2010-12-29 18:36

You have 2 systems and you want to set up a secure backup with rsync + SSH of one system to the other.

Very simply, you can use:

backup.example.com# rsync -avz --numeric-ids --delete root@myserver.example.com:/path/ /backup/myserver/

To do the backup, you have to be root on the remote server, because some files are only root readable.

Problem: you will allow backup.example.com to do anything on myserver.example.com, where just read only access on the directory is sufficient.

To solve it, you can use the command="" directive in the authorized_keys file to filter the command.

To find this command, start rsync adding the -e'ssh -v' option:

rsync -avz -e'ssh -v' --numeric-ids --delete root@myserver.example.com:/path/ /backup/myserver/ 2>&1 | grep "Sending command"

You get a result like:

debug1: Sending command: rsync --server --sender -vlogDtprze.iLsf --numeric-ids . /path/

Now, just add the command before the key in /root/.ssh/authorized_keys:

command="rsync --server --sender -vlogDtprze.iLsf --numeric-ids . /path/" ssh-rsa AAAAB3NzaC1in2EAAAABIwAAABio......

And for even more security, you can add an IP filter, and other options:

from="backup.example.com",command="rsync --server --sender -vlogDtprze.iLsf --numeric-ids . /path/",no-agent-forwarding,no-port-forwarding,no-pty,no-user-rc,no-X11-forwarding ssh-rsa AAAAB3NzaC1in2EAAAABIwAAABio......

Now try to open a ssh shell on the remote server.. and try some unauthorized rsync commands…

Notes:

  • Beware that if you change rsync command options, change also the authorized_keys file.
  • No need for complex chroot anymore. Forget my previous article: sftp-chroot-rsync

See also:

  • man ssh #/AUTHORIZED_KEYS FILE FORMAT
  • man rsync
  • view /usr/share/doc/rsync/scripts/rrsync.gz (restricted rsync, allows you to manage allowed options precisely)
2010-12-29 18:36 · Tags: , ,

2 Comments

  1. my user is no login, how can I use this for that user?
    do I have to change my user setting on the server to be able to allow login?

    Reply

  2. Thanks for the above – it was just what I needed.

    (I didn’t know you could have a ‘from’ bit in authorized_keys).

    Reply

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