Securely automating rsync to archive sensitive files

Before I begin, please note that sync is not backup. If you plan to use this sort of method for backups, you should look into using something like duplicity or rsnapshot instead. In the latter case, you can implement the rsync side of what I'm suggesting here using --rsync-path='sudo rsync' as part of rsync_long_args in your rsnapshot config, but I don't know if such a thing is possible with duplicity.

Obviously, any "secure" implementation using rsync would require that rsync itself was secure. Whilst it has a lot of talented eyes on it, there have been fairly bad exploits in the past (including CVE-2011-1097 and CVE-2007-6200, among others). In any case, the less stuff that you do, the more secure your system is likely to be (especially since there is probably far less scrutiny of your work than there is of rsync).

I have some large datasets on some of my servers that I often need to process locally (ie. on my laptop). It's much better for me to have these datasets already locally available, because it's not efficient for me to process the data on the server, and the data sets are too large to access them live the network without incurring significant speed penalties.

My solution to this is to automatically sync them every day to my local machine, which is easy enough just using rsync and a cron job, but there is a problem. Some of the files I need are system files that are owned by root, so I need to elevate to root to get them. This poses a few problems:

My solution to these problems is as follows:

This method limits the possibility that an attacker can effectively use any privilege she would gain from getting the SSH key (which should be securely stored, anyway).

It's common to see two major mistakes when people implement setups like this (other than just logging in directly as root on the remote):

The proper way to handle these issues is to use ssh-agent, and further limit the commands accepted by sudo to allow unprompted elevation.


rsync user

On the remote, create the new "rsync" user that we will use exclusively to elevate to root when we do the sync:

useradd -m -d /srv/rsync -r -s /bin/bash -p NP rsync

We need to use "-p NP" so that the account does not become locked, because without it, sshd will not allow us to log in. This does not mean that the password actually is "NP", but that that is what the crypt(3) output is compared to. Since crypt can never return "NP", this denies direct logins using a password to the account, whilst still allowing us to log into that account using our SSH key.

Generate key

On your local machine, generate a new key that will be used exclusively for syncing (do this as the same user as you are going to be syncing as):

mkdir -pm700 ~/.ssh/keys
ssh-keygen -f ~/.ssh/keys/rsync_archive -C rsync-archive

Use a good passphrase that you can remember, you'll need it when adding the key to the SSH agent. You might as well start up an agent and add it now, since you're going to need it for the next steps:

mkdir -m700 ~/.ssh/agents
eval "$(ssh-agent | tee ~/.ssh/agents/rsync_archive)"

Work out what command is going to be run on the server

We need to work out what command is going to be run on the server, so that we can lock sudo access down to that single command. We could also do something like "rsync –server –sender *" in /etc/sudoers, but unless you have a reason to do that, you might as well further limit the opportunities for exploitation if someone got access to the rsync user.

Run your normal command, but this time, add --rsync-path="printf '%s\n' >&2". You'll see something like this printed out:

rsync: connection unexpectedly closed (0 bytes received so far) [Receiver]
rsync error: error in rsync protocol data stream (code 12) at io.c(601) [Receiver=3.0.7]

Don't worry about the rsync errors – they're expected, you did just give it a fake rsync path, after all. What you really want is given on the previous lines; this is the command that rsync is running on the remote after it connects to the server, and this is the command that you need to allow elevation for.

Allow the user to become root when acting as an rsync sender

Append the following line in /etc/sudoers (preferably using visudo so that a syntax check is performed before it is saved):

rsync ALL=(ALL) NOPASSWD: rsync --server --sender -lHogDtpAXrze.iLsf --numeric-ids . /var/lib/couchdb

You can test this by running the same command prepended with "sudo -K" as the rsync user. If nothing appears to happen, it's working (if it wasn't, you would receive the password prompt from sudo).

A caveat of this method is that if rsync ever changes the command used by the command you are running locally, this will start to fail. In practise, I have not had this happen whilst I have been running this setup, but even if it did happen, it's easy to fix (just redo these steps).

Limit the rsync user to running this single command

If you are only ever going to allow the rsync user to run this one command (ie. you only do one type of archive on the server as this user), you can force the SSH daemon on the remote to use this command when the user logs in, instead of the command requested by the user. This further limits the vectors for attack.

To do this, first add the public key from the keypair you generated to the remote rsync user's ~/.ssh/authorized_keys. Then, prepend the same command you just added to sudoers, except with "sudo" prepended, so it looks something like this:

command="sudo rsync --server --sender -lHogDtpAXrze.iLsf --numeric-ids . /var/lib/couchdb" [public key]

You should also consider using the no-port-forwarding, no-X11-forwarding, and no-pty options, to further limit the opportunities for exploitation. In that case, it would look something like this:


If you now ssh to the rsync user on the remote machine, you should get nothing returned. You can make it more clear that it's not still connecting by using the various levels of ssh's -v flags (-vv should be clear enough).

Create a cron job

We now need to create a cron job that connects to the ssh agent, and uses it to connect to the server you are syncing. I suggest you do this as two parts (although it can be done in one, it will look pretty messy). Assuming that you created the agent as root earlier to preserve ownership information (you could also run the agent as your own user and source that, instead, as long as you are running the job as root), the following should work to sync every hour:

cat > /usr/local/bin/sync-foo << 'EOF'

. ~/.ssh/agents/rsync_archive
rsync -avzHAX --numeric-ids --rsync-path='sudo rsync' rsync@foo:/var/lib/couchdb /sync/foo-couch
chmod a+x /usr/local/bin/sync-foo

cat > /etc/cron.d/sync-foo << 'EOF'
0 * * * * root sync-foo

Obviously, this requires that your SSH agent has the key added and unlocked. That's your responsibility (when you boot the server), otherwise rsync will time out. You can immediately fail the connection on such conditions if you disable password authentication in your sshd_config on the remote.

You should now have a fully working setup. It goes without saying that you should keep the passphrase to the key (and, of course, the key itself) secure.