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OVERLAY-BOOT(8)   OVERLAY-BOOT(8)

NAME

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

SYNOPSIS

overlay-boot conf

DESCRIPTION

overlay-boot is the main script in a small collection of administration scripts for containerizing services with minimal ado. The script starts a "subhost" whose root filesystem is an overlay on the main host filesystem, and with separate mount, network and pid namespaces. In effect the default use case is merely an administrative sandboxing that is pre-populated with a copy of the overlaid filesystem.

Each subhost is defined by means of a configuration file, conf, that is a simple text file with small collection of "variables" telling how the subhost is set up. overlay-boot spawns a subprocess that performs the boot-up of the "subhost" as an init script that ends with a pid 1 reaper that simply "reaps" any terminated child processes.

An administrator may "enter" the subhost execution environment to perform adminstrative tasks by means of overlay-go, which uses chroot to start an interactive shell within the subhost namespaces. Such a shell however is not a child of the subhost pid 1 and it would normally only be used initially for configuring the subhost’s network, and set up network based services, such as sshd, for subsequent access.

Subhost execution is stopped with overlay-stop.

OPTIONS

An overlay-boot subhost is defined in a configuration file that 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. But if that value startes with an exclamation mark, then the line is a command to run during start-up 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.

A typical subhost setup is defined by a dedicated directory that contains the configuration file and the three mount points "root", "work" and "live".

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 optionally a MAC address or VLAN tag to the right. See the section on Networking below for more details.

INIT

This variable is a command to run (with environment variable CONFIG set) that outputs on its standard output the series of commands for the running subhost. The default value for this variable is /var/lib/overlay-boot/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 one of

1.a pre-mounted directory,

2.bind mounted with UPPER, or

3.and overlay mount of UPPER (and WORK) over LOWER. See also 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, and 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 environment variable CONFIG set) following the setup of the subhost root filesystem and before the services are started. The default for this variable is /var/lib/overlay-mount/overlay-postmount. Note that this command is executed within the mount namespace of the subhost, and still with the main host as root filesystem. The POSTMOUNT command is executed for all of the three different root filesystem setup variants.

PREMOUNT

This variable is a command line to run (with environment 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 Note that this command is executed within the mount namespace of the subhost, and still with the main host as root filesystem. The PREMOUNT command is executed for both the overlay and bind mount variants of root filesystem setup but not for plain root filesystem, which does not involve mounting LIVE.

RAM_SIZE

This variable configures the subhost /run directory which by default is mounted as a tmpfs of 50M. Use none to avoid that mount, and otherwise the desired tmpfs size.

START

This variable names the services to 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.

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 overlay. Rather, if UPPER is different from LIVE, the root filesystem is set up as a bind mount, and if UPPER and LIVE are also the same, then LIVE is used as root filesystem without additional mounting.

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.

SHARE

This variable nominates a pathname under $LOWER to bind-mount onto the same pathname under $LIVE so as to share that directory tree into the overlay. The SHARE variable is a special configuration variable in that the value does not allow exclamation mark evaluation, and that it may occur many times for declaring multiple shared subdirectories.

NETWORKING

overlay-boot sets up a nework namespace named by the subhost $NAME, and uses the CABLES variable to set up veth type 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 is a space separated list of cable specifiers, each consisting of an optional bridge interface name, an optional MAC adddress or VLAN tag and with a required equal sign ("=") between them.

The bridge interface name, when given, will be given control of the host end cable interface. When the bridge interface name is omitted (empty) then the host end interface attracts an ifup $IF attempt instead.

The MAC address, if given, is used for the subhost end cable interface, which otherise gets its MAC address from the kernel, possibly a different one upon each start.

A VLAN tag has the format ".N" where N is a number between 1 and 4095. This modifies the cable function to set up a VLAN host interface on the prior host interface, and skip subhost side setup. E.g. if the prior host interface is example1 and the tag is .302 then the VLAN interface would be example1.302. The host side setup uses ifup $IF and thus, the host needs to have a supporting configuration entry in /etc/network/interfaces for the VLAN interface.

Note that it may be a good practice to keep a local interfaces file as sibling to the subhost configuration file on the host and use a source statement to include this into the system networking configuration.

EXAMPLES

Smallest possible

# mkdir -p /opt/subhost/copy/{root,work,live}
# cat << EOF > /opt/subhost/copy/copy.conf
BASE=.
EOF

This setup has a minimal configuration for a subhost that overlays the root filesystem but is without networking. The subhost must be entered with overlay-go, although the default start might have started sshd listening on a loopback interface in the subhost’s network namespaces.

Note that overlay-go runs a shell within the namespaces, but not as a child of the "subhost init" (aka .reaper).

/opt/subhost/tiny/tiny.conf

BASE=.
CABLES= =
START= none
LOWER= base

The tiny subhost would be for overlaying a separate debootstrap root filesystem, without any services (since START is "none"). This gets started with a dummy_service to hold the overlay for access via overlay-go. The dummy_service sets up and listens on a pipe at /run/dummy_service, and exits when anything is written to that.

/opt/subhost/mta/mta.conf

BASE=.
CABLES= =
START= rsyslog networking ssh saslauthd postfix dovecot

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. Further, the running subhost will feature a virtual cable to the main host, where the subhost end is named eth0 and the main host end is named mta0, and upon start, an ifup mta0 is attempted at the host end while the subhost end is handled via its neworking service.

SEE ALSO

overlay-stop, overlay-go

2025-04-15