Here's how I caused 25 minutes of downtime to my web server today.
Upon restart of the http deamon, Apache runs an action called configtest, which parses all files in conf.d/ the
name of which ends in .conf, and decides if they are valid configuration files. This is defined in your server's
configuration file, in a line that should be something similar to:
Include conf.d/*.conf
What I didn't know was that the server will refuse to start altogether, if it doesn't like an instruction in those
files, so, not only had I placed textfiles in that folder that were not valid apache config files, but at some point I
recommended this to others also. In my case, the files were combinations of credentials used for authentication by
.htaccess rules, files produced with htpasswd.
The problem only came up when I restarted httpd to enable new configuration elsewhere. Of course Apache offered a
useful explanation (where USERNAME and aSdFG123456ASdFG are the username and the hashed password used for access by
.htaccess):
Invalid command 'USERNAME:aSdFG123456ASdFG', perhaps misspelled or
defined by a module not included in the server configuration
Action 'configtest' failed.
The Apache error log may have more information.
...fail!
It seemed like a logical and convenient place to put the credentials' files upon creation, but it made apache refuse to
launch, because I had the inspiration to name those files something.conf. I checked some older web servers, with
prehistoric versions of apache -some 5 years old- and I estimate that this is not an issue for those versions, I assume
that maybe older versions of Apache ignored unknown instructions and started anyway.