backup.sh

backup.sh is a POSIX compliant, modular and lightweight backup utility to save and encrypt your files. This tool is intended to be used on small scale UNIX environments such as VPS, personal servers and workstations. backup.sh uses rsync, tar, gpg and sha256sum to copy, compress, encrypt the backup and verify the backup.

Installation

backup.sh consists in a single source file, to install it you can copy the script wherever you want. Alternatively, you can install the script, the default sources file and the man file using the following command:

$> sudo make install

This will copy backup.sh into /usr/local/bin/backup.sh, sources.bk into /usr/local/etc/sources.bk and backup.sh.1 into /usr/share/man/man1/backup.sh.1. To uninstall the program along with the sample sources file and the manual page, you can issue sudo make uninstall.

At this point you still need to install the following dependencies:

• Bash
• rsync
• tar
• gpg

Usage

To show the available options, you can run backup.sh --help, which will print out the following message:

backup.sh - POSIX compliant, modular and lightweight backup utility.

Syntax: ./backup.sh [-b|-c|-e|-h]
options:
-b|--backup   SOURCES DEST PASS  Backup folders from SOURCES file.
-c|--checksum                    Generate/check SHA256 of a backup.
-e|--extract  ARCHIVE PASS       Extract ARCHIVE using PASS.
-h|--help                        Show this helper.

General help with the software: https://github.com/ceticamarco/backup.sh
Report bugs to: Marco Cetica(<email@marcocetica.com>)

As you can see, backup.sh supports three options: backup creation, backup extraction and checksum to verify the integrity of a backup. The first option requires root permissions, while the second one does not. The checksum option must be used in combination of one of the previous options.

Backup creation

To specify the directories to back up, backup.sh uses an associative array defined in a text file(called sources file) with the following syntax:

<LABEL>=<PATH>

Where <LABEL> is the name of the backup and <PATH> is its path. For example, if you want to back up /etc/nginx and /etc/ssh, add the following entries to the sources file:

nginx=/etc/nginx/
ssh=/etc/ssh/

backup.sh will create two folders inside the backup archive with the following syntax:

backup-<LABEL>-<YYYYMMDD>

In the previous example, this would be:

backup-nginx-<YYYYMMDD>
backup-ssh-<YYYYMMDD>

You can add as many entries as you want, just be sure to use the proper syntax. In particular, the sources file, should not include:

• Spaces between the label and the equal sign;
• Empty lines;
• Comments.

You can find a sample sources file at sources.bk(or at /usr/local/etc/sources.bk).

After having defined the sources file, you can invoke backup.sh using the following syntax:

$> sudo ./backup.sh --backup <SOURCES_FILE> <DEST> <ENCRYPTION_PASSWORD>

Where <SOURCES_FILE> is the sources file, <DEST> is the absolute path of the output of the backup without trailing slashes and <ENCRYPTION_PASSWORD> is the password to encrypt the compressed archive.

In the previous example, this would be:

$> sudo ./backup.sh --backup sources.bk /home/john badpw1234

You can also tell backup.sh to generate a SHA256 file containing the hash of each file using the -c option. In the previous example, this would be:

$> sudo ./backup.sh --checksum --backup sources.bk /home/john badpw1234

The backup utility will begin to copy the files defined in the sources file:

Copying nginx(1/2)
Copying ssh(2/2)
Compressing backup...
Encrypting backup...
File name: /home/john/backup-<HOSTNAME>-<YYYYMMDD>.tar.gz.enc
Checksum file: /home/john/backup-<HOSTNAME>-<YYYYMMDD>.sha256
File size: 7336400696(6.9G)
Elapsed time: 259 seconds.

After that, you will find the backup archive and the checksum file in /home/john/backup-<HOSTNAME>-<YYYYMMDD>.tar.gz.enc and /home/john/backup-<HOSTNAME>-<YYYYMMDD>.sha256, respectively.

You can also use backup.sh from a crontab rule:

$> sudo crontab -e
30 03 * * 6 EKEY=$(cat /home/john/.ekey) bash -c '/usr/local/bin/backup.sh -b /usr/local/etc/sources.bk /home/john $EKEY' > /dev/null 2>&1

This will automatically run backup.sh every Saturday morning at 03:30 AM. In the example above, the encryption key is stored in a local file(with fixed permissions) to avoid password leaking in crontab logs. You can also adopt this practice while using the --extract option to avoid password leaking in shell history.

