rsync

rsync

Original author(s)	Andrew Tridgell, Paul Mackerras
Developer(s)	Wayne Davison
Initial release	June 19, 1996 (1996-06-19)[1]
Stable release	3.0.7  (December 31, 2009; 8 months ago (2009-12-31))[2] [+/−]
Platform	Unix
Type	Data transfer/ differential backup
License	GNU General Public License
Website	http://rsync.samba.org

rsync is a software application for Unix systems which synchronizes files and directories from one location to another while minimizing data transfer using delta encoding when appropriate. An important feature of rsync not found in most similar programs/protocols is that the mirroring takes place with only one transmission in each direction. rsync can copy or display directory contents and copy files, optionally using compression and recursion.

In daemon mode, rsync listens on the default TCP port of 873, serving files in the native rsync protocol or via a remote shell such as RSH or SSH. In the latter case, the rsync client executable must be installed on both the local and the remote host.

Released under the GNU General Public License, rsync is free software. It is widely used.^[3][4][5][6]

[edit] Algorithm

The rsync utility uses an algorithm invented by the Australian computer programmer Andrew Tridgell for efficiently transmitting a structure (such as a file) across a communications link when the receiving computer already has a different (but similar) version of the same structure.

The recipient splits its copy of the file into fixed-size non-overlapping chunks and computes two checksums for each chunk: the MD4 hash, and a weaker 'rolling checksum'. It sends these checksums to the sender. Version 30 of the protocol (released with rsync version 3.0.0) now uses MD5 hashes rather than MD4.^[7]

The sender computes the rolling checksum for every chunk of size S in its own version of the file, even overlapping chunks. This can be calculated efficiently because of a special property of the rolling checksum: if the rolling checksum of bytes n through n + S − 1 is R, the rolling checksum of bytes n + 1 through n + S can be computed from R, byte n, and byte n + S without having to examine the intervening bytes. Thus, if one had already calculated the rolling checksum of bytes 1–25, one could calculate the rolling checksum of bytes 2–26 solely from the previous checksum, and from bytes 1 and 26.

The rolling checksum used in rsync is based on Mark Adler's adler-32 checksum, which is used in zlib, and which itself is based on Fletcher's checksum.

The sender then compares its rolling checksums with the set sent by the recipient to determine if any matches exist. If they do, it verifies the match by computing the hash for the matching block and by comparing it with the hash for that block sent by the recipient.

The sender then sends the recipient those parts of its file that did not match the recipient's blocks, along with information on where to merge these blocks into the recipient's version. This makes the copies identical. However, there is a small probability that differences between chunks in the sender and recipent are not detected, and thus remains uncorrected. This requires a simultaneous hash collision in MD5 and the rolling checksum. It is possible to generate MD5 collisions, and the rolling checksum is not cryptographically strong, but the chance for this to occur by accident is nevertheless extremely remote. With 128 bits from MD5 plus 32 from the rolling checksum, and assuming maximum entropy in these bits, the possibility of a hash collision with this combined checksum is 2^−(128+32) = 2⁻¹⁶⁰. The actual possibility is a few times higher, since good checksums approach maximum output entropy but very rarely achieve it.

If the sender's and recipient's versions of the file have many sections in common, the utility needs to transfer relatively little data to synchronize the files.

While the rsync algorithm forms the heart of the rsync application that essentially optimizes transfers between two computers over TCP/IP, the rsync application supports other key features that aid significantly in data transfers or backup. They include compression and decompression of data block by block using zlib at sending and receiving ends, respectively, and support for protocols such as ssh that enables encrypted transmission of compressed and efficient differential data using rsync algorithm. Instead of ssh, stunnel can also be used to create an encrypted tunnel to secure the data transmitted.

Finally, rsync is capable of limiting the bandwidth consumed during a transfer, a useful feature that few other standard file transfer protocols offer.

[edit] Uses

rsync was originally written as a replacement for rcp and scp. As such, it has a similar syntax to its parent programs.^[8] Like its predecessors, it still requires a source and a destination to be specified, one of which may be remote. Because of the flexibility, speed and script-ability of rsync, it has become popular with many system administrators. This has resulted in rsync being ported to Windows, Mac and Linux operating systems.

Possible uses:

rsync [OPTION] … SRC [SRC] … [USER@]HOST:DEST
rsync [OPTION] … [USER@]HOST:SRC [DEST]

One of the earliest applications of rsync was to implement mirroring or backup for multiple Unix clients onto a central Unix server using rsync/ssh and standard Unix accounts.

With a scheduling utility such as cron, one can even schedule automated encrypted rsync-based mirroring between multiple host computers and a central server.

An alternative to scripting rsync is using an Libre Software (FLOSS) GUI program like BackupPC, which uses rsync to perform automatic, scheduled backups of Linux and PCs to other rsync servers.

[edit] Variations

A utility called rdiff uses the rsync algorithm to generate delta files with the difference from file A to file B (like the utility diff, but in a different delta format). The delta file can then be applied to file A, turning it into file B (similar to the patch utility).

Unlike diff, the process of creating a delta file has two steps: first a signature file is created from file A, and then this (relatively small) signature and file B is used to create the delta file. Also unlike diff, rdiff works well with binary files.

Using rdiff, a utility called rdiff-backup has been created, capable of maintaining a backup mirror of a file or directory either locally or remotely over the network, on another server. rdiff-backup stores incremental rdiff deltas with the backup, with which it is possible to recreate any backup point.

duplicity is a variation on rdiff-backup that allows for backups without cooperation from the storage server, as with simple storage services like Amazon S3. It works by generating the hashes for each block in advance, encrypting them, and storing them on the server, then retrieving them when doing an incremental backup. The rest of the data is also stored encrypted for security purposes.

rsyncrypto is a utility to encrypt files in an rsync-friendly fashion. The rsyncrypto algorithm ensures that two almost identical files, such as the same file before and after a change, when encrypted using rsyncrypto and the same key, will produce almost identical encrypted files. This allows for the low-overhead data transfer achieved by rsync while providing encryption for secure transfer and storage of sensitive data in a remote location.

[edit] GUIs

[edit] Linux

GUIs for rsync are: GADMIN RSYNC (as part of Gadmintools), Grsync and QtdSync (since v0.6.11).

[edit] Windows

Windows GUIs with client and server support are DeltaCopy or QtdSync. Another (client only) implementation is RsyncBackup. Note: RsynBackup's website & installer are in German, however the program itself is in English only.

[edit] Mac OS X

On Mac OS X there is Backuplist+, RipCord Backup, RsyncX and the Open-Source software arRsync and Grsync.

[edit] History

Andrew Tridgell and Paul Mackerras wrote the original rsync. Tridgell discusses the design, implementation and performance of rsync in chapters 3 through 5 of his Australian National University PhD thesis^[9].

rsync was first announced on 19 June 1996.^[1]

Rsync 3.0 was released on 1 March 2008.^[10]

[edit] See also

Free software portal

• CVSup
• Unison (file synchronizer)
• Zsync, (client-side implementation of rsync algorithm for file transfer)
• Grsync, Graphical User Interface (GUI) for rsync
• DeltaCopy, Windows rsync
• Remote Differential Compression

[edit] References

[edit] External links

• rsync homepage
• rsync algorithm
• Using rsync on windows
• Official rdiff-backup site
• rsyncrypto home page
• QtdSync download

[edit] Tutorials

• rsync tutorial
• Easy to follow rsync tutorial
• Installing Rsync on Linux and Windows Servers

[edit] Examples

• Official rsync examples
• A short practical example
• Unified script for easy rsync & rdiff backup