Sysadmin Ramblings

You can do this with rdist, but I prefer the rsync method.

You wish to backup a directory on machine A by making a copy on machine B. (In our case, machine B is a netapp, so we get further backup for free).

On machine A, create a new key using ssh-keygen
[user@A]: cd; cd .ssh
[user@A]: ssh-keygen -t dsa -C 'rsync@A' -f rsync_dsa
Generating public/private dsa key pair.
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in rsync_dsa.
Your public key has been saved in rsync_dsa.pub.
The key fingerprint is:
fe:fe:fe:fe:fe:fe:fe:fe:fe:fe:fe:fe:fe:fe:fe:fe rsync@A
[user@A]:
Next, send the public key rsync_dsa.pub to user@B (machine B).
[user@A]: scp rsync_dsa.pub user@B:.ssh/
Now setup A to send updates from your directory to machine B. You can do this with crontab, or create a script in /etc/cron.daily. (We show the script here) #!/bin/bash

HOST=ssh.math.ias.edu
SRC=/home/user
DEST=/home/user/A
USER=user
RSH="ssh -x -i /home/user/.ssh/rsync_dsa -l $USER"

pushd $SRC
/usr/bin/rsync -v \
        --checksum \
        --recursive \
        --relative \
        --links \
        --perms \
        --times \
        --progress \
        --backup \
        --rsh="$RSH" \
        . $HOST:$DEST
popd
Now, move over to machine B. On B, we need to add the public key for A to our authorized_keys. More importantly we need to make sure A can only do rsync, we do this by adding a command to the authorized_keys
[user@B]: cd; cd .ssh
[user@B]: echo 'command="rsync --suffix .`date +%s` --server -vbltprcR /home/user/A" ' > command
[user@B]: cat command rsync_dsa.pub >>authorized_keys
[user@B]: chmod 600 authorized_keys
Your authorized keys should then have a line with command=”<code>” ssh-dss KEYDATAHERE…AQ== rsync@A. If not, edit by hand and make it look like it should. If you’re having issues, check my ssh-keys troubleshooting page. Go back to machine A and test your script.
[user@A]: ./rsync_dir
~/work /etc/cron.daily
building file list ...
2 files to consider
IGotaNewFileNow
          21 100%    0.00kB/s    0:00:00  (1, 100.0% of 2)
 
sent 138 bytes  received 40 bytes  356.00 bytes/sec
total size is 21  speedup is 0.12
/etc/cron.daily
[user@A]:
Done.

Figuring out what to put in your ssh-key is easy, try running your script without the command in the authorized_keys. Then suspend it. Go to the remote machine and ps -aef |grep rsync, you’ll see the correct arguments there…

This entry was posted on Wednesday, April 5th, 2006 at 9:58 am and is filed under Linux. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.