There are many web pages describing how to enable the rsync daemon on Mac OS X using launchd/launchctl mechanism. But I had to use a different (and simpler) plist file in LaunchDaemons to make it work across reboots on Lion (10.7.4).

(I started by following this guide , and this very similar one. And I also read this and this. In the end, what helped me getting the plist file right was this thread. Particularly this post: “For one you have both a Program and a ProgramArguments key, when you should have only one or the other (you use Program if there is just one element to the command, or ProgramArguments if there are multiple.” And this one.)

This is the .plist file I used in /Library/LaunchDaemons/org.samba.rsync.plist : 

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
    <key>Disabled</key>
    <false/>
    <key>Label</key>
    <string>org.samba.rsync</string>
    <key>ProgramArguments</key>
    <array>
        <string>/usr/bin/rsync</string>
        <string>--daemon</string>
    </array>
    <key>RunAtLoad</key>
    <true/>
    <key>KeepAlive</key>
    <dict>
        <key>SuccessfulExit</key>
        <false/>
    </dict>
</dict>
</plist>

This is an example /etc/rsyncd.conf file:

secrets file = /etc/rsyncd.secrets
hosts allow = 192.168.1.0/24 10.0.0.1 *.cust.isp.tld

pid file = /var/run/rsyncd.pid
uid = nobody
gid = nobody
list = yes
read only = yes

[shared]
path = /Users/Shared
comment = Users-Shared
uid = someuser
gid = admin
auth users = user_in_secrets

The file /etc/rsyncd.secrets looks like:

some_rsync_user:password
other_user:other_password

To install it:

sudo -s
chown root:wheel /etc/rsyncd.*
chmod 644 /etc/rsyncd.conf
chmod 600 /etc/rsyncd.secrets
launchctl load /Library/LaunchDaemons/org.samba.rsync.plist
launchctl start org.samba.rsync ## (this last command is probably unneeded)

To check if it is installed and running:

launchctl list | grep rsync
808  -    0x7fddb4806c10.anonymous.rsync
-    0    org.samba.rsync

ps ax | grep [r]sync
  808   ??  Ss     0:00.00 /usr/bin/rsync --daemon

rsync --stats someuser@localhost::

To remove it:

sudo launchctl unload /Library/LaunchDaemons/org.samba.rsync.plist
sudo killall rsync

For logging transfers, add

log file = /var/log/rsyncd.log
transfer logging = yes

to /etc/rsyncd.conf. And to have the log rotated, create a file like /etc/newsyslog.d/rsyncd.conf and add

# logfilename          [owner:group]    mode count size when  flags [/pid_file] [sig_num]
/var/log/rsyncd.log   644  5    5000 *     J