This is intended for net.*.conf.*.foo files. Setting just "default" is not very
useful because any interfaces present before systemd-sysctl is invoked are not
affected. Setting "all" is too harsh, because the kernel takes the stronger of
the device-specific setting and the "all" value, so effectively having a weaker
setting for specific interfaces is not possible. Let's add a way in which can
set "default" first and then all the others without "all".
This section is loaded in a bunch of places, so this affects many
man pages.
1. point the reader to the synopsis section, which has the exact paths
that are used to load files.
2. put the "reference" part first, and recommendations later, in separate
paragraphs.
3. describe how individual settings and whole files are replaces.
Closes#12791.
(This also changes the suggested /etc/nsswitch.conf line to use for
hooking up nss-system to use glibc's [SUCCESS=merge] feature so that we
can properly merge group membership lists).
This also updates the suggested PAM snippet in a number of way:
1. Be closer to the logic nowadays implemented in Fedora where the
auth/account/password stacks are all finished off with
pam_{deny|permit}.so
2. Make pam_unix.so just "sufficient" instead of "required" (paving
ground for pam_systemd_home.so being hooked in as additional
sufficient module.
3. Only do pam_nologin in the "account" stack, since it's about account
validity really.
4. Use modern parameters to pam_unix when changing passwords, i.e.
sha512 and shadow, and use already set up passwords (preparing ground
for pam_systemd_home again)
There are sometimes users who put unit files in a location that is inaccessible
when systemd starts although they are not found and thus not started because
the corresponding mount units have not activated yet.
There is already a warning for such issue in man 8 systemctl:
link PATH...
...<snip>...
The file system where
the linked unit files are located must be accessible
when systemd is started (e.g. anything underneath /home
or /var is not allowed, unless those directories are
located on the root file system).
However, it looks that it's difficult to find the warning because introductory
users typically doesn't know systemctl link.
Although there is a description in UNIT FILE LOAD PATH pointing to systemctl
link, symlink is now not explicitly mentioned there and thus users doesn't
easily get aware of they should read it.
To deal with this, let's describe "symlink" and "systemctl link" more
explicitly in UNIT FILE LOAD PATH.
The document is rather huge, and a specific link is easier to consume. The form
is a bit strange because troff puts the symlink at the bottom, keyed by title,
so we need to use the same link target in all places.
Let's clarify that scope units can fail, but not due to process exit
statusses.
This hopefully clears up some confusion that manifested in #14142: scope
units can fail, but for other reasons than assumed there.
Fixes: #14142
