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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2//EN">
<HTML>
<HEAD>
<TITLE>rsync firewall tunneling</TITLE>
</HEAD>
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<H2 align="center">Using rsync through a firewall</H2>

<p>If you have a setup where there is no way to directly connect two
systems for an rsync transfer, there are several ways to get a firewall
system to act as an intermediary in the transfer.

<p>This first method should work for any remote-shell (e.g. ssh, rsh, etc).
The other methods are all targeted at ssh (which has a lot of flexibility
in making a tunneled connection possible).

<h4>Method 1 -- should work with any remote-shell</h4>

<p>Use your remote shell (e.g. ssh) to access the middle system and have it
use a remote shell to hop over to the actual target system.

<p>To effect this extra hop, you'll need to make sure that the remote-shell
connection from the middle system to the target system does not involve any
tty-based user interaction (such as prompting for a password) because there
is no way for the middle system to access the local user's tty.

<p>One way that should work for all remote-shell programs is to enable host-based
authentication, which would allow all connections from the middle system to
the target system to succeed (when the username remains the same).
However, this may not be a desirable setup.

<p>A better method that works with ssh (and is very safe) is to setup an ssh
key (see the ssh-keygen manpage) and ensure that ssh-agent forwarding is turned
on in your ssh client config (e.g. "ForwardAgent&nbsp;yes").  You would put
the public version of your key onto the middle and target systems (in the
~/.ssh/authorized_keys file), and the private key on your local system (which
I recommend you encrypt).  With this setup, a series of ssh connections that
starts from the system where your private key is available will auto-authorize
(after a pass-phrase prompt on the first system if your key is encrypted).
See also ssh-agent for a way to keep a public key unlocked in memory for an
extended time, and the keychain project for a way to manage a system-wide,
per-user ssh-agent.

<p>To test that you have things setup right, first test a series of
remote-shell connections outside of rsync.  A command such as the following
should work without issuing multiple prompts (use the appropriate remote-shell
and the substitute the real hostnames for "middle" and "target", of course):

<blockquote><pre>ssh middle ssh target uptime</pre></blockquote>

<p>If you get a password/passphrase prompt to get into the first ("middle")
system that's fine, but the extra hop needs to occur without any extra user
interaction.

<p>Once that's done, you can do an rsync copy like this:

<blockquote><pre>rsync -av -e "ssh middle ssh" target:/src/ /dest/</pre></blockquote>

<h4>Method 2 -- requires ssh and nc (netcat)</h4>

<p>Assuming you're using ssh as your remote shell, you can configure ssh to
use a proxy command to get to the remote host you're interested in reaching.
Doing this will allow the multi-hop connection to work with rsync, even if
both hosts prompt for a password -- this is because both ssh connections
originate from the localhost, and thus both instances of ssh have access to
the local tty to use for an out-of-band password prompt.

<p>Here is an example config for your ~/.ssh/config file (substitute "target",
"target_user", and "middle" as appropriate):

<blockquote><pre>Host target
  ProxyCommand nohup ssh middle nc -w1 %h %p
  User target_user
</pre></blockquote>

<p>This proxy setup uses "ssh" to login to the firewall system ("middle") and
uses "nc" (netcat) to connect to the target host ("target") using the default
port number.  The use of "nohup" silences a warning at the end of the run, and
the "-w1" option tells nc to shut down when the connection closes.

<p>With this done, you can run a normal-looking rsync command to "target" that
will run the proxy command to get through the firewall system:

<blockquote><pre>rsync -av /src/ target:/dest/</pre></blockquote>

<h4>Method 3 -- an alternate ssh method for those without nc (netcat)</h4>

<p>Assuming you're using ssh as your remote shell, you can configure ssh to
forward a local port through your middle system to the ssh port (22) on the
target system.  This method does not require the use of nc (it uses only
ssh to effect the extra hop), but otherwise it is similar to, but slightly
less convenient than, method 2.

<p>The first thing we need is an ssh configuration that will allow us to
connect to the forwarded port as if we were connecting to the target
system, and we need ssh to know what we're doing so that it doesn't
complain about the host keys being wrong.  We can do this by adding this
section to your ~/.ssh/config file (substitute "target" and "target_user"
as appropriate, but leave "localhost" unchanged):

<blockquote><pre>Host target
  HostName localhost
  Port 2222
  HostKeyAlias target
  User target_user
</pre></blockquote>

<p>Next, we need to enable the port forwarding:

<blockquote><pre>ssh -fN -l middle_user -L 2222:target:22 middle</pre></blockquote>

<p>What this does is cause a connection to port 2222 on the local system to
get tunneled to the middle system and then turn into a connection to the
target system's port 22.  The -N option tells ssh not to start a shell on
the remote system, which works with modern ssh versions (you can run a
sleep command if -N doesn't work).  The -f option tells ssh to put the
command in the background after any password/passphrase prompts.

<p>With this done, you could run a normal-looking rsync command to "target"
that would use a connection to port 2222 on localhost automatically:

<blockquote><pre>rsync -av target:/src/ /dest/</pre></blockquote>

<p><b>Note:</b> starting an ssh tunnel allows anyone on the source system
to connect to the localhost port 2222, not just you, but they'd still need
to be able to login to the target system using their own credentials.

<h4>Method 4 -- for using rsync in daemon-mode (requires nc)</h4>

<p>Install and configure an rsync daemon on the target and use ssh and nc
to send the socket data to the remote host.

<blockquote><pre>RSYNC_CONNECT_PROG='ssh -l middle_user middle nc %H 873' \
    rsync daemonuser@target::module/src/ /dest/</pre></blockquote>

<p>(You can also export that variable into your environment if you want to
perform a series of daemon rsync commands through the same middle host.)

<p>This command takes advantage of the RSYNC_CONNECT_PROG environment
variable, which tells rsync to pipe its socket data to an external program
in place of making a direct socket connection.  The command specifed above
uses ssh to run the nc (netcat) command on the middle host, which forwards
all socket data to port 873 on the target host (%H).  The "%H" will be
substituted with the target host from the rsync command as long as you're
running rsync 3.0.0 or newer (you'll need to replace %H with the actual
target hostname for earlier rsync versions, which makes the hostname
specified in the rsync command superfluous).

<h4>Method 5 -- for using rsync in daemon-mode (for those without nc)</h4>

<p>Install and configure an rsync daemon on the target and use an ssh
tunnel to reach the rsync sever.  This is similar to method 3, but it
tunnels the daemon port for those that prefer to use an rsync daemon.

<p>(Note that this method also works to tunnel a socket connection
directly to a destination system if you use "localhost" as the target
hostname and your destination system's name as the middle hostname.)

<p>Installing the rsync daemon is beyond the scope of this document, but
see the rsyncd.conf manpage for more information.  Keep in mind that you
don't need to be root to run an rsync daemon as long as you don't use a
protected port.

<p>Once your rsync daemon is up and running, you build an ssh tunnel
through your middle system like this:

<blockquote><pre>ssh -fN -l middle_user -L 8873:target:873 middle</pre></blockquote>

<p>What this does is cause a connection to port 8873 on the local system to
turn into a connection from the middle system to the target system on port
873.  (Port 873 is the normal port for an rsync daemon.) The -N option
tells ssh not to start a shell on the remote system, which works with
modern ssh versions (you can run a sleep command if -N doesn't work).  The
-f option tells ssh to put the command in the background after any
password/passphrase prompts.

<p>Now when an rsync command is executed with a daemon-mode command-line
syntax to the local system, the conversation is directed to the target
system.  For example:

<blockquote><pre>rsync -av --port 8873 localhost::module/src/ dest/
rsync -av rsync://localhost:8873/module/src/ dest/</pre></blockquote>

<p><b>Note:</b> starting an ssh tunnel allows anyone on the source system
to connect to the localhost port 8873, not just you, so you may want to
enable username/password restrictions on your rsync daemon.

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