What Is Error 451, and How Can It Make a Difference?

You’ve probably seen a number of different HTTP errors over the years while browsing the web: 401 Unauthorized, 403 Forbidden, 404 Not Found, 500 Internal Server Error, 503 Service Unavailable, and so on.

But there’s a new error message that you probably haven’t seen yet: 451 Unavailable for Legal Reasons. It’s a very interesting error message, and you’re likely to start seeing it more often. Here’s what you need to know about this mysterious HTTP error.

What Does It Mean?

“Unavailable for Legal Reasons” is a bit of a euphemism—the idea behind this error is to indicate government censorship. The group behind the 451 Unavailable movement hopes that Internet service providers (ISPs) will show this error when they’ve been forced by the government to block a specific website or page. At the moment, it’s difficult or impossible to tell when this is happening, as ISPs often serve a 403 Forbidden error, which doesn’t tell you anything about why you’re not seeing the site.


To be clear, it’s up to the ISP to show the 451 error; if a site has been censored, there’s no way to reach it through your ISP. The website that’s been blocked doesn’t have a say in which error is shown if someone tries to get to it.

Compounding the issue is the fact that court orders detailing the reasons behind the censoring of a specific site are rarely published and are very hard to find. The ultimate goal of the movement is for the inclusion of links to court orders from the rulings that necessitated the blocking of a site or page right on the 451 error page itself, making it very clear not only that a site has been blocked, but why.


If you’re wondering why “451” was chosen for this error, it’s a nod to Ray Bradbury’s classic dystopian novel Fahrenheit 451, in which books have been banned. It’s a not-so-subtle way of showing disapproval for censorship of the Internet.

Why We Need Error 451

According to 451unavailable.org,

A really good Error 451 message would tell their customers how to challenge a block, how long the block’s expected to last, where the relevant legal documents are and which legal authority imposed the blocking order.

In short, the error message would make it clear why the site is being blocked (with a link to a court order, like the one below), be it for for copyright infringement, political dissent, or other reasons; who ordered the block; and how to speak out against it. It would give web users all the tools they need to fight back against censorship.


In addition to showing the average user that their government is censoring a specific site, the new error message would also allow bots to crawl the Internet at large to catalog examples of censorship, giving us a better idea of what Internet freedom looks like around the world. It’s a win-win, though it’s a loss for goverments who wish to keep their censoring a secret.

Will It Work?

Will 451 Unavailable really make a difference? It’s hard to say. It’s been approved by the Internet Engineering Steering Group, which means it’s ready to go live on servers around the world, and it’s starting to make appearances in the wild. We could start seeing it pop up more often in censorious countries, and even in the US and UK, which compel ISPs to block sites on copyright grounds (though it’s easy to bypass these blocks).

But if a government is widely censoring the Internet, will they allow ISPs to show the 451 error? That’s a big question we don’t yet have the answer to. It certainly seems likely that the governments of countries like China and Saudi Arabia wouldn’t let that stand, and that they would compel ISPs to continue showing 403 or other less useful errors. There’s really nothing to stop them from doing that except pressure from the citizenry, which is semi-effective at best in those countries.


In countries where censorship isn’t as widespread, it does seem likely to broaden the conversation about Internet censorship and get more people thinking about who really controls the Internet and why they make the choices they do. That, at least, is something.

Whether 451 Unavailable will make a big splash remains to be seen, but the more support and attention it gets, the more likely it is to be useful. Spreading the word and supporting the cause will help.

What to Do If You See Error 451

If you see 451 Unavailable for Legal Reasons, you should know that you very well may be under surveillance for visiting that particular site. Your IP address may be added to a list somewhere, and your visit may be recorded. After you realize that, if you still want to continue, your best bet is to close the tab, fire up a VPN or another censorship-bypassing tool, and access the restricted website that way. It might not always work, but it’s probably your best bet.

What do you think about 451 Unavailable? Will it change the public perception of Internet censorship? Or will it fizzle out and fade away? Share your thoughts below!