subhost scripts

This is the script collection for a bespoke "subhost" platform for running sandboxed services directly on the infrastructure hosts rather than within virtual machines (VM).

pretend-boot

The pretend-boot script runs a subhost almost as if booted in a virtual machine, except of course, that it really is merely an "unshared sandboxing" executed by the the host kernel.

Example

# ./pretend-boot mysubhost

pretend-boot sets up the /opt/sub/mysubhost/live directory that it uses as root filesystem mount point. It requires the variables DISKS and BRIDGES to be set, either as environment variables, or via a configuration file that it by default expects to find at /opt/sub/mysubhost/config That config would be a textfile, or in fact a bash source file, that should set the variables DISKS and BRIDGES for running the subhost.

The DISKS variable is an array declaring the disks in terms the partitions to mount. Each array element has three parts with ":" between them:

1. the device node name to use in the subhost

2. the partition index

3. the disk image file on the host

The very first element of DISKS is the subhost root filesystem, which gets loop-mounted during the "pretend boot".

Example of DISKS

DISKS=( vda:1:/disks/disk1.raw vdb:1:/disks/disk2.raw )

The BRIDGES variable is an array declaring the network interfaces in terms of their host appearance. Each array element has two parts with ":" between them:

1. the host bridge to link up with

2. the subhost interface’s MAC address

Example of BRIDGES

BRIDGES=( homenet:06:a7:dd:b4:02:f2 )

Obviously the first ":" separates the bridge name from the MAC address, which in itself includes 5 (more) ":" characters.

subhost-init

The subhost-init script is copied into the subhost root filesystem, and it first performs the rudimentary pre-pivot tasks (mounting "proc" etc.), the runs rcS and rc2 before entering into a plain wait for anything to be written to /run/initctl which works as an immediate poweroff for the subhost.

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control

The control script is an alternative subhost run script used for experimentation and debugging.

It sets up the subhosts fully within the $SUBHOST directory tree, originally /opt/subhost, and each subhost system is represented there as its own directory tree whose top level contains the following:

• config is a bash include file that declares the subhost setup for the control script.

• live is the root of the subhost filesystem when activated. It gets set up either as an rbind mount of the subhost root or as an overlay mount placing the subhost root over a common, read-only base filesystem.

• root is the directory that contains the subhost filesystem (root filesystem). This may be the file system in full, or the writable part of the overlay filesystem.

• work is the work directory for the overlay mount, unless the subhost is equipped with an image file for its root filesystem (in which case that image file offers the work directory)

• mnt is optional. It is used as the mount target for the image file when the subhost has one.

• $SUB.img (for the subhost named $SUB)is the optional image file for a subhost. The image file must itself have top directories /root and /work which then are used instead of those of the subhost tree. I.e. with an image file, the overlay root portion is at $SUB/mnt/root and the associated work directory is at $SUB/mnt/work (following the mounting of the image $SUB.img onto mnt.

Networking

Each subhost $SUB gets set up with a network namespace $SUB, with veth virtual cabling for the $BRIDGES it is declared to attach to.

The cable interfaces show up inside the $SUB namespace as eth0, eth1, etc., while the host side interfaces have $SUB as prefix, followed be the same number.

Examples

start a subhost named future

# ./control start future

Starting a subhost will enter a bash shell "inside" the subhost. The subhost is "running" as long as that shell exists; it is basically like the "init" of the sandboxed subhost.

When exiting a subhost, its mounts and network namespace hang around, but all its processes have got killed. This is like a "hibernation state" for the subhost, which may be restarted (without stopping) using the start command again.

stop a subhost named future

# ./control stop future

The stop command unmounts everything for the subhost and deletes the network namespace.