httpd: Could not reliably determine the server's fully qualified domain name

The following line in your httpd.conf file is correct:

ServerName localhost

The problem is that on macOS it is the wrong file (not /private/etc/httpd.conf).

To find the right location of your httpd.conf Apache configuration file, run:

apachectl -t -D DUMP_INCLUDES

then double check whether ServerName is uncommented and set to localhost.

Ignore ALL of the advice about setting ServerNames. IMHO this is a red herring. Apache is trying to use standard system components to determine a FQDN and not finding anything suitable. Sure, you could subvert this process by adding a directive globally to Apache, but methinks this is treating the symptom. You should have ServerName directives in your virtualhost blocks, of course.

Instead, edit your hosts file and be sure that you have a FQDN listed there. I have seen various suggestions on the syntax for the particular 127* line, but I think that it doesn't matter what order things are in, so long as there is a FQDN listed:

127.0.0.1       localhost.localdomain   localhost       foo.example.com

This worked for me and seemed much more to the point than all of the other "worked for me!" upvotes on posts recommending you edit httpd.conf and such. I hope this helps and that I don't get downvoted. If you disagree, please state why with examples. Thanks.

http://wiki.apache.org/httpd/CouldNotDetermineServerName

Once Apache can determine the system's FQDN, it will then read your specific ServerName directives for your NameBasedHosts.

I recently had the same problem on a mac and though I did edit my host file as Spanky suggests (and I think this is also necessary), I also needed to edit my httpd.conf.

in case anyone has this in the future the following may work

in your /etc/apache2/httpd.conf add this line after the ServerAdmin [email protected] line

ServerName your-favorite-server-name

source: https://blog.cloudtroopers.com/upgrade-osx-1010-yosemite-and-keep-apache-functional