The RewriteBase directive specifies the URL prefix to be used for per-directory (htaccess) RewriteRule directives that substitute a relative path.

This directive is required when you use a relative path in a substitution in per-directory (htaccess) context unless any of the following conditions are true:

• The original request, and the substitution, are underneath the DocumentRoot (as opposed to reachable by other means, such as Alias ).
• The filesystem path to the directory containing the RewriteRule , suffixed by the relative substitution is also valid as a URL path on the server (this is rare).
• In Apache HTTP Server 2.4.16 and later, this directive may be omitted when the request is mapped via Alias or mod_userdir .

In the example below, RewriteBase is necessary to avoid rewriting to http://example.com/opt/myapp-1.2.3/welcome.html since the resource was not relative to the document root. This misconfiguration would normally cause the server to look for an "opt" directory under the document root.

DocumentRoot "/var/www/example.com"
AliasMatch "^/myapp" "/opt/myapp-1.2.3"
<Directory "/opt/myapp-1.2.3">
 RewriteEngine On
 RewriteBase "/myapp/"
 RewriteRule "^index\.html$" "welcome.html"
</Directory>