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