cnx-network-clan/modules/mail.nix

# Declarative mail stack for mx1 (Simple NixOS Mailserver: Postfix + Dovecot +
# Rspamd + OpenDKIM). Imported by machines/mx1 alongside the SNM flake module.
#
# Mailboxes are virtual (not system users): each address below is a login account
# whose password is auto-generated by a clan vars generator as a four-word
# passphrase with a trailing number (e.g. otter-lantern-cobalt-driftwood-42). The
# generator stores both the passphrase and its sha-512 hash. To add a mailbox:
# append the address to `accounts`, run `clan vars generate mx1`, redeploy mx1,
# then hand the passphrase to the user:
#   clan vars get mx1 mail-passwd-<addr>/passphrase
# (addr with @ and . replaced by -at- and -, e.g. mail-passwd-postmaster-at-cnx-email)
{
  config,
  lib,
  pkgs,
  ...
}:
let
  hosts = import ./hosts.nix;
  fqdn = "mx1.cnx.email";
  mtaStsHost = "mta-sts.cnx.email";
  # Client-facing alias (CNAME -> mx1) so Thunderbird etc. can use mail.cnx.email
  # for submission/IMAP; added as a cert SAN so TLS validates against that name.
  clientHost = "mail.cnx.email";

  # MTA-STS policy served at https://mta-sts.cnx.email/.well-known/mta-sts.txt.
  # enforce = a sending MTA that fetched this must use a valid, MX-matching TLS
  # cert or refuse to deliver. Bump the _mta-sts TXT id (in the zone) whenever
  # this changes.
  mtaStsPolicy = pkgs.writeText "mta-sts.txt" ''
    version: STSv1
    mode: enforce
    mx: ${fqdn}
    max_age: 604800
  '';

  # The mailboxes mx1 serves. postmaster is required by RFC 5321.
  accounts = [
    "postmaster@cnx.email"
  ];

  genName = addr: "mail-passwd-" + lib.replaceStrings [ "@" "." ] [ "-at-" "-" ] addr;

  passwdGenerators = lib.listToAttrs (
    map (addr: {
      name = genName addr;
      value = {
        files."passphrase".secret = true; # retrievable to hand to the user
        files."hash".secret = true; # consumed by SNM's hashedPasswordFile
        runtimeInputs = [
          pkgs.xkcdpass
          pkgs.mkpasswd
        ];
        script = ''
          pass="$(xkcdpass --numwords=4 --delimiter=- --case=lower)-$((RANDOM % 90 + 10))"
          printf '%s' "$pass" > "$out"/passphrase
          printf '%s' "$pass" | mkpasswd -s -m sha-512 > "$out"/hash
        '';
      };
    }) accounts
  );

  loginAccounts = lib.listToAttrs (
    map (addr: {
      name = addr;
      value.hashedPasswordFile = config.clan.core.vars.generators.${genName addr}.files."hash".path;
    }) accounts
  );
in
{
  imports = [ ./dns/acme-mx1-secret.nix ];

  clan.core.vars.generators = passwdGenerators // {
    # Render the shared acme_mx1 TSIG secret into a lego rfc2136 env file. lego
    # (via security.acme below) uses it to write the _acme-challenge.mx1.cnx.email
    # TXT record to ns1, which authorizes the acme_mx1 key for exactly that owner.
    dns-acme-rfc2136 = {
      files."rfc2136.env".secret = true; # root-owned; systemd reads it as root
      dependencies = [ "dns-acme-mx1-secret" ];
      script = ''
        printf 'RFC2136_NAMESERVER=${hosts.ns1.ipv4}:53\nRFC2136_TSIG_ALGORITHM=hmac-sha256.\nRFC2136_TSIG_KEY=acme_mx1\nRFC2136_TSIG_SECRET=%s\n' \
          "$(cat "$in"/dns-acme-mx1-secret/secret)" > "$out"/rfc2136.env
      '';
    };
  };

  mailserver = {
    enable = true;
    # Fresh install: declare the latest layout the nixos-25.11 branch ships (3),
    # so SNM uses the current dovecot mail directory layout with nothing to migrate.
    stateVersion = 3;
    inherit fqdn;
    domains = [ "cnx.email" ];
    inherit loginAccounts;

    # Consume a security.acme cert we obtain ourselves via DNS-01 (below); no
    # web server and no inbound HTTP needed, so port 80 stays closed. Add the
    # MTA-STS host as a SAN so the one cert also covers the policy endpoint.
    certificateScheme = "acme";
    certificateDomains = [
      mtaStsHost
      clientHost
    ];

    dkimSelector = "mail";
  };

  security.acme = {
    acceptTerms = true;
    defaults.email = "postmaster@cnx.email";
    certs.${fqdn} = {
      dnsProvider = "rfc2136";
      environmentFile = config.clan.core.vars.generators.dns-acme-rfc2136.files."rfc2136.env".path;
      # ns1 is the only nameserver that accepts the acme_mx1 UPDATE; check
      # propagation against it directly rather than a public resolver.
      dnsResolver = "${hosts.ns1.ipv4}:53";
      # Keep the private key fixed across renewals so the DANE TLSA "3 1 1"
      # record (public-key digest, published in the zone) stays valid.
      extraLegoRenewFlags = [ "--reuse-key" ];
    };
  };

  # The mail cert is owned group=acme (SNM adds postfix/dovecot); Caddy serves the
  # MTA-STS endpoint from the same cert, so it needs to read the key too.
  users.users.caddy.extraGroups = [ "acme" ];

  # MTA-STS policy endpoint, served by Caddy (same web server as control's docs).
  # The explicit `tls cert key` points at the lego-issued mail cert (which carries
  # mta-sts.cnx.email as a SAN) and disables Caddy's automatic ACME, so no extra
  # issuance happens and the DANE TLSA key stays stable. Only :443 is opened.
  services.caddy = {
    enable = true;
    virtualHosts.${mtaStsHost}.extraConfig = ''
      tls /var/lib/acme/${fqdn}/cert.pem /var/lib/acme/${fqdn}/key.pem
      root * ${pkgs.writeTextDir ".well-known/mta-sts.txt" (builtins.readFile mtaStsPolicy)}
      file_server
    '';
  };

  # DKIM private keys are generated on first start under this dir. They're
  # regenerable (rotate + republish the TXT), but declaring the path as clan
  # state lets a borg client back it up to avoid a needless DNS round-trip on
  # restore. Wiring mx1 into the borgbackup instance is a separate step.
  clan.core.state.mail-dkim.folders = [ config.mailserver.dkimKeyDirectory ];
}