === 0x80) || // fe80::/10 link-local
    (b[0] === 0xfe && (b[1] & 0xc0) === 0xc0) || // fec0::/10 site-local
    b[0] === 0xff; // multicast
}

/**
 * Checks whether `ip` is an IP literal outside the public internet. Non-IPs
 * are false.
 */
export function isPrivateAddress(ip: string): boolean {
  const v4 = parseIPv4(ip);
  if (v4) return isPrivateIPv4(v4);
  const v6 = parseIPv6(ip);
  return v6 ? isPrivateIPv6(v6) : false;
}

function isIpLiteral(host: string): boolean {
  return parseIPv4(host) !== null || parseIPv6(host) !== null;
}

/**
 * Returns `url`'s hostname without the brackets `URL.hostname` keeps around
 * an IPv6 literal.
 */
function bareHostname(url: URL): string {
  return url.hostname.replace(/^\[(.*)\]$/, "$1");
}

/**
 * Checks whether `url`'s host is private without DNS: a private IP literal,
 * or a name that is local by definition. Names still need resolving (see
 * resolvePublicAddress) — this only rejects what is obvious without DNS.
 */
export function isPrivateHostname(url: URL): boolean {
  const host = bareHostname(url).toLowerCase();
  if (host === "localhost" || host.endsWith(".localhost")) return true;
  return isPrivateAddress(host);
}

/**
 * Resolves `url`'s host and returns one of its addresses, or throws if the
 * host is private or any address it resolves to is (a name with both a public
 * and a private record is refused outright, since the fetch may pick either).
 *
 * The fetch itself resolves again, so a DNS server answering differently the
 * second time (rebinding) isn't fully excluded; the curl fallback pins the
 * address this returns.
 *
 * @throws {Error} When the host is private, can't be resolved, or resolves to
 * a private address.
 */
export async function resolvePublicAddress(url: URL): Promise<string> {
  if (isPrivateHostname(url)) {
    throw new Error(`Refusing to fetch private address ${url.hostname}`);
  }
  const host = bareHostname(url);
  if (isIpLiteral(host)) return host;

  const results = await Promise.allSettled([
    Deno.resolveDns(host, "A"),
    Deno.resolveDns(host, "AAAA"),
  ]);
  const addresses = results.flatMap((r) =>
    r.status === "fulfilled" ? r.value : []
  );
  if (addresses.length === 0) {
    throw new Error(`Could not resolve ${host}`);
  }
  const blocked = addresses.find(isPrivateAddress);
  if (blocked) {
    throw new Error(`Refusing to fetch ${host}: resolves to ${blocked}`);
  }
  return addresses[0];
}