rsync, Part II

Setting up rsync modules at the filesystem level and making connections.
Using rsync to Connect to an rsync Server

Lest I forget, I haven't explained how to connect to an rsync server as a client. This is a simple matter of syntax; when specifying the remote host, use a double colon rather than a single colon and use a path relative to the desired module, not an absolute path.

For instance, to revisit the scenario in last month's example, in which the client system is called near and the remote system is called far, suppose you wish to retrieve the file newstuff.tgz and far is running rsync in dæmon mode. Suppose further that you can't remember the name of the module on far in which new files are stored. First, you can query far for a list of its available modules, as shown below:

[root@near darthelm ]# rsync far::
public          Nobody home but us tarballs
incoming        You can put, but you can't take

(Not coincidentally, these are the same modules we set up in this month's examples; as I predicted in the previous section, the module Audiofreakz is omitted.) The directory you need is named public. Assuming you're right, the command to copy newstuff.tgz to your current working directory would look like this:

[yodeldiva@near ~]# rsync far::public/newstuff.tgz .
Both the double colon and the path format differ from SSH mode. Whereas SSH expects an absolute path after the colon, the rsync dæmon expects a module name, which acts as the “root” of the file's path. To illustrate, let's look at the same command using SSH mode:
[yodeldiva@near ~]# rsync -e ssh \
far:/home/public_rsync/newstuff.tgz .
These two aren't exactly equivalent, of course; whereas the rsync dæmon process on far is configured to serve files in this directory to anonymous users (i.e., without authentication), SSH always requires authentication (although this can be automated using null-passphrase RSA or DSA keys, described in Chapter 4 of Building Secure Servers with Linux). But it does show the difference between how paths are handled.

Tunneling rsync with Stunnel

The last rsync usage I'll mention is the combination of rsync, running in dæmon mode, with Stunnel. Stunnel is a general-purpose TLS or SSL wrapper that can be used to encapsulate any simple TCP transaction in an encrypted and optionally X.509-certificate-authenticated session. Although rsync gains encryption when you run it in SSH mode, it loses its dæmon features, most notably anonymous rsync. Using Stunnel gives you encryption as good as SSH's, while still supporting anonymous transactions.

What About Recursion?

Stunnel is covered in-depth in Chapter 5 of Building Secure Servers with Linux, using rsync in most examples. Suffice it to say that this method involves the following steps on the server side:

  1. Configure rsyncd.conf as you normally would.

  2. Invoke rsync with the --port option, specifying some port other than 873 (e.g., rsync --daemon --port=8730).

  3. Set up a Stunnel listener on TCP port 873 to forward all incoming connections on TCP 873 to the local TCP port specified in the previous step.

  4. If you don't want anybody to connect “in the clear”, configure hosts.allow to block nonlocal connections to the port specified in Step 2. In addition, or instead, you can configure iptables to do the same thing.

On the client side, the procedure is as follows:

  1. As root, set up a Stunnel listener on TCP port 873 (assuming you don't have an rsync server on the local system already using it), which forwards all incoming connections on TCP 873 to TCP port 873 on the remote server.

  2. When you wish to connect to the remote server, specify localhost as the remote server's name. The local Stunnel process now opens a connection to the server and forwards your rsync packets to the remote Stunnel process. The remote Stunnel process decrypts your rsync packets and delivers them to the remote rsync dæmon. Reply packets, naturally, are sent back through the same encrypted connection.

As you can see, rsync itself isn't configured much differently in this scenario than anonymous rsync would be—most of the work is in setting up Stunnel forwarders.

Resources

Mick Bauer (mick@visi.com) is a network security consultant for Upstream Solutions, Inc., based in Minneapolis, Minnesota. He is the author of the O'Reilly book Building Secure Servers with Linux, composer of the “Network Engineering Polka” and a proud parent (of children).

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