From the rsync documentation:
It is also possible to use rsync without using rsh or ssh as the transport.
In this case you will connect to a remote rsync server running on TCP port 873.
You may establish the connection via a web proxy by setting the environment variable
RSYNC_PROXY to a hostname:port pair pointing to your web proxy.
Note that your web proxy's configuration must allow proxying to port 873.
Using rsync in this way is the same as using it with rsh or ssh except that:
# you use a double colon :: instead of a single colon to separate the hostname
from the path.
# the remote server may print a message of the day when you connect.
# if you specify no path name on the remote server then the list of accessible
paths on the server will be shown.
