2020-10-24
Why configuration file snippets in a directory should have some extension
After a great deal of painful experience with the combination of local configuration tweaks combined with vendor upgrades, many systems and people have adopted an approach of splitting monolithic configuration files apart into multiple snippets that sit in a directory. One of the latest I've run into is Fedora with sshd configuration, with is especially relevant to me because I've been customizing mine for years (and carefully re-merging my customizations after various upgrades). However, there is an important thing to bear in mind when setting up such a system.
When you do this split and support including snippets from a
directory, you should always require that the snippets have a
specific extension, conventionally .conf, instead of just
accepting any old file there. A big reason for this is that many
Linux packaging systems may wind up creating or leaving oddly named
files there when a package is added, upgraded, or removed under the
right circumstances; for example, RPM (used on Red Hat Enterprise
Linux among others) can create <something>.rpmnew and .rpmsave
files. These variously created files should not be treated as live
configuration snippets.
(Similarly, some systems for automatically modifying files will
leave backup versions of the file around with some extension like
.bak. You can usually turn this off, but you have to remember to
do so; mistakes are inevitable.)
Requiring a specific extension also makes it easier to temporarily
deactivate a snippet (just rename it to add a suffix on the extension),
put in a README file to explain what you're doing, and so on.
Other methods of marking which snippets should be active don't cooperate as well with common package managers and generally aren't as obvious. If you're writing or modifying local software, you may not care about package managers (although you never know, you may want to put your software in one someday), but there's value in the other advantages of requiring an extension and other things on your systems are probably already working this way.
(Fedora's modification of their sshd_config to move it to being
modified through snippets in /etc/ssh/sshd_config.d unsurprisingly
requires all of the snippets to have a .conf extension.)
PS: This may be a standard new OpenSSH thing, since Ubuntu has it as well, and thus presumably Debian too. If anything Fedora is late to this party.