Uploads an object to a bucket by using an HTML form.

Usage notes

  • The size of the object that is uploaded by calling the PostObject operation cannot exceed 5 GB.
  • To initiate a PostObject request to a bucket, you must have write permissions on the bucket. If the access control list (ACL) of the bucket to which you want to initiate a PostObject request is public-read-write, you do not need to sign the PostObject request. Otherwise, Object Storage Service (OSS) verifies the signature information contained in the request.
  • Unlike the PutObject operation, the PostObject operation uses an AccessKey secret to calculate the signature for the policy form field. The calculated signature string is used as the value of the Signature form field. OSS checks this value to verify the validity of the signature.
  • The URL of the submitted form is the domain name of the bucket. You do not need to specify the object that you want to upload in the URL. In other words, the request line is in the format of POST / HTTP/1.1 instead of POST /ObjectName HTTP/1.1.
  • OSS does not check the signature information that is contained in headers or URLs in PostObject requests.

Versioning

If you initiate a PostObject request to a versioning-enabled bucket, OSS generates a unique version ID for the uploaded object and returns the version ID as the x-oss-version-id header in the response.

If you initiate a PostObject request to a versioning-suspended bucket, OSS generates a version ID of null for the uploaded object and returns the version ID as the x-oss-version-id header in the response. An object can have only one version whose version ID is null.

Request syntax

POST / HTTP/1.1 
Host: BucketName.oss-cn-hangzhou.aliyuncs.com
User-Agent: browser_data
Content-Length: ContentLength
Content-Type: multipart/form-data; boundary=9431149156168
--9431149156168
Content-Disposition: form-data; name="key"
key
--9431149156168
Content-Disposition: form-data; name="success_action_redirect"
success_redirect
--9431149156168
Content-Disposition: form-data; name="Content-Disposition"
attachment;filename=oss_download.jpg
--9431149156168
Content-Disposition: form-data; name="x-oss-meta-uuid"
myuuid
--9431149156168
Content-Disposition: form-data; name="x-oss-meta-tag"
mytag
--9431149156168
Content-Disposition: form-data; name="OSSAccessKeyId"
access-key-id
--9431149156168
Content-Disposition: form-data; name="policy"
encoded_policy
--9431149156168
Content-Disposition: form-data; name="Signature"
signature
--9431149156168
Content-Disposition: form-data; name="file"; filename="MyFilename.jpg"
Content-Type: image/jpeg
file_content
--9431149156168
Content-Disposition: form-data; name="submit"
Upload to OSS
--9431149156168--

Request headers

Notice
  • The message body of a PostObject request is encoded in the multipart/form-data format. In the PostObject operation, the parameters are passed in as form fields in the request message body, whereas the parameters are passed as HTTP request headers in the PutObject operation.
  • You cannot add tags to objects by passing in x-oss-tagging request headers during the PostObject operation. You can call the PutObjectTagging operation to add tags to objects after the PostObject operation is complete.
Header Type Required Description
Content-Type String No The type of the object that you want to upload and encoding of the web page. Browsers determine how to read and encode the object based on the value of the parameter.

The form submitted by the PostObject operation must be encoded in the multipart/form-data format. For example, the Content-Type header must be in the multipart/form-data;boundary=xxxxxx format.

In this format, boundary is a boundary string that is randomly generated by the form. You do not need to specify the boundary string. If you use an OSS SDK to create a form, the SDK also generates a random boundary string.

x-oss-object-acl String No The ACL of the object that you want to upload.

If you specify x-oss-object-acl in both of the request header and the form field, the ACL that is specified in the form field takes priority over the ACL that is specified in the request header. Default value: default.

Valid values:
  • default: The ACL of the object is the same as that of the bucket in which the object is stored.
  • private: The ACL of the object is private. Only the owner of the object and authorized users can read and write this object.
  • public-read: The ACL of the object is public-read. Only the owner of the object and authorized users can read and write this object. Other users can only read the object. Exercise caution when you set the object ACL to this value.
  • public-read-write: The ACL of the object is public-read-write. All users can read and write this object. Exercise caution when you set the object ACL to this value.

For more information about object ACLs, see Object ACL.

For more information about the common request headers contained in PostObject requests such as Host and Date, see Common request headers.

Request elements

Notice The key of a form field cannot exceed 8 KB, and the value of a form field cannot exceed 2 MB.
Element Type Required Description
Cache-Control String No The caching behavior of the web page when the object is downloaded. For more information, see RFC 2616.

Default value: null

Content-Disposition String No The name of the object when the object is downloaded. For more information, see RFC 2616.

