mdg-filius/packages/fls/src/dnsHosts.ts

// Java's DNS server keeps its records in the text file /dns/hosts and reads
// them on every query (DNSServer.leseRecordListe, ResourceRecord(String)).
// Measured on test/dns-hosts.cases.txt against Java (dns-hosts.java.json)
// and Swift (dns-hosts.swift.json). Java's format decides where the two
// differ:
//  - fields are split at single spaces only: a tab or NBSP joins two fields;
//  - there are no comments, no "IN" and no zone-file order;
//  - a TTL that is not a number silently becomes 3600;
//  - a name that is not a domain name becomes the empty name, and the record
//    still counts - it just never answers, like one of another type;
//  - duplicates stay.
// Java's errors are not copied (each named in dnsHosts.test.ts): a label may
// start with a digit and holds no backslash (RFC 1123 - Java's pattern has
// both wrong; see isDomainName), the type A, MX or NS is read in any case
// (RFC 4343), and a negative TTL is invalid (RFC 2181).

import { javaIpCheck, javaParseInt, type DnsResourceRecord } from '@mdg-filius/core';
import { javaFileContent } from './javaFileSystem.js';
import { javaTrim } from './javaString.js';
import type { FlsFileTree } from './systemSoftware.js';

/** A record of /dns/hosts, as Java's ResourceRecord holds it (the getters). */
export interface HostsRecord {
  readonly domainName: string;
  readonly type: string;
  readonly ttl: number;
  readonly rdata: string;
}

export const DNS_HOSTS_PATH = '/dns/hosts';

// Labels of letters, digits and inner hyphens, an optional final dot; the
// empty string counts too.
const DOMAIN_LABELS = /^([a-zA-Z0-9]([a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?(\.[a-zA-Z0-9]([a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?)*)?\.?$/;
/**
 * A domain name as EingabenUeberpruefung.musterDomain means it (the empty
 * string included). Java's pattern wants every label to start with a letter
 * and lets a backslash into the labels after the first; instead a label may
 * start with a digit (RFC 1123), but the last one must not be all digits
 * (RFC 3696), so that no IPv4 address passes for a name.
 */
export function isDomainName(text: string): boolean {
  if (!DOMAIN_LABELS.test(text)) return false;
  const labels = text.split('.').filter((label) => label !== '');
  return labels.length === 0 || !/^[0-9]+$/.test(labels[labels.length - 1]!);
}

// EingabenUeberpruefung.musterIpAdresse.
const JAVA_IP_ADDRESS =
  /^(0*([1-9]?[0-9]?|1[0-9]{1,2}|2[0-4]?[0-9]|25[0-5]))\.((0*(1?[0-9]{1,2}|2[0-4]?[0-9]|25[0-5]))\.){2}(0*(1?[0-9]{1,2}|2[0-4]?[0-9]|25[0-5]))$/;

/** The content of /dns/hosts as Java's DNS server finds it (see javaFileSystem.ts). */
export function javaHostsFileContent(root: FlsFileTree | undefined): string | undefined {
  return javaFileContent(root, DNS_HOSTS_PATH);
}

const ANSWERED_TYPES = new Set(['A', 'MX', 'NS']);

/** `DNSServer.leseRecordListe` on the content of /dns/hosts. */
export function readDnsHosts(content: string): HostsRecord[] {
  const records: HostsRecord[] = [];
  for (const token of content.split('\n')) {
    const line = javaTrim(token);
    // line.split(" ", 5).length < 4: fewer than three single spaces.
    if (line === '' || line.split(' ').length < 4) continue;
    const record = hostsRecord(line);
    if (isValidRecord(record)) records.push(record);
  }
  return records;
}

/** `new ResourceRecord(line)`: up to four space-separated fields. */
function hostsRecord(line: string): HostsRecord {
  const fields = line.split(' ').filter((field) => field !== '').map(javaTrim);
  let domainName = '';
  let type = 'A';
  let ttl = 3600;
  let rdata = '';
  if (fields[0] !== undefined && isDomainName(fields[0])) {
    domainName = fields[0].endsWith('.') ? fields[0] : `${fields[0]}.`;
  }
  if (fields[1] !== undefined) {
    const upper = fields[1].replace(/[a-z]/g, (letter) => letter.toUpperCase());
    type = ANSWERED_TYPES.has(upper) ? upper : fields[1];
  }
  if (fields[2] !== undefined) {
    const parsed = javaParseInt(fields[2]);
    if (parsed !== undefined && parsed >= 0) ttl = parsed;
  }
  if (fields[3] !== undefined) {
    // Java tests ".*\." (no line terminator before the dot); a target with
    // one fails the domain check anyway, so endsWith says the same.
    const needsDot = (type === 'NS' || type === 'MX') && !fields[3].endsWith('.');
    rdata = needsDot ? `${fields[3]}.` : fields[3];
  }
  return { domainName, type, ttl, rdata };
}

function isValidRecord(record: HostsRecord): boolean {
  switch (record.type) {
    case 'A':
      return isDomainName(record.domainName) && JAVA_IP_ADDRESS.test(record.rdata);
    case 'MX':
    case 'NS':
      return isDomainName(record.domainName) && isDomainName(record.rdata);
    default:
      return true;
  }
}

/**
 * The records Java answers with, in the model's form: Java looks them up by
 * type (A, MX, NS) and by name ignoring case, so a record with the empty
 * name or another type never answers and is left out. The file itself stays
 * as it is.
 */
export function dnsRecordsOf(records: readonly HostsRecord[], warn: (message: string) => void): DnsResourceRecord[] {
  const answered: DnsResourceRecord[] = [];
  for (const record of records) {
    if (record.domainName === '') continue;
    const name = record.domainName === '.' ? '.' : record.domainName.slice(0, -1).toLowerCase();
    switch (record.type) {
      case 'A': {
        const address = javaIpCheck(record.rdata);
        if (address === null) {
          warn(`DNS record '${name} A ${record.rdata}' has no usable IPv4 address, left out (it stays in ${DNS_HOSTS_PATH})`);
          continue;
        }
        answered.push({ name, type: 'A', ttlSeconds: record.ttl, target: address });
        break;
      }
      case 'MX':
      case 'NS':
        answered.push({ name, type: record.type, ttlSeconds: record.ttl, target: record.rdata.toLowerCase() });
        break;
      default:
        break;
    }
  }
  return answered;
}