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 Using Rsync and SSH
Keys, Validating, and Automation
This document covers using cron, ssh, and rsync to backup files over a local network or the Internet. Part of my goal is to ensure no user intervention is required when the computer is restarted (for passwords, keys, or key managers).
I like to backup some logging, mail, and configuration information sometimes on hosts across the network and Internet, and here is a way I have found to do it. You'll need these packages installed:
  • rsync
  • openssh
  • cron (or vixie-cron)
Please note these instructions may be specific to Red Hat Linux versions 7.3, 9, and Fedora Core 3, but I hope they won't be too hard to adapt to almost any *NIX type OS. The man pages for 'ssh' and 'rsync' should be helpful to you if you need to change some things (use the "man ssh" and "man rsync" commands).
First, I'll define some variables. In my explanation, I will be synchronizing files (copying only new or changed files) one way, and I will be starting this process from the host I want to copy things to. In other words, I will be syncing files from /remote/dir/ on remotehost, as remoteuser, to /this/dir/ on thishost, as thisuser.
I want to make sure that 'rsync' over 'ssh' works at all before I begin to automate the process, so I test it first as thisuser:
$ rsync -avz -e ssh remoteuser@remotehost:/remote/dir /this/dir/
and type in remoteuser@remotehost's password when prompted. I do need to make sure that remoteuser has read permissions to /remote/dir/ on remotehost, and that thisuser has write permissions to /this/dir/ on thishost. Also, 'rsync' and 'ssh' should be in thisuser's path (use "which ssh" and "which rsync"), 'rsync' should be in remoteuser's path, and 'sshd' should be running on remotehost.
Configuring thishost

If that all worked out, or I eventually made it work, I am ready for the next step. I need to generate a private/public pair of keys to allow a 'ssh' connection without asking for a password. This may sound dangerous, and it is, but it is better than storing a user password (or key password) as clear text in the script [0]. I can also put limitations on where connections made with this key can come from, and on what they can do when connected. Anyway, I generate the key I will use on thishost (as thisuser):
$ ssh-keygen -t dsa -b 2048 -f /home/thisuser/cron/thishost-rsync-key
Generating public/private dsa key pair.
Enter passphrase (empty for no passphrase): [press enter here]
Enter same passphrase again: [press enter here]
Your identification has been saved in /home/thisuser/cron/thishost-rsync-key.
Your public key has been saved in /home/thisuser/cron/
The key fingerprint is:
2e:28:d9:ec:85:21:e7:ff:73:df:2e:07:78:f0:d0:a0 thisuser@thishost
and now we have a key with no password in the two files mentioned above [1]. Make sure that no other unauthorized user can read the private key file (the one without the '.pub' extension).
This key serves no purpose until we put the public portion into the 'authorized_keys' file [2] on remotehost, specifically the one for remoteuser:
I use scp to get the file over to remotehost:
$ scp /home/thisuser/cron/ remoteuser@remotehost:/home/remoteuser/
and then I can prepare things on remotehost.
Configuring remotehost

I 'ssh' over to remotehost:
$ ssh remoteuser@remotehost
remoteuser@remotehost's password: [type correct password here]
$ echo I am now $USER at $HOSTNAME
I am now remoteuser at remotehost
to do some work.
I need to make sure I have the directory and files I need to authorize connections with this key [3]:
$ if [ ! -d .ssh ]; then mkdir .ssh ; chmod 700 .ssh ; fi
$ mv .ssh/
$ cd .ssh/
$ if [ ! -f authorized_keys ]; then touch authorized_keys ; chmod 600 authorized_keys ; fi
$ cat >> authorized_keys
Now the key can be used to make connections to this host, but these connections can be from anywhere (that the ssh daemon on remotehost allows connections from) and they can do anything (that remoteuser can do), and I don't want that. I edit the 'authorized_keys' file (with vi) and modify the line with '' information on it. I will only be adding a few things in front of what is already there, changing the line from this:
ssh-dss AAAAB3NzaC1kc3MAAAEBAKYJenaYvMG3nHwWxKwlWLjHb77CT2hXwmC8Ap+fG8wjlaY/9t4u
yJNej2Sia70fu3XLHj2yBgN5cy8arlZ80q1Mcy763RjYGkR/FkLJ611HWIA= thisuser@thishost
to this [4]:
from="",command="/home/remoteuser/cron/validate-rsync" ssh-dss AAAAB3Nza
j2yBgN5cy8arlZ80q1Mcy763RjYGkR/FkLJ611HWIA= thisuser@thishost
where "" is the IP (version 4 [5]) address of thishost, and "/home/remoteuser/cron/validate-rsync" (which is just one of a few options [6], including customization [7] to enhance security) is a script that looks something like this :

