Temporary URL middleware

To discover whether your Object Storage system supports this feature, check with your service provider or send a GET request using the /info path.

A temporary URL gives users temporary access to objects. For example, a website might want to provide a link to download a large object in Object Storage, but the Object Storage account has no public access. The website can generate a URL that provides time-limited GET access to the object. When the web browser user clicks on the link, the browser downloads the object directly from Object Storage, eliminating the need for the website to act as a proxy for the request.

Furthermore, a temporary URL can be prefix-based. These URLs contain a signature which is valid for all objects which share a common prefix. They are useful for sharing a set of objects.

Ask your cloud administrator to enable the temporary URL feature. For information, see TempURL in the Source Documentation.

Note

To use POST requests to upload objects to specific Object Storage locations, use Form POST middleware instead of temporary URL middleware.

Temporary URL format

A temporary URL is comprised of the URL for an object with added query parameters:

Example Temporary URL format

https://swift-cluster.example.com/v1/my_account/container/object
?temp_url_sig=da39a3ee5e6b4b0d3255bfef95601890afd80709
&temp_url_expires=1323479485
&filename=My+Test+File.pdf

The example shows these elements:

Object URL: Required. The full path URL to the object.

temp_url_sig: Required. An HMAC-SHA1 cryptographic signature that defines the allowed HTTP method, expiration date, full path to the object, and the secret key for the temporary URL.

temp_url_expires: Required. An expiration date as a UNIX Epoch timestamp, which is an integer value. For example, 1390852007 represents Mon, 27 Jan 2014 19:46:47 GMT.

For more information, see Epoch & Unix Timestamp Conversion Tools.

filename: Optional. Overrides the default file name. Object Storage generates a default file name for GET temporary URLs that is based on the object name. Object Storage returns this value in the Content-Disposition response header. Browsers can interpret this file name value as a file attachment to be saved.

A prefix-based temporary URL is similar but requires the parameter temp_url_prefix, which must be equal to the common prefix shared by all object names for which the URL is valid.

https://swift-cluster.example.com/v1/my_account/container/my_prefix/object
?temp_url_sig=da39a3ee5e6b4b0d3255bfef95601890afd80709
&temp_url_expires=1323479485
&temp_url_prefix=my_prefix

Secret Keys

The cryptographic signature used in Temporary URLs and also in Form POST middleware uses a secret key. Object Storage allows you to store two secret key values per account, and two per container. When validating a request, Object Storage checks signatures against all keys. Using two keys at each level enables key rotation without invalidating existing temporary URLs.

To set the keys at the account level, set one or both of the following request headers to arbitrary values on a POST request to the account:

  • X-Account-Meta-Temp-URL-Key
  • X-Account-Meta-Temp-URL-Key-2

To set the keys at the container level, set one or both of the following request headers to arbitrary values on a POST or PUT request to the container:

  • X-Container-Meta-Temp-URL-Key
  • X-Container-Meta-Temp-URL-Key-2

The arbitrary values serve as the secret keys.

For example, use the swift post command to set the secret key to ``MYKEY``:

$ swift post -m "Temp-URL-Key:MYKEY"

Note

Changing these headers invalidates any previously generated temporary URLs within 60 seconds, which is the memcache time for the key.

HMAC-SHA1 signature for temporary URLs

Temporary URL middleware uses an HMAC-SHA1 cryptographic signature. This signature includes these elements:

  • The allowed method. Typically, GET or PUT.
  • Expiry time. In the example for the HMAC-SHA1 signature for temporary URLs below, the expiry time is set to 86400 seconds (or 1 day) into the future.
  • The path. Starting with /v1/ onwards and including a container name and object. The path for prefix-based signatures must start with prefix:/v1/. Do not URL-encode the path at this stage.
  • The secret key. Use one of the key values as described in Secret Keys.

These sample Python codes show how to compute a signature for use with temporary URLs:

Example HMAC-SHA1 signature for object-based temporary URLs

import hmac
from hashlib import sha1
from time import time
method = 'GET'
duration_in_seconds = 60*60*24
expires = int(time() + duration_in_seconds)
path = '/v1/my_account/container/object'
key = 'MYKEY'
hmac_body = '%s\n%s\n%s' % (method, expires, path)
signature = hmac.new(key, hmac_body, sha1).hexdigest()

Example HMAC-SHA1 signature for prefix-based temporary URLs

import hmac
from hashlib import sha1
from time import time
method = 'GET'
duration_in_seconds = 60*60*24
expires = int(time() + duration_in_seconds)
path = 'prefix:/v1/my_account/container/my_prefix'
key = 'MYKEY'
hmac_body = '%s\n%s\n%s' % (method, expires, path)
signature = hmac.new(key, hmac_body, sha1).hexdigest()

Do not URL-encode the path when you generate the HMAC-SHA1 signature. However, when you make the actual HTTP request, you should properly URL-encode the URL.

The ``MYKEY`` value is one of the key values as described in Secret Keys.

For more information, see RFC 2104: HMAC: Keyed-Hashing for Message Authentication.

swift-temp-url script

Object Storage provides the swift-temp-url script that auto-generates the ``temp_url_sig`` and ``temp_url_expires`` query parameters. For example, you might run this command:

$ bin/swift-temp-url GET 3600 /v1/my_account/container/object MYKEY

This command returns the path:

/v1/my_account/container/object
?temp_url_sig=5c4cc8886f36a9d0919d708ade98bf0cc71c9e91
&temp_url_expires=1374497657

To create the temporary URL, prefix this path with the Object Storage storage host name. For example, prefix the path with https://swift-cluster.example.com, as follows:

https://swift-cluster.example.com/v1/my_account/container/object
?temp_url_sig=5c4cc8886f36a9d0919d708ade98bf0cc71c9e91
&temp_url_expires=1374497657

Note that if the above example is copied exactly, and used in a command shell, then the ampersand is interpreted as an operator and the URL will be truncated. Enclose the URL in quotation marks to avoid this.