transmission: apply RFC0042 and harden the service

nixos/doc/manual/release-notes/rl-2009.xml

@@ -680,6 +680,37 @@ systemd.services.nginx.serviceConfig.ReadWritePaths = [ "/var/www" ];
      was removed, as udev gained native support to handle FIDO security tokens.
     </para>
    </listitem>
+   <listitem>
+    <para>
+     The <literal>services.transmission</literal> module
+     was enhanced with the new options:
+     <xref linkend="opt-services.transmission.credentialsFile"/>,
+     <xref linkend="opt-services.transmission.openFirewall"/>,
+     and <xref linkend="opt-services.transmission.performanceNetParameters"/>.
+    </para>
+    <para>
+     <literal>transmission-daemon</literal> is now started with additional systemd sandbox/hardening options for better security.
+     Please <link xlink:href="https://github.com/NixOS/nixpkgs/issues">report</link>
+     any use case where this is not working well.
+     In particular, the <literal>RootDirectory</literal> option newly set
+     forbids uploading or downloading a torrent outside of the default directory
+     configured at <link linkend="opt-services.transmission.settings">settings.download-dir</link>.
+     If you really need Transmission to access other directories,
+     you must include those directories into the <literal>BindPaths</literal> of the service:
+<programlisting>
+systemd.services.transmission.serviceConfig.BindPaths = [ "/path/to/alternative/download-dir" ];
+</programlisting>
+    </para>
+    <para>
+     Also, connection to the RPC (Remote Procedure Call) of <literal>transmission-daemon</literal>
+     is now only available on the local network interface by default.
+     Use:
+<programlisting>
+services.transmission.settings.rpc-bind-address = "0.0.0.0";
+</programlisting>
+     to get the previous behavior of listening on all network interfaces.
+    </para>
+   </listitem>
    <listitem>
     <para>
      With this release <literal>systemd-networkd</literal> (when enabled through <xref linkend="opt-networking.useNetworkd"/>)

nixos/modules/services/torrent/transmission.nix

@@ -1,52 +1,51 @@
-{ config, lib, pkgs, ... }:
+{ config, lib, pkgs, options, ... }:
 
 with lib;
 
 let
   cfg = config.services.transmission;
+  inherit (config.environment) etc;
   apparmor = config.security.apparmor.enable;
-
+  rootDir = "/run/transmission";
-  homeDir = cfg.home;
+  homeDir = "/var/lib/transmission";
-  downloadDirPermissions = cfg.downloadDirPermissions;
+  settingsDir = ".config/transmission-daemon";
-  downloadDir = "${homeDir}/Downloads";
+  downloadsDir = "Downloads";
-  incompleteDir = "${homeDir}/.incomplete";
+  incompleteDir = ".incomplete";
-
+  # TODO: switch to configGen.json once RFC0042 is implemented
-  settingsDir = "${homeDir}/config";
+  settingsFile = pkgs.writeText "settings.json" (builtins.toJSON cfg.settings);
-  settingsFile = pkgs.writeText "settings.json" (builtins.toJSON fullSettings);
-
-  # for users in group "transmission" to have access to torrents
-  fullSettings = { umask = 2; download-dir = downloadDir; incomplete-dir = incompleteDir; } // cfg.settings;
-
-  preStart = pkgs.writeScript "transmission-pre-start" ''
-    #!${pkgs.runtimeShell}
-    set -ex
-    cp -f ${settingsFile} ${settingsDir}/settings.json
-  '';
 in
 {
   options = {
     services.transmission = {
-      enable = mkOption {
+      enable = mkEnableOption ''the headless Transmission BitTorrent daemon.
-        type = types.bool;
-        default = false;
-        description = ''
-          Whether or not to enable the headless Transmission BitTorrent daemon.
 
