HTTP Headers for Dummies

Whether you're a programmer or not, you have seen it everywhere on the web. At this moment your browsers address bar shows something that starts with "https://". Even your first Hello World script sent HTTP headers without you realizing it. In this article we are going to learn about the basics of HTTP headers and how we can use them in our web applications.

1. What are HTTP Headers?

HTTP stands for "Hypertext Transfer Protocol". The entire World Wide Web uses this protocol. It was established in the early 1990's. Almost everything you see in your browser is transmitted to your computer over HTTP. For example, when you opened this article page, your browser probably have sent over 40 HTTP requests and received HTTP responses for each.

HTTP headers are the core part of these HTTP requests and responses, and they carry information about the client browser, the requested page, the server and more

Example

When you type a url in your address bar, your browser sends an HTTP request and it may look like this:

GET /tutorials/other/top-20-mysql-best-practices/ HTTP/1.1

Host: net.tutsplus.com

User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US; rv:1.9.1.5) Gecko/20091102 Firefox/3.5.5 (.NET CLR 3.5.30729)

Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8

Accept-Language: en-us,en;q=0.5

Accept-Encoding: gzip,deflate

Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7

Keep-Alive: 300

Connection: keep-alive

Cookie: PHPSESSID=r2t5uvjq435r4q7ib3vtdjq120

Pragma: no-cache

Cache-Control: no-cache

First line is the "Request Line" which contains some basic info on the request. And the rest are the HTTP headers.

After that request, your browser receives an HTTP response that may look like this:

HTTP/1.x 200 OK

Transfer-Encoding: chunked

Date: Sat, 28 Nov 2009 04:36:25 GMT

Server: LiteSpeed

Connection: close

X-Powered-By: W3 Total Cache/0.8

Pragma: public

Expires: Sat, 28 Nov 2009 05:36:25 GMT

Etag: "pub1259380237;gz"

Cache-Control: max-age=3600, public

Content-Type: text/html; charset=UTF-8

Last-Modified: Sat, 28 Nov 2009 03:50:37 GMT

X-Pingback: https://net.tutsplus.com/xmlrpc.php

Content-Encoding: gzip

Vary: Accept-Encoding, Cookie, User-Agent

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">

<html xmlns="http://www.w3.org/1999/xhtml">

<head>

<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />

<title>Top 20+ MySQL Best Practices - Nettuts+</title>

The first line is the "Status Line", followed by "HTTP headers", until the blank line. After that, the "content" starts (in this case, an HTML output).

When you look at the source code of a web page in your browser, you will only see the HTML portion and not the HTTP headers, even though they actually have been transmitted together as you see above.

These HTTP requests are also sent and received for other things, such as images, CSS files, JavaScript files etc. That is why I said earlier that your browser has sent at least 40 or more HTTP requests as you loaded just this article page.

Now, let's start reviewing the structure in more detail.

How to See HTTP Headers

I use the following Firefox extensions to analyze HTTP headers:

Firebug

Live HTTP Headers

Waptimeloaded

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