This directive lets Apache adjust the URL in the Location header on HTTP redirect responses. This is essential when Apache is used as a reverse proxy to avoid bypassing the reverse proxy because of HTTP redirects on the backend servers. It is generally used in conjunction with the ProxyPass directive to build a complete frontend proxy server.

ProxyPass          /perl/  http://backend.example.com/perl/
ProxyPassReverse   /perl/  http://backend.example.com/perl/

When a user initiates a request to http://www.example.com/perl/foo, the request is proxied to http://backend.example.com/perl/foo. Let's say the backend server responds by issuing a redirect for http://backend.example.com/perl/foo/ (adding a trailing slash). The response will include a Location header:

Location: http://backend.example.com/perl/foo/

ProxyPassReverse on the frontend server will rewrite this header to:

Location: http://www.example.com/perl/foo/

This happens completely transparently. The end user is never aware of the URL rewrites happening behind the scenes.

Note that this ProxyPassReverse directive can also be used in conjunction with the proxy pass-through feature of mod_rewrite, described later in this chapter.