Hola "Better Internet" is an extremely popular free VPN. How it works is not very clear to all its users though, as I quickly became aware in the past week when 8chan was hit by multiple denial of service attacks from their network.
When a user installs Hola, he becomes a VPN endpoint, and other users of the Hola network may exit through his internet connection and take on his IP. This is what makes it free: Hola does not pay for the bandwidth that its VPN uses at all, and there is no user opt out for this. On the other hand, with the Tor onion router, users must specifically opt in to be exit nodes and are aware that completely anonymous traffic can pass through their connections, which means they should be ready for abuse reports for child porn, spam, copyrighted content and other ills that come with the territory.
Hola was created by the Israeli corporation Hola Networks Limited at the end of 2012, and at first was just the VPN service. However, Hola has gotten greedy. They recently (late 2014) realized that they basically have a 9 million IP strong botnet on their hands, and they began selling access to this botnet (right now, for HTTP requests only) at https://luminati.io .
Luminati boasts of having "More than 9,761,015 exit nodes" on their website, and based on what I saw in the past week I have no reason to doubt it. The only silver lining is their greed: they charge $20/GB to use lines that cost them nothing, their software simply mooches off of the unfortunate users who have installed the proprietary Hola software.
Hola is the most unethical VPN I have ever seen.
So far as I can tell, there is no way to tell if an IP has the Hola VPN software installed or not: no tell tale open port, no special header from Luminati, and no specific range.
This is a huge issue for 8ch, which allows posters to post completely anonymously, and has some protections in place for typically abused ranges (like Tor and VPN ranges) but still allows posts through. An attacker used the Luminati network to send thousands of legitimate-looking POST requests to 8chan's post.php in 30 seconds, representing a 100x spike over peak traffic and crashing PHP-FPM.
I have had to regretfully turn on the 24 hour CAPTCHA for all users until a solution can be found, but I'm not sure how quickly that will happen. I hope that Luminati takes my advice and rejects POST requests through their service, or allows domains to pay them off for such a rejection.