Apache Organization: https://www.apache.org/
Apache Week: https://www.apacheweek.com/
Netcraft: https://www.netcraft.com/
Red Hat: https://www.redhat.com/
Digest Authentication Method: https://www.apache.org/docs/mod/mod_digest.html
HTTP RFC: https://www.w3.org/Protocols/rfc2616/rfc2616.html
man httpd
man db
man dbmmanage
man htpasswd
Apache: The Definitive Guide, Second Edition by Ben Laurie and Peter Laurie (ISBN: 1-56592-528-9)
Apache Web-Server, by Lars Eilebrecht (ISBN: 3-8266-0612-4)
Apache Quick Reference Card, by Andrew Ford (ISBN: 0-9534897-0-1)
Apache Server Administrators Handbook, by Mohammed J. Kabir (ISBN: 0-7645-3306-1)
comp.infosystems.www.servers.unix