Rsync

rsync is an open source utility that provides fast incremental file transfer. rsync is freely available under the GNU General Public License and is currently being maintained by Wayne Davison.

rsync uses the "rsync algorithm" which provides a very fast method for bringing remote files into sync. It does this by sending just the differences in the files across the link, without requiring that both sets of files are present at one of the ends of the link beforehand.

Some features of rsync include

• Can update whole directory trees and filesystems
• Optionally preserves symbolic links, hard links, file ownership, permissions, devices and times
• Requires no special privileges to install
• Internal pipelining reduces latency for multiple files
• Can use rsh, ssh or direct sockets as the transport
• Supports anonymous rsync which is ideal for mirroring

Using rsync over SSH

Sometimes it's useful to do a one-off backup of a file structure from one host to another, and since all the hosts (in our system) are guaranteed to be able to connect to each other with SSH (after adding appropriate RSA keys), using rsync over SSH is a good way to do this.

The transfer syntax is then done very similarly to SCP, for example to pull new changes from a remote directory to a local one, use:

<bash>rsync -avz -e ssh remoteuser@remotehost:/remote/dir /this/dir/</bash>

After the systems are confirmed as being able to connect over SSH you may want to lock them down so that the connection between them can only be used for rsync. The IP and command can be prepended to the key in the remote hosts ~/.ssh/authorized_keys file.

from="1.2.3.4",no-agent-forwarding,no-port-forwarding,no-pty,no-user-rc,no-X11-forwarding ssh-rsa AAAAB...

For more security, the command allowed can be restricted to just that specific rsync command. This can be done by manually running the rsync command with the -e'ssh -v' option which will output the exact command sent that can be used in the remote hosts authorized_keys file instead of just "rsync".

Backing up Maildirs with Rsync

Backing up Maildirs can be a problem with many target systems (even non-Windows ones) because many filesystems don't allow colons in file names. This problem occurs for us using the ADrive backup service.

We've over come this problem using a two step solution. First we install rsync from source with the transliterate patch applied which adds a --tr=BAD/GOOD option for mapping bad characters to good ones. And then second, we use the CurlFtpFS utility which allows one to mount a remote FTP storage resource to a local mount point. This allows one to use a patched local rsync to synchronise with a remote storage facility with the colons replaced with a more suitable character such as a semicolon.

To install the patch you need to download and unpack the latest source and the patches, then change into the source directory and do the following:

<bash> patch -p1 <patches/transliterate.diff ./configure make make install </bash>

Next, the CurlFtpFS utility can be installed via apt-get on Debian-based systems, then a script run on from crontab which mounts the remote resource and synchronises the Maildirs. This example script synchronises all the home directories. It assumes that a directory called /root/adrive already exists.

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