How are parameters sent in an HTTP POST request?

Question

In an HTTP GET request, parameters are sent as a query string:

http://example.com/page?parameter=value&also=another

In an HTTP POST request, the parameters are not sent along with the URI.

Where are the values? In the request header? In the request body? What does it look like?

Answer 1

The values are sent in the request body, in the format that the content type specifies.

Usually the content type is application/x-www-form-urlencoded, so the request body uses the same format as the query string:

parameter=value&also=another 

When you use a file upload in the form, you use the multipart/form-data encoding instead, which has a different format. It's more complicated, but you usually don't need to care what it looks like, so I won't show an example, but it can be good to know that it exists.

Answer 2

The content is put after the HTTP headers. The format of an HTTP POST is to have the HTTP headers, followed by a blank line, followed by the request body. The POST variables are stored as key-value pairs in the body.

You can see this in the raw content of an HTTP Post, shown below:

POST /path/script.cgi HTTP/1.0 From: [email protected] User-Agent: HTTPTool/1.0 Content-Type: application/x-www-form-urlencoded Content-Length: 32  home=Cosby&favorite+flavor=flies 

You can see this using a tool like Fiddler, which you can use to watch the raw HTTP request and response payloads being sent across the wire.