Backup extraction

backup.sh can also be used to extract and to verify the encrypted backup.
To do so, use the following commands:

$> ./backup.sh --extract <ENCRYPTED_ARCHIVE> <ARCHIVE_PASSWORD>

Where <ENCRYPTED_ARCHIVE> is the encrypted backup and <ARCHIVE_PASSWORD> is the backup password.

For instance:

$> ./backup.sh --extract backup-<hostname>-<YYYYMMDD>.tar.gz.enc badpw1234

This will create a new folder called backup.sh.tmp in your local directory with the following content:

backup-nginx-<YYYYMMDD>
backup-ssh-<YYYYMMDD>

note: be sure to rename any directory with that name to avoid collisions.

If you also want to verify the integrity of the backup data, use the following commands:

$> ./backup.sh --checksum --extract <ENCRYPTED_ARCHIVE> <ARCHIVE_PASSWORD> <CHECKSUM_ABSOLUTE_PATH>

For instance:

$> ./backup.sh --checksum --extract backup-<hostname>-<YYYYMMDD>.tar.gz.enc badpw1234 backup-<hostname>-<YYYYMMDD>.sha256

How does backup.sh work?

backup.sh uses rsync to copy the files, tar to compress the backup, gpg to encrypt it and sha256sum to verify it. By default, rsync is being used with the following parameters:

$> rsync -aPhrq --delete

That is:

- a: archive mode: rsync copies files recursively while preserving as much metadata as possible;  
- P: progress/partial: allows rsync to resume interrupted transfers and to shows progress information;  
- h: human readable output, rsync shows output numbers in a more readable way;  
- r: recursive mode: forces rsync to copy directories and their content;  
- q: quiet mode: reduces the amount of information rsync produces;  
- delete: delete mode: forces rsync to delete any extraneous files at the destination dir.

If specified(--checksum option), backup.sh can also generate the checksum of each file of the backup. To do so, it uses sha256sum(1) to compute the hash of every single file using the SHA256 hashing algorithm. The checksum file contains nothing but the checksums of the files, no other information about the files stored on the backup archive is exposed on the unencrypted checksum file. This may be an issue if you want plausible deniability(see privacy section for more information).

After that the backup folder is being encrypted using gpg. By default, it is used with the following parameters:

$> gpg -a \
        --symmetric \
        --cipher-algo=AES256 \
        --no-symkey-cache \
        --pinentry-mode=loopback \
        --batch --passphrase "$PASSWORD" \
        --output "$OUTPUT" \
        "$INPUT"

This command encrypts the backup using the AES-256 symmetric encryption algorithm with a 256bit key. Here is what each flag do:

• --symmetric: Use symmetric encryption;
• --cipher-algo=AES256: Use AES256 algorithm;
• --no-symkey-cache: Do not save password on GPG's cache;
• --pinentry-mode=loopback --batch: Do not prompt the user;
• --passphrase-fd 3 3<< "$PASSWORD": Read password without revealing it on ps;
• --output: Specify output file;
• $INPUT: Specify input file.

Plausible Deniability

While backup.sh provide some pretty strong security against bruteforce attack(assuming a strong passphrase is being used) it should by no means considered a viable tool against a cryptanalysis investigation. Many of the copying, compressing and encrypting operations made by backup.sh during the backup process can be used to invalidate plausible deniability. In particular, you should pay attention to the following details:

1. The --checksum option generates an UNENCRYPTED checksum file containing the digests of EVERY file in your backup archive. If your files are known to your adversary(e.g., a banned book), they may use a rainbow table attack to determine whether you own a given file, voiding your plausible deniability;
2. Since backup.sh is essentially a set of shell commands, an eavesdropper could monitor the whole backup process to extract the name of the files or the encryption password.

Unit tests

backup.sh provides some unit tests inside the tests.sh script. This script generates some dummy files inside the following directories:

• /var/log
• /var/www
• /etc/nginx
• /etc/ssh

For this reason, this script should NOT be used in non-testing environments. To run all tests, issue the following command:

$> sudo ./tests.sh I_HAVE_READ_THE_HELPER

License

This software is released under GPLv3, you can obtain a copy of this license by cloning this repository or by visiting this page.