What is better than a [Pi-hole](https://github.com/pi-hole/pi-hole) blocking ads via DNS on your network? That's right, Two Pi-hole! (Redundency is key in any network infrastucture.) But if you have more than one Pi-hole in your network you'll want a simple way to keep the list configurations and local DNS settings identical between the two. That's where Gravity Sync comes in.
- Sync the Adlist database (allow/block list) configurations stored in `gravity.db` between multiple Pi-hole.
- Sync the Local DNS Settings stored in `custom.list` between multiple Pi-hole.
- Provide an easy way to keep this happening in the background.
Ideally you set up Gravity Sync and forget about it -- and in the long term, it would be awesome if the Pi-hole team made this entire script unnecessary.
Gravity Sync is not developed by or affiliated with the Pi-hole project. This is a community effort that seeks to implement replication, which is currently not a part of the core Pi-hole product. The code has been well tested across multiple user environments but there always is an element of risk involved with running any arbitrary software you find on the Internet.
- Pi-hole 5.0 (or higher) must already be installed on at least two systems, using any of the Linux distribution that Pi-hole is [certified to run on](https://docs.pi-hole.net/main/prerequesites/#supported-operating-systems).
- While it is possible to leverage container/Docker deployments of Pi-hole and Gravity Sync, this configuration is currently not officially supported. Instructions here assume a "native" installation of Pi-hole.
- You will need to make sure that you have password-less `SUDO` enabled for the user accounts on both the primary and secondary Pi-hole. Most of the pre-built images available for the Raspberry Pi already have this configured, but if you have your Pi-hole running in a virtual machine built from a generic ISO, you may need to [adjust this manually](https://linuxize.com/post/how-to-run-sudo-command-without-password/).
- Make sure `SSH` and `RSYNC` are installed on both the primary and secondary Pi-hole prior to installation. These two binaries are what does the heavy lifting between your Pi-hole nodes. In the past, Dropbear was supported but this has proven problematic. If you're using a ultra-lightweight Pi distribution (such as DietPi) that uses Dropbear by default, you will need to convert to OpenSSH as of Gravity Sync version 2.2.
You will want to designate one Pi-Hole as primary and at least one as secondary.
- The primary Pi-hole is where you'll make most of your configuration changes through the Web UI, doing things such as; manual allow-listing, adding block-lists, device/group management, configuring custom/local network DNS, and changing other list settings.
- The secondary Pi-hole(s) are where you will install and configure Gravity Sync.
For more information and for reference architectures, please [refer to this document](https://github.com/vmstan/gravity-sync/blob/master/ADVANCED.md#reference-architectures)
Starting with version 2.0, Gravity Sync will attempt to sync the Adlist database and Local DNS Settings on each Pi-hole regardless of where the data was changed. Previous versions only pulled data one way as the standard operation.
- Perform a backup of the existing Pi-hole database.
The configuration will be saved as `gravity-sync.conf` in the same folder as the script. If you need to make adjustments to your settings in the future, you can edit this file or run the configuration tool to generate a new one.
Now, test Gravity Sync. You can run a comparison between primary and secondary databases, which will be non-disruptive, and see if everything has been configured correctly.
Assuming Gravity Sync runs successfully, it will indicate if there are changes pending between the two databases. If not, make a subtle change to a allow/block list on your primary Pi-hole, such as changing a description field or disabling a allow list item, and then running `./gravity-sync.sh compare` again to validate your installation is working correctly.
Gravity Sync will perform some checks to help ensure success and then stop before making changes if it detects an issue. It will also perform the same `compare` function outlined above, and if there are no changes pending, it will exit without making an attempt to copy data.
**Example:** If the `gravity.db` has been modified on the primary Pi-hole, but the `custom.list` file has been changed on the secondary, Gravity Sync will now do a pull of the `gravity.db` then push `custom.list` and finally restart the correct components on each server. It will also now only perform a sync of each component if there are changes within each type to replicate. So if you only make a small change to your Local DNS settings, it doesn't kickoff the larger `gravity.db` replication.
This allows you to be more flexible in where you make your configuration changes to block/allow lists and local DNS settings being made on either the primary or secondary, but it's best practice to continue making changes on one side where possible. In the event there are configuration changes to the same element (example, `custom.list` changes at both sides) then Gravity Sync will attempt to determine based on timestamps on what side the last changed happened, in which case the latest changes will be considered authoritative and overwrite the other side. Gravity Sync does not merge the contents of the files when changes happen, it simply overwrites the entire content.
If the execution completes, you will now have a synchronized copy of your running `gravity.db` and `custom.list` on the both Pi-hole after creating a time-stamped copy of the running files (with `.backup` appended) in the `backup` subfolder located with your script, on the secondary Pi-hole.
Finally, a file called `gravity-sync.log` will be created in the `gravity-sync` folder along side the script with the date the script was last executed appended to the bottom.
Automation of sync is accomplished by adding an execution of the script to the user's crontab file. As Gravity Sync won't make any changes if it doesn't detect a difference to sync, then the impact should be minor to your systems.
```bash
./gravity-sync.sh automate
```
Select the frequency per hour (in minutes) that you'd like to sync and that's it.
Now, make another small adjustment to your primary settings and wait until anointed time to see if your changes have been synchronized. If so, profit! If not, start from the beginning. From this point forward any block list changes you make to the primary will reflect on the secondary within the frequency you select.
If you'd like to see the log of what was run the last crontab, you can view that output by running:
```bash
./gravity-sync.sh cron
```
Keep in mind if your cron task has never run, you will not see any valid output from this command.
### Adjusting Automation
You can verify your existing automation entry by running `crontab -l` and see it listed at the bottom of the crontab file. If you decide to remove or change your frequency (as of version 1.8.3) you can run `./gravity-sync.sh automate` again and pick a new timing, including setting it to 0 to disable automation.
If you'd like to know what version of the script you have running, check the built in version checker. It will notify you if there are updates available.
You can then run the built-in updater to get the latest version of all the files. Both the `version` and `update` commands reach out to GitHub, so outbound access to GitHub.com is required.
Your copy of the `gravity-sync.conf` file, logs and backups should not be be impacted by this update, as they are specifically ignored. The main goal of Gravity Sync is to be simple to execute and maintain, so any additional requirements should also be called out when it's executed. After updating, be sure to manually run a `./gravity-sync.sh compare` or `./gravity-sync.sh pull` to validate things are still working as expected.
- If the update script fails, make sure you did your original deployment via `git clone` and not a manual install. Refer to [ADVANCED.md](https://github.com/vmstan/gravity-sync/blob/master/ADVANCED.md) for more details.
This function will totally wipe out your existing Gravity Sync installation and reset it to the default state for the version you are running. If all troubleshooting of a bad installation fails, this is the command of last resort.
**This will not impact any of the Pi-hole binaries, configuration files, directories, services, etc.** Your Adlist database and Local Custom DNS records will no longer sync, but they will be in the status they were when Gravity Sync was removed.
