F-Droid’s collection of apps and files are run on servers run by the core F-Droid contributors. Originally, this main repository was hosted only on f-droid.org, but as F-Droid grew f-droid.org alone was no longer able to handle the entire load. F-Droid now supports “mirror” servers that replicate a full copy of the repositories. Hosting a mirror involves running an HTTPS webserver that has a full copy of the repository synchronized using rsync.

Primary and Secondary Mirrors

Whenever F-Droid has built a new index, it proactively pushes it to a selected set of mirrors, called the “primary mirrors”. In other words, primary mirrors are updated nearly immediately. As of September 2024, the primary mirrors are (latest list):

• ftp.agdsn.de
• ftp.lysator.liu.se
• plug-mirror.rcac.purdue.edu

Most other mirrors are “secondary mirrors”. These mirrors sync periodically (e.g., every 4 hours) from one of the primary mirrors.

Therefore, secondary mirrors usually have a small propagation delay. Nevertheless, note that in practice the time between any two index updates is dominated by the time to update and build the apps (in the order of days). Not having too many primary mirrors also keeps the load on the F-Droid origin server small.

Technically, you can also have a “tertiary mirror” by syncing from a secondary mirror, and so on.

Requirements

There are two official F-Droid repository sections, the “repo” and the “archive”. It’s most important to mirror the “repo” since it is used much more than the “archive”.

The primary resources required by a mirror are disk space and upload bandwidth. Bandwidth requirements are reduced with each new mirror, but disk requirements grow at a reasonable rate. In March 2019, the primary repository required just over 60GB of disk space in 24K files, and the archive required 300GB of disk space in 52K files. In February 2024, the primary repository required 450GB in 180K files, and the archive required 1.9TB in 430K files. The amount of disk space required grows with every new app release.

There are many mirror servers which offer an rsync connection, make sure to select the mirror closest to your mirror server:

You can find current information on disk space requirements by running the following in your terminal:

$ rsync --dry-run --recursive --stats --human-readable ftp.fau.de::fdroid .

Setup

This guide assumes the use of Nginx with a deb-based distribution, and mirroring the primary repository plus the archive. Please adjust accordingly if you’re using alternatives or don’t intend to mirror the archive. Also substitute the examples paths and domains for your own.

For assistance with this process, feel free to reach out to us.

1. Create appropriate directories

$ sudo mkdir -p /var/www/fdroid/fdroid/repo
$ sudo mkdir -p /var/www/fdroid/fdroid/archive
$ sudo chown -R www-data.www-data /var/www/fdroid

2. Synchronize the repositories.

These commands are best run in a terminal multiplexer (screen, tmux etc) as they will take some time to complete. With --info=progress2 you can see the progress.

$ sudo -u www-data -E /usr/bin/rsync -aHS  --delete --delete-delay --info=progress2 ftp.fau.de::fdroid/repo/ /var/www/fdroid/fdroid/repo/
$ sudo -u www-data -E /usr/bin/rsync -aHS  --delete --delete-delay --info=progress2 ftp.fau.de::fdroid/archive/ /var/www/fdroid/fdroid/archive/

3. Establish a cronjob to keep the repositories up to date

Create a cronjob file in /etc/cron.d

$ vi /etc/cron.d/fdroid

Fill the file with entries to update the repositories. These commands will run at minute 35 past every 6th hour, you can change it to fit your needs.

35 */6 * * * www-data /usr/bin/rsync -aHS  --delete --delete-delay ftp.fau.de::fdroid/repo/ /var/www/fdroid/fdroid/repo/
35 */6 * * * www-data /usr/bin/rsync -aHS  --delete --delete-delay ftp.fau.de::fdroid/archive/ /var/www/fdroid/fdroid/archive/

4. Configure your webserver

This is an example server block for nginx. If used, it should be copied to /etc/nginx/sites-available/ and symlinked to /etc/nginx/sites-enabled. Note that it is important that your URI be /fdroid/repo so that the app can automatically add your mirror.

server {
  listen [::]:80 ipv6only=off;

  server_name fdroidmirror.example.com;

  rewrite ^ https://$host$request_uri permanent;
}

server {
  listen [::]:443 ssl http2 ipv6only=off;

  server_name fdroidmirror.example.com;

  root /var/www/fdroid/;

  ssl_certificate /etc/letsencrypt/live/fdroidmirror.example.com/fullchain.pem;
  ssl_certificate_key /etc/letsencrypt/live/fdroidmirror.example.com/privkey.pem;

  # Insert here a TLS config from Mozilla SSL config generator https://mozilla.github.io/server-side-tls/ssl-config-generator/
}

5. Test that your mirror works

Manually add your mirror to F-Droid to test that it works.

Go to Settings -> Repositories, click the “+” button, and then manually enter your repository URL (don’t use the QR code): https://fdroidmirror.example.com/fdroid/repo?fingerprint=43238D512C1E5EB2D6569F4A3AFBF5523418B82E0A3ED1552770ABB9A9C9CCAB

Then disable the other mirrors except your new one, refresh the index with pull-to-refresh, and browse and install some apps!

6. Set up a privacy policy

Please include a privacy policy so that users can understand what happens with their metadata when using your mirror. For inspiration, see:

• FAU: https://ftp.fau.de/datenschutz
• Lysator: https://ftp.lysator.liu.se/datahanteringspolicy.txt
• Purdue PLUG: https://plug-mirror.rcac.purdue.edu/info.html

7. Submit your mirror for inclusion

• Fork the mirror monitor repo, add your mirror to the list in the README, and open a merge request.
• Open an issue on the admin repo, including any pertinent information, requesting the inclusion of your mirror.
• Once the core contributor team deems your mirror trustworthy and reliable, it will be accepted into the official list.

Other considerations

• Forward emails from cronjob failures so you know if the synchronization fails
• Monitor disk usage to prevent it from getting full
• Monitor your mirror so you know if it goes down (ideally keyword on /srv/mymirror.org/htdocs/fdroid/repo/index-v1.jar)
• Harden your SSH server config (disable password authentication, install fail2ban)
• Enable unattended security upgrades (in Debian, just apt-get install unattended-upgrades)

Running a Primary Mirror (receiving syncs via push)

The preferred setup is for the F-Droid updates to be pushed to the primary mirror via rsync over ssh with SSH Key authentication. This is the same as Debian, the key difference is that there currently is no script used for the command="", but instead, there is a hard-coded rsync command. This really nicely restricts the security interaction to only want needs to happen (Least Authority!).

command="rsync --server -logDtpre.iLsfx --log-format=X --delete --delay-updates . /srv/fdroid-mirror.at.or.at/htdocs/fdroid/"

The only piece of that command that is customizable is the final path. It can be any path but it must point to the /fdroid/ directory and must have the trailing slash. If any of the rsync options are changed, it will break the sync setup.

As an extra precaution, there should be a user account (e.g. fdroid) dedicated to receiving the rsync/ssh connection. It should have as little access as possible. It should definitely not have write access to the authorized_keys file, since that would allow an attacker who gains write access to add a separate key configuration line which circumvents all the restrictions listed there. This can be done simply by doing:

$ sudo chown root.root /home/fdroid/.ssh /home/fdroid/.ssh/authorized_keys
$ sudo chmod 0755 /home/fdroid/.ssh
$ sudo chmod 0644 /home/fdroid/.ssh/authorized_keys