Default value: null

Content-Encoding String No The content encoding format of the object when the object is downloaded. For more information, see RFC 2616.

Default value: null

Expires String No The expiration time. For more information, see RFC 2616.

Default value: null

OSSAccessKeyId String Conditional The AccessKey ID of the account used by the bucket owner.

Default value: null

Constraint: This form field is required if the bucket ACL is not public-read-write or the policy or Signature form field is specified in the request.

policy String Conditional The validity of the form fields in the request. A request that does not contain the policy form field is considered an anonymous request and can only be used to access buckets whose ACLs are public-read-write.

Default value: null

Constraint: This form field is required if the bucket ACL is not public-read-write or the OSSAccessKeyId or Signature form field is specified in the request.

Notice The form and the policy form field must be UTF-8-encoded. The policy form field must also be Base64-encoded.
Signature String Conditional The signature information that is calculated based on the AccessKey secret and the policy form field. OSS checks the signature information to verify the validity of the PostObject request. For more information, see Appendix: Signature

Default value: null

Constraint: This form field is required if the bucket ACL is not public-read-write or the OSSAccessKeyId or policy form field is specified in the request.

Note Form fields are not case-sensitive, but their values are case-sensitive.
x-oss-server-side-encryption-key-id String No The ID of the customer master key (CMK) managed by Key Management Service (KMS). This form field is valid only when x-oss-server-side-encryption is set to KMS.
x-oss-content-type String No You can add the x-oss-content-type form field to the message body of a PostObject request to specify the content type of the object that you want to upload. The content type specified by the x-oss-content-type form field takes priority over the content type specified by the form field that is automatically generated by browsers.

The content types are prioritized in the following order: x-oss-content-type form field > Content-Type specified in the form field that is automatically generated by browsers > Content-Type header.

Default value: null

x-oss-forbid-overwrite String No Specifies whether the PostObject operation overwrites the existing object that has the same name.

When versioning is enabled or suspended for the bucket to which you want to upload the object, the x-oss-forbid-overwrite header does not take effect. In this case, the object that is uploaded by calling the PostObject operation overwrites the existing object that has the same name.

  • If x-oss-forbid-overwrite is not specified or x-oss-forbid-overwrite is set to false, the object uploaded by calling the PostObject operation overwrites the existing object that has the same name.
  • If x-oss-forbid-overwrite is set to true, the object uploaded by calling the PostObject operation cannot overwrite the existing object that has the same name.

If you specify the x-oss-forbid-overwrite request header, the queries per second (QPS) performance of OSS may be degraded. If you want to use the x-oss-forbid-overwrite request header to perform a large number of operations (QPS greater than 1,000), contact technical support.

x-oss-object-acl String No The ACL of the object that you want to upload. You can specify the ACL of the object that you want to upload in both the request header and the form field. Default value: default.

If you specify x-oss-object-acl in both of the request header and the form field, the ACL that is specified in the form field takes priority over the ACL that is specified in the request header. Default value: default.

Valid values:
  • default: The ACL of the object is the same as that of the bucket in which the object is stored.
  • private: The ACL of the object is private. Only the owner of the object and authorized users can read and write this object.
  • public-read: The ACL of the object is public-read. Only the owner of the object and authorized users can read and write this object. Other users can only read the object. Exercise caution when you set the object ACL to this value.
  • public-read-write: The ACL of the object is public-read-write. All users can read and write this object. Exercise caution when you set the object ACL to this value.

For more information about object ACLs, see Object ACL.

x-oss-storage-class String No The storage class of the object that you want to upload.

If you specify this form field when you upload an object, the storage class of the uploaded object is the specified value regardless of the storage class of the bucket to which the object is uploaded. For example, if you set x-oss-storage-class to Standard when you upload an object to an IA bucket, the object is stored as a Standard object.

Valid values:
  • Standard
  • IA
  • Archive
  • ColdArchive

For more information about storage classes, see Overview.

key String Yes The name of the object that you want to upload. You do not need to encode the object name. If the object name contains a path such as destfolder/example.jpg, OSS automatically creates the corresponding directory.

Default value: null

success_action_redirect String No The URL to which the client is redirected after the object is uploaded. If this form field is not specified, the returned result is specified by success_action_status. If the upload fails, OSS returns an error code, and the client is not redirected to a URL.

Default value: null

success_action_status String No The HTTP status code that is returned to the client when the success_action_redirect form field is not specified and the object is uploaded. Default value: 204.

Valid values: 200, 201, and 204.

