Tag Archives: rsync

`rsync` and FAT File-Systems

I’m using rsync to sync files from my computer to a FAT formatted SD card. Using the --update flag to rsync makes it “skip files that are newer on the receiver”. It seems that it should work as follows: After the first sync, any subsequent syncs will only transfer those files which changed in the meantime. However, I noticed that transfer times are usually longer than expected, which led me to think that things are not working as they should.

As --update relies only on modification date, obviously something is wrong with it. After ruling out several other possibilities, I’ve guessed that the modification date of some files get mangled. A quick check about FAT revealed that FAT can only save modification times in 2 second granularity. I’ve also used rsync’s --archive flag, which among other things attempts to preserve modifications times of the files it copies. But what about FAT’s lower granularity for modification time? That was apparently the culprit. Some files when copied got a modification time which was up to 1 second before the original modification time! Hence, every time I’ve synced, from rsync’s perspective, the target was older than the source, and hence needs to be overwritten.

This can be demonstrated by the following session:
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Restricting SSH Access to rsync

Passphrase-less SSH keys allows one to automate remote tasks by not requiring user intervention to enter a passphrase to decrypt the key. While this is convenient, is posses a security risk as the plain key can be used by anyone who gets hold of it to access the remote server. To this end, the developers of SSH allowed to restrict via the .ssh/authorized_keys the commands that can be executed of specific keys. This works great for simple commands, but as using rsync requires executing remote commands withe different arguments on the remote end, depending on the invocation on the local machine, it gets quite complicated to properly restrict it via .ssh/authorized_keys.

Luckily, the developers of rsync foresaw this problem and wrote a script called rrsync (for restricted rsync) specifically to ease the restricting keys to be used only for rsync via .ssh/authorized_keys. If you have rsync installed, rrsync should have been distributed along side it. In Debian/Ubuntu machines it can be found under /usr/share/doc/rsync/scripts/rrsync.gz. If you can find it there, you can download the script directly from here. On the remote machine, copy the script, unpacking if needed, and make it executable:

user@remote:~$ gunzip /usr/share/doc/rsync/scripts/rrsync.gz -c > ~/bin/rrsync
user@remote:~$ chmod +x ~/bin/rrsync

On the local machine, create a new SSH key and leave the passphrase empty (this will allow you to automate the rsync via cron). Copy the public key to the remote server.

user@local:~$ ssh-keygen -f ~/.ssh/id_remote_backup -C "Automated remote backup"
user@local:~$ scp ~/.ssh/id_remote_backup.pub user@remote:~/

Once the public key is on the remote server edit ~/.ssh/authorized_keys and append the public key.

user@remote:~$ vim ~/.ssh/authorized_keys

(Vim tip: Use :r! cat id_remote_backup.pub to directly insert the contents of id_remote_backup.pub into a new line). Now prepend to the newly added line

command="$HOME/bin/rrsync -ro ~/backups/",no-agent-forwarding,no-port-forwarding,no-pty,no-user-rc,no-X11-forwarding

The command="..." restricts access of that public key by executing the given command and disallowing others. All the other no-* stuff further restrict what can be done with that particular public key. As the SSH daemon will not start the default shell when accessing the server using this public key, the $PATH environment variable will be pretty empty (similar to cron), hence you should specify the full path to the rrsync script. The two arguments to rrsync are -ro which restricts modifying the directory (drop it if you want to upload stuff to the remote directory) and the path to the directory you want to enable remote access to (in my example ~/backups/).

The result should look something like:

command="$HOME/bin/rrsync -ro ~/backups/",no-agent-forwarding,no-port-forwarding,no-pty,no-user-rc,no-X11-forwarding ssh-rsa AAA...vp Automated remote backup

After saving the file, you should be able to rsync files from the remote server to the local machine, without being prompted to for a password.

user@local:~$ rsync -e "ssh -i $HOME/.ssh/id_remote_backup" -av user@remote: etc2/

To things are needed to be noted:

  1. You need to specify the passphrase-less key in the rsync command (the -e "ssh -i $HOME/.ssh/id_remote_backup" part).
  2. The remote directory is always relative to the directory given to rrsync in the ~/.ssh/authorized_keys file.