Full Code

Transmission app is already included in FreedomBox. Here is the full source for the module for reference.

plinth/modules/transmission/__init__.py

# SPDX-License-Identifier: AGPL-3.0-or-later
"""
FreedomBox app to configure Transmission server.
"""

from django.urls import reverse_lazy
from django.utils.translation import gettext_lazy as _

from plinth import app as app_module
from plinth import cfg, frontpage, menu
from plinth.daemon import Daemon
from plinth.modules.apache.components import Webserver
from plinth.modules.backups.components import BackupRestore
from plinth.modules.firewall.components import (Firewall,
                                                FirewallLocalProtection)
from plinth.modules.users import add_user_to_share_group
from plinth.modules.users.components import UsersAndGroups
from plinth.package import Packages
from plinth.utils import format_lazy

from . import manifest, privileged

_description = [
    _('Transmission is a BitTorrent client with a web interface.'),
    _('BitTorrent is a peer-to-peer file sharing protocol. '
      'Note that BitTorrent is not anonymous.'),
    _('Please do not change the default port of the Transmission daemon.'),
    format_lazy(
        _('Compared to <a href="{deluge_url}">'
          'Deluge</a>, Transmission is simpler and lightweight but is less '
          'customizable.'), deluge_url=reverse_lazy('deluge:index')),
    format_lazy(
        _('It can be accessed by <a href="{users_url}">any user</a> on '
          '{box_name} belonging to the bit-torrent group.'),
        box_name=_(cfg.box_name), users_url=reverse_lazy('users:index')),
    format_lazy(
        _('<a href="{samba_url}">Samba</a> shares can be set as the '
          'default download directory from the dropdown menu below.'),
        samba_url=reverse_lazy('samba:index')),
    format_lazy(
        _('After a download has completed, you can also access your files '
          'using the <a href="{sharing_url}">Sharing</a> app.'),
        sharing_url=reverse_lazy('sharing:index'))
]

SYSTEM_USER = 'debian-transmission'


class TransmissionApp(app_module.App):
    """FreedomBox app for Transmission."""

    app_id = 'transmission'

    _version = 5

    DAEMON = 'transmission-daemon'

    def __init__(self):
        """Create components for the app."""
        super().__init__()

        groups = {
            'bit-torrent': _('Download files using BitTorrent applications')
        }

        info = app_module.Info(
            app_id=self.app_id, version=self._version, name=_('Transmission'),
            icon_filename='transmission',
            short_description=_('BitTorrent Web Client'),
            description=_description, manual_page='Transmission',
            clients=manifest.clients,
            donation_url='https://transmissionbt.com/donate/')
        self.add(info)

        menu_item = menu.Menu('menu-transmission', info.name,
                              info.short_description, info.icon_filename,
                              'transmission:index', parent_url_name='apps')
        self.add(menu_item)

        shortcut = frontpage.Shortcut(
            'shortcut-transmission', info.name,
            short_description=info.short_description, icon=info.icon_filename,
            url='/transmission', clients=info.clients, login_required=True,
            allowed_groups=list(groups))
        self.add(shortcut)

        packages = Packages('packages-transmission', ['transmission-daemon'])
        self.add(packages)

        firewall = Firewall('firewall-transmission', info.name,
                            ports=['http', 'https',
                                   'transmission-client'], is_external=True)
        self.add(firewall)

        firewall_local_protection = FirewallLocalProtection(
            'firewall-local-protection-transmission', ['9091'])
        self.add(firewall_local_protection)

        webserver = Webserver('webserver-transmission', 'transmission-plinth',
                              urls=['https://{host}/transmission'])
        self.add(webserver)

        daemon = Daemon(
            'daemon-transmission', self.DAEMON, listen_ports=[
                (9091, 'tcp4'),
                (51413, 'tcp4'),
                (51413, 'tcp6'),
                (51413, 'udp4'),
            ])
        self.add(daemon)

        users_and_groups = UsersAndGroups('users-and-groups-transmission',
                                          reserved_usernames=[SYSTEM_USER],
                                          groups=groups)
        self.add(users_and_groups)

        backup_restore = BackupRestore('backup-restore-transmission',
                                       **manifest.backup)
        self.add(backup_restore)

    def setup(self, old_version):
        """Install and configure the app."""
        super().setup(old_version)

        if old_version and old_version <= 3 and self.is_enabled():
            self.get_component('firewall-transmission').enable()

        new_configuration = {
            'rpc-whitelist-enabled': False,
            'rpc-authentication-required': False
        }
        privileged.merge_configuration(new_configuration)
        add_user_to_share_group(SYSTEM_USER, TransmissionApp.DAEMON)
        self.enable()

plinth/modules/transmission/forms.py

# SPDX-License-Identifier: AGPL-3.0-or-later
"""
FreedomBox app for configuring Transmission.
"""

from django.utils.translation import gettext_lazy as _

from plinth.modules.storage.forms import (DirectorySelectForm,
                                          DirectoryValidator)