  • If the value of this form field is set to 200 or 204, OSS returns an empty file and HTTP status code 200 or 204.
  • If the value of this form field is set to 201, OSS returns an XML file and HTTP status code 201.
  • If the value of this form field is not specified or set to an invalid value, OSS returns an empty file and HTTP status code 204.
x-oss-meta-* String No The user metadata of the object that you want to upload.

Default value: null

If the request contains a form field prefixed with x-oss-meta-, the form field is considered the user metadata of the object. Example: x-oss-meta-location.
Note An object can have multiple form fields whose names contain the x-oss-meta- prefix. However, the total size of all user metadata of the object cannot exceed 8 KB.
x-oss-security-token String No The security token for temporary access authorization. If you obtain a temporary access credential from Security Token Service (STS) and use the temporary access credential for the request, you must set this form field to the value of the security token and set OSSAccessKeyId to the value of the AccessKey ID in the temporary access credential. You can calculate the signature for a request that uses a temporary AccessKey ID the same way you calculate the signature for a request that uses an AccessKey ID.

Default value: null

For more information about how to set up STS, see Use a temporary credential provided by STS to access OSS in OSS Developer Guide. You can call the AssumeRole operation or use STS SDKs for various programming languages to obtain temporary access credentials. For more information, see STS SDK overview.

file String Yes The file or text content. You do not need to encode the content. Browsers automatically add the Content-Type form field based on the file type and overwrite the content type specified in the Content-Type header. You can upload only one object by using a PostObject request.

Default value: null

Notice This form field must be the last field in the form.

Response headers

Header Type Description
x-oss-server-side-encryption String If x-oss-server-side-encryption is specified in the request, the response contains this header to indicate the algorithm that is used to encrypt the object at the server side.

For more information about the common response headers contained in the response to a PostObject request, see Common response headers.

Response elements

Element Type Description
PostResponse Container The container that stores the result of the PostObject request.

Child nodes: Bucket, ETag, Key, and Location

Bucket String The name of the bucket.

Parent node: PostResponse

ETag String The entity tag (ETag) that is created when the object is created. If an object is created by calling the PostObject operation, the ETag value is not the MD5 hash of the object content but a unique value calculated based on a rule. The ETag value of an object can be used to check whether the object content is modified.

Parent node: PostResponse

Location String The URL that is used to access the uploaded object.

Parent node: PostResponse

Examples

  • Sample requests
    POST / HTTP/1.1
    Host: oss-example.oss-cn-hangzhou.aliyuncs.com
    Content-Length: 344606
    Content-Type: multipart/form-data; boundary=9431149156168
    --9431149156168
    Content-Disposition: form-data; name="key"
    /user/a/objectName.txt
    --9431149156168
    Content-Disposition: form-data; name="success_action_status"
    200
    --9431149156168
    Content-Disposition: form-data; name="Content-Disposition"
    content_disposition
    --9431149156168
    Content-Disposition: form-data; name="x-oss-meta-uuid"
    uuid
    --9431149156168
    Content-Disposition: form-data; name="x-oss-meta-tag"
    metadata
    --9431149156168
    Content-Disposition: form-data; name="OSSAccessKeyId"
    44CF9590006BF252****
    --9431149156168
    Content-Disposition: form-data; name="policy"
    eyJleHBpcmF0aW9uIjoiMjAxMy0xMi0wMVQxMjowMDowMFoiLCJjb25kaXRpb25zIjpbWyJjb250ZW50LWxlbmd0aC1yYW5nZSIsIDAsIDEwNDg1NzYwXSx7ImJ1Y2tldCI6ImFoYWhhIn0sIHsiQSI6ICJhIn0seyJrZXkiOiAiQUJDIn1dfQ==
    --9431149156168
    Content-Disposition: form-data; name="Signature"
    kZoYNv66bsmc10+dcGKw5x2P****
    --9431149156168
    Content-Disposition: form-data; name="file"; filename="MyFilename.txt"
    Content-Type: text/plain
    abcdefg
    --9431149156168
    Content-Disposition: form-data; name="submit"
    Upload to OSS
    --9431149156168--
  • Sample responses
    HTTP/1.1 200 OK
    x-oss-request-id: 61d2042d-1b68-6708-5906-33d81921362e 
    Date: Fri, 24 Feb 2014 06:03:28 GMT
    ETag: "5B3C1A2E053D763E1B002CC607C5****"
    Connection: keep-alive
    Content-Length: 0  
    Server: AliyunOSS

