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manual: document how to create a full backup without running restic as root

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Dennis Stosberg 2017-12-14 17:58:10 +01:00
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@ -270,3 +270,81 @@ restored:
The snapshot was successfully restored. This concludes the tutorial.
*****************************************************
Backing up your system without running restic as root
*****************************************************
Motivation
==========
Creating a complete backup of a machine requires a privileged process
that is able to read all files. On UNIX-like systems this is
traditionally the ``root`` user. Processes running as root have
superpower. They cannot only read all files but do also have the power
to modify the system in any possible way.
With great power comes great responsibility. If a process running as
root malfunctions, is exploited, or simply configured in a wrong way it
can cause any possible damage to the system. This means you only want
to run programs as root that you trust completely. And even if you
trust a program, it is good and common practice to run it with the
least possible privileges.
Capabilities on Linux
=====================
Fortunately, Linux has functionality to divide root's power into
single separate *capabilities*. You can remove these from a process
running as root to restrict it. And you can add capabilities to a
process running as a normal user, which is what we are going to do.
Full backup without root
========================
To be able to completely backup a system, restic has to read all the
files. Luckily Linux knows a capability that allows precisely this. We
can assign this single capability to restic and then run it as an
unprivileged user.
First we create a new user called ``restic`` that is going to create
the backups:
.. code-block:: console
root@a3e580b6369d:/# useradd -m restic
Then we download and install the restic binary into the user's home
directory.
.. code-block:: console
root@a3e580b6369d:/# mkdir ~restic/bin
root@a3e580b6369d:/# curl -L https://github.com/restic/restic/releases/download/v0.8.0/restic_0.8.0_linux_amd64.bz2 | bunzip2 > ~restic/bin/restic
Before we assign any special capability to the restic binary we
restrict its permissions so that only root and the newly created
restic user can execute it. Otherwise another - possibly untrusted -
user could misuse the privileged restic binary to circumvent file
access controls.
.. code-block:: console
root@a3e580b6369d:/# chown root:restic ~restic/bin/restic
root@a3e580b6369d:/# chmod 750 ~restic/bin/restic
Finally we can use ``setcap`` to add an extended attribute to the
restic binary. On every execution the system will read the extended
attribute, interpret it and assign capabilities accordingly.
.. code-block:: console
root@a3e580b6369d:/# setcap cap_dac_read_search=+ep ~restic/bin/restic
From now on the user ``restic`` can run restic to backup the whole
system.
.. code-block:: console
root@a3e580b6369d:/# sudo -u restic /opt/restic/bin/restic --exclude={/dev,/media,/mnt,/proc,/run,/sys,/tmp,/var/tmp} -r /tmp backup /