-          Transmission daemon can be controlled via the RPC interface using
+        Transmission daemon can be controlled via the RPC interface using
-          transmission-remote or the WebUI (http://localhost:9091/ by default).
+        transmission-remote, the WebUI (http://127.0.0.1:9091/ by default),
+        or other clients like stig or tremc.
 
-          Torrents are downloaded to ${downloadDir} by default and are
+        Torrents are downloaded to ${homeDir}/${downloadsDir} by default and are
-          accessible to users in the "transmission" group.
+        accessible to users in the "transmission" group'';
-        '';
-      };
 
-      settings = mkOption {
+      settings = mkOption rec {
+        # TODO: switch to types.config.json as prescribed by RFC0042 once it's implemented
         type = types.attrs;
+        apply = recursiveUpdate default;
         default =
           {
-            download-dir = downloadDir;
+            download-dir = "${cfg.home}/${downloadsDir}";
-            incomplete-dir = incompleteDir;
+            incomplete-dir = "${cfg.home}/${incompleteDir}";
             incomplete-dir-enabled = true;
+            message-level = 1;
+            peer-port = 51413;
+            peer-port-random-high = 65535;
+            peer-port-random-low = 49152;
+            peer-port-random-on-start = false;
+            rpc-bind-address = "127.0.0.1";
+            rpc-port = 9091;
+            script-torrent-done-enabled = false;
+            script-torrent-done-filename = "";
+            umask = 2; # 0o002 in decimal as expected by Transmission
+            utp-enabled = true;
           };
         example =
           {
@@ -56,11 +55,12 @@ in
             rpc-whitelist = "127.0.0.1,192.168.*.*";
           };
         description = ''
-          Attribute set whos fields overwrites fields in settings.json (each
+          Attribute set whose fields overwrites fields in
-          time the service starts). String values must be quoted, integer and
+          <literal>.config/transmission-daemon/settings.json</literal>
+          (each time the service starts). String values must be quoted, integer and
           boolean values must not.
 
-          See https://github.com/transmission/transmission/wiki/Editing-Configuration-Files
+          See <link xlink:href="https://github.com/transmission/transmission/wiki/Editing-Configuration-Files">Transmission's Wiki</link>
           for documentation.
         '';
       };
@@ -70,22 +70,32 @@ in
         default = "770";
         example = "775";
         description = ''
-          The permissions to set for download-dir and incomplete-dir.
+          The permissions set by <literal>systemd.activationScripts.transmission-daemon</literal>
-          They will be applied on every service start.
+          on the directories <link linkend="opt-services.transmission.settings">settings.download-dir</link>
+          and <link linkend="opt-services.transmission.settings">settings.incomplete-dir</link>.
+          Note that you may also want to change
+          <link linkend="opt-services.transmission.settings">settings.umask</link>.
         '';
       };
 
       port = mkOption {
-        type = types.int;
+        type = types.port;
-        default = 9091;
+        description = ''
-        description = "TCP port number to run the RPC/web interface.";
+          TCP port number to run the RPC/web interface.
+
+          If instead you want to change the peer port,
+          use <link linkend="opt-services.transmission.settings">settings.peer-port</link>
+          or <link linkend="opt-services.transmission.settings">settings.peer-port-random-on-start</link>.
+        '';
       };
 
       home = mkOption {
         type = types.path;
-        default = "/var/lib/transmission";
+        default = homeDir;
         description = ''
-          The directory where transmission will create files.
+          The directory where Transmission will create <literal>${settingsDir}</literal>.
+          as well as <literal>${downloadsDir}/</literal> unless <link linkend="opt-services.transmission.settings">settings.download-dir</link> is changed,
+          and <literal>${incompleteDir}/</literal> unless <link linkend="opt-services.transmission.settings">settings.incomplete-dir</link> is changed.
         '';
       };
 
