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/ 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 to directly insert the contents of 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.

19 thoughts on “Restricting SSH Access to rsync

  1. Thank you for this tutorial. I found it very useful. Also quite rare: there’s very few guidelines out there about how to allow a third party to access a single directory. This was the only usefu one I found.


  2. VIM tip: just use :r There is no need for the exclamation mark to run ‘cat’, as ‘r’ by itself will already read the file.

  3. I found it helpful to specify the single restrict' option rather than a bunch ofno-*’ options (which might not be enough in the future, as new restrictions become necessary); perhaps `restrict’ is relatively new.

    Anyway, thanks a lot! man sshd' describes this stuff in section "AUTHORIZED_KEYS FILE FORMAT", but it is nice to see a concrete example, especially one that deals withrrsync’ explicitly.

  4. Thanks for this tutorial! Exactly what I needed! 🙂

    Two annotations have to be made:

    The argument “command=…” in authorized_keys has to be put in front of the other arguments. I tried different varieties, but neither worked.

    When you try to “ssh user@remotemachine -i keyfile”, you are presented this message:

    PTY allocation request failed on channel 0
    /home/backup/bin/rrsync: Not invoked via sshd
    Use ‘command=”/home/backup/bin/rrsync [-ro|-wo] SUBDIR”‘
    in front of lines in /home/backup/.ssh/authorized_keys
    Connection to 192.x.x.x closed.

    I consider this a bit of a security breach, because of the information being given, e.g. the internal IP, the directory structure and the involved files. Can this message be suppressed somehow?

  5. yeah definitely not a security breach. There’s no information there that’s useful to an attacker.

  6. Unfortunalty doesn’t work when using internal-sftp. Any idea to have sftp AND rsync WITHOUT a interactive shell ? All solutions that I have seen seems to be outdated… (scponly, rssh, …)

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.