The rsync software that comes with Mac OS X 10.4, and newer releases, supports extended attributes (HFS+ resource forks). This means it can sync files from a local HFS+ filesystem to a remote volume which does not suport HFS+ resource forks by using AppleDouble encoding.

The AppleDouble encoding splits a native HFS+ file in two parts:

1. The data fork, which is the one that holds the real contents of the file, like the contents of a document, the pixels from a bitmap, and so on. It receives the same name as the original HFS+ file.
2. The resource fork, which is built of data held on the resource forks and Finder data, like the Spotlight comments and so on. It receives a filename which consists on prepending a dot and a slash characters to the original HFS+ filename.

Thus, for a HFS+ file named MyDocument.webloc, when stored in the AppleDouble format, it is splitted in two files: MyDocument.webloc and ._MyDocument.webloc.

By default, Mac OS X rsync implementation does not enable extended attributes support. This must be explicitly enabled by supplying the -E command-line switch to the rsync command. The problem is, however, that few rsync implementations (I don’t know of any besides Apple’s Mac OS X 10.4 one) support neither this kind of functionality nor the command-line switch that activates it.

The solution was pretty easy, by the way. I downloaded Darwin rsync source code for the rsync-20 from the Darwin Projects Directory and extracted the file patches/EA.diff from within it. This patch file includes the extended attributes (HFS+ resource forks) functionality I was seeking. This patch, at the moment of this writing, is agaist rsync-2.6.3.

Thus, I only had to grab the sources for rsync-2.6.3, which are also included inside the rsync-20.tar.gz file I downloaded before, extracted them, patched, configured, made and installed:

# tar zxvf rsync-20.tar.gz
# cd rsync-20
# tar zxvf rsync-2.6.3.tar.gz
# cd rsync-2.6.3
# patch < ../patches/EA.diff
# CFLAGS="-O -pipe" \
  ./configure --prefix=/usr/local \
              --disable-debug --enable-ea-support \
               --with-rsyncd-conf=/usr/local/etc/rsyncd.conf
# make
# make install