@@ -100,32 +110,174 @@ in
         default = "transmission";
         description = "Group account under which Transmission runs.";
       };
+
+      credentialsFile = mkOption {
+        type = types.path;
+        description = ''
+          Path to a JSON file to be merged with the settings.
+          Useful to merge a file which is better kept out of the Nix store
+          because it contains sensible data like <link linkend="opt-services.transmission.settings">settings.rpc-password</link>.
+        '';
+        default = "/dev/null";
+        example = "/var/lib/secrets/transmission/settings.json";
+      };
+
+      openFirewall = mkEnableOption "opening of the peer port(s) in the firewall";
+
+      performanceNetParameters = mkEnableOption ''tweaking of kernel parameters
+        to open many more connections at the same time.
+
+        Note that you may also want to increase
+        <link linkend="opt-services.transmission.settings">settings.peer-limit-global</link>.
+        And be aware that these settings are quite aggressive
+        and might not suite your regular desktop use.
+        For instance, SSH sessions may time out more easily'';
     };
   };
 
   config = mkIf cfg.enable {
-    systemd.tmpfiles.rules = [
+    # Note that using systemd.tmpfiles would not work here
-      "d '${homeDir}' 0770 '${cfg.user}' '${cfg.group}' - -"
+    # because it would fail when creating a directory
-      "d '${settingsDir}' 0700 '${cfg.user}' '${cfg.group}' - -"
+    # with a different owner than its parent directory, by saying:
-      "d '${fullSettings.download-dir}' '${downloadDirPermissions}' '${cfg.user}' '${cfg.group}' - -"
+    # Detected unsafe path transition /home/foo → /home/foo/Downloads during canonicalization of /home/foo/Downloads
-      "d '${fullSettings.incomplete-dir}' '${downloadDirPermissions}' '${cfg.user}' '${cfg.group}' - -"
+    # when /home/foo is not owned by cfg.user.
+    # Note also that using an ExecStartPre= wouldn't work either
+    # because BindPaths= needs these directories before.
+    system.activationScripts.transmission-daemon = ''
+      install -d -m 700 '${cfg.home}/${settingsDir}'
+      chown -R '${cfg.user}:${cfg.group}' ${cfg.home}/${settingsDir}
+      install -d -m '${cfg.downloadDirPermissions}' -o '${cfg.user}' -g '${cfg.group}' '${cfg.settings.download-dir}'
+      '' + optionalString cfg.settings.incomplete-dir-enabled ''
+      install -d -m '${cfg.downloadDirPermissions}' -o '${cfg.user}' -g '${cfg.group}' '${cfg.settings.incomplete-dir}'
+      '';
+
+    assertions = [
+      { assertion = builtins.match "^/.*" cfg.home != null;
+        message = "`services.transmission.home' must be an absolute path.";
+      }
+      { assertion = types.path.check cfg.settings.download-dir;
+        message = "`services.transmission.settings.download-dir' must be an absolute path.";
+      }
+      { assertion = types.path.check cfg.settings.incomplete-dir;
+        message = "`services.transmission.settings.incomplete-dir' must be an absolute path.";
+      }
+      { assertion = cfg.settings.script-torrent-done-filename == "" || types.path.check cfg.settings.script-torrent-done-filename;
+        message = "`services.transmission.settings.script-torrent-done-filename' must be an absolute path.";
+      }
+      { assertion = types.port.check cfg.settings.rpc-port;
+        message = "${toString cfg.settings.rpc-port} is not a valid port number for `services.transmission.settings.rpc-port`.";
+      }
+      # In case both port and settings.rpc-port are explicitely defined: they must be the same.
+      { assertion = !options.services.transmission.port.isDefined || cfg.port == cfg.settings.rpc-port;
+        message = "`services.transmission.port' is not equal to `services.transmission.settings.rpc-port'";
+      }
     ];
 
+    services.transmission.settings =
+      optionalAttrs options.services.transmission.port.isDefined { rpc-port = cfg.port; };
+
     systemd.services.transmission = {
       description = "Transmission BitTorrent Service";
       after = [ "network.target" ] ++ optional apparmor "apparmor.service";
-      requires = mkIf apparmor [ "apparmor.service" ];
+      requires = optional apparmor "apparmor.service";
       wantedBy = [ "multi-user.target" ];
+      environment.CURL_CA_BUNDLE = etc."ssl/certs/ca-certificates.crt".source;
 
