CentOS - Mar 2017 - How rc-local.service works?

Hi,

Let me cite the service file here:

[Unit]
Description=/etc/rc.d/rc.local Compatibility
ConditionFileIsExecutable=/etc/rc.d/rc.local
After=network.target

[Service]
Type=forking
ExecStart=/etc/rc.d/rc.local start
TimeoutSec=0
RemainAfterExit=yes

I basically don't understand two things here:

1. What makes it run? Sure, when I make /etc/rc.d/rc.local executable,
it runs. But for all I know, for it to run some other service must
pull in it as a dependency. Am I wrong?

2. Why Type=forking?

Thanks in advance.

Regards,
Yuri

> Let me cite the service file here:
> 
> [Unit]
> Description=/etc/rc.d/rc.local Compatibility
> ConditionFileIsExecutable=/etc/rc.d/rc.local
> After=network.target
> 
> [Service]
> Type=forking
> ExecStart=/etc/rc.d/rc.local start
> TimeoutSec=0
> RemainAfterExit=yes
> 
> I basically don't understand two things here:
> 
> 1. What makes it run? Sure, when I make /etc/rc.d/rc.local executable,
> it runs. But for all I know, for it to run some other service must
> pull in it as a dependency. Am I wrong?
It will be run according to the status of the rc-local service - i.e.
what does 

  systemctl status rc-local

say. It's enabled by default, but it only runs if rc.local is
executable (as defined in the [Unit] section).
> 
> 2. Why Type=forking?
Err, because it forks! (i.e. it can leave processes running after the
script has finished executing.)

P.

> It will be run according to the status of the rc-local service - i.e.
what does
>   systemctl status rc-local
> say. It's enabled by default, but it only runs if rc.local is
executable (as defined in the [Unit] section).

When booted with /etc/rc.d/rc.local being not executable, it says:

? rc-local.service - /etc/rc.d/rc.local Compatibility
   Loaded: loaded (/usr/lib/systemd/system/rc-local.service; static;
vendor preset: disabled)
   Active: inactive (dead)

After making it executable:

? rc-local.service - /etc/rc.d/rc.local Compatibility
   Loaded: loaded (/usr/lib/systemd/system/rc-local.service; static;
vendor preset: disabled)
   Active: active (exited) since Sun 2017-03-12 11:08:47 EDT; 10s ago
  Process: 1012 ExecStart=/etc/rc.d/rc.local start (code=exited,
status=0/SUCCESS)

Mar 12 11:08:47 localhost.localdomain systemd[1]: Starting
/etc/rc.d/rc.local Compatibility...
Mar 12 11:08:47 localhost.localdomain systemd[1]: Started
/etc/rc.d/rc.local Compatibility.

But I dug into it more, and it revealed that systemd introduces the
concept of generators:

https://www.freedesktop.org/software/systemd/man/systemd.generator.html

Which are run early in the boot process (before unit files are
processed). And their purpose is to dynamically generate dependencies.
Particularly, rc-local-generator dynamically makes rc-local service a
dependency of multi-user.target if /etc/rc.d/rc.local is executable:

https://github.com/systemd/systemd/blob/v233/src/rc-local-generator/rc-local-generator.c#L89

Regards,
Yuri