overlay-boot(8) Manual Page
overlay-boot - Start a subhost with overlay root filesystem.
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
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.
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.
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.confby default names its subhost
foounless there is a
NAMEvariable says differently.
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.
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.
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
This variable nominates the mount point for the running subhost’s root file system. It defaults to
$BASE/liveThe nominated directory must exist, and depending on the directory pathnames in the
LOWERvariables, 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.
This variable nominates the logfile to use by
overlay-bootwhen running the subhost. The default is
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.
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
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
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.
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.
This variable tells which services should be started on the overlay-boot startup. The default value is
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
This variable may be used to configure the
/rundirectory which by defult gets mounted on an overlay mount as a
tmpfsof 50M. Use
noneto avoid that mount, and otherwise the desired
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
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.
BASE=. START= rsyslog newtorking ssh postfix
The above example assumes a directory
/opt/subhost/mta that contains
the configuration file
mta.conf and directories
live. overlay-boot will set up an overlay mount on
root as UPPER,
work as WORK and
/ as LOWER, i.e. an overlay of
the main host filesystem.