-      # 1) Only the "transmission" user and group have access to torrents.
+      serviceConfig = {
-      # 2) Optionally update/force specific fields into the configuration file.
+        # Use "+" because credentialsFile may not be accessible to User= or Group=.
-      serviceConfig.ExecStartPre = preStart;
+        ExecStartPre = [("+" + pkgs.writeShellScript "transmission-prestart" ''
-      serviceConfig.ExecStart = "${pkgs.transmission}/bin/transmission-daemon -f --port ${toString config.services.transmission.port} --config-dir ${settingsDir}";
+          set -eu${lib.optionalString (cfg.settings.message-level >= 3) "x"}
-      serviceConfig.ExecReload = "${pkgs.coreutils}/bin/kill -HUP $MAINPID";
+          ${pkgs.jq}/bin/jq --slurp add ${settingsFile} '${cfg.credentialsFile}' |
-      serviceConfig.User = cfg.user;
+          install -D -m 600 -o '${cfg.user}' -g '${cfg.group}' /dev/stdin \
-      serviceConfig.Group = cfg.group;
+           '${cfg.home}/${settingsDir}/settings.json'
-      # NOTE: transmission has an internal umask that also must be set (in settings.json)
+        '')];
-      serviceConfig.UMask = "0002";
+        ExecStart="${pkgs.transmission}/bin/transmission-daemon -f";
+        ExecReload = "${pkgs.coreutils}/bin/kill -HUP $MAINPID";
+        User = cfg.user;
+        Group = cfg.group;
+        # Create rootDir in the host's mount namespace.
+        RuntimeDirectory = [(baseNameOf rootDir)];
+        RuntimeDirectoryMode = "755";
+        # Avoid mounting rootDir in the own rootDir of ExecStart='s mount namespace.
+        InaccessiblePaths = ["-+${rootDir}"];
+        # This is for BindPaths= and BindReadOnlyPaths=
+        # to allow traversal of directories they create in RootDirectory=.
+        UMask = "0066";
+        # Using RootDirectory= makes it possible
+        # to use the same paths download-dir/incomplete-dir
+        # (which appear in user's interfaces) without requiring cfg.user
+        # to have access to their parent directories,
+        # by using BindPaths=/BindReadOnlyPaths=.
+        # Note that TemporaryFileSystem= could have been used instead
+        # but not without adding some BindPaths=/BindReadOnlyPaths=
+        # that would only be needed for ExecStartPre=,
+        # because RootDirectoryStartOnly=true would not help.
+        RootDirectory = rootDir;
+        RootDirectoryStartOnly = true;
+        MountAPIVFS = true;
+        BindPaths =
+          [ "${cfg.home}/${settingsDir}"
+            cfg.settings.download-dir
+          ] ++
+          optional cfg.settings.incomplete-dir-enabled
+            cfg.settings.incomplete-dir;
+        BindReadOnlyPaths = [
+          # No confinement done of /nix/store here like in systemd-confinement.nix,
+          # an AppArmor profile is provided to get a confinement based upon paths and rights.
+          builtins.storeDir
+          "-/etc/hosts"
+          "-/etc/ld-nix.so.preload"
+          "-/etc/localtime"
+          ] ++
+          optional (cfg.settings.script-torrent-done-enabled &&
+                    cfg.settings.script-torrent-done-filename != "")
+            cfg.settings.script-torrent-done-filename;
+        # The following options are only for optimizing:
+        # systemd-analyze security transmission
+        AmbientCapabilities = "";
+        CapabilityBoundingSet = "";
+        # ProtectClock= adds DeviceAllow=char-rtc r
+        DeviceAllow = "";
+        LockPersonality = true;
+        MemoryDenyWriteExecute = true;
+        NoNewPrivileges = true;
+        PrivateDevices = true;
+        PrivateMounts = true;
+        PrivateNetwork = mkDefault false;
+        PrivateTmp = true;
+        PrivateUsers = true;
+        ProtectClock = true;
+        ProtectControlGroups = true;
+        # ProtectHome=true would not allow BindPaths= to work accross /home,
+        # and ProtectHome=tmpfs would break statfs(),
+        # preventing transmission-daemon to report the available free space.
+        # However, RootDirectory= is used, so this is not a security concern
+        # since there would be nothing in /home but any BindPaths= wanted by the user.
+        ProtectHome = "read-only";
+        ProtectHostname = true;
+        ProtectKernelLogs = true;
+        ProtectKernelModules = true;
+        ProtectKernelTunables = true;
+        ProtectSystem = "strict";
+        RemoveIPC = true;
+        # AF_UNIX may become usable one day:
+        # https://github.com/transmission/transmission/issues/441
+        RestrictAddressFamilies = [ "AF_UNIX" "AF_INET" "AF_INET6" ];
+        RestrictNamespaces = true;
+        RestrictRealtime = true;
+        RestrictSUIDSGID = true;
+        SystemCallFilter = [
+          "@system-service"
+          # Groups in @system-service which do not contain a syscall
+          # listed by perf stat -e 'syscalls:sys_enter_*' transmission-daemon -f
+          # in tests, and seem likely not necessary for transmission-daemon.
+          "~@aio" "~@chown" "~@keyring" "~@memlock" "~@resources" "~@setuid" "~@timer"
+          # In the @privileged group, but reached when querying infos through RPC (eg. with stig).
+          "quotactl"
+        ];
+        SystemCallArchitectures = "native";
+        SystemCallErrorNumber = "EPERM";
+      };
     };
 
