Errors when pointing to domainname, no error with local IP

Warning: longish post.
I have this setup with 4 VMs.

• Internet->Local Network Varnish router->firstdomain1.com (VM1)
• Internet->Local Network Varnish router->seconddomain2.com (VM2)
• Internet->Local Network Varnish router->Internal IP3 (VM3)
• Internet->Local Network Varnish router->Internal IP4 (VM4)

WordPress is installed on VM3 and has an internal IP. On the network, I see no fileupload errors. The goal is to have wordpress on IP3 behave as thirddomain3.com

Step 1:
I point thirddomain3.com to IP3 using Varnish, and I can see wordpress from the internet. However all links for a user from the internet point to a local ip3, and as soon as a user clicks on comments or a home page the server tries to go to local IP3 and times out. I can still get to wordpress and upload images if I am on the local network, with no problems.

Step 2:
So I went to wordpress and update the Settings->General->WordPress Address (URL) and Settings->General->Site Address (URL) to thirddomain3.com. All external users can browse the site but i now get errors on the local network. When I try to upload an image on a post I get aync-upload.php 403 forbidden error even though the mod_Security is off in the htaccess file !! Cant seem to win!

How do I configure wordpress , apache or varnish or all 3 somehow, so I can access the site over the internet using thirddomain3.com and also be able to upload the images and not get the 403 error ?

Here is my htaccess file

<IfModule mod_security.c>
<Files async-upload.php>
SecFilterEngine Off
SecFilterScanPOST Off
</Files>
</IfModule>

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>
# END WordPress