Encrypted cloud backups with Duplicity

Safe in the Cloud

If you want to use Ubuntu One, you need an active session of X to use Duplicity, because a window appears with the password prompt. This option is thus primarily of interest to users of the Déjà Dup graphical interface. Ubuntu One sets a target folder relative to the user's home directory for the path specified in Duplicity.

If you use Amazon's S3 cloud storage (again, a free usage tier is available), you first need to create a user account, wait for your credentials, and create what is known as a bucket. Duplicity requires the name of this bucket as a path. If it is on Amazon's European availability zone (AZ), Duplicity also expects the parameters --s3-use-new-style and --s3-european-buckets.

To test the backup mechanism first, you can set the -v8 and -v9 switches to elicit more detailed information from Duplicity. If you will be using an SSH server or Ubuntu One for your backups, you also need the python-paramiko package; users of S3 additionally need to install python-boto.

The passwords are passed in as environment variables in Listing 1 and deleted again after the process. Duplicity expects FTP and SSH passwords in the FTP_PASSWORD variable. If you prefer an unencrypted backup, you can use the --no-encryption parameter.

Fine Tuning

If you do not want to back up all the home directories (Listing 1), you can exclude files from the backup. The parameters --include=${SOURCE}/carl, --include=${SOURCE}/lenny, and --exclude='**' ensure that only the home directories belonging to carl and Lenny are backed up.

Duplicity also saves files in the form of volumes with a default size of 26MB. If your backup is somewhere in the gigabyte range, this action will quickly produce an impressive collection of files. You might want to change the volume size to approximately 250MB using --volsize 250, but you should use this option with caution because it requires a huge amount of main memory.

Check It Out

In the next step, Duplicity checks to see whether the signatures of the backed up files match those of the originals. If they do not, the software displays an error message. You can evaluate this with the script here (-v increases the verbosity level of verify):

$ duplicity verify -v4 --s3-use-new-style --s3-european-buckets --encrypt-key ${GPG_KEY} ${AMAZON_S3} ${SOURCE}

The Duplicity user does not need to enter a GPG password here because the software typically retrieves it from .cache/duplicity. You can output the report to a file (--log-file), which can be mailed to you or sent by sync software in the event of an irregularity. Thus, the backup creator is always notified if a backup fails and can then take any necessary steps.

At this point, you might also consider setting up a routine that regularly checks the free or used space, or both, on the external storage and reports bottlenecks.

If you want to retrieve a specific file from the backup, you can view all of the files from the last backup:

$ duplicity list-current-files --s3-use-new-style --s3-european-buckets --encrypt-key ${GPG_KEY} {AMAZON_S3}

If you want the list to show you the state at a specific point in time, you can set the --time parameter, which expects a time in date-time format [3] (e.g., 1997-07-16T19:20:30+01:00) or as YYYY/MM/DD.

This parameter comes into play when restoring files:

$ duplicity --time 2013/06/08 --s3-use-new-style --s3-european-buckets --file-to-restore path/to/<sourcefile> ${AMAZON_S3} path/to/<targetfolder>/<sourcefile>

Note that the --time parameter must come before the restore command. In addition to the recovery path, you must define the name of the source file; the first path refers to the source file in the Amazon bucket.

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