How to recognize Facebook User-Agent

Question

When sharing one of my pages on FB, I want to display something different. Problem is, I prefer not to use the og: elements, but to recognize FB user-agent.

What is

Answer 1

"Facebook's user-agent string is facebookexternalhit/1.1 (+http://www.facebook.com/externalhit_uatext.php)..."

Hi

Small, yet important, correction -> Facebook external hit uses 2 different user agents:

facebookexternalhit/1.0 (+http://www.facebook.com/externalhit_uatext.php)
facebookexternalhit/1.1 (+http://www.facebook.com/externalhit_uatext.php) 

Setting you fitler to 1.1 only may cause filtering issues with 1.0 version.

For more information about Facebook Bot (and other bots) please refer to Botopedia.org - a Comunity-Sourced bot directory, powered by Incapsula.

Besides user-agent data, the directory also offers an IP verification option, allowing you to cross-verify an IP/User-Agent, thus helping to prevent impersonation attempts.

Answer 2

For list of user-agent strings, look up here. The most used, as of September 2015, are facebookexternalhit/* and Facebot. As you haven't stated what language you're trying to recognize the user-agent in, I can't tell you more information. If you do want to recognize Facebook bot in PHP, use

if (
    strpos($_SERVER["HTTP_USER_AGENT"], "facebookexternalhit/") !== false ||          
    strpos($_SERVER["HTTP_USER_AGENT"], "Facebot") !== false
) {
    // it is probably Facebook's bot
}
else {
    // that is not Facebook
}

UPDATE: Facebook has added Facebot to list of their possible user-agent strings, so I've updated my code to reflect the change. Also, code is now more predictible to possible future changes.

Answer 3

You already have the answer for Facebook above, but one way to get any user agent is to place a script on your site that will mail you when there is a visit to it. For example, create this file on your domain at, say, https://example.com/user-agent.php :

<?php
    mail('you@youremail.com', 'User Agent', $_SERVER['HTTP_USER_AGENT']);

Then, visit Facebook, and type the link to the script there, and hit space bar. You don't actually have to share anything, just typing the link in and a space will cause Facebook to fetch a preview. You should then get an email with Facebook's user agent.

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Answer 4

Please do note that sometimes the agent is visionutils/0.2 . You should check for it too.

Answer 5

In the perspective of user-agent modifications on FB side, it is maybe safer to use a regex like that :

<?php
if (preg_match("/facebook|facebot/i", $_SERVER['HTTP_USER_AGENT'])){
   do_something();
}
?>

You can find more information about Facebook crawler on their doc: https://developers.facebook.com/docs/sharing/webmasters/crawler