Secure Automated Backups of a Linux Web Server with Rrsync and Passwordless Key Based Authentication

Backups Automated and Secure

Backing up data is an essential task, yet it can be cumbersome and requires some work. As most people are lazy and avoid tedious tasks wherever possible, automation is the key, as it allows us dealing with more interesting work instead. In this article, I describe how a Linux Web server can be backed up in a secure way by using restricted SSH access to the rsync tool. I found a great variety of useful blog posts, which I will reuse in this article.

This is what we want to achieve:

  • Secure data transfer via SSH
  • Passwordless authentication via keys
  • Restricted rsync access
  • Backup of all files by using a low privileged user

In this article, I will denote the client which should be backed up WebServer. The WebServer contains all the important data that we want to keep. The BackupServer is responsible for fetching the data in a pull manner from the WebServer.

On the BackupServer

On the BackupServer, we create a key pair without a password which we can use for authenticating with the WebServer. Details about passwordless authentication are given here.

On the WebServer

We are going to allow a user who authenticated with her private key to rsync sensitive data from our WebServer to the BackupServer, This user should have a low privileged account and still being able to backup data which belongs to other users. This capability comes with a few security threats which need to be mitigated. The standard way to backup data is rsync. The tool can be potentially dangerous, as it allows the user to write data to an arbitrary location if not handled correctly. In order to deal with this issue, a restricted version of rsync exists, which locks the usage of the tool to a declared directory: rrsync.

Obtain Rrsync

You can obtain rrsync from the developer page or extract it from your Ubuntu/Debian distribution as described here. With the following command you can download the file from the Web page and store it as executable.

Add a Backup User

First, we create a new user and verify the permissions for the SSH directory.

Create a Read Only of the Data You Want to Backup

I got this concept from this blog post. As we want to backup also data from other users, our backup user (rsync-backup) needs to have read access to this data. As we do not want to change the permissions for the rsync-backup user directly in the file system, we use bindfs to create read only view of the data we want to backup. We will create a virtual directory containing all the other directories that we want to backup. This directory is called /mnt/Backups-Rsync-Readonly . Instead of copying all the data into that directory, which would be a waste of space, we link all the other directories into the backup folder and then sync this folder to the BackupServer.

One Time Steps:

The following steps create the directory structure for the backup and set the links to the actual data that we want backup. With this method, we neither need root, sudo or any advanced permissions. We simply create a readonly view of the data where the only user with access is rsync-backup.

Testrun

In order to use these directories, we need to mount the folders. We set the permissions for bindfs and establish the link between the data and our virtual backup folders.

These commands mount the data directories and create a view. Note that these commands are only valid until you reboot. If the above works and the rsync-backup user can access the folder, you can add the mount points to fstab to automatically mount them at boot time. Unmount the folders before you continue with sudo umount /mnt/Backups-Rsync-Readonly/* .

Permanently Add the Virtual Folders

You can add the folders to fstab like this:

Remount the directories with sudo mount -a .

Adding the Keys

In the next step we add the public key from the BackupServer to the authorized_keys file from the rsync-backup user at the WebServer. On the BackupServer, cat the public key and copy the output to the clipboard.

Switch to the WebServer and login as rsync-backup user. Then add the key to the file ~/.ssh/authorized_keys.
The file now looks similar like this:

We then prepend the key with the only command this user should be able to execute: rrsync. We add additional limitations for increasing the security of this account. We can provide an IP address and limit the command execution further. The final file contains the following information:

Now whenever the user rsync-backup connects, the only possible command is rrsync. Rrsync itself is limited to the directory provided and only has read access. We also verify the IP address and restrict the source of the command.

Hardening SSH

Additionall we can force the rsync-backup user to use the keybased authentication only. Additionally we set the IP address restriction for all SSH connections in the sshd_config as well.

Backing Up

Last but not least we can run the backup. To start synching we login into the BackupServer and execute the following command. There is no need to provide paths as the only valid path is already defined in the authorized_key file.

Conclusion

This article covers how a backup user can create backups of data owned by other users without having write access to the data. The backup is transferred securely via SSH and can run unattended. The backup user is restricted to using rrsync only and we included IP address verification. The backup user can only create backups of directories we defined earlier.

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