nixos-router/modules/router/dns.nix

{ config, lib, ... }:
let
  cfg = config.router;
  lans = lib.attrValues cfg.lans;
  ipv4 = (import ./lib.nix { inherit lib; }).ipv4;

  # Tunnels that opted into DNS: listen on each of their addresses.
  dnsAddrs = lib.concatMap (w: w.addresses) (lib.filter (w: w.dns) (lib.attrValues cfg.wireguard));
  addrOf = a: builtins.head (lib.splitString "/" a);
  # IPv4 gets the proper network address; IPv6 netblocks are masked by Unbound.
  netOf =
    a:
    let
      parts = lib.splitString "/" a;
    in
    if lib.hasInfix ":" a then
      a
    else
      ipv4.cidr (builtins.head parts) (lib.toInt (builtins.elemAt parts 1));

  dns = cfg.dns;
  inherit (dns) domain;
  fqdn = name: if lib.hasInfix "." name || domain == null then name else "${name}.${domain}";
  isV6 = lib.hasInfix ":";
  rrOf = a: if isV6 a then "AAAA" else "A";

  # name -> addresses: explicit hosts plus DHCP reservations (A records only).
  reservations = lib.concatMap (
    l: lib.mapAttrsToList (n: r: lib.nameValuePair n [ r.address ]) l.dhcp.reservations
  ) lans;
  # The router's own name resolves to its addresses on the LANs (and their
  # ULAs), so clients can reach it by name; with a domain also bare, for
  # clients that have no search domain.
  selfName = config.networking.hostName;
  selfAddrs =
    map (l: l.ipv4.address) lans
    ++ map (l: addrOf l.ipv6.address) (lib.filter (l: l.ipv6.address != null) lans);
  # Unless dns.hosts names the router itself.
  selfDefined = lib.hasAttr selfName dns.hosts || lib.hasAttr (fqdn selfName) dns.hosts;
  selfRecord = lib.optional (!selfDefined) (lib.nameValuePair (fqdn selfName) selfAddrs);
  # (No reverse record for the bare name: the address has one already.)
  selfBare = lib.optional (!selfDefined && fqdn selfName != selfName) (
    lib.nameValuePair selfName selfAddrs
  );

  records =
    selfRecord
    ++ lib.mapAttrsToList (n: a: lib.nameValuePair (fqdn n) a) dns.hosts
    ++ map (r: lib.nameValuePair (fqdn r.name) r.value) reservations
    ++ lib.mapAttrsToList (n: a: lib.nameValuePair n a) dns.overrides;

  # Unbound reads these as one quoted argument.
  localData = lib.concatMap (r: map (a: ''"${r.name}. ${rrOf a} ${a}"'') r.value) (records ++ selfBare);
  # Reverse records for hosts and reservations (not for wildcard overrides).
  ptrs = lib.concatMap (r: map (a: ''"${a} ${r.name}."'') r.value) (
    lib.filter (r: !(lib.hasAttr r.name dns.overrides)) records
  );
  privateZones = lib.attrNames dns.forwardZones ++ lib.optional (domain != null) domain;

  # Peers of tunnels that use the resolver may be remote networks with no
  # address of their own on the tunnel (site-to-site); allow what they route.
  dnsPeerNets = lib.filter (n: !lib.hasSuffix "/0" n) (
    lib.concatMap (w: lib.concatMap (p: p.allowedIPs) (lib.attrValues w.peers)) (
      lib.filter (w: w.dns) (lib.attrValues cfg.wireguard)
    )
  );
in
{
  config = lib.mkIf cfg.enable {
    services.unbound = {
      enable = true;
      settings = {
        server = {
          interface = [
            "127.0.0.1"
          ]
          ++ map (l: l.ipv4.address) lans
          ++ map (l: addrOf l.ipv6.address) (lib.filter (l: l.ipv6.address != null) lans)
          ++ map addrOf dnsAddrs;
          # LAN addresses are assigned by networkd, possibly after Unbound starts.
          ip-freebind = true;
          access-control = [
            "127.0.0.0/8 allow"
          ]
          ++ map (l: "${l.ipv4.subnet} allow") lans
          ++ map (l: "${l.ipv6.address} allow") (lib.filter (l: l.ipv6.address != null) lans)
          ++ map (a: "${netOf a} allow") dnsAddrs
          ++ map (n: "${n} allow") dnsPeerNets;
          local-zone =
            lib.optional (domain != null) "${domain}. static"
            # The router's bare name is a zone of its own (not under the domain).
            ++ lib.optional (!lib.hasInfix "." selfName) "${selfName}. transparent"
            ++ map (n: "${n}. redirect") (lib.attrNames dns.overrides);
          local-data = localData;
          local-data-ptr = ptrs;
          domain-insecure = privateZones ++ lib.attrNames dns.overrides;
        };
      }
      // lib.optionalAttrs (cfg.dns.forwarders != [ ] || dns.forwardZones != { }) {
        forward-zone =
          lib.mapAttrsToList (name: addrs: {
            inherit name;
            forward-addr = addrs;
          }) dns.forwardZones
          ++ lib.optional (cfg.dns.forwarders != [ ]) {
            name = ".";
            forward-addr = cfg.dns.forwarders;
          };
      };
    };
  };
}