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add examples/var_service/README

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Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
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Denys Vlasenko 2010-12-06 15:13:58 +01:00
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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
or 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.
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 -
An *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.
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.