Manual:$wgNoFollowLinks

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Parser: $wgNoFollowLinks
If true, external URL links in wiki text will be given the `rel="nofollow"` attribute.
Introduced in version:	1.4.0 (r7174)
Removed in version:	Still in use
Allowed values:	(boolean)
Default value:	`true`
Other settings: Alphabetical \| By function

Details

If true, external URL links in wiki text will be given the rel="nofollow" attribute as a hint to search engines that they should not be followed for ranking purposes as they are user-supplied and thus subject to spamming. Defaults to true.

Setting nofollow for red links

It may be desirable to configure MW to append rel="nofollow" to internal links that point to pages that haven't yet been written (so-called "red links") for various reasons that include avoiding unnecessary crawler traffic to non-extant pages or for the possibility of improved SEO by avoiding punitive action against a site's ranking due to the presence of "broken links" that aren't broken, just not yet authored.

This may be accomplished by using the HtmlPageLinkRendererEnd hook as follows:

// Add rel="nofollow" to links to pages that don't exist (redlinks)
$wgHooks['HtmlPageLinkRendererEnd'][] = 'noFollowRedLinks';
function noFollowRedLinks(
    $linkRenderer, $target, $isKnown, &$text, &$attribs, &$ret)
{
    if (!$isKnown && preg_match('/\bnew\b/S', $attribs['class'] ?? "")) {
        $attribs['rel'] = 'nofollow';
    }
    return true;
}

$wgNoFollowDomainExceptions
$wgNoFollowNsExceptions
Extension:Interwiki - the easiest way to manage whitelisting (since nofollow is not applied to interwiki links)
Manual:Nofollow
meta:nofollow, Meta-Wiki discussion on nofollow's use on Wikimedia sites. Some of the arguments raised there may also applicable to other wikis' decisions concerning this configuration setting.

Manual:$wgNoFollowLinks

Details

Setting nofollow for red links

See also