Trouble with serving local content while using RewriteRule [P] - apache

Question

I'm trying to write rules that will serve files on the new server, while proxying files that don't exist, to the old server. The proxy rules work fine, sending missing files and directories to the old server.
The problem I'm having is new-server.example.com/blah (/blah/ exists on the new server) is passed through to the proxy rules and routed to the old server.
The reason for attempting the skip is because new-server.example.com/blah falls through to the "proxy section" and passes new-server.example.com/blah/index.php. Index.php doesn't exist on the new server, index.html does. The "proxy section" doesn't follow all the index file possibilities in DirectoryIndex, it just looks at the first one.
So I end up with error message, "The requested URL /blah/index.php was not found on this server."
How can this be written so it actually works? -- Thanks.
RewriteEngine On
# skip, proxy section
RewriteCond /var/www/html%{REQUEST_FILENAME} -f [OR]
RewriteCond /var/www/html%{REQUEST_FILENAME} -d
RewriteRule .? - [S=4]
# proxy section
RewriteCond /var/www/html%{REQUEST_FILENAME} !-f
RewriteCond /var/www/html%{REQUEST_FILENAME} !-d
RewriteRule ^/(.*) http://old-server.example.com/$1 [P]
ProxyPassReverse / http://old-server.example.com/
# rewrite log
pass through /index.php
init rewrite engine with requested uri /
applying pattern '.?' to uri '/'
RewriteCond: input='/var/www/html/' pattern='-f' => not-matched
RewriteCond: input='/var/www/html/' pattern='-d' => matched
pass through /
init rewrite engine with requested uri /index.php
applying pattern '.?' to uri '/index.php'
RewriteCond: input='/var/www/html/index.php' pattern='-f' => matched
pass through /index.php
init rewrite engine with requested uri /blah/
applying pattern '.?' to uri '/blah/'
RewriteCond: input='/var/www/html/blah/' pattern='-f' => not-matched
RewriteCond: input='/var/www/html/blah/' pattern='-d' => matched
pass through /blah/
init rewrite engine with requested uri /blah/index.php
applying pattern '.?' to uri '/blah/index.php'
RewriteCond: input='/var/www/html/blah/index.php' pattern='-f' => not-matched
RewriteCond: input='/var/www/html/blah/index.php' pattern='-d' => not-matched
applying pattern '^/(.*)' to uri '/blah/index.php'
RewriteCond: input='/var/www/html/blah/index.php' pattern='!-f' => matched
RewriteCond: input='/var/www/html/blah/index.php' pattern='!-d' => matched
rewrite '/blah/index.php' -> 'http://old-server.example.com/blah/index.php'
forcing proxy-throughput with http://old-server.example.com/blah/index.php
go-ahead with proxy request proxy:http://old-server.example.com/blah/index.php [OK]

Answer 1

This is the way Apache has worked since version 2.0: It gets a request, runs that request through all of the RewriteRules, then if the resulting request is a directory, it tries to see if the first DirectoryIndex will work, so it runs that through the RewriteRules and tries to serve the result. If the first DirectoryIndex entry doesn't work (i.e. it gets to the point where Apache would have served a 404), the server tries again with the next entry and so on. Your problem, obviously, is that the proxy rules latch on to the nonexistent first DirectoryIndex value and try to send it through the reverse proxy.
I haven't found a general way to fix this behavior server-wide, but there is a workaround. Use either a <Location> section in your httpd.conf file or a .htaccess file and override DirectoryIndex for that location.
In your case, create the file /var/www/html/blah/.htaccess and put the following into it:
DirectoryIndex index.html