abus 8

Abingdon Backup Script

ABuS is a script for backing up (and restoring) your files to a local disk.

The backups are encrypted, compressed, and deduplicated. It is assumed that another program (e.g. rsync) is used to make off-site copies of the backups (see below).

ABuS only backs up file content. In particular the backups do not include permissions, symbolic links, hard links, or special files.

Content of this document:

• Installation

• Documentation

  • Purging

  • Off-site copies

  • Index Database

  • Configuration file

  • Command line switches

• History

Installation

1. Install Python 3.6 from python.org

   • include pip

   • it helps to add python to path

2. From the command line, “as administrator” if python has been installed “for all users”:

   c:\path\to\python36\scripts\pip install abus

3. Create minimal config file, e.g.:

   logfile c:/my/home/abus.log
   archive e:/backups
   password password1234 just kidding!
   [include]
   c:/my/home

4. Initialise the backup directory and the index database with:

   c:\path\to\python36\scripts\abus.exe -f c:/my/home/abus.cfg --init

5. Add to Task Scheduler:

   c:\path\to\python36\scripts\abus.exe -f c:/my/home/abus.cfg --backup

   If there are any problems that prevent ABuS from getting as far as opening the log file (and Windows permissions can cause many such problems), then use cmd.exe to allow redirection:

   cmd /c
     c:\path\to\python36\scripts\abus.exe -f c:/my/home/abus.cfg --backup
     >c:\abus.err 2>&1

Documentation

Overview

ABuS is a single script for handling backups. Its command line parameters determine whether the backups are to be created, listed, or restored. The backups are stored in subdirectories of the backup directory which must be on a local filesystem. For off-site copies another program is to be used, for example rsync.

Warning: Off-site copies must be made correctly to minimise the risk of propagating any local corruption (see below).

A configuration file is used to point to the backup directory, define the backup set, and some options. ABuS finds the configuration file either via a command line parameter or an environment variable.

Purging

Old backup files are deleted after every backup. In order to determine which backups are deleted, time is divided into slots and only the latest version of a file in each slot is retained while the others are subject to purging. As slots get old they are combined into bigger slots.

The configuration file defines the slot sizes using freq/age pairs of numbers, which define that 1 version in freq days is to be retained for backups up to age days old.

For example, if the retention values are 1 7, 7 30, 28 150, then for each file one version a day is kept from the versions that are up to 7 days old, one a week is kept for versions up to 30 days old, and one every four weeks is kept up to 150 days.

There is also a single slot older than the highest age defined, called “slot 0”. In the example above one file older than 150 days will be kept as well.

Purging of deleted files

The time that a file deletion is detected (i.e. a file previously backed up no longer exists) must fall into slot 0 before the last backup of the file is purged. E.g. with default retention values, 150 days after a file is deleted its backups will be purged.

Off-site copies

ABuS only backs up to local filesystems. This means that the backups themselves are at risk of corruption, for example from ramsomware. It is important that another copy of the backup is made and that it fulfills these criteria:

• It must not be on a locally accessible filesystem or network share, so that the machine being backed up cannot corrupt it.

• Files must never be overwritten, once created, so that any local corruption does not propagate.

• As a consequence, partially transferred files must be removed at the destination.

The following is an example of an rsync command that would copy the local backup directory to an off-site location:

rsync --recursive --ignore-existing            \
      --exclude index.sl3 --exclude '*.part'   \
      /my/local/backups/  me@offsite:/backups/

index.sl3 need not be transferred because it changes and it can be rebuilt from the static files. Files with .part extension are backup files that are currently being written and will be renamed once complete. Excluding them ensures that incomplete backup files are not transferred.

Off-site purging

Since it is not advisable to propagate changed files - and therefor deletions - to the off-site copy of the backup files, these must be purged independently.

