This change guards against possible regressions introduced by
f3766990f9 if rfkill were to
prevent netlink carrier up events from being delivered while
rfkill is in effect for the interface.
If no 'script-file = SCRIPTFILE' is specified in the configuration
file and if no '-X SCRIPTFILE' or '--script-file SCRIPTFILE'
command argument is provided, then this functionality is entirely
inactive and no associated subprocess is spawned.
Otherwise, ndhc will spawn a subprocess that runs as root that has the
sole job of forking off a subprocess that exec's the specified script in
a sanitized and fixed-state environment whenever a new DHCPv4 lease is
acquired.
Note that this script is provided no information about ndhc or the
DHCP state in the environment or in any argument fields; it is the
responsibility of this script to gather whatever information it needs
from either the filesystem or syscalls. This design is intended to
avoid the historical problems that are associated with dhcp clients
invoking scripts.
The path of the scriptfile cannot be changed after ndhc is initially
run; ndhc forks off the privsep script subprocess that executes scripts
after it has read the configuration file and command arguments, but
before it begins processing network data; thus, it is impossible for the
network-handling process to modify or influence the script assuming
proper OS memory protection.
The privsep channel communicates that the script should be run by simply
writing a newline; anything else will result in ndhc terminating itself.
Before the recommended way to update system state after a change in
lease information was to run the fcactus program and watch the
associated leasefile for the interface for modification; now no external
program is needed for this job.
This can be enabled via the s6-notify configure option; see
http://www.skarnet.org/software/s6/notifywhenup.html for details.
ndhc will signal that it is ready when the first valid lease is
obtained. Programs dependent on a working network interface
can then simply use s6-wait on the associated ndhc service dir.
A typical command line option assuming the s6 service directory
notification-fd contains '3' would be '--s6-notify 3', and
a typical configure file option would be 's6-notify 3'.
