Go to your
sources/Subs.php file in your SMF folder and go to somewhere around lines 1578. Add the
nofollow to the definition of the array elements.
array(
'tag' => 'url',
'type' => 'unparsed_content',
'content' => '<a href="$1" target="_blank"
rel="nofollow">$1</a>',
'validate' => create_function('&$tag, &$data, $disabled', '$data = strtr($data, array(\'<br />\' => \'\'));'),
),
array(
'tag' => 'url',
'type' => 'unparsed_equals',
'before' => '<a href="$1" target="_blank"
rel="nofollow">',
'after' => '</a>',
'disallow_children' => array('email', 'ftp', 'url', 'iurl'),
'disabled_after' => ' ($1)',
),
Note, this will add nofollow to all a href links whether they point outside the server or to another topic in your forum.
There is also the
NoFollow All Links v1.0 module that can do the code change for you:
http://custom.simplemachines.org/mods/index.php?mod=1236
