You’d think that HTTP is so common that most people should have no problem with getting basics of protocol right. Even if you know next to nothing about computers you still probably heard about the meaning of basic HTTP codes such as 404 or 200. Despite its popularity, or maybe because of its popularity HTTP is one of the most frequently abused and misunderstood protocols. This is clearly paradoxical, every job ad these days speaks about REST-ful apis, there are millions of apis deployed around the web, yet so many of them openly violate semantics of HTTP. And those violations happen not only in some small apps created by rookie web developes, even the biggest websites violate standard sometimes.

Maybe one thing that plays a role here is popular misunderstanding that HTTP semantics is only important for API-s returning json or xml. Some people seem to think that if they don’t have api returning JSON they dont need to care about HTTP semantincs. This is clearly wrong. If you have a blog with 3 html pages you’re usually serving it over HTTP so you should respect semantics of HTTP. Every web page should respect HTTP since this is the standard that powers the web.

In this post I’d like to take a look at most blatant and most frequent abuses of HTTP that I’ve found throughout last 1,5 years when building web crawlers for various purposes (mostly indexing web).

Use 200 instead of 404

Every child knows 404 means page not found. Every web developer should know that there is a difference between showing huge sign “404” in html body (with some optional cool humourous text or whatever) and returning actual HTTP response with status code 404. What really matters here
is not the body, but response status code. I lost track of how many web pages return 200 with stupid 404 sign in html body do this. Even biggest US stores with millions of visitors return do this. You can check this out yourself now, go to your favorite websites, put some rubbish in url and see at response status code.

I don’t really know why people do this. Maybe someone with more knowledge about this would tell me. I don’t really see any reason why you would do this. (You can always return 404 response with same body no problem). I see bunch of reasons why you should never do this.

First thing that every search engine or bot checks is response status code, if status code is 200 it means “all clear this is best content I have under requested resource location”, this implies lost of things. First off all it implies that this content should be indexed (assuming of course you want to be indexed). Do you really want your cool “page not found” body to be treated as legit content? It will be treated as legit content if you return response 200. Response 200 also implies that content can and perhaps should be cached. This means that crawler will actually create cache of your cool “page not found” response body, and it will keep cache for some specified time. When you put valid content in this page later, bot may ignore it and just take content from cache.

Use 200 instead of 5xx

Exceptions can always happen. Every server is down once in a while, every request may return error code once in a while. HTTP has specific semantics to deal with that. When your server is down you should return one of 5xx codes (500, 502, 503, 504) Usually the best response is just 503 service unavailable with ‘Retry-After’ header. If you check HTTP specs 503 means that:

The server is currently unable to handle the request due to a temporary overloading or maintenance of the server. The implication is that this is a temporary condition which will be alleviated after some delay. If known, the length of the delay MAY be indicated in a Retry-After header. If no Retry-After is given, the client SHOULD handle the response as it would for a 500 response.

This seems really simple, right? Why is it then that so many web sites don’t do that?

As with 404 most frequent abuse is returning 200 with some cool error message instead of proper response code. “Ooops something went wrong, we’re working hard to fix it” in html body. It’s great you’re working hard to fix it but first thing you should fix is returning proper HTTP codes on errors. If you return 200 it communicates that there is no error. Most non-human visitors will think that this ‘Oops something went wrong” is something you want to show to the world, some product you’re selling or service you offer. Are you really selling “oopses”?

When server responds with 5xx most clients are going to retry request after some specified wait time. If you specify ‘retry-after’ headers you’re being really nice and most bots will retry after value from this header. 90% of 5xx errors are temporary, and retry helps most of the time. If you have responded with 200 retry is not going to happen. Content will be lost to bot, it will assume it got content from your response and it will continue it’s journey around the web visiting other places.

302 instead of 5xx

Other frequent and irritating abuse of HTTP is redirect on exception. Server is going down for millisecond but instead of returning proper 500 on url requested you respond with 301 or 302 with ‘Location’ header leading to some generic error handling page ‘”. This is bad and harmful for you. Most clients encountering one of 5xx codes will retry request. When you redirect to some errorpage with different url crawler is going to retry this exact error page and not original page it requested. In worse case when this happens frequently and you have lots of bots visiting your site (which is generally sign that your site is really popular) you are creating problems for yourself because bunch of bots can be retrying this stupid error page instead of getting actual url they requested.

5xx instead of 400

Many apis and webpages have some required parameters in url querystring part. If some required parameter is not present in querystring server should return 400 Bad Request code with friendly error message advising client what it did wrong. HTTP specs are clear on that

The 4xx class of status code is intended for cases in which the client seems to have erred. Except when responding to a HEAD request, the server SHOULD include an entity containing an explanation of the error situation, and whether it is a temporary or permanent condition. These status codes are applicable to any request method. User agents SHOULD display any included entity to the user.

As usual this is simple, but so many developers forget that. Most frequent abuse here is returning 500 server error when some url param is missing. This is either intended or perhaps unintented (someone developing app forgot to check required params in url handling function and server crashes when user mistypes param name). You should generally never trust user input, and url querystring is just a form of user input. Everyone can manipulate your url in browser, and many web clients will access your api if it’s public. Web clients can get your url params wrong. If you want to keep your web clients happy please tell them what they are doing wrong. If they miss some parameter give them proper 400 and tell them what they are missing. If you are returning 500 you are telling clients that there is some temporary error in your application. Faced with this message most clients will simply retry after some delay. Do you really want them to retry request that was bad and incorrect?

POST instead of GET

POST requests should be used for posting new data, GET requests should be used to retrieve data. Most good crawlers will never use POST. I’m not talking about spam bots or about some malicious bots scanning your site for vulnerabities. I’m talking about search engines (there are many, not only one) and bots indexing content for all types of purposes. Legit bots will not use POST. If you use POST for navigation or for retrieving content it’s like putting a tag ‘robots, no-follow’.

I’m pretty sure most developers know this, but I see suspiously large amount of sites that abuse POST. I noticed that this happens often with old .net applications. Instead of proper GET to get resource they just use POST-s with some huge amount of parameters in formbody. If you have this kind of application and you wonder why you’re doing poorly in search results look no further - you’re practically hiding your content from non-human visitors.

this ain’t no rocket science man

HTTP is not rocket science, and you don’t have to be genius to get it right. If you have still have 5 minutes now go read all definitions of HTTP status codes, and use them properly.