     # It's useful to have transmission in path, e.g. for remote control
@@ -133,70 +285,153 @@ in
 
     users.users = optionalAttrs (cfg.user == "transmission") ({
       transmission = {
-        name = "transmission";
         group = cfg.group;
         uid = config.ids.uids.transmission;
         description = "Transmission BitTorrent user";
-        home = homeDir;
+        home = cfg.home;
-        createHome = true;
       };
     });
 
     users.groups = optionalAttrs (cfg.group == "transmission") ({
       transmission = {
-        name = "transmission";
         gid = config.ids.gids.transmission;
       };
     });
 
-    # AppArmor profile
+    networking.firewall = mkIf cfg.openFirewall (
+      if cfg.settings.peer-port-random-on-start
+      then
+        { allowedTCPPortRanges =
+            [ { from = cfg.settings.peer-port-random-low;
+                to   = cfg.settings.peer-port-random-high;
+              }
+            ];
+          allowedUDPPortRanges =
+            [ { from = cfg.settings.peer-port-random-low;
+                to   = cfg.settings.peer-port-random-high;
+              }
+            ];
+        }
+      else
+        { allowedTCPPorts = [ cfg.settings.peer-port ];
+          allowedUDPPorts = [ cfg.settings.peer-port ];
+        }
+    );
+
+    boot.kernel.sysctl = mkMerge [
+      # Transmission uses a single UDP socket in order to implement multiple uTP sockets,
+      # and thus expects large kernel buffers for the UDP socket,
+      # https://trac.transmissionbt.com/browser/trunk/libtransmission/tr-udp.c?rev=11956.
+      # at least up to the values hardcoded here:
+      (mkIf cfg.settings.utp-enabled {
+        "net.core.rmem_max" = mkDefault "4194304"; # 4MB
+        "net.core.wmem_max" = mkDefault "1048576"; # 1MB
+      })
+      (mkIf cfg.performanceNetParameters {
+        # Increase the number of available source (local) TCP and UDP ports to 49151.
+        # Usual default is 32768 60999, ie. 28231 ports.
+        # Find out your current usage with: ss -s
+        "net.ipv4.ip_local_port_range" = "16384 65535";
+        # Timeout faster generic TCP states.
+        # Usual default is 600.
+        # Find out your current usage with: watch -n 1 netstat -nptuo
+        "net.netfilter.nf_conntrack_generic_timeout" = 60;
+        # Timeout faster established but inactive connections.
+        # Usual default is 432000.
+        "net.netfilter.nf_conntrack_tcp_timeout_established" = 600;
+        # Clear immediately TCP states after timeout.
+        # Usual default is 120.
+        "net.netfilter.nf_conntrack_tcp_timeout_time_wait" = 1;
+        # Increase the number of trackable connections.
+        # Usual default is 262144.
+        # Find out your current usage with: conntrack -C
+        "net.netfilter.nf_conntrack_max" = 1048576;
+      })
+    ];
+
     security.apparmor.profiles = mkIf apparmor [
       (pkgs.writeText "apparmor-transmission-daemon" ''
-        #include <tunables/global>
+        include <tunables/global>
 
