Access Control Lists (ACLs)

Access Control Lists (ACLs)

Normally to create, read and modify containers and objects, you must have the appropriate roles on the project associated with the account, i.e., you must be the owner of the account. However, an owner can grant access to other users by using an Access Control List (ACL).

There are two types of ACLs:

  • Container ACLs. These are specified on a container and apply to that container only and the objects in the container.
  • Account ACLs. These are specified at the account level and apply to all containers and objects in the account.

Container ACLs

Container ACLs are stored in the X-Container-Write and X-Container-Read metadata. The scope of the ACL is limited to the container where the metadata is set and the objects in the container. In addition:

  • X-Container-Write grants the ability to perform PUT, POST and DELETE operations on objects within a container. It does not grant the ability to perform POST or DELETE operations on the container itself. Some ACL elements also grant the ability to perform HEAD or GET operations on the container.
  • X-Container-Read grants the ability to perform GET and HEAD operations on objects within a container. Some of the ACL elements also grant the ability to perform HEAD or GET operations on the container itself. However, a container ACL does not allow access to privileged metadata (such as X-Container-Sync-Key).

Container ACLs use the “V1” ACL syntax which is a comma separated string of elements as shown in the following example:

.r:*,.rlistings,7ec59e87c6584c348b563254aae4c221:*

Spaces may occur between elements as shown in the following example:

.r : *, .rlistings, 7ec59e87c6584c348b563254aae4c221:*

However, these spaces are removed from the value stored in the X-Container-Write and X-Container-Read metadata. In addition, the .r: string can be written as .referrer:, but is stored as .r:.

While all auth systems use the same syntax, the meaning of some elements is different because of the different concepts used by different auth systems as explained in the following sections:

Common ACL Elements

The following table describes elements of an ACL that are supported by both Keystone auth and TempAuth. These elements should only be used with X-Container-Read (with the exception of .rlistings, an error will occur if used with X-Container-Write):

Element Description
.r:* Any user has access to objects. No token is required in the request.
.r:<referrer> The referrer is granted access to objects. The referrer is identified by the Referer request header in the request. No token is required.
.r:-<referrer> This syntax (with “-” prepended to the referrer) is supported. However, it does not deny access if another element (e.g., .r:*) grants access.
.rlistings Any user can perform a HEAD or GET operation on the container provided the user also has read access on objects (e.g., also has .r:* or .r:<referrer>. No token is required.

Keystone Auth ACL Elements

The following table describes elements of an ACL that are supported only by Keystone auth. Keystone auth also supports the elements described in Common ACL Elements.

A token must be included in the request for any of these ACL elements to take effect.

Element Description
<project-id>:<user-id> The specified user, provided a token scoped to the project is included in the request, is granted access. Access to the container is also granted when used in X-Container-Read.
<project-id>:* Any user with a role in the specified Keystone project has access. A token scoped to the project must be included in the request. Access to the container is also granted when used in X-Container-Read.
*:<user-id> The specified user has access. A token for the user (scoped to any project) must be included in the request. Access to the container is also granted when used in X-Container-Read.
*:* Any user has access. Access to the container is also granted when used in X-Container-Read. The *:* element differs from the .r:* element because *:* requires that a valid token is included in the request whereas .r:* does not require a token. In addition, .r:* does not grant access to the container listing.
<role_name> A user with the specified role name on the project within which the container is stored is granted access. A user token scoped to the project must be included in the request. Access to the container is also granted when used in X-Container-Read.

Note

Keystone project (tenant) or user names (i.e., <project-name>:<user-name) must no longer be used because with the introduction of domains in Keystone, names are not globally unique. You should use user and project ids instead.

For backwards compatibility, ACLs using names will be granted by keystoneauth when it can be established that the grantee project, the grantee user and the project being accessed are either not yet in a domain (e.g. the X-Auth-Token has been obtained via the Keystone V2 API) or are all in the default domain to which legacy accounts would have been migrated.

TempAuth ACL Elements

The following table describes elements of an ACL that are supported only by TempAuth. TempAuth auth also supports the elements described in Common ACL Elements.

Element Description
<user-name> The named user is granted access. The wildcard (“*”) character is not supported. A token from the user must be included in the request.

Container ACL Examples

Container ACLs may be set by including X-Container-Write and/or X-Container-Read headers with a PUT or a POST request to the container URL. The following examples use the swift command line client which support these headers being set via its --write-acl and --read-acl options.

Example: Public Container

The following allows anybody to list objects in the www container and download objects. The users do not need to include a token in their request. This ACL is commonly referred to as making the container “public”. It is useful when used with StaticWeb:

swift post www --read-acl ".r:*,.rlistings"

Example: Shared Writable Container

The following allows anybody to upload or download objects. However, to download an object, the exact name of the object must be known since users cannot list the objects in the container. The users must include a Keystone token in the upload request. However, it does not need to be scoped to the project associated with the container:

swift post www --read-acl ".r:*" --write-acl "*:*"

Example: Sharing a Container with Project Members

The following allows any member of the 77b8f82565f14814bece56e50c4c240f project to upload and download objects or to list the contents of the www container. A token scoped to the 77b8f82565f14814bece56e50c4c240f project must be included in the request:

swift post www --read-acl "77b8f82565f14814bece56e50c4c240f:*" \
               --write-acl "77b8f82565f14814bece56e50c4c240f:*"

Example: Sharing a Container with Users having a specified Role

The following allows any user that has been assigned the my_read_access_role on the project within which the www container is stored to download objects or to list the contents of the www container. A user token scoped to the project must be included in the download or list request:

swift post www --read-acl "my_read_access_role"

Example: Allowing a Referrer Domain to Download Objects

The following allows any request from the example.com domain to access an object in the container:

swift post www --read-acl ".r:.example.com"

However, the request from the user must contain the appropriate Referer header as shown in this example request:

curl -i $publicURL/www/document --head -H "Referