{ config, lib, pkgs, ... }:
let
  cfg = config.sbruder.mailserver;

  certDir = config.security.acme.certs."${cfg.fqdn}".directory;
in
{
  options.sbruder.mailserver = with lib; with lib.types; {
    enable = mkEnableOption "simple mail server";
    fqdn = mkOption {
      type = str;
      description = ''
        FQDN of the mail server

        It needs to have a matching reverse DNS record. Also, an acme
        certificate with this name has to be present.
      '';
      example = "mail.example.com";
    };
    storage = mkOption {
      type = path;
      description = "Location of the storage for mails";
      default = "/var/vmail";
    };
    domains = mkOption {
      type = listOf str;
      description = "Domains to serve";
      example = [ "example.com" "example.org" ];
    };
    users = mkOption {
      type = listOf (submodule {
        options = {
          address = mkOption {
            type = str;
            description = "Primary e-mail address of the user";
            example = "jdoe@example.com";
          };
          passwordHash = mkOption {
            type = str;
            description = ''
              Bcrypt hash of the user’s password. Please note that it will be
              world-readable in the nix store.

              You can generate a password with `nix run nixpkgs.apacheHttpd -c
              htpasswd -nBC 12 "" | cut -d: -f2`
            '';
            example = "$2y$05$SHxhwVGx.XCd19HAcb1NKuidUxW1BwU7GeO0ZIcMTc5t2uZoYLVRK";
          };
          aliases = mkOption {
            type = listOf str;
            description = ''
              A list of aliases for the user.

              If multiple users have the same alias defined, mail will be
              delivered to both of them.
            '';
            default = [ ];
            example = [
              "j.doe@example.com"
              "jane.doe@example.com"
              "postmaster@example.com"
            ];
          };
        };
      });
      description = "Users of the mail server";
    };
    cleanHeaders = mkOption {
      type = listOf str;
      description = "A list of regular expressions that define what headers are filtered";
      default = [
        "/^\\s*Received:/"
        "/^\\s*User-Agent:/"
        "/^\\s*X-Mailer:/"
        "/^\\s*X-Originating-IP:/"
      ];
    };
    rejectSenders = mkOption {
      type = listOf str;
      description = "A list of senders to reject mails from";
      default = [ ];
      example = [
        "newsletter@example.com"
        "spammer@example.com"
      ];
    };
  };

  config = lib.mkIf cfg.enable {
    # Users and groups
    users.users.vmail = {
      uid = 10000;
      group = "vmail";
      home = cfg.storage;
      createHome = true;
    };

    users.groups.vmail.gid = 10000;

    # Firewall
    networking.firewall.allowedTCPPorts = [
      143 # IMAP
      25 # SMTP
      587 # SMTP submission
    ];

    # Service dependencies
    systemd.services.dovecot2 = {
      wants = [ "acme-finished-${cfg.fqdn}.target" ];
      after = [ "acme-finished-${cfg.fqdn}.target" ];
    };
    systemd.services.postfix = {
      wants = [ "acme-finished-${cfg.fqdn}.target" ];
      requires = [ "dovecot2.service" ];
      after = [ "acme-finished-${cfg.fqdn}.target" "dovecot2.service" ];
    };

    # Reload on certificate renewal
    security.acme.certs."${cfg.fqdn}".postRun = ''
      if systemctl is-active dovecot2; then
          systemctl --no-block reload dovecot2
      fi
      if systemctl is-active postfix; then
          systemctl --no-block reload postfix
      fi
    '';

    # Postfix
    security.dhparams.params.postfix = { };
    services.postfix =
      let
        listToString = lib.concatStringsSep ",";

        valiases =
          let
            # List of attribute sets with single key-value pair
            plainAliases = (lib.flatten
              (map
                ({ address, aliases, ... }:
                  map
                    (alias: { "${alias}" = address; })
                    (aliases ++ lib.singleton address))
                cfg.users));

            # Attribute set with every alias mapped to a list of receivers
            mergedAliases = (lib.attrsets.foldAttrs
              (val: col: lib.singleton val ++ col)
              [ ]
              plainAliases);

            # Contents of the aliases file
            aliasesString = (lib.concatStringsSep
              "\n"
              (lib.mapAttrsToList
                (alias: addresses: "${alias} ${listToString addresses}")
                mergedAliases));
          in
          pkgs.writeText
            "valiases"
            aliasesString;

        access_sender = pkgs.writeText
          "access_sender"
          (lib.concatMapStringsSep
            "\n"
            (sender: "${sender} REJECT")
            cfg.rejectSenders);

        submissionHeaderCleanupRules = pkgs.writeText "submission_header_cleanup_rules"
          (lib.concatMapStringsSep
            "\n"
            (regex: "${regex} IGNORE")
            cfg.cleanHeaders);
      in
      {
        enable = true;

        enableSubmission = true;

        hostname = cfg.fqdn;
        networksStyle = "host";
        sslCert = "${certDir}/fullchain.pem";
        sslKey = "${certDir}/key.pem";

        recipientDelimiter = "+";

        mapFiles = {
          inherit access_sender valiases;
        };

        config = {
          # General
          smtpd_banner = "${cfg.fqdn} ESMTP NO UCE";
          disable_vrfy_command = true; # disable check if mailbox exists
          enable_long_queue_ids = true; # better for debugging
          strict_rfc821_envelopes = true; # only accept properly formatted envelope
          message_size_limit = "50331648"; # 48 MiB

