Wade's Blog

Rsync

Rsync, for those who aren’t familiar, is a file copy tool, which, after the first copy, will only send changes during subsequent updates. This makes it a very efficient tool, especially when used over an internet connection.

Anyway, to enable rsync from server A to server B, it is common to perform the login via key. This means that on Server A you’d generate a SSH keypair for your backup user, then copy the public key that was generated into the ~/.ssh/authorized_keys file for your backup user on Server B.

Because rsync is going to be executed automatically via cron script, it is necessary to create the key file without a password.

Jail

• Configure your SSH server
  • Open up /etc/ssh/sshd_config
  • At the end of the file, tell SSH to create a chroot jail for your backup user:

    ChrootDirectory %h
    AllowTcpForwarding no
    PermitTunnel no
    X11Forwarding no

    Note, because of the way chroot works, you’ll need to make sure the chroot directory is owned by ROOT, even if it’s actually the home directory of your backup user.

• Save, and restart your SSH server.

This gets you part of the way, you should now be able to SSH/SFTP into Server B using your backup user, and when connected, you will be restricted to the location set in ChrootDirectory.

Unfortunately, rsync needs more than this, and in order to copy files it’ll need access to the shell (I’m assuming bash), as well as the rsync application itself, together with whatever libraries are required.

Therefore, it becomes necessary to create a partial chroot image in the backup user’s chroot directory. You could do this the traditional way (e.g. by using something like debootstrap), which will create a mirror of your base operating system files in the chroot jail. However, this generally takes a few hundred megabytes at least, and if all you want is to copy some files, you don’t want to give access to more than you need.

Instead, I opt to create a skeleton chroot jail by hand.

The goal here is to mirror the filesystem of your server inside the chroot jail, so that if a file exists in /foo/bar, then you need to copy it to /home/backup-user/foo/bar, and make sure it’s owned by root.

• Copy bash from /bin/bash to the directory /home/backup-user/bin/
• Copy rsync (on my system this was in /usr/bin)
• Next, you need to copy the symbolic link libraries to which these files are linked against. You can use the tool ldd to interrogate the executable and get a list of files to copy, e.g:

  root@server-b:/home/backup-user# ldd /bin/bash
      linux-vdso.so.1 =>  (0x00007fff52bff000)
      libtinfo.so.5 => /lib/x86_64-linux-gnu/libtinfo.so.5 (0x00007f412810a000)
      libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007f4127f06000)
      libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f4127b79000)
      /lib64/ld-linux-x86-64.so.2 (0x00007f4128340000)

  Copy the files which have directories into the appropriate locations, e.g./lib/x86_64-linux-gnu/libtinfo.so.5 should go into/home/backup-user/lib/x86_64-linux-gnu/

• Do the same for /usr/bin/rsync