Very strange Drupal 7 404 behaviour

Question

I am having some very strange behaviour in D7 that I have not seen before. I have migrated a new site from my dev machine to a test machine, so code and DB are identical. On the dev machine (Ubuntu 13.04, Apache, PHP 5.5.3), everything works fine, but on the test box which is a Raspberry Pi running PHP 5.4.4 on Raspbian, I seem to get 404 errors on form submission. What seems to happen is that the form is not processed and the original page is returned but with a 404 status code when you look at the HTTP response.

This is affecting everything that uses POST, including login. I did manage to get it to log in by changing the 'action' attribute of the login form to point to '/' using Firefox's web developer tools, but lots of other things didn't work and the admin menu occasionally disappeared.

Thought it might be a caching issue so manually truncated all the cache tables but that made no difference. There appeared to be an issue with a non-existent temp folder so I created the one it was expecting with 777 perms (as I couldn't change the setting because form submission wouldn't work...) but it wasn't this either. Clean URLs are working fine.

Never seen anything like this before. Any ideas most appreciated!

Answer 1

Here is what seems to be the answer to this question (taken from a comment from OPer below the question):

I'd forgotten to enable mod_rewrite and mod_headers! What I was amazed about was that clean URLs were working at all without them. I think this is because .htaccess points 404s to Drupal, which will then interpret the requested URL and show the appropriate page, but of course Apache is sending a 404 header.