[Andreas]

A maybe easier, generalized and more failsafe approach:

In functions_newpost.php
FIND

PHP Code:

// ###################### Start convert_url_to_bbcode_callback ####################### 


ABOVE that ADD

PHP Code:

function autoparse_onsite($url)
{
    $parsed = parse_url($url);
    if (stripos($parsed['host'], getenv('HTTP_HOST')) !== false)
    {
        return '[url]' . $url . '[/url]';
    }
    else
    {
        return $url;
    }
} 


FIND

PHP Code:

$urlSearchArray = array(
    "#(^|(?<=[^_a-z0-9-=\]\"'/@]|(?<=" . $taglist . ")\]))((https?|ftp|gopher|news|telnet)://|www\.)((\[(?!/)|[^\s[^$!`\"'|{}<>])+)(?!\[/url|\[/img)(?=[,.]*(\)\s|\)$|[\s[]|$))#siU"
);

$urlReplaceArray = array(
    "[url]\\2\\4[/url]"
); 


REPLACE that with

PHP Code:

$urlSearchArray = array(
    "#(^|(?<=[^_a-z0-9-=\]\"'/@]|(?<=" . $taglist . ")\]))((https?|ftp|gopher|news|telnet)://|www\.)((\[(?!/)|[^\s[^$!`\"'|{}<>])+)(?!\[/url|\[/img)(?=[,.]*(\)\s|\)$|[\s[]|$))#siUe"
);

$urlReplaceArray = array("autoparse_onsite('\\2\\4')"); 


This should take care of only autoparsing "onsite" URLs everywhere - no need to mess with Checkbox settings.
Please not that it, if for xample the Board is running on http://forum.yourdomain.com and a User posts http://www.yourdomain.com, this will not be autoparsed.
Also, if a User posts http://forum.yourdomain.com.mydomain...esntmatter.php or http://www.foo.bar.forum.yourdomain....esntmatter.php it will be autoparsed.

@Merk
There are no security issues with these Modifications.
convert_url_to_bbcode() just wraps [url] around URLs - that's it.