from . import SYSTEM_USER


class TransmissionForm(DirectorySelectForm):
    """Transmission configuration form"""

    def __init__(self, *args, **kw):
        validator = DirectoryValidator(username=SYSTEM_USER,
                                       check_creatable=True)
        super().__init__(title=_('Download directory'),
                         default='/var/lib/transmission-daemon/downloads',
                         validator=validator, *args, **kw)

plinth/modules/transmission/manifest.py

# SPDX-License-Identifier: AGPL-3.0-or-later

from django.utils.translation import gettext_lazy as _

clients = [{
    'name': _('Transmission'),
    'platforms': [{
        'type': 'web',
        'url': '/transmission'
    }]
}]

backup = {
    'data': {
        'directories': ['/var/lib/transmission-daemon/.config']
    },
    'secrets': {
        'files': ['/etc/transmission-daemon/settings.json']
    },
    'services': ['transmission-daemon']
}

plinth/modules/transmission/privileged.py

# SPDX-License-Identifier: AGPL-3.0-or-later
"""
Configuration helper for Transmission daemon.
"""

import json
import pathlib
from typing import Union

from plinth import action_utils
from plinth.actions import privileged

_transmission_config = pathlib.Path('/etc/transmission-daemon/settings.json')


@privileged
def get_configuration() -> dict[str, str]:
    """Return the current configuration in JSON format."""
    return json.loads(_transmission_config.read_text(encoding='utf-8'))


@privileged
def merge_configuration(configuration: dict[str, Union[str, bool]]):
    """Merge given JSON configuration with existing configuration."""
    current_configuration = _transmission_config.read_bytes()
    current_configuration = json.loads(current_configuration)

    new_configuration = current_configuration
    new_configuration.update(configuration)
    new_configuration = json.dumps(new_configuration, indent=4, sort_keys=True)

    _transmission_config.write_text(new_configuration, encoding='utf-8')
    action_utils.service_reload('transmission-daemon')

plinth/modules/transmission/urls.py

# SPDX-License-Identifier: AGPL-3.0-or-later
"""
URLs for the Transmission module.
"""

from django.urls import re_path

from .views import TransmissionAppView

urlpatterns = [
    re_path(r'^apps/transmission/$', TransmissionAppView.as_view(),
            name='index'),
]

plinth/modules/transmission/views.py

# SPDX-License-Identifier: AGPL-3.0-or-later
"""
FreedomBox app for configuring Transmission Server.
"""

import logging
import socket

from django.contrib import messages
from django.utils.translation import gettext as _

from plinth import views

from . import privileged
from .forms import TransmissionForm

logger = logging.getLogger(__name__)


class TransmissionAppView(views.AppView):
    """Serve configuration page."""
    form_class = TransmissionForm
    app_id = 'transmission'

    def get_initial(self):
        """Get the current settings from Transmission server."""
        status = super().get_initial()
        configuration = privileged.get_configuration()
        status['storage_path'] = configuration['download-dir']
        status['hostname'] = socket.gethostname()

        return status

    def form_valid(self, form):
        """Apply the changes submitted in the form."""
        old_status = form.initial
        new_status = form.cleaned_data
        if old_status['storage_path'] != new_status['storage_path']:
            new_configuration = {
                'download-dir': new_status['storage_path'],
            }
            privileged.merge_configuration(new_configuration)
            messages.success(self.request, _('Configuration updated'))

        return super().form_valid(form)

plinth/modules/transmission/data/etc/plinth/modules-enabled/transmission

plinth.modules.transmission

plinth/modules/transmission/data/etc/apache2/conf-available/transmission-plinth.conf

##
## On all sites, provide Transmission on a default path: /transmission
##
## Requires the following Apache modules to be enabled:
##   mod_headers
##   mod_proxy
##   mod_proxy_http
##
<Location /transmission>
    ProxyPass        http://localhost:9091/transmission
    Include          includes/freedombox-single-sign-on.conf
    <IfModule mod_auth_pubtkt.c>
        TKTAuthToken "admin" "bit-torrent"
    </IfModule>
    ## Send the scheme from user's request to enable Transmission to
    ## redirect URLs, set cookies, set absolute URLs (if any)
    ## properly.
    RequestHeader    set X-Forwarded-Proto 'https' env=HTTPS

    # Make redirects to avoid 409 Conflict errors. See: #2219. Upstream issue:
    # https://github.com/transmission/transmission/pull/857 . Drop this
    # workaround with Transmission >= 4.0.
    <IfModule mod_rewrite.c>
        RewriteEngine On
        RewriteCond %{REQUEST_URI} ^/transmission/$
        RewriteRule .* /transmission/web/ [R=302,L]
        RewriteCond %{REQUEST_URI} ^/transmission/web$
        RewriteRule .* /transmission/web/ [R=302,L]
    </IfModule>
</Location>

plinth/modules/transmission/tests/__init__.py