nixConfig/nixos/modules/mail.nix

/*
* Using the web admin interface: Set your SSH to do portforwarding of some local port to port 80 of the VM and
*   and use your personal admin account or create one using the fallback admin password.
* Create users with mail boxes: Go to the admin interface and create them.
* Stalwart mailserver docs can be found at https://stalw.art/docs
* DNS-Records: Collect the right DNS entries from the management interface and copy them to the DNS hoster. Caution:
*   Not all entries are applicable since we relay via HRZ.
*/
{
  config,
  lib,
  ...
}: let
  inherit
    (lib)
    mkIf
    mkEnableOption
    mkOption
    ;
  inherit (lib.types) listOf strMatching str path;
  cfg = config.services.mathebau-mail;
in {
  options.services.mathebau-mail = {
    enable = mkEnableOption "mathebau mail service";
    stalwartAdminHash = mkOption {
      type = str;
      description = "String containing the hashed fallback admin password";
    };
  };

  config = mkIf cfg.enable {
    services = {
      stalwart-mail = {
        enable = true;
        openFirewall = true;
        settings = {
          server = {
            lookup.default.hostname = "fb04184.mathematik.tu-darmstadt.de"; # Because the DNS PTR of 130.83.2.184 is this and this should be used in SMTP EHLO.
            listener = {
              "smtp" = {
                bind = ["[::]:25"];
                protocol = "smtp";
              };
              "submissions" = {
                bind = ["[::]:465"];
                protocol = "smtp";
                tls.implicit = true;
              };
              "imaptls" = {
                bind = ["[::]:993"];
                protocol = "imap";
                tls.implicit = true;
              };
              "management" = {
                # Cthulhu forwards requests for http://fb04184.mathematik.tu-darmstadt.de/.well-known/acme-challenge/ http://imap.mathebau.de/.well-known/acme-challenge/ and http://smtp.mathebau.de/.well-known/acme-challenge/
                # for TLS certificate challenge validation
                # whereas the rest of the management interface is not available publically.
                # It can be reached via SSH and portforwarding.
                bind = ["[::]:80"];
                protocol = "http";
              };
            };
          };
          acme.letsencrypt = {
            directory = "https://acme-v02.api.letsencrypt.org/directory"; # This setting is necessary for this block to be activated
            challenge = "http-01";
            contact = ["root@mathebau.de"];
            domains = ["fb04184.mathematik.tu-darmstadt.de" "imap.mathebau.de" "smtp.mathebau.de"];
            default = true;
          };
          spam.header.is-spam = "Dummyheader"; # disable moving to spam which would conflict with forwarding
          auth = {
            # TODO check if HRZ conforms to these standards and we can validate them strictly
            dkim.verify = "relaxed";
            arc.verify = "relaxed";
            dmarc.verify = "relaxed";
            iprev.verify = "relaxed";
            spf.verify.ehlo = "relaxed";
            spf.verify.mail-from = "relaxed";
          };

          # Forward outgoing mail to HRZ or mail VMs.
          # see https://stalw.art/docs/smtp/outbound/routing/ relay host example
          queue.outbound = {
            next-hop = [
              {
                "if" = "rcpt_domain = 'lists.mathebau.de'";
                "then" = "'mailman'";
              }
              {
                "if" = "is_local_domain('', rcpt_domain)";
                "then" = "'local'";
              }
              {"else" = "'hrz'";}
            ];
            tls = {
              # we only talk to HRZ and our own VMs anyway
              mta-sts = "disable";
              dane = "disable";
              starttls = "optional"; # e.g. Lobon does not offer starttls
            };
          };
          remote."hrz" = {
            address = "mailout.hrz.tu-darmstadt.de";
            port = 25;
            protocol = "smtp";
            tls.implicit = false; # Don't assume TLS on this port but use STARTTLS
          };
          remote."mailman" = {
            address = "lobon.mathebau.de"; # must be created in DNS as a MX record because this field does not accept ip addresses.
            port = 25;
            protocol = "smtp";
            tls.implicit = false; # Don't assume TLS on this port but use STARTTLS
          };

          session.rcpt = {
            # In order to accept mail that we only forward
            # without having to generate an account.
            # Invalid addresses are filtered by DFN beforehand.
            catch-all = true;
            relay = [
              {
                "if" = "!is_empty(authenticated_as) || rcpt_domain == 'lists.mathebau.de' || starts_with(remote_ip, '192.168.0.')"; #TODO restrict trust by IP
                "then" = true;
              }
              {"else" = false;}
            ];
          };

          authentication.fallback-admin = {
            user = "admin";
            # see passwd on azathoth for plaintext or machine secret in encoded format for HTTP Basic AUTH
            secret = cfg.stalwartAdminHash;
          };
          store = {
            # structured data in SQLite, blobs on filesystem
            db.type = "sqlite";
            db.path = "/var/lib/stalwart-mail/data/index.sqlite3";
            fs.type = "fs";
            fs.path = "/var/lib/stalwart-mail/data/blobs";
          };
        };
      };
    };
    environment.persistence.${config.impermanence.name} = {
      directories = [
        "/var/lib/stalwart-mail"
      ];
      files = ["/root/.ssh/known_hosts"]; # for the backup server bragi
    };

    # Backups
    services.borgbackup.jobs.mail = {
      paths = [
        "/var/lib/stalwart-mail/data"
      ];
      encryption.mode = "none"; # Otherwise the key is next to the backup or we have human interaction.
      environment = {
        BORG_RSH = "ssh -i /run/secrets/backupKey";
        # “Borg ensures that backups are not created on random drives that ‘just happen’ to contain a Borg repository.”
        # https://borgbackup.readthedocs.io/en/stable/deployment/automated-local.html
        # We don't want this in order to not need to persist borg cache and simplify new deployments.
        BORG_UNKNOWN_UNENCRYPTED_REPO_ACCESS_IS_OK = "yes";
      };
      repo = "borg@192.168.1.11:kaluut"; # TODO for https://gitea.mathebau.de/Fachschaft/nixConfig/issues/33
      startAt = "daily";
      user = "root";
      group = "root";
    };
  };
}