overlay-boot/overlay-boot.8.adoc

overlay-boot(8)
===============
:doctype: manpage
:revdate: {sys:date "+%Y-%m-%d %H:%M:%S"}
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:EQUALS: =

NAME
----
overlay-boot - Start a subhost with overlay root filesystem.

SYNOPSIS
--------
*overlay-boot* _conf_

DESCRIPTION
-----------
*overlay-boot* is the main script on a small collection of
administration scripts for containerizing services with minimal ado.
The script starts a "subhost" with a dedicated network namespace, and
the mount and pid namespaces separated from the main host by means of
+unshare+. A subhost root file system may in particular be set up as
an overlay of the main host filesystem to keep the specifics of a
service distinctly separate from the main host while sharing files
wherever sensible.

A subhost is started by identifyinf its configuration file on the
command line for *overlay-boot*. The configuration file is a plain
text file with a small collection of "variables" that tell how the
subhost is set up. When all is good, *overlay-boot* spawns a
subprocess that invokes a command shell within an chroot into
"unshared" subhost root filesystem, all similar to the bootup of any
odd computer.

The subhost execution environment may be "entered" to perform
adminstrative tasks with *overlay-go*, and it is later stopped with
*overlay-stop*.

OPTIONS
-------

An overlay-boot subhost is defined in the configuration file, which is
a plain text file with a number of variable assignments. Each
assignment is written with the varable name flush left and immediately
followed by an equal sign, The rest of that line (ignoring leading and
trailing spaces) is its value, or if that value startes with an
exclamation mark, then the line is a command to run so as to generate
the value. See examples below.

*NAME*::

This variable declares a short name for the subhost, and should be no
more than 12 printable ascii characters. The base name of the
configuration file is used by default. I.e., a configuration file
named +foo.conf+ by default names its subhost +foo+ unless there is a
+NAME+ variable says differently.

*BASE*::

This variable declares a pathname for a directory that is considered
to be a "base" for the subhost setup. This is the only required
variable.

*CABLES*::

This variable declares the subhost networking in terms of its virtual
cables. The value is a space separated list of "virtual cable
specifiers", each consisting of an equal sign optionally with a bridge
name to the left and optinally a MAC address to the right. See the
section on Networking below for more details.

INIT::

This variable is a command line to run, with envirnment variable
CONFIG set, for producing the initial commands to the running subhost,
similar to the initrd phase of a computer bootup. The default value
for this variable is +overlay-init+.

*LIVE*::

This variable nominates the mount point for the running subhost's root
file system. It defaults to +$BASE/live+ The nominated directory must
exist, and depending on the directory pathnames in the +UPPER+ and
+LOWER+ variables, the subhost root filesystem is either of a
pre-mounted directory, bind mounted or overlay mounted directory. See
the details of this with the UPPER variable below.

*LOG*::

This variable nominates the logfile to use by +overlay-boot+ when
running the subhost. The default is +/tmp/overlay-$NAME.log+.

*LOWER*::

This variable nominates the "lower" filesystem of an overlay mount.
This will be accessed read-only, an it is intended to be the operating
system root file system. The default is +/+, i.e. the main host root
filesystem. When overlay is not desired, then LOWER should be the smae
as UPPER.

*POSTMOUNT*::

This variable is a command line to run, with envirnment variable
CONFIG set, just after the setup of the subhost root filesystem and
before the services are started. The default for this variable is
+overlay-postmount+

*PREMOUNT*::

This variable is a command line to run, with envirnment variable
CONFIG set, just before the setup of the subhost root filesystem and
before the services are started. The default for this variable is
+overlay-premount+

*UPPER*::

This variable nominates the "upper" filesystem for an overlay mount.
This will be accessed read-write and it constitutes the "private"
files of the subhost.

If UPPER is different from LOWER, then the root file system is set up
as an overlay mount. In that case WORK (below) must be defined as a
directory outside of but on the same mount as UPPER.

If UPPER is the same as LOWER, then the subhost root filesystem is not
setup as an overla. Rather it is set up as a bind mount, if UPPER is
different from LIVE, or just as a pre-mounted LIVE filesystem.

*WORK*::

This variable nominates the "work" directory for an overlay mount. It
has to be a writable directory on the same mount device as the UPPER
directory.

*START*::

This variable tells which services should be started on the
overlay-boot startup. The default value is +networking ssh+.

Note that if no services are started, then +overlay-init+ starts a
+dummy_service+ so as to keep +/.reaper+ from running out of child
processes, as it otherwise will exit and thereby terminate
+overlay-boot+.

*RAM_SIZE*::

This variable may be used to configure the +/run+ directory which by
defult gets mounted on an overlay mount as a +tmpfs+ of 50M. Use
+none+ to avoid that mount, and otherwise the desired +tmpfs+ size.

Networking
----------

*overlay-boot* sets up a nework namespace named by the subhost NAME,
and uses the CABLES variable to set up +veth+ virtual cables. The host
end of such cables are named by NAME followed by a number from 0 and
up while the subhost end are named by +eth+ followed by the same
number.

As mentioned above, CABLES consists of a space separated list of cable
specifiers, each consisting of a bridge interface name and a with an
equal sign ("=") between them. The equal sign is required while either
or both of the bridge and MAC address may be left empty.

The bridge interface name, if given, will be given control of the host
end cable interface. A cable specification with empty bridge name part
results in that the host end is brought up at link level and then
+ifup $IF+ is attempted.

The MAC address, if given, is used for the subhost end cable
interface. A cable specification with empty MAC address part results
in that the interface get its MAC address from the kernel, possibly a
different one upon each start.

EXAMPLES
--------

./opt/subhost/mta/mta.conf
****
BASE=.
START= rsyslog newtorking ssh postfix
****

The above example assumes a directory +/opt/subhost/mta+ that contains
the configuration file +mta.conf+ and directories +root+, +work+ and
+live+. *overlay-boot* will set up an overlay mount on +live+ with
+root+ as UPPER, +work+ as WORK and +/+ as LOWER, i.e. an overlay of
the main host filesystem.