{ pkgs, lib, ... }: let 
  config = {
    id = "heisenbridge";
    url = "http://127.0.0.1:9898";
    as_token = "$APPSERVICE_TOKEN";
    hs_token = "$HOMESERVER_TOKEN";
    rate_limited = false;
    sender_localpart = "heisenbridge";
    namespaces = {
      users = [{ regex = "@heisenbridge_.*"; exclusive = true; }];
      aliases = [ ];
      rooms = [ ];
    };
  };
  configFile = pkgs.writeText "heisenbridge.yml" (lib.generators.toYAML {} config);
  package = pkgs.heisenbridge.overrideAttrs (oA: let
    version = "1.8.2";
  in {
    name = "${oA.pname}-${version}";

    src = pkgs.fetchFromGitHub {
      owner = "hifi";
      repo = oA.pname;
      rev = "v${version}";
      sha256 = "173prcd56rwlxjxlw67arnm12k1l317xi5s6m7jhmp8zbbrj5vwr";
    };

    patches = [];
  });
in {
  systemd.services.heisenbridge = {
    description = "Heisenbridge Matrix IRC bridge";
    before = [ "matrix-synapse.service" ];
    wantedBy = [ "multi-user.target" ];
    stopIfChanged = false;

    preStart = ''
      umask 027
      ${pkgs.envsubst}/bin/envsubst -i ${configFile} -o /run/heisenbridge/appservice.yml
      chgrp matrix-synapse /run/heisenbridge/appservice.yml
    '';

    sandbox = 2;
    serviceConfig = {
      Restart = "always";
      EnvironmentFile = "/run/secrets/heisenbridge/env";
      ExecStart = "${package}/bin/heisenbridge --config /run/heisenbridge/appservice.yml --identd http://localhost:8080";

      RuntimeDirectory = "heisenbridge";
      RuntimeDirectoryMode = "0755";

      User = "heisenbridge";
      Group = "heisenbridge";

      PrivateNetwork = false;
      SystemCallFilter = "@system-service";
      # needed for ident
      AmbientCapabilities = [ "CAP_CHOWN" "CAP_NET_BIND_SERVICE" ];
      PrivateUsers = false;
    };
    apparmor = {
      enable = true;
      extraConfig = ''
        @{PROC}@{pid}/fd/ r,

        network udp,
        network tcp,
        network netlink raw,

        capability chown,
        capability net_bind_service,
      '';
    };
  };

  users.users.heisenbridge = {
    description = "Heisenbridge service user";
    isSystemUser = true;
    group = "heisenbridge";
  };
  users.groups.heisenbridge = {};

  # identd
  helsinki.firewall.ports.tcp = [ 113 ];

  helsinki.helsinkey.private."heisenbridge/env" = {
    services = [ "heisenbridge" ];
  };
}

Relevant part of that module:

config = mkMerge [
          (mkIf (config.sandbox >= 2) {
            serviceConfig = mapAttrs (_: mkDefaulter) {
              # Filesystem stuff
              ProtectSystem = "strict"; # Prevent writing to most of /
              ProtectHome = true; # Prevent accessing /home and /root
              PrivateTmp = true; # Give an own directory under /tmp
              PrivateDevices = true; # Deny access to most of /dev
              ProtectKernelTunables = true; # Protect some parts of /sys
              ProtectControlGroups = true; # Remount cgroups read-only
              RestrictSUIDSGID = true; # Prevent creating SETUID/SETGID files
              PrivateMounts = true; # Give an own mount namespace
              RemoveIPC = true;
              UMask = "0077";

              # Capabilities
              CapabilityBoundingSet = ""; # Allow no capabilities at all
              NoNewPrivileges = true; # Disallow getting more capabilities. This is also implied by other options.

              # Kernel stuff
              ProtectKernelModules = true; # Prevent loading of kernel modules
              SystemCallArchitectures = "native"; # Usually no need to disable this
              ProtectKernelLogs = true; # Prevent access to kernel logs
              ProtectClock = true; # Prevent setting the RTC

              # Networking
              RestrictAddressFamilies = ""; # Example: "AF_UNIX AF_INET AF_INET6"
              PrivateNetwork = true; # Isolate the entire network

              # Misc
              LockPersonality = true; # Prevent change of the personality
              ProtectHostname = true; # Give an own UTS namespace
              RestrictRealtime = true; # Prevent switching to RT scheduling
              MemoryDenyWriteExecute = true; # Maybe disable this for interpreters like python
              PrivateUsers = true; # If anything randomly breaks, it's mostly because of this
              RestrictNamespaces = true;
            };
          })
        ];

In reply to @janne.hess:helsinki-systems.de
The sandbox module is basically the opt-out approach to sandboxing that hifi was talking about

In reply to @sumner:nevarro.space
https://github.com/NixOS/nixpkgs/pull/153279 would be great to get merged

In reply to @janne.hess:helsinki-systems.de
DId you test this?

In reply to @sumner:nevarro.space
yes, added screenshot. Also using analogous fix in my home-manager config: https://github.com/sumnerevans/home-manager-config/blob/master/modules/communication.nix#L5 and that works fine

In reply to @janne.hess:helsinki-systems.de
Looks like adding the backport label after merging doesn't do anything :(

In reply to @sumner:nevarro.space
I'm pretty sure it's supposed to, maybe it's just slow?