echo "Rejected"
echo "Rejected"
echo "Rejected"
echo "Rejected"
echo "Rejected"
echo "Rejected"
echo "Rejected"
rsync\ --server*)
echo "Rejected"
If thishost has a variable address, or shares its address (via NAT or something similar) with hosts you do not trust, omit the 'from="",' part of the line (including the comma), but leave the 'command' portion. This way, only the 'rsync' will be possible from connections using this key. Make certain that the 'validate-rsync' script is executable by remoteuser on remotehost and test it.
PLEASE NOTE: The private key, though now somewhat limited in what it can do (and hopefully where it can be done from), allows the possessor to copy any file from remotehost that remoteuser has access to. This is dangerous, and I should take whatever precautions I deem necessary to maintain the security and secrecy of this key. Some possibilities would be ensuring proper file permissions are assigned, consider using a key caching daemon, and consider if I really need this process automated verses the risk.
ALSO NOTE: Another security detail to consider is the SSH daemon configuration on remotehost. This example focuses on a user (remoteuser) who is not root. I recommend not using root as the remote user because root has access to every file on remotehost. That capability alone is very dangerous, and the penalties for a mistake or misconfiguration can be far steeper than those for a 'normal' user. If you do not use root as your remote user (ever), and you make security decisions for remotehost, I recommend either:
PermitRootLogin no
PermitRootLogin forced-commands-only
be included in the '/etc/ssh/sshd_config' file on remotehost. These are global settings, not just related to this connection, so be sure you do not need the capability these configuration options prohibit. [8].
The 'AllowUsers', 'AllowGroups', 'DenyUsers', and 'DenyGroups' key words can be used to restrict SSH access to particular users and groups. They are documented in the man page for "sshd_config", but I will mention that they all can use '*' and '?' as wildcards to allow and deny access to users and groups that match patterns. 'AllowUsers' and 'DenyUsers' can also restrict by host when the pattern is in USER@HOST form.

Now that I have the key with no password in place and configured, I need to test it out before putting it in a cron job (which has its own small set of baggage). I exit from the ssh session to remotehost and try:
$ rsync -avz -e "ssh -i /home/thisuser/cron/thishost-rsync-key" remoteuser@remotehost:/remote/dir /this/dir/
If this doesn't work, I will take off the "command" restriction on the key and try again. If it asks for a password, I will check permissions on the private key file (on thishost, should be 600), on 'authorized_keys' and (on remotehost, should be 600), on the '~/.ssh/' directory (on both hosts, should be 700), and on the home directory ('~/') itself (on both hosts, should not be writeable by anyone but the user). If some cryptic 'rsync' protocol error occurs mentioning the 'validate-rsync' script, I will make sure the permissions on 'validate-rsync' (on remotehost, may be 755 if every remotehost user is trusted) allow remoteuser to read and execute it.
If things still aren't working out, some useful information may be found in log files. Log files usually found in the /var/log/ directory on most linux hosts, and in the /var/log/secure log file on Red Hat-ish linux hosts. The most useful logfiles in this instance will be found on remotehost, but localhost may provide some client side information in its logs [9] . If you can't get to the logs, or are just impatient, you can tell the 'ssh' executable to provide some logging with the 'verbose' commands: '-v', '-vv', '-vvv'. The more v's, the more verbose the output. One is in the command above, but the one below should provide much more output:
$ rsync -avvvz -e "ssh -i /home/thisuser/cron/thishost-rsync-key" remoteuser@remotehost:/remote/dir /this/dir/
Hopefully, it will always just work flawlessly so I never have to extend the troubleshooting information listed here [10] .
Cron Job Setup

The last step is the cron script. I use something like this:


because it is easy to modify the bits and pieces of the command line for different hosts and paths. I will usually call it something like 'rsync-remotehost-backups' if it contains backups. I test the script too, just in case I carefully inserted an error somewhere.
When I get the script running successfully, I use 'crontab -e' to insert a line for this new cron job:
0 5 * * * /home/thisuser/cron/rsync-remotehost-backups
for a daily 5 AM sync, or:
0 5 * * 5 /home/thisuser/cron/rsync-remotehost-backups
for a weekly (5 AM on Fridays). Monthly and yearly ones are rarer for me, so look at "man crontab" or here for advice on those.  

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