Using Rsnapshot and SSH
Keys and Configuration
This document covers the details of setting up the
SSH part of an automated remote Rsnapshot backup configuration,
so I do not have to relearn how to do it next time.
It is assumed that the starting point here is a correctly installed
setup, so you will need these packages installed:
I have completed my configuration on a
Red Hat Fedora Core 3
server, but I will try to keep this instructional as "distribution
generic" as possible. There is much that is not covered here regarding
the setup of
because it is covered so well in the
Please grow to know and love that document before reading this one.
Much of what is explained here is explained in a more general way
in another document I wrote on
Using Rsync and SSH
but it has no
In the examples I will be configuring
Rsnapshot as the
root user on
localhost.example.com to do backups as
user to run
is more dangerous than using an unprivileged user to do the same,
but I think it is the default configuration (for a few good reasons
considerably more dangerous, and should be avoided where possible,
so I will not connect as
in these examples.
Creating A SSH Key With No Password
key without a password is not something one should
consider lightly. If you can use the key to automatically connect
and copy important files from a remote host (which is the point of
all this), the potential exists for others to do this too.
Seriously consider if you want to endure the risk this entails,
and protect the key from all eyes if you do.
To generate the passwordless
key, use this command or something
like it (using 'man ssh-keygen' as your guide):
[root@localhost ~]# ssh-keygen -t dsa -b 2048 -f /root/cron/localhost-rsnapshot-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 /root/cron/localhost-rsnapshot-key.
Your public key has been saved in /root/cron/localhost-rsnapshot-key.pub.
The key fingerprint is:
and the two files will be created in the above mentioned locations
The public key (the one with the '.pub' file extension) will be copied
and inserted in a
file (that belonging to
, specifically), as can be seen
The private key file will be used to make the
, and is the key
that requires the utmost security.
Making A SSH Host Configuration Entry
file you can create a text label for a connection, and assign
various configuration options to it, without interfereing with
the options set for other connections. The default file for
Create the file if it does not exist already, and add the following
to the end of the file (using your hostnames and file locations
in place of the example contents):
You can use these commands to make sure the file has the correct
permissions and contents (again, using your hostnames and file
# if [ ! -d ~/.ssh ]; then mkdir ~/.ssh ; chmod 700 ~/.ssh ; fi
# cd ~/.ssh/
# if [ ! -f config ]; then touch config ; chmod 600 config ; fi
# echo Host remotehost-rsnapshot >> config
# echo Hostname remotehost.example.com >> config
# echo IdentityFile /root/cron/localhost-rsnapshot-key >> config
These configuration options will make sure the host designation
in the Rsnapshot configuration file will use the correct key
and will not have pass anything to 'ssh' (via the '-i' command
line option) to make sure that key use used. The
'remotehost-rsnapshot' host designation can be used on the
command line, but the 'rsnapshot' in the name should remind
future configuration file editors for what the host and key are
Adding To known_hosts
To avoid any problems with interactive host key recognition, a
host key for the
must be added to the SSH initiator's
On a system configured to add keys automatically to
upon first connection, after prompting the user to do so, all
you must do is connect to the remote system as the user who
will be running Rsnapshot (
in this case):
[root@localhost ~]# ssh email@example.com
The authenticity of host 'remotehost.example.com (18.104.22.168)' can't be established.
RSA key fingerprint is 34:c1:29:42:67:b3:8d:ad:ba:50:34:d1:ab:d5:9a:aa.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added 'remotehost.example.com,22.214.171.124' (RSA) to the list of known hosts.
Last login: Sun Apr 9 22:21:09 2006 from localhost.example.com
It is important to add
to the known_hosts
file for root on
will fail because the connection will not be allowed until the
question is answered.
That file is located here:
. The reason the
question is posed is the SSH configuration option
'StrictHostKeyChecking' which defaults to 'ask' in many
installations. The easy (and less secure) way of avoiding this
problem is to set 'StrictHostKeyChecking' to 'no'. This can be
done under the 'remotehost-rsnapshot' part of the
If SSH host keys must be added to the known_hosts file manually,
'man' pages for
sshd for formatting information.
