An Automated Reliable Backup Solution
After setting up SSH key authentication, I created a GnuPG key that Duplicity would use to sign and encrypt the backups. I created a key as my normal user on the client machine. Having the GnuPG key associated with a normal user account prevents backing up the entire filesystem. If I decided at some point that I wanted to back up the entire filesystem, I simply would create a GnuPG key as the root user on the client machine. To generate a GPG key, I used the following command:
$ gpg --gen-key
Once both the GnuPG and SSH keys were created, the first thing I did was make a CD containing copies of both my SSH and GnuPG keys. Then I installed and set up Keychain. Keychain is an application that manages long-lived instances of ssh-agent and gpg-agent to provide a mechanism that eliminates the need for password entry for every command that requires either the GnuPG or SSH keys. On a Debian client machine, I first had to install the keychain and ssh-askpass packages. Then I edited the /etc/X11/Xsession.options file and commented out the use-ssh-agent line so that the ssh-agent was not started every time I logged in with an Xsession. Then I added the following lines to my .bashrc file to start up Keychain properly:
/usr/bin/keychain ~/.ssh/id_dsa 2> /dev/null source ~/.keychain/`hostname`-sh
After that, I added an xterm instantiation to my gnome-session so that an xterm in turn starts an instance of bash, which reads in the .bashrc file and runs Keychain. When Keychain is executed, it checks to see whether the key is already cached; if it is not, it prompts me once for my key passwords every time I start my computer and log in.
Once Keychain was installed and configured, I was able to make unattended backups of directories simply by configuring cron to execute Duplicity. I backed up my home directory with the following command:
$ duplicity --encrypt-key AA43E426 \ --sign-key AA43E426 /home/username \ scp://user@backup_serv/backup/home
After backing up my home directory, I verified the backup with the following command:
$ duplicity --verify --encrypt-key AA43E426 \ --sign-key AA43E426 \ scp://user@backup_serv/backup/home \ /home/username
Suppose that I accidentally removed my home directory on my client machine. To recover it from the backup server, I would use the following command:
$ duplicity --encrypt-key AA43E426 \ --sign-key AA43E426 \ scp://user@backup_serv/backup/home \ /home/username
However, my GnuPG and SSH keys are normally stored in my home directory. Without the keys I cannot recover my backups. Hence, I first recovered my GPG and SSH keys from the CD on which I previously saved my keys.
This solution also provides the capability of cleaning up files on the backup server for a specified date and time. Given this capability, I also added the following command to my cron tab to remove any backups more than two months old:
$ duplicity --remove-older-than 2M \ --encrypt-key AA43E426 --sign-key AA43E426 \ scp://user@backup_serv/backup/home \ /home/username
This command conserves disk space, but it limits how far back I can recover data.
This solution has worked very well for me. It provides the key functionality that I need and meets all of my requirements. It is not perfect, however. Duplicity currently does not support hard-links; it treats them as individual files. Hence, in a backup recovery that contains hard-links, individual files are produced rather than one file with associated hard-links.
Despite Duplicity's lack of support for hard-links, this is still my choice of backup solution. It seems that development of Duplicity has recently picked up, and maybe this phase of development will add hard-link support. Maybe I will find the time to add this support myself. Either way, this provides an unattended, encrypted, redundant network backup solution that takes very little money or effort to set up.
Andrew J. De Ponte is a security professional and avid software developer. He has worked with a variety of UNIX-based distributions since 1997 and believes the key to success in general is the balance of design and productivity. He awaits comments and questions at email@example.com.
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