          virtual_mailbox_domains = listToString cfg.domains;
          virtual_mailbox_maps = "hash:/var/lib/postfix/conf/valiases";
          virtual_alias_maps = "hash:/var/lib/postfix/conf/valiases";
          virtual_transport = "lmtp:unix:/run/dovecot2/dovecot-lmtp";

          smtpd_recipient_restrictions = listToString [
            "reject_non_fqdn_recipient"
            "reject_rbl_client ix.dnsbl.manitu.net"
            "reject_unknown_recipient_domain"
            "reject_unverified_recipient"
          ];

          smtpd_client_restrictions = listToString [
            "reject_rbl_client ix.dnsbl.manitu.net"
            "reject_unknown_client_hostname"
          ];

          smtpd_sender_restrictions = listToString [
            "check_sender_access hash:/var/lib/postfix/conf/access_sender"
            "reject_non_fqdn_sender"
            "reject_unknown_sender_domain"
          ];

          # generated 2021-02-04, Mozilla Guideline v5.6, Postfix 3.5.6, OpenSSL 1.1.1i, intermediate configuration
          # https://ssl-config.mozilla.org/#server=postfix&version=3.5.6&config=intermediate&openssl=1.1.1i&guideline=5.6
          smtpd_tls_security_level = "may";
          smtpd_tls_auth_only = "yes";
          smtpd_tls_mandatory_protocols = "!SSLv2, !SSLv3, !TLSv1, !TLSv1.1";
          smtpd_tls_protocols = "!SSLv2, !SSLv3, !TLSv1, !TLSv1.1";
          smtpd_tls_mandatory_ciphers = "medium";
          smtpd_tls_loglevel = "1";

          tls_medium_cipherlist = listToString [
            "ECDHE-ECDSA-AES128-GCM-SHA256"
            "ECDHE-RSA-AES128-GCM-SHA256"
            "ECDHE-ECDSA-AES256-GCM-SHA384"
            "ECDHE-RSA-AES256-GCM-SHA384"
            "ECDHE-ECDSA-CHACHA20-POLY1305"
            "ECDHE-RSA-CHACHA20-POLY1305"
            "DHE-RSA-AES128-GCM-SHA256"
            "DHE-RSA-AES256-GCM-SHA384"
          ];
          tls_preempt_cipherlist = "no";

          smtpd_tls_dh1024_param_file = config.security.dhparams.params.postfix.path;
        };

        submissionOptions = {
          smtpd_tls_security_level = "encrypt";
          smtpd_sasl_auth_enable = "yes";
          smtpd_sasl_type = "dovecot";
          smtpd_sasl_path = "/run/dovecot2/auth";

          smtpd_sender_login_maps = "hash:/etc/postfix/valiases";

          smtpd_recipient_restrictions = listToString [ ];

          smtpd_client_restrictions = listToString [
            "permit_sasl_authenticated"
            "reject"
          ];

          smtpd_sender_restrictions = listToString [
            "reject_sender_login_mismatch"
          ];

          cleanup_service_name = "submission-header-cleanup";
        };

        masterConfig = {
          submission-header-cleanup = {
            private = false;
            maxproc = 0;
            command = "cleanup";
            args = [ "-o" "header_checks=pcre:${submissionHeaderCleanupRules}" ];
          };
        };
      };

    # Dovecot
    services.dovecot2 =
      let
        postfixCfg = config.services.postfix;

        passdb = pkgs.writeText "dovecot-users"
          (lib.concatMapStringsSep
            "\n"
            ({ address, passwordHash, ... }: "${address}:{BLF-CRYPT}${passwordHash}")
            cfg.users);
      in
      {
        enable = true;

        enableLmtp = true;
        enablePAM = false;

        mailUser = "vmail";
        mailGroup = "vmail";
        mailLocation = "maildir:${cfg.storage}/%d/%n";

        sslServerCert = "${certDir}/fullchain.pem";
        sslServerKey = "${certDir}/key.pem";

        extraConfig = ''
          # generated 2021-02-04, Mozilla Guideline v5.6, Dovecot 2.3.13, OpenSSL 1.1.1i, intermediate configuration
          # https://ssl-config.mozilla.org/#server=dovecot&version=2.3.13&config=intermediate&openssl=1.1.1i&guideline=5.6
          ssl = required
          ssl_min_protocol = TLSv1.2
          ssl_cipher_list = ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:DHE-RSA-AES128-GCM-SHA256:DHE-RSA-AES256-GCM-SHA384
          ssl_prefer_server_ciphers = no

          service imap-login {
            inet_listener imap {
            }
          }

          service lmtp {
            unix_listener dovecot-lmtp {
              mode = 0600
              user = ${postfixCfg.user}
              group = ${postfixCfg.group}
            }
          }

          passdb {
            driver = passwd-file
            args = username_format=%u ${passdb}
          }

          userdb {
            driver = static
            args = uid=vmail gid=vmail home=${cfg.storage}/%d/%n
          }

          service auth {
            unix_listener auth {
              mode = 0660
              user = ${postfixCfg.user}
              group = ${postfixCfg.group}
            }
          }

          lda_mailbox_autosubscribe = yes
          lda_mailbox_autocreate = yes
        '';
      };
  };
}