Restricting The Key
key restrictions are made on a per-key basis, and they reside in
from="10.1.1.1",command="/home/remoteuser/cron/validate-rsync" ssh-dss AAAAB3Nza
There are 4 fields here, all on a single line, all separated by spaces:
restriction, key type, key text, and key comment.
Originally, when the key was first generated by "ssh-keygen",
there were just the last three fields. The first is added onto
the front to restrict what kind of connections can be made with this
key. Two separate restrictions are in this example's restriction field to
emphasize that the separator between them is a comma character (,) and
not a space. You will not be able to connect with a key that has too
many, or too few fields, due to a mis-editted key line. SSH will simply
fail to authenticate with that key, then move on and ask you for your
The 'from="10.1.1.1"' part only allows connections to come from
a certain IP Address, "10.1.1.1" in this example. The
"command=/home/remoteuser/cron/validate-rsync" part only executes
the designated command. This may seem very limiting, but you
may write the script yourself and allow many different commands.
You even have the actual command line given to SSH to work with
in the form of the
An example script is located here:
and other options are included in surrounding footnotes.
configuration file is, by default,
It contains quite a few configuration options, most of them in
"option [tab character] value" format. Remove the 'comment'
character ('#') to the left of the 'cmd_ssh' option so that part of
the file looks like:
# Uncomment this to enable remote ssh backups over rsync.
Confirm that the 'ssh' command is where the file says it should be
(with the 'which ssh' command).
Scroll down a bit and make sure the 'ssh_args' configuration option
is uncommented and contains the following
ssh_args -o BatchMode=yes
The only reason I can see for not wanting to do this is if you plan
on using Rsnapshot interactively and you want to provide SSH
credentials for some connections. That is an unusual situation and
probably best addressed with separate and customized 'rsnapshot.conf'
Further down in the file, insert a new 'backup' line that looks
similar to this:
backup remoteuser@remotehost-rsnapshot:/usr/local/apache/htdocs/ remotehost/
Notice the 'remotehost-rsnapshot' text from the
file, where normally a hostname would be found.
To make sure there are no
related errors in the configuration file, run the following command
when editting is completed:
# rsnapshot configtest
The second line should appear as above if everything is correct.
SSH Server Options
SSH server options, in the context of this Rsnapshot configuration,
are set on
server configuration file will typically be found at
and by default contains many of the configuration options
set to their default values and commented out.
This file controls
connections on this server, so be careful
not to "secure away" functions you or others actually need.
PermitRootLogin can be used to make a
installation more secure
in one of two ways. When set to "no":
no connections can be made as the
user. When set to "forced-commands-only":
connections may be made as
, but only from connections using
authorized keys associated with a command in the
file (as mentioned above).
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.
You could create a group called sshusers
, add only necessary users to it, put this:
into your /etc/ssh/sshd_config
file, then give your SSH server a restart and test it out.
Run an hourly backup to test the configuration:
# rsnapshot hourly
If things don't work
out, you may have to try running an hourly backup multiple times.
To keep from messing up Rsnapshot backups you may already have in
place, you may wish to move the current hourly out of the way
before testing. Remove (or move) each test before running the
next so as not to displace normal hourly backups. If you have
a long running Rsnapshot configuration, you may want to comment
out some of the backups, or possibly use a separate testing
Rsnapshot configuration file altogether.
I think Rsnapshot is usually configured to run as the root user
so that all files, especially those only accessable by the root
user, are backed up without trouble. Many problems, and a
considerable amount of fooling around, is avoided by running
Rsnapshot as root.
The '/root/' directory is the root user's home directory. To use
another user, replace '/root/' where you see it with the location
of the (hopefully) unprivileged user's home directory (like
'/home/thisuser/' for the 'thisuser' user).
You can see what fails and what succeeds during a SSH connection
by using the 'verbose' command line options: '-v', '-vv', or '-vvv'
(more v's meaning more verbosity).
Thanks to Martin Shroder for pointing this out.
To affect settings for the SSH client on 'localhost' changes should
be made to the '/root/.ssh/config' file mentioned above, or if more
global changes are necessary, the '/etc/ssh/ssh_config' file.
This document is really not complete, but hopefully it can be of
some help for now.
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