To that end ABuS creates a content file in the backup directory which lists all backup files. The content file is compressed with gzip and its file name is that of the last backup run with a .gz extension. When such a file is written, the previous one is removed. Since the run names are basically ISO dates, a script on the off-site server can easily pick up the latest and remove all backup files that are not listed in it.

N.B.: The following is only an outline of such a script to convey the idea. You must not use it without checking it first:

cd .../offsite-copy
keep_list=$(ls *.gz | tail -n 1)
(find -type f -printf '%P\n'; zcat $keep_list $keep_list) | sort | uniq -u >/tmp/remove
[[ $(wc -l /tmp/remove) -lt 50 ]] || exit # sanity check
xargs rm </tmp/remove

Index Database

The index database duplicates backup meta data for quicker access. Since it is changed during normal operation, it cannot be included in the off-site copy. There are therefore command line options to rebuild the index database from the backup files.

Important: Before rebuilding the index database, check the integrity of the content file, for example by comparing it with its off-site copy.

It is important that the index database be not rebuilt from corrupt backup data. Since the backup files are encrypted, corruption would normally show, but a missing backup file would not. The integrity of the content file (see Off-site purging above), which is not encrypted, must therefore be ascertained before rebuilding the index database.

Configuration file

The file has three sections

• parameters at the beginning

• inclusions

• exclusions

ABuS uses slashes as path separators internally. All filenames given in the config file or on the command line may use backslashes or slashes; all backslashes are converted to slashes.

Parameters

The first word of each line is a parameter name, the following words form the value. Leading and trailing spaces are trimmed while spaces within the value are preserved.

logfile

Specifies the path of a file to which all log entries are made. The parameter should be given first so that any subsequent errors in the configuration can be reported to the log.

archive

Specifies the path to the root backup directory containing all backup files.

indexdb

Specifies the path to the index database. By default this is index.sl3 inside the backup directory, but it might be preferable to place it on a faster disk, for example.

password

Specifies the encryption password to be used for all backup files. The encryption allows copying the backup archive to an off-site location.

retain

Specifies how old backups are pruned. The keyword is followed by a space-separated list of numbers forming freq and age pairs, meaning: “keep one backup per freq days for files up to age days old”. See Purging above.

The age values must not repeat and the freq values must be multiples of each other. freq can be a float, e.g. 0.25 for six hours.

The rentention values default to:

retain  1 7  56 150

Inclusions

A line containing the header [include] starts the inclusion section, each line of which is a directory path which will be backed up recursively. There must be at least one inclusion.

Exclusions

A line containing the header [exclude] starts the exclusion section, each line of which is a shell global pattern. All file paths that would be backed up (or directory paths that would be searched for files) are skipped if they match any of the patterns.

A * in the patterns also matches the directory separators. *.bak ignores any file with the extension .bak; */~* ignores any file or directory starting with a tilde.

Command line switches

Run abus --help for detailed command line switch help.

History

v8 (beta) 2017-12-10

• purges backups of deleted files (see above)

• much reduced size of index database

v7 (beta) 2017-11-19

• fix: index database on different drive caused exception at purge

• fix: restore could not handle paths from different drives

• fix: exception for u64 file numbers

v6 (beta) 2017-11-12

• retries if file changes while reading

• config file option “indexdb” to set location of index database

• improved restore performance

• progress indicators during restore

• fix: exception when no files matched during restore

v5 (beta) 2017-11-05

• feature: content files allow safe purging of off-site copies

• index database upgrades ifself on startup

• fix: spaces in filenames caused index-rebuild to fall over

v4 (alpha) 2017-10-22

• feature: purging of old backups

• fix: -a and -d options didn’t work with –list

• fix: timestamp rounding error at index-rebuild

• fix: –init could not create backup directory

v3 (alpha) 2017-10-15

• feature: rebuilding of index database from backup meta data

v2 (alpha) 2017-10-07

• not excruciatingly slow any more

v1 (alpha) 2017-10-04

• first version