How are parameters sent in an HTTP POST request?

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.

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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: frog@jmarshall.com
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.

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Short answer: in POST requests, values are sent in the "body" of the request. With web-forms they are most likely sent with a media type of application/x-www-form-urlencoded or multipart/form-data. Programming languages or frameworks which have been designed to handle web-requests usually do "The Right Thing™" with such requests and provide you with easy access to the readily decoded values (like $_REQUEST or $_POST in PHP, or cgi.FieldStorage(), flask.request.form in Python).

Now let's digress a bit, which may help understand the difference ;)

The difference between GET and POST requests are largely semantic. They are also "used" differently, which explains the difference in how values are passed.

GET (relevant RFC section)

When executing a GET request, you ask the server for one, or a set of entities. To allow the client to filter the result, it can use the so called "query string" of the URL. The query string is the part after the ?. This is part of the URI syntax.

So, from the point of view of your application code (the part which receives the request), you will need to inspect the URI query part to gain access to these values.

Note that the keys and values are part of the URI. Browsers may impose a limit on URI length. The HTTP standard states that there is no limit. But at the time of this writing, most browsers do limit the URIs (I don't have specific values). GET requests should never be used to submit new information to the server. Especially not larger documents. That's where you should use POST or PUT.

POST (relevant RFC section)

When executing a POST request, the client is actually submitting a new document to the remote host. So, a query string does not (semantically) make sense. Which is why you don't have access to them in your application code.

POST is a little bit more complex (and way more flexible):

When receiving a POST request, you should always expect a "payload", or, in HTTP terms: a message body. The message body in itself is pretty useless, as there is no standard (as far as I can tell. Maybe application/octet-stream?) format. The body format is defined by the Content-Type header. When using a HTML FORM element with method="POST", this is usually application/x-www-form-urlencoded. Another very common type is multipart/form-data if you use file uploads. But it could be anything, ranging from text/plain, over application/json or even a custom application/octet-stream.

In any case, if a POST request is made with a Content-Type which cannot be handled by the application, it should return a 415 status-code.

Most programming languages (and/or web-frameworks) offer a way to de/encode the message body from/to the most common types (like application/x-www-form-urlencoded, multipart/form-data or application/json). So that's easy. Custom types require potentially a bit more work.

Using a standard HTML form encoded document as example, the application should perform the following steps:

Read the Content-Type field If the value is not one of the supported media-types, then return a response with a 415 status code otherwise, decode the values from the message body.

Again, languages like PHP, or web-frameworks for other popular languages will probably handle this for you. The exception to this is the 415 error. No framework can predict which content-types your application chooses to support and/or not support. This is up to you.

PUT (relevant RFC section)

A PUT request is pretty much handled in the exact same way as a POST request. The big difference is that a POST request is supposed to let the server decide how to (and if at all) create a new resource. Historically (from the now obsolete RFC2616 it was to create a new resource as a "subordinate" (child) of the URI where the request was sent to).

A PUT request in contrast is supposed to "deposit" a resource exactly at that URI, and with exactly that content. No more, no less. The idea is that the client is responsible to craft the complete resource before "PUTting" it. The server should accept it as-is on the given URL.

As a consequence, a POST request is usually not used to replace an existing resource. A PUT request can do both create and replace.

Side-Note

There are also "path parameters" which can be used to send additional data to the remote, but they are so uncommon, that I won't go into too much detail here. But, for reference, here is an excerpt from the RFC:

Aside from dot-segments in hierarchical paths, a path segment is considered opaque by the generic syntax. URI producing applications often use the reserved characters allowed in a segment to delimit scheme-specific or dereference-handler-specific subcomponents. For example, the semicolon (";") and equals ("=") reserved characters are often used to delimit parameters and parameter values applicable to that segment. The comma (",") reserved character is often used for similar purposes. For example, one URI producer might use a segment such as "name;v=1.1" to indicate a reference to version 1.1 of "name", whereas another might use a segment such as "name,1.1" to indicate the same. Parameter types may be defined by scheme-specific semantics, but in most cases the syntax of a parameter is specific to the implementation of the URIs dereferencing algorithm.

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You cannot type it directly on the browser URL bar.

You can see how POST data is sent on the Internet with Live HTTP Headers for example. Result will be something like that

http://127.0.0.1/pass.php
POST /pass.php HTTP/1.1

Host: 127.0.0.1
User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:18.0) Gecko/20100101 Firefox/18.0
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
DNT: 1
Referer: http://127.0.0.1/pass.php
Cookie: passx=87e8af376bc9d9bfec2c7c0193e6af70; PHPSESSID=l9hk7mfh0ppqecg8gialak6gt5
Connection: keep-alive
Content-Type: application/x-www-form-urlencoded
Content-Length: 30
username=zurfyx&pass=password

Where it says

Content-Length: 30
    username=zurfyx&pass=password

will be the post values.

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The default media type in a POST request is application/x-www-form-urlencoded. This is a format for encoding key-value pairs. The keys can be duplicate. Each key-value pair is separated by an & character, and each key is separated from its value by an = character.

For example:

Name: John Smith
Grade: 19

Is encoded as:

Name=John+Smith&Grade=19

This is placed in the request body after the HTTP headers.

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Form values in HTTP POSTs are sent in the request body, in the same format as the querystring.

For more information, see the spec.

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Some of the webservices require you to place request data and metadata separately. For example a remote function may expect that the signed metadata string is included in a URI, while the data is posted in a HTTP-body.

The POST request may semantically look like this:

POST /?AuthId=YOURKEY&Action=WebServiceAction&Signature=rcLXfkPldrYm04 HTTP/1.1
Content-Type: text/tab-separated-values; charset=iso-8859-1
Content-Length: []
Host: webservices.domain.com
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Encoding: identity
User-Agent: Mozilla/3.0 (compatible; Indy Library)

name    id
John    G12N
Sarah   J87M
Bob     N33Y

This approach logically combines QueryString and Body-Post using a single Content-Type which is a "parsing-instruction" for a web-server.

Please note: HTTP/1.1 is wrapped with the #32 (space) on the left and with #10 (Line feed) on the right.

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First of all, let's differentiate between GET and POST

Get: It is the default HTTP request that is made to the server and is used to retrieve the data from the server and query string that comes after ? in a URI is used to retrieve a unique resource.

this is the format

GET /someweb.asp?data=value HTTP/1.0

here data=value is the query string value passed.

POST: It is used to send data to the server safely so anything that is needed, this is the format of a POST request

POST /somweb.aspHTTP/1.0
Host: localhost
Content-Type: application/x-www-form-urlencoded //you can put any format here
Content-Length: 11 //it depends
Name= somename

Why POST over GET?

In GET the value being sent to the servers are usually appended to the base URL in the query string,now there are 2 consequences of this

The GET requests are saved in browser history with the parameters. So your passwords remain un-encrypted in browser history. This was a real issue for Facebook back in the days.

Usually servers have a limit on how long a URI can be. If have too many parameters being sent you might receive 414 Error - URI too long

In case of post request your data from the fields are added to the body instead. Length of request params is calculated, and added to the header for content-length and no important data is directly appended to the URL.

You can use the Google Developer Tools' network section to see basic information about how requests are made to the servers.

and you can always add more values in your Request Headers like Cache-Control , Origin , Accept.

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There are many ways/formats of post parameters

formdata

raw data

json

encoded data

file

xml

They are controlled by content-type in Header that are representes as mime-types.

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In CGI Programming on the World Wide Web the author says:

Using the POST method, the server sends the data as an input stream to the program. ..... since the server passes information to this program as an input stream, it sets the environment variable CONTENT_LENGTH to the size of the data in number of bytes (or characters). We can use this to read exactly that much data from standard input.

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