Libre Things

marcmaurice.fr Diaspora
fr

Tag - rsync

Entries feed - Comments feed

Wednesday 29 December 2010

Rsync command restriction over SSH

By dooblem on Wednesday 29 December 2010, 18h36

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: /post/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)

one comment

Friday 9 October 2009

SFTP chroot + rsync

By dooblem on Friday 9 October 2009, 17h12 - Linux

Here is howto make sftp shares with chroot.

In /etc/ssh/sshd_config:

# we use openssh internal sftp
# because /usr/lib/openssh/sftp-server won't be available in chroot
Subsystem sftp internal-sftp

Match group sftp
        ChrootDirectory %h
        X11Forwarding no
        AllowTcpForwarding no
        ForceCommand internal-sftp

UPDATE 17/06/2010: Beware with the syntax! Comments must start at the line beginning, and no spaces at the end of the ForceCommand internal-sftp line.

Now just create users belonging to sftp group, and that's it. Test it with:

sftp user@myserver.com

Problem: we cannot use the rsync command to send files, because rsync is not available in the chroot.

First, we allow other commands, commenting the line:

#ForceCommand internal-sftp

Then, we build the following tree in the chroot directory:

bin/
bin/bash
bin/rsync
lib/
lib/libncurses.so.5
lib/ld-linux.so.2
lib/libacl.so.1
lib/libpopt.so.0
lib/libattr.so.1
lib/i686
lib/i686/cmov
lib/i686/cmov/libdl.so.2
lib/i686/cmov/libc.so.6

We must put both bash and rsync commands, and all their librairies (you can display them with the ldd command).

Note: the user must have /bin/bash as default shell.

Note2: the chroot dir must belong to root, even if it's the user's folder. To allow the user to write in it, you have to create a subfolder with appropriate permissions. According to OpenSSH programers, it's a big constraint, but very important for a chroot's security.

References :

one comment