zsync is a file transfer program. It allows you to download a file from a web server, where you have an older version of the file on your computer already. zsync downloads only the new parts of the file. It uses the same algorithm as rsync.
zsync does not require any special server software or a shell account on the remote system at download time (rsync requires that you have an rsh or ssh account, or that the remote system runs rsyncd). Instead, it uses a control file - a .zsync file - that describes the file to be downloaded and enables zsync to work out which blocks it needs. This file can be created by the admin of the web server hosting the download, and placed alongside the file to download - it is generated once, then any downloaders with zsync can use it. Alternatively, anyone can download the file, make a .zsync and provide it to other users.
The zsync web site is at http://zsync.moria.org.uk/ .
zsync is free software. There is no implied support, no implied fitness for purpose, no warranty. You use it at your own risk. See the included LICENSE for details.
To build zsync:
go build -o zsync ./cmd/zsync
go build -o zsyncmake ./cmd/zsyncmake
You can use zsync and zsyncmake without installing them. If you want to
install them then, as root, run:
install zsync zsyncmake /usr/local/bin
install -D man/zsync.1 man/zsyncmake.1 /usr/local/man/man1/
In its simplest form, as an end-user:
zsync https://cdimage.ubuntu.com/ubuntu/daily-live/current/resolute-desktop-amd64.iso.zsync
Someone has to make a .zsync file for the download before you can use zsync.
You have to have an older version of the file around - or a related file that contains a lot of the same content - otherwise there is little point in using zsync. zsync normally looks in the current directory for a file of the same name as the one being downloaded to use as a source of data. If your local older copy is in a different directory, or you have another file with relevant source data for zsync to use, you can specify it with -i.
Simple example:
Suppose you have http://example.com/dl/some-image-0.2.iso ; which is in
/var/www/downloads/ on your server.
cd /var/www/downloads/
zsyncmake some-image-0.2.iso
This creates some-image-0.2.iso.zsync in the same directory, a zsync control file. A zsync user can then download the original file by running:
zsync -i some-image-0.1.iso http://example.com/dl/some-image-0.2.iso.zsync
Users still need to access to the full download file - zsync merely allows then to save time by only downloading parts of the file.) A user with v0.1 of the same file can now use zsync to download only the new bits.
By default zsyncmake will include a relative URL in the zsync control file, so
the client program will access the full file from the same server and directory
as the .zsync file. You can instead specify a URL to include with -u if you
are putting the .zsync file in a different directory or on another server.
zsync is not very actively developed in recent years, but you can submit pull requests or issues on Github https://github.com/cph6/zsync/. Or you can reach out to me personally at cph@moria.org.uk.
zsync is based on the rsync algorithm, by Andrew Tridgell. It also incorporates a number of optimisations, based on ideas in academic papers by Utku Irmak, Svilen Mihaylov and Torsten Suel (primarily "Improved Single-Round Protocols for Remote File Synchronization", Sept 2004).
zsync is copyright 2004,2005,2007,2009,2025,2026 Colin Phipps cph@moria.org.uk. zsync is made available under the Artistic License 2.0 - see the file LICENSE for details.
Also, thanks to Dennis Schridde, Timothy Lee, Richard Kiss, Érsek László, James Montgomerie, James Antill, saul@alien-science.org, Kent Mein, Marc Lehmann, Robert Lemmen, Mark Adler, Ricardo Correia, Karl Kalleberg, Michael Stone, Richard Lucassen, Duncan Mac-Vicar, Jari Aalto, Marcin Mirosław, Jan Varho, Loïc Minier, Gian Merlino and S Page for useful feedback and bug reports for previous versions.
$ claude mcp add zsync \
-- python -m otcore.mcp_server <graph>