         ${pkgs.transmission}/bin/transmission-daemon {
-          #include <abstractions/base>
+          include <abstractions/base>
-          #include <abstractions/nameservice>
+          include <abstractions/nameservice>
 
-          ${getLib pkgs.glibc}/lib/*.so                    mr,
+          # NOTE: https://github.com/NixOS/nixpkgs/pull/93457
-          ${getLib pkgs.libevent}/lib/libevent*.so*        mr,
+          # will remove the need for these by fixing <abstractions/base>
-          ${getLib pkgs.curl}/lib/libcurl*.so*             mr,
+          r ${etc."hosts".source},
-          ${getLib pkgs.openssl}/lib/libssl*.so*           mr,
+          r /etc/ld-nix.so.preload,
-          ${getLib pkgs.openssl}/lib/libcrypto*.so*        mr,
+          ${lib.optionalString (builtins.hasAttr "ld-nix.so.preload" etc) ''
-          ${getLib pkgs.zlib}/lib/libz*.so*                mr,
+            r ${etc."ld-nix.so.preload".source},
-          ${getLib pkgs.libssh2}/lib/libssh2*.so*          mr,
+            ${concatMapStrings (p: optionalString (p != "") ("mr ${p},\n"))
-          ${getLib pkgs.systemd}/lib/libsystemd*.so*       mr,
+              (splitString "\n" config.environment.etc."ld-nix.so.preload".text)}
-          ${getLib pkgs.xz}/lib/liblzma*.so*               mr,
-          ${getLib pkgs.libgcrypt}/lib/libgcrypt*.so*      mr,
-          ${getLib pkgs.libgpgerror}/lib/libgpg-error*.so* mr,
-          ${getLib pkgs.nghttp2}/lib/libnghttp2*.so*       mr,
-          ${getLib pkgs.c-ares}/lib/libcares*.so*          mr,
-          ${getLib pkgs.libcap}/lib/libcap*.so*            mr,
-          ${getLib pkgs.attr}/lib/libattr*.so*             mr,
-          ${getLib pkgs.lz4}/lib/liblz4*.so*               mr,
-          ${getLib pkgs.libkrb5}/lib/lib*.so*              mr,
-          ${getLib pkgs.keyutils}/lib/libkeyutils*.so*     mr,
-          ${getLib pkgs.utillinuxMinimal.out}/lib/libblkid.so.* mr,
-          ${getLib pkgs.utillinuxMinimal.out}/lib/libmount.so.* mr,
-          ${getLib pkgs.utillinuxMinimal.out}/lib/libuuid.so.* mr,
-          ${getLib pkgs.gcc.cc.lib}/lib/libstdc++.so.* mr,
-          ${getLib pkgs.gcc.cc.lib}/lib/libgcc_s.so.* mr,
-
-          @{PROC}/sys/kernel/random/uuid   r,
-          @{PROC}/sys/vm/overcommit_memory r,
-
-          ${pkgs.openssl.out}/etc/**                     r,
-          ${pkgs.transmission}/share/transmission/** r,
-
-          owner ${settingsDir}/** rw,
-
-          ${fullSettings.download-dir}/** rw,
-          ${optionalString fullSettings.incomplete-dir-enabled ''
-            ${fullSettings.incomplete-dir}/** rw,
           ''}
+          r ${etc."ssl/certs/ca-certificates.crt".source},
+          r ${pkgs.tzdata}/share/zoneinfo/**,
+          r ${pkgs.stdenv.cc.libc}/share/i18n/**,
+          r ${pkgs.stdenv.cc.libc}/share/locale/**,
+
