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HTML Forms

Topics

When communicating with Amazon S3, you normally use the REST or SOAP APIs to perform put, get, delete, and other operations. With POST, users upload data directly to Amazon S3 through their browsers, which do not understand SOAP APIs or how to make a REST PUT request.

To allow users to upload content to Amazon S3 using their browsers, you use HTML forms. HTML Forms consist of a form declaration and form fields. The form declaration contains high level information about the request. The form fields contain detailed information about the request as well as the policy that is used to authenticate it and make sure that it meets conditions that you specify.

	Note
	The form data and boundaries (excluding the contents of the file) cannot exceed 20K.

This section describes how to use HTML forms.

HTML Form Encoding

The form and policy must be UTF-8 encoded. You can apply UTF-8 encoding to the form by specifying it in the HTML heading or as a request header.

	Note
	The HTML form declaration does not accept query string authentication parameters. For information about query string authentication, see Using Query String Authentication.

Following is an example of UTF-8 encoding in the HTML heading.

<html>
  <head>
    ...
    <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
    ...
  </head>
  <body>

Following is an example of UTF-8 encoding in a request header.


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

HTML Form Declaration

The form declaration has three components: the action, the method, and the enclosure type. If any of these values is improperly set, the request fails.

The action specifies the URL that processes the request, which must be set to the URL of the bucket. For example, if the name of your bucket is "johnsmith", the URL is "http://johnsmith.s3.amazonaws.com/".

	Note
	The key name is specified in a form field.

The method must be POST.

The enclosure type (enctype) must be specified and must be set to multipart/form-data (go to RFC 1867) for both file uploads and text area uploads.

Example

This is a form declaration for the bucket "johnsmith".

<form action="http://johnsmith.s3.amazonaws.com/" method="post"

enctype="multipart/form-data">

HTML Form Fields

Following is a table that describes a list of fields that can be used within a form.

Note

The variable ${filename} is automatically replaced with the name of the file provided by the user and is recognized by all form fields. If the browser or client provides a full or partial path to the file, only the text following the last slash (/) or backslash (\) will be used (e.g., "C:\Program Files\directory1\file.txt" will be interpreted as "file.txt"). If no file or filename is provided, the variable is replaced with an empty string.

Element Name Description Required

AWSAccessKeyId

The AWS Access Key ID of the owner of the bucket who grants an Anonymous user access for a request that satisfies the set of constraints in the Policy. This is required if a policy document is included with the request.

Conditional

acl

Specifies an Amazon S3 access control list. If an invalid access control list is specified, an error is generated. For more information on ACLs, see Amazon S3 ACLs.

Type: String

Default: private

Cache-Control, Content-Type, Content-Disposition, Content-Encoding, Expires

REST-specific headers. For more information, see PUT Object.

key

The name of the uploaded key.

To use the filename provided by the user, use the ${filename} variable. For example, if the user Betty uploads the file the file lolcatz.jpg and you specify /user/betty/${filename}, the file is stored as /user/betty/lolcatz.jpg.

For more information, see Using Keys .

Yes

policy

Security Policy describing what is permitted in the request. Requests without a security policy are considered anonymous and only work on publicly writable buckets.

success_action_redirect, redirect

The URL to which the client is redirected upon successful upload. Amazon S3 appends the bucket, key and etag values as query string parameters to the URL.

If success_action_redirect is not specified, Amazon S3 returns the empty document type specified in the success_action_status field.

If Amazon S3 cannot interpret the URL, it acts as if the field is not present.

If the upload fails, Amazon S3 displays an error and does not redirect the user to a URL.

For more information, see Redirection.

	Note
	The redirect field name is deprecated and support for the redirect field name will be removed in the future.

success_action_status

The status code returned to the client upon successful upload if success_action_redirect is not specified.

Accepts the values 200, 201, or 204 (default).

If the value is set to 200 or 204, Amazon S3 returns an empty document with a 200 or 204 status code.

If the value is set to 201, Amazon S3 returns an XML document with a 201 status code. For information on the content of the XML document, see POST Object.

If the value is not set or if it is set to an invalid value, Amazon S3 returns an empty document with a 204 status code.

	Note
	Some versions of the Adobe Flash player do not properly handle HTTP responses with an empty body. To support uploads through Adobe Flash, we recommend setting `success_action_status` to 201.

signature

The HMAC signature constructed using the secret key of the provided AWSAccessKeyId. This is required if a policy document is included with the request.

For more information, see Using Auth Access .

Conditional

x-amz-security-token

Amazon DevPay security token.

Each request that uses Amazon DevPay requires two x-amz-security-token form fields: one for the product token and one for the user token.

For more information, see Using DevPay .

Other field names prefixed with x-amz-meta-

User-specified metadata.

Amazon S3 does not validate or use this data.

For more information, see PUT Object.

file

File or text content.

The file or content must be the last field in the form, as any fields below it are ignored.

You cannot upload more than one file at a time.

Yes

Policy Construction

Topics

The policy is a UTF-8 and Base64 encoded JSON document that specifies conditions which the request must meet and is used to authenticate the content. Depending on how you design your policy documents, you can use them per-upload, per-user, for all uploads, or according to other designs that meet your needs.

	Note
	Although the policy document is optional, we highly recommend it over making a bucket publicly writable.

Following is an example of a policy document.

{ "expiration": "2007-12-01T12:00:00.000Z",

  "conditions": [

    {"acl": "public-read" },

    {"bucket": "johnsmith" },

    ["starts-with", "$key", "user/eric/"],

  ]

}

The policy document contains the expiration and conditions.

Expiration

The expiration specifies the expiration date of the policy in ISO8601 GMT date format. For example, "2007-12-01T12:00:00.000Z" specifies that the policy is not valid after 12:00 GMT on 2007-12-01. Expiration is required in a policy.

Conditions

The conditions in the policy document are used to validate the contents of the uploaded object. Each form field that you specify in the form (except AWSAccessKeyId , signature, file, policy, and field names that have an x-ignore- prefix) must be included in the list of conditions.

Note

If you have multiple fields with the same name, the values must be separated by commas. For example, if you have two fields named "x-amz-meta-tag" and the first one has a value of "Ninja" and second has a value of "Stallman", you would set the policy document to Ninja,Stallman.

All variables within the form are expanded prior to validating the policy. Therefore, all condition matching should be against the expanded fields. For example, if you set the key field to user/betty/${filename}, your policy might be [ "starts-with", "$key", "user/betty/" ]. Do not enter [ "starts-with", "$key", "user/betty/${filename}" ]. For more information, see Condition Matching.

Policy document conditions are described in the following table.

Element Name	Description
acl	Specifies conditions the ACL must meet. Supports exact matching and `starts-with`.
content-length-range	Specifies the minimum and maximum allowable size for the uploaded content. Supports range matching.
Cache-Control, Content-Type, Content-Disposition, Content-Encoding, Expires	REST-specific headers. Supports exact matching and `starts-with`.
key	The name of the uploaded key. Supports exact matching and `starts-with`.
success_action_redirect, redirect	The URL to which the client is redirected upon successful upload. Supports exact matching and `starts-with`.
success_action_status	The status code returned to the client upon successful upload if success_action_redirect is not specified. Supports exact matching.
x-amz-security-token	Amazon DevPay security token. Each request that uses Amazon DevPay requires two `x-amz-security-token` form fields: one for the product token and one for the user token. As a result, the values must be separated by commas. For example, if the user token is `eW91dHViZQ==` and the product token is `b0hnNVNKWVJIQTA=`, you set the policy entry to: `{ "x-amz-security-token": "eW91dHViZQ==,b0hnNVNKWVJIQTA=" }`. For more information about Amazon DevPay, see Using DevPay .
Other field names prefixed with x-amz-meta-	User-specified metadata. Supports exact matching and `starts-with`.

	Note
	If your toolkit adds additional fields (e.g., Flash adds filename), you must add them to the policy document. If you can control this functionality, prefix `x-ignore-` to the field so Amazon S3 ignores the feature and it won't affect future versions of this feature.

Condition Matching

Following is a table that describes condition matching types. Although you must specify one condition for each form field that you specify in the form, you can create more complex matching criteria by specifying multiple conditions for a form field.

Condition	Description
Exact Matches	Exact matches verify that fields match specific values. This example indicates that the ACL must be set to public-read: {"acl": "public-read" } This example is an alternate way to indicate that the ACL must be set to public-read: [ "eq", "$acl", "public-read" ]
Starts With	If the value must start with a certain value, use starts-with. This example indicates that the key must start with user/betty: ["starts-with", "$key", "user/betty/"]
Matching Any Content	To configure the policy to allow any content within a field, use starts-with with an empty value. This example allows any success_action_redirect: ["starts-with", "$success_action_redirect", ""]
Specifying Ranges	For fields that accept ranges, separate the upper and lower ranges with a comma. This example allows a file size from 1 to 10 megabytes: ["content-length-range", 1048579, 10485760]

Character Escaping

Characters that must be escaped within a policy document are described in the following table.

Escape Sequence	Description
\\	Backslash
\$	Dollar symbol
\b	Backspace
\f	Form feed
\n	New line
\r	Carriage return
\t	Horizontal tab
\v	Vertical tab
\u`xxxx`	All Unicode characters

Constructing a Signature

Step	Description
1	Encode the policy using UTF-8.
2	Encode those UTF-8 bytes using Base64.
3	Sign the policy with your Secret Access Key using HMAC SHA-1.
4	Encode the SHA-1 signature using Base64.

For general information about authentication, see Using Auth Access .

Redirection

This section describes how to handle redirects.

General Redirection

On completion of the POST, the user is redirected to the location that you specified in the success_action_redirect field. If Amazon S3 cannot interpret the URL, it ignores the success_action_redirect field.

If success_action_redirect is not specified, Amazon S3 returns the empty document type specified in the success_action_status field.

If the POST fails, Amazon S3 displays an error and does not provide a redirect.

Pre-Upload Redirection

If your bucket was created using <CreateBucketConfiguration>, your end-users might require a redirect. If this occurs, some browsers might handle the redirect incorrectly. This is relatively rare, but is most likely to occur right after a bucket is created.

Document Conventions
Terms of Use

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