busybox/examples/var_service/README

In many cases, network configuration makes it necessary to run several daemons:
dhcp, zeroconf, ppp, openvpn and such. They need to be controlled,
and in many cases you also want to babysit them. runsvdir is a good tool for this.
examples/var_service directory provides a few examples. It is meant to be used
this way: copy it somewhere (say, /var/service) and run something like

env - PATH=... <other vars=...> runsvdir /var/service &

from one of system startup scripts. (Google "man runsvdir" and "man runsv"
for more info about these tools).

Some existing examples:

var_service/dhcp_if -
controls a udhcpc instance which provides dhpc-assigned IP
address on interface named "if". Copy/rename this directory as needed to run
udhcpc on other interfaces (var_service/dhcp_if/run script uses _foo suffix
of the parent directory as interface name). When IP address is obtained or lost,
var_service/dhcp_if/dhcp_handler is run. It saves new config data to
/var/run/service/fw/dhcp_if.ipconf and (re)starts /var/service/fw service.
This example can be used as a template for other dynamic network link services
(ppp/vpn/zcip).

var_service/ifplugd_if -
watches link status of interface if. Downs and ups /var/service/dhcp_if
service accordingly. In effect, it allows you to unplug/plug-to-different-network
and have your IP properly re-negotiated at once.

var_service/dhcp_if_pinger -
Uses var_service/dhcp_if's data (/var/service/dhcp_if/dhcp_if.out file)
to determine router IP. Pings it. If ping fails, restarts /var/service/dhcp_if
service. Basically, an example of watchdog service for networks
which are not reliable and need babysitting.

var_service/fw -
A *one-shot* service which reconfigures network based on current known state
of ALL interfaces. Uses conf/*.ipconf (static config) and /var/run/service/fw/*.ipconf
(dynamic config from dhcp/ppp/vpn/etc) to determine what to do.
One-shot-ness of this service means that it shuts itself off after single run.
IOW: it is not a constantly running daemon sort of thing.
It starts, it configures the network, it shuts down, all done
(unlike infamous NetworkManagers which sit in RAM forever, doing hell knows what).

However, any dhcp/ppp/vpn or similar service can restart it anytime
when it senses the change in network configuration.
This even works while fw service runs: if dhcp signals fw to (re)start
while fw runs, fw will not stop after its execution, but will re-execute once,
picking up dhcp's new configuration.
This is achieved very simply by having
# Make ourself one-shot
sv o .
at the very beginning of fw/run script, not at the end.
Therefore, any "sv u /var/run/service/fw" command by any other
script "undoes" o(ne-shot) command if fw still runs, thus
runsv will rerun it; or start it in a normal way if fw is not running.

System administrators are expected to edit fw/run script, since
network configuration needs are likely to be very complex and different
for non-trivial installations.