As explained on Debian's documentation on this subject:

> Push mirroring is a form of mirroring that minimizes the time it takes 
> for changes to the main archive to reach mirrors. The server mirror uses
> a triggering mechanism to immediately inform the client mirror that it
> needs to be updated.
>
> Push mirroring takes a little more effort to set up since the maintainers
> of the upstream and downstream mirror must exchange information. The 
> benefit is that the upstream mirror initiates the mirror process immediately
> after its archive has been updated. This allows changes to the archive to
> propagate extremely quickly. 

So instead of relying on cron jobs to keep things in sync, mirrors can be asked to sync when it is necessary. SSH is usually used, however we can offer HTTP triggers if preferred.

Setup

Your mirror should have previously been set up to use cron jobs and mirroring scripts. Such scripts can be found here.

We recommend that you create an ubuntu user and grant it permissions to the directory where your mirror is stored. Then place the keys below (or from whichever mirror you are syncing from) into that user's authorized_keys file.

When the upstream mirror connects as the ubuntu user to your mirror, it will run the script and background the process. Meanwhile the rsync command called by the script, will connect to the upstream mirror and sync any changes as needed.

When this setup has been completed, please let us know and we'll set up the necessary trigger commands on our end.

If the command is not run in the background, you can try redirecting stdin, stdout and stderr to /dev/null, i.e. use the following command :

command="~/archive-sync </dev/null >/dev/null 2>/dev/null &"

Keys

These are the SSH keys used for triggering archive and releases mirrors respectively. Please be sure to change the command= option to the location of your mirroring script and do not remove the backgrounding character (&) after it.

The keys below are for mirrors pushed by Canonical-only, these mirrors should be syncing from rsync.archive.ubuntu.com (for packages) and rsync.releases.ubuntu.com (for CD release images). Connections from Canonical will be originating from the IP address: 91.189.88.154 and 185.125.188.80 for archive, and 91.189.88.156 and 185.125.188.81 for releases - so please ensure that your firewalls allow this.

For authenticity; these keys are signed by Haw Loeung with the key ID 0xD10ACEB3227B53FC.

If you are not being pushed by Canonical, please contact the mirror administrator of the mirror you're being pushed from for their keys and be sure to have your sync source set to their mirror.

Archive trigger key

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA384

Ubuntu Archive trigger SSH key for Canonical.

SSH key fingerprint: c8:ea:0f:db:86:da:64:86:de:76:64:b8:84:33:4b:23

no-port-forwarding,no-X11-forwarding,no-agent-forwarding,no-pty,command="~/archive-sync &",from="91.189.88.154,185.125.188.80" ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEAt8xHRbCVFT3Uw/B+TavIlDYRoLMxOKlN3HnBeniFUJTto5Im52FbT3ODfMszz5/BIAnXBf1baWDljHErx4huohh9MxyovZ0h8GYCmMy7dZzsrV5eYhLXd2idCOKIl6gr0BTgTlJOKOgVEoZ2YtiU9MnNzRk3gkBeCMDJrnQOCC8Sko0F0RUJnrzLXOdtvDfNu7Ff+tRNb4PwrU3inbm2YJRnOoZI9vIsv/9DwsMm9d+YIIOz/7y5jLGhZ34nXzhmI6cJO92+Ve5ubhbbpKUFQAh2L1PP6A+I7jHvoWHToSaZlt+DCN4Kg+JlZuf2FXk8MeHkEc6qWWHQTFF8/ArKew== archvsync@syowa
-----BEGIN PGP SIGNATURE-----

iHUEARYJAB0WIQRlU8JZ1ISAlY6/xTT0TupTmP8ZlgUCYmeDUwAKCRD0TupTmP8Z
lkBLAQCbcDkL77iDOtpildPE334qHcjaKlnMimpZ/MIkI4YgEQD/cKxEv6GnVuXD
Fz7eaN0W/O7fwpAFPLrpASseraXILwE=
=fayr
-----END PGP SIGNATURE-----

Releases trigger key

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA384

Ubuntu Releases trigger SSH key for Canonical.

SSH key fingerprint: ff:79:41:eb:c7:7d:00:d4:78:34:28:d1:d2:f0:ae:90

no-port-forwarding,no-X11-forwarding,no-agent-forwarding,no-pty,command="~/releases-sync &",from="91.189.88.156,185.125.188.81" ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEA7l6nWM6Z2KfR4KmwF29Fv9nAgTLwHM5H/TWhinl7DZDG+Jn+TC9kll3cuyGByhwh/mNTwbyvsyiDSXFtbglowQoPSW4rhOEVy6s+/lDjDBGTDsgk8wyBqlNJRlppODsl+kqX0IqAIc3XJ9luDl894tD5rxhiXzqXL3c8r8CuhPkGdUCCMbWU4OUAIjIAs8DClYzjrAZ54IVbk5gTjDYUtlSLNXjm1rZ788h65waKBn4/LV+8nEaFIPA9FxPZI6VLmKGO/RQqZrLPNKOzotmkofV1jV2OmQNHzIwu2seV6HGYqZc3U9jE2+Eat86C6IMYS7KPxVoQd6AnHjRMlhyb6Q== archvsync@syowa

-----BEGIN PGP SIGNATURE-----

iHUEARYJAB0WIQRlU8JZ1ISAlY6/xTT0TupTmP8ZlgUCYmeDvgAKCRD0TupTmP8Z
ltuCAQDnzCcZ5wTOV+Pxg+YQQihC5R64kslnjVI+CnGMzLeuzQD7B5pkoeRneVJp
JR9bXlpTBqjGCTaLVRFF4faLrLef5Qk=
=cdwz
-----END PGP SIGNATURE-----

HTTP triggers

While SSH triggers above are the preferred trigger method, it is also possible for the master Ubuntu archive to send HTTP triggers. HTTP triggers can be nearly any format, for example:

http://ubuntu-archive:8BBsmDsLXpjJvSjM4nZv@cctld.mirrors.example.com/trigger
http://ubuntu-releases:zT99CGf9V499RH9SQFtV@cctld.mirrors.example.com/trigger

or:

http://cctld.mirrors.example.com/ubuntu-archive/53cfa724-f75d-44a1-b9d2-a14e637887bc/trigger
http://cctld.mirrors.example.com/ubuntu-releases/f4b95974-9862-40ba-a476-f751c30c5f31/trigger

The first example uses usernames and (long random) passwords to a trigger destination, and would decide which component to begin syncing based on the authenticated username. The second example places the component name and (long random) UUID in the path itself. Treat these URLs as sensitive, to avoid outside users triggering unnecessary syncs.

It is up to the mirror administrator to build logic into the HTTP endpoint which authenticates the master Ubuntu archive and begins a sync. This logic varies wildly based on your specific platform. Output format is undefined; the master simply loads the URL and moves on without examining the output.