+          mr ${getLib pkgs.stdenv.cc.cc}/lib/*.so*,
+          mr ${getLib pkgs.stdenv.cc.libc}/lib/*.so*,
+          mr ${getLib pkgs.attr}/lib/libattr*.so*,
+          mr ${getLib pkgs.c-ares}/lib/libcares*.so*,
+          mr ${getLib pkgs.curl}/lib/libcurl*.so*,
+          mr ${getLib pkgs.keyutils}/lib/libkeyutils*.so*,
+          mr ${getLib pkgs.libcap}/lib/libcap*.so*,
+          mr ${getLib pkgs.libevent}/lib/libevent*.so*,
+          mr ${getLib pkgs.libgcrypt}/lib/libgcrypt*.so*,
+          mr ${getLib pkgs.libgpgerror}/lib/libgpg-error*.so*,
+          mr ${getLib pkgs.libkrb5}/lib/lib*.so*,
+          mr ${getLib pkgs.libssh2}/lib/libssh2*.so*,
+          mr ${getLib pkgs.lz4}/lib/liblz4*.so*,
+          mr ${getLib pkgs.nghttp2}/lib/libnghttp2*.so*,
+          mr ${getLib pkgs.openssl}/lib/libcrypto*.so*,
+          mr ${getLib pkgs.openssl}/lib/libssl*.so*,
+          mr ${getLib pkgs.systemd}/lib/libsystemd*.so*,
+          mr ${getLib pkgs.utillinuxMinimal.out}/lib/libblkid.so*,
+          mr ${getLib pkgs.utillinuxMinimal.out}/lib/libmount.so*,
+          mr ${getLib pkgs.utillinuxMinimal.out}/lib/libuuid.so*,
+          mr ${getLib pkgs.xz}/lib/liblzma*.so*,
+          mr ${getLib pkgs.zlib}/lib/libz*.so*,
+
+          r @{PROC}/sys/kernel/random/uuid,
+          r @{PROC}/sys/vm/overcommit_memory,
+          # @{pid} is not a kernel variable yet but a regexp
+          #r @{PROC}/@{pid}/environ,
+          r @{PROC}/@{pid}/mounts,
+          rwk /tmp/tr_session_id_*,
+
+          r ${pkgs.openssl.out}/etc/**,
+          r ${config.systemd.services.transmission.environment.CURL_CA_BUNDLE},
+          r ${pkgs.transmission}/share/transmission/**,
+
+          owner rw ${cfg.home}/${settingsDir}/**,
+          rw ${cfg.settings.download-dir}/**,
+          ${optionalString cfg.settings.incomplete-dir-enabled ''
+            rw ${cfg.settings.incomplete-dir}/**,
+          ''}
+          profile dirs {
+            rw ${cfg.settings.download-dir}/**,
+            ${optionalString cfg.settings.incomplete-dir-enabled ''
+              rw ${cfg.settings.incomplete-dir}/**,
+            ''}
+          }
+
+          ${optionalString (cfg.settings.script-torrent-done-enabled &&
+                            cfg.settings.script-torrent-done-filename != "") ''
+            # Stack transmission_directories profile on top of
+            # any existing profile for script-torrent-done-filename
+            # FIXME: to be tested as I'm not sure it works well with NoNewPrivileges=
+            # https://gitlab.com/apparmor/apparmor/-/wikis/AppArmorStacking#seccomp-and-no_new_privs
+            px ${cfg.settings.script-torrent-done-filename} -> &@{dirs},
+          ''}
+
+          # FIXME: enable customizing using https://github.com/NixOS/nixpkgs/pull/93457
+          # include <local/transmission-daemon>
         }
       '')
     ];
   };
 
+  meta.maintainers = with lib.maintainers; [ julm ];
 }

nixos/tests/bittorrent.nix

@@ -19,6 +19,7 @@ let
   externalClient2Address = "80.100.100.2";
   externalTrackerAddress = "80.100.100.3";
 
+  download-dir = "/var/lib/transmission/Downloads";
   transmissionConfig = { ... }: {
     environment.systemPackages = [ pkgs.transmission ];
     services.transmission = {
@@ -26,6 +27,7 @@ let
       settings = {
         dht-enabled = false;
         message-level = 3;
+        inherit download-dir;
       };
     };
   };
@@ -117,12 +119,12 @@ in
       router.wait_for_unit("miniupnpd")
 
       # Create the torrent.
-      tracker.succeed("mkdir /tmp/data")
+      tracker.succeed("mkdir ${download-dir}/data")
       tracker.succeed(
-          "cp ${file} /tmp/data/test.tar.bz2"
+          "cp ${file} ${download-dir}/data/test.tar.bz2"
       )
       tracker.succeed(
-          "transmission-create /tmp/data/test.tar.bz2 --private --tracker http://${externalTrackerAddress}:6969/announce --outfile /tmp/test.torrent"
+          "transmission-create ${download-dir}/data/test.tar.bz2 --private --tracker http://${externalTrackerAddress}:6969/announce --outfile /tmp/test.torrent"
       )
       tracker.succeed("chmod 644 /tmp/test.torrent")
 
@@ -133,18 +135,16 @@ in
 
       # Start the initial seeder.
       tracker.succeed(
-          "transmission-remote --add /tmp/test.torrent --no-portmap --no-dht --download-dir /tmp/data"
+          "transmission-remote --add /tmp/test.torrent --no-portmap --no-dht --download-dir ${download-dir}/data"
       )
 
       # Now we should be able to download from the client behind the NAT.
       tracker.wait_for_unit("httpd")
       client1.wait_for_unit("network-online.target")
+      client1.succeed("transmission-remote --add http://${externalTrackerAddress}/test.torrent >&2 &")
+      client1.wait_for_file("${download-dir}/test.tar.bz2")
       client1.succeed(
-          "transmission-remote --add http://${externalTrackerAddress}/test.torrent --download-dir /tmp >&2 &"
+          "cmp ${download-dir}/test.tar.bz2 ${file}"
-      )
-      client1.wait_for_file("/tmp/test.tar.bz2")
-      client1.succeed(
-          "cmp /tmp/test.tar.bz2 ${file}"
       )
 
       # Bring down the initial seeder.
@@ -154,11 +154,11 @@ in
       # the first client created a NAT hole in the router.
       client2.wait_for_unit("network-online.target")
       client2.succeed(
-          "transmission-remote --add http://${externalTrackerAddress}/test.torrent --no-portmap --no-dht --download-dir /tmp >&2 &"
+          "transmission-remote --add http://${externalTrackerAddress}/test.torrent --no-portmap --no-dht >&2 &"
       )
-      client2.wait_for_file("/tmp/test.tar.bz2")
+      client2.wait_for_file("${download-dir}/test.tar.bz2")
       client2.succeed(
-          "cmp /tmp/test.tar.bz2 ${file}"
+          "cmp ${download-dir}/test.tar.bz2 ${file}"
       )
     '';
 })