Automating Remote Backups
Linux users are a diverse group because of the wide swath of choices they have at their fingertips. But, whether they choose Ubuntu, Fedora or Debian, or KDE, GNOME or Xfce, they all have one thing in common: a lot of data. Losing data through hard disk failure or simply by overwriting is something all users must face at some point. Yet, these are not the only reasons to do backups. With a little planning, backups are not nearly as hard as they might seem.
Hard disk prices have dropped to the point where USB storage easily replaces the need for off-line tape storage for the average user. Pushing your data nightly to external USBs, either local or remote, is a fairly inexpensive and simple process that should be part of every user's personal system administration.
In this article, I describe a process for selecting files to back up, introduce the tools you'll need to perform your backups and provide simple scripts for customizing and automating the process. I have used these processes and scripts both at home and at work for a number of years. No special administrative skills are required, although knowledge of SSH will be useful.
Before proceeding, you should ask yourself the purpose of the backup. There are two reasons to perform a backup. The first is to recover a recent copy of a file due to some catastrophic event. This type of recovery makes use of full backups, where only a single copy of each file is maintained in the backup archive. Each file that is copied to the archive replaces the previous version in the archive.
This form of backup is especially useful if you partition your system with a root partition for the distribution of choice (Fedora, Ubuntu and so forth) and a user partition for user data (/home). With this configuration, distribution updates are done with re-installs instead of upgrades. Installing major distributions has become fairly easy and nearly unattended. Re-installing using a separate root partition allows you to wipe clean the old installation without touching user data. All that is required is to merge your administrative file backups—a process made easier with tools like meld (a visual diff tool).
The second reason to perform a backup is to recover a previous version of a file. This type of recovery requires the backup archive to maintain an initial full backup and subsequent incremental changes. Recovery of a particular version of a file requires knowing the time between when the full backup was performed and the date of the version of the file that is desired in order to rebuild the file at that point. Figure 1 shows the full/incremental backup concepts graphically.
Incremental backups will use up disk space on the archive faster than full backups. Most home users will be more concerned with dealing with catastrophic failure than retrieving previous versions of a file. Because of this, home users will prefer full backups without incremental updates, so this article focuses on handling only full backups. Fortunately, adding support for incremental backups to the provided scripts is not difficult using advanced features of the tools described here.
In either case, commercial environments often keep backups in three locations: locally and two remote sites separated by great distance. This practice avoids the possibility of complete loss of data should catastrophe be widespread. Home users might not go to such lengths, but keeping backups on separate systems, even within your home, is highly recommended.
The primary tool for performing backups on Linux systems is rsync. This tool is designed specifically for handling copying of large numbers of files between two systems. It originally was designed as a replacement for rcp and scp, the latter being the file copy tool provided with OpenSSH for doing secure file transfers.
As a replacement for scp, rsync is able to utilize the features provided by OpenSSH to provide secure file transfers. This means a properly installed SSH configuration can be utilized when using rsync. In fact, SSH transfers are used by default using standard URI formats for source or destination files (such as user@host:/path). Alternatively, rsync provides a standalone server that rsync clients can connect to for file transfers. To use the rsync server, use a double colon in the URI instead of a single colon.
SSH (secure shell), is a client/server system for performing operations across a network using encrypted data. This means what you're transferring can't be identified easily. SSH is used to log in securely to remote Linux systems, for example. It also can be used to open a secure channel, called a tunnel, through which remote desktop applications can be run and displayed on the local system.
SSH configuration can be fairly complex, but fortunately, it doesn't have to be. For use with rsync, configure the local and remote machines for the local machine to log in to the remote machine without a password. To do this, on the local machine, change to $HOME/.ssh and generate a public key file:
$ cd $HOME/.ssh $ ssh-keygen -t dsa
ssh-keygen will prompt you for various information. For simplicity's sake, press Enter to take the default for each prompt. For higher security, read the ssh-keygen and ssh man pages to learn what those prompts represent.
ssh-keygen generates two files, id_dsa and id_dsa.pub. The latter file must be copied to the remote system under $HOME/.ssh and appended to the file $HOME/.ssh/authorized_keys. In this code, remoteHost is the name of the remote computer and localHost is the name of the local computer:
$ scp id_dsa.pub \ remoteHost:$HOME/.ssh/id_dsa.pub.localHost $ ssh remoteHost $ cd $HOME/.ssh $ cat id_dsa.pub.localHost >> authorized_keys
In this article, I assume a proper SSH configuration with no password required in order to perform the rsync-based backups. These automated backup scripts are intended to be run from cron and require a proper SSH configuration.
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