Trouble with mod_rewrite, subdomain and codeigniter - .htaccess

Question

I have a codeigniter installation at example.com/ci.
I have a subdomain foo.example.com. The document root for the foo subdomain is set to be home/public_html/ci.
I'm using the following rule in .htaccess to send requests for foo.example.com to example.com/ci/city/foo.
RewriteCond %{HTTP_HOST} !^(www)\. [NC]
RewriteCond %{HTTP_HOST} ^(.*)\.example\.com [NC]
RewriteRule (.*) http://example.com/ci/city/%1/$1 [L]
It all works like I want it to except that the address bar url changes from foo.example.com to example.com/ci/city/foo. I would like it to remain foo.example.com. There is no R=301 in the RewriteRule (used to be but I removed it). The .htaccess file is in the ci/ folder and the rule is above all the codeigniter stuff.
The redirect works perfectly and the url remains foo.example.com with (Jon Lin's answer)
RewriteCond %{HTTP_HOST} !^(www)\. [NC]
RewriteCond %{HTTP_HOST} ^(.*)\.example\.com [NC]
RewriteCond %{REQUEST_URI} !/city/
RewriteRule (.*) /city/%1/$1 [L]
but the codeigniter default controller is called instead of the foo method in the city controller.
Any help is appreciated.

Answer 1

When your rewrite rule's target has an http://example.com in it, a 302 redirect is implicit regardless of whether an R flag is used or not. You need to provide the URI path based on the subdomain's document root, so I'm assuming you want something like:
RewriteCond %{HTTP_HOST} !^(www)\. [NC]
RewriteCond %{HTTP_HOST} ^(.*)\.example\.com [NC]
RewriteCond %{REQUEST_URI} !/city/
RewriteRule (.*) /city/%1/$1 [L]
If the subdomain's document root is in the /ci/ directory.
The other option is to use the P flag to reverse proxy the request:
RewriteCond %{HTTP_HOST} !^(www)\. [NC]
RewriteCond %{HTTP_HOST} ^(.*)\.example\.com [NC]
RewriteRule (.*) http://example.com/ci/city/%1/$1 [L,P]

Answer 2

Your mileage may vary with this (might need to finesse it to fit your server and conditions), but doing some testing on my Mac, here's what I had mild success with:
Directory Structure
public_html/
ci/
application/
system/
.htaccess
index.php
I'm assuming that you have other stuff in your root public_html directory. So I'm letting the .htaccess focus on the CodeIgniter-related stuff by leaving it in the ci dir.
.htaccess
DirectoryIndex index.php
RewriteEngine on
RewriteCond %{HTTP_HOST} ^(.*)\.ciwildsub\.dev [NC]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.+)$ index.php/city/%1/$1 [L,QSA]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ index.php/$1 [L,QSA]
It's fairly self explanatory, but the first block is your subdomain check. I didn't bother excluding www but you may want to (as I said, your mileage may vary). The second block is a standard CodeIgniter index.php removal.
These rules will only apply to sub.example.com or example.com/ci/ URLs, since as I said, I assume your root has stuff that shouldn't be disturbed by rewrites.
CodeIgniter Config
$config['uri_protocol'] = 'PATH_INFO';
Because of the way Apache handles a URL like example.com/index.php/controller/method, it bypasses the index.php and handles it like any other directory segment. Also, mod_rewrite doesn't necessarily stop at the [L] tag -- it stops processing the .htaccess at that point, passes through the RewriteRule, and then runs that URL through the .htaccess. Setting PATH_INFO helps make sure CodeIgniter pulls the current URI correctly, and our .htaccess doesn't get stuck in a validation loop.
I will note, though, that I'm not entirely happy with what I see in my RewriteLog output -- there has to be a way to optimize this further, I'm just not sure of it yet (I'm done tinkering with this for today!). Sorry if any of the explanation here is a little out of whack - I'm not a server admin or mod_rewrite expert, I've just had fun tinkering with this. If I manage to find a better solution, I'll be sure to update this.
Looks like the END flag would be perfect for situations like this (to prevent [L] loops), but it's only available in Apache 2.3.9+. The search continues.

Answer 3

I got it to work correctly using the following rewrite rule
RewriteCond %{HTTP_HOST} !^(www)\. [NC]
RewriteCond %{HTTP_HOST} ^(.*)\.example\.com [NC]
RewriteCond %{REQUEST_URI} !/city/
RewriteRule (.*) /city/%1/$1 [L]
and by setting
$config['uri_protocol'] = 'ORIG_PATH_INFO';
in the codeigniter config file. Thanks for all the help.

Answer 4

This worked for me
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /file_path/to/subdomain
RewriteCond $1 !^(index\.php|images|robots\.txt)
RewriteRule ^(.*)$ /index.php/$1 [L]
RewriteCond %{REQUEST_URI} ^application.*
RewriteRule ^(.*)$ /index.php?/$1 [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ index.php?/$1 [L]
</IfModule>
<IfModule !mod_rewrite.c>
ErrorDocument 404 /index.php
</IfModule>