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:
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.
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:
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.
The first line mounts the adrive storage resource to the local directory /root/adrive using the FTP protocol. The second then performs an rsync synchronisation of the local home directories to the local mount point ensuring that all colons will be replaced semicolons. And the final line unmounts adrive from the local mount point after the rsync command has finished running. The --inplace option on the rsync command is required to prevent the fatal error that would otherwise occur when it tries to execute the mkstemp commands. It will also issue errors when trying to set the ownership of the target directories, but these are just non-fatal warnings.