Error codes

Error code HTTP status code Description
FieldItemTooLong 400 The error message returned because the size of the key or the value of a form field exceeds the limit.
InvalidArgument 400 The error message returned because one of the OSSAccessKeyId, policy, and Signature form fields is specified but the other two form fields are not specified in the request regardless of whether the ACL of the bucket is public-read-write.
InvalidDigest 400 The error message returned because the Content-MD5 value that is calculated by OSS based on the request body is different from the Content-MD5 value that is specified in the request header.
EntityTooLarge 400 The error message returned because the total size of the PostObject request body exceeds 5 GB.
InvalidEncryptionAlgorithmError 400 The error message returned because the x-oss-server-side-encryption request header is set to a valueother than AES256 and KMS. The x-oss-server-side-encryption request header can be set only to AES256 or KMS.
IncorrectNumberOfFilesInPOSTRequest 400 The error message returned because the key form field is not specified in the request.
FileAlreadyExists 409 The error message returned because the request contains the x-oss-forbid-overwrite=true header, but an existing object has the same name as the object that you want to upload.
KmsServiceNotEnabled 403 The error message returned because x-oss-server-side-encryption is set to KMS, but KMS is not activated in advance.
FileImmutable 409 The error message returned because the data that you want to delete or modify is protected by a retention policy.

Appendix: policy

The policy form field in a PostObject request is used to verify the validity of the request. The value of the policy form field is a JSON string that is encoded in UTF-8 and Base64. This value declares the conditions that a PostObject request must meet. The policy form field is optional when you upload an object to a bucket whose ACL is public-read-write. However, we recommend that you specify this form field to limit PostObject requests.

The following example shows the format of the policy form field:

{ "expiration": "2014-12-01T12:00:00.000Z",
  "conditions": [
    {"bucket": "johnsmith" },
    ["starts-with", "$key", "user/eric/"]
  ]
}

The policy form field must contain the expiration and conditions parameters.

  • Expiration

    The expiration parameter specifies the expiration time of the request. The time must follow the ISO 8601 standard and must be in GMT. For example, 2014-12-01T12:00:00.000Z indicates that the PostObject request must be sent before 12:00 on December 1, 2014.

  • Conditions
    The conditions parameter is a list that specifies the valid values of the form fields in the PostObject request.
    Note The value of a form field is extended after OSS checks the policy. Therefore, the valid value of the form field set in policy is equivalent to the value of the form field before extension.

    The following table describes the conditions that can be configured in the policy field.

    Element Description
    content-length-range The minimum and maximum sizes of the object that you want to upload.
    Cache-Control, Content-Type, Content-Disposition, Content-Encoding, Expires HTTP request headers. This condition supports the exact matching and starts-with matching modes.
    Notice To prevent malicious modifications to the Content-Type header during form upload, we recommend that you include the Content-Type parameter in the policy form field.
    key The name of the object. This condition supports the exact matching and starts-with matching modes.
    success_action_redirect The URL to which the client is redirected after the object is uploaded. This condition supports the exact matching and starts-with matching modes.
    success_action_status The HTTP status code to return after the object is uploaded if success_action_redirect is not specified. This condition supports the exact matching and starts-with matching modes.
    x-oss-meta-* The user metadata of the object that you want to upload. This condition supports the exact matching and starts-with matching modes.

    If the PostObject request contains additional form fields, OSS adds these form fields to the conditions of the policy and checks their validity.

  • Condition matching modes
    Matching mode Description
    Exact matching The value of a form field must be exactly the same as the value that is declared in conditions. For example, if the value of the key form field must be a in the format of {"key":"a"}, the condition must be ["eq", "$key", "a"].
    Starts With The value of a form field must start with the specified value. For example, if the value of the key form field must start with user/user1, the condition must be ["starts-with", "$key", "user/user1"].
    Specified object size The size of the object that you want to upload must be within the range of allowed object sizes. For example, if the acceptable object size is 1 to 10 bytes, the condition must be ["content-length-range", 1, 10].

In the policy form field of a PostObject request, the dollar sign ($) indicates a variable. To describe the dollar sign ($), you must use the following escape character: \$. The following table describes the escape characters used in the JSON string of the policy form field in a PostObject request.

Escape character Description
\/ Forward slash
\\ Backslash
\" Double quotation marks
\$ Dollar sign
\b Backspace
\f Form feed
\n New line
\r Carriage return
\t Horizontal tab
\uxxxx Unicode character

Appendix: Signature

To verify a PostObject request, you must include the policy and Signature form fields in the HTML form. The policy form field specifies the values that are acceptable in the request.

You can perform the following steps to calculate the value of the Signature form field:

  1. Create a policy encoded in UTF-8.
  2. Encode the policy in Base64. The result is the value of the policy form field, and this value is used as the string to sign.
  3. Use the AccessKey secret to sign the string in the following format: Signature = base64(hmac-sha1(base64(policy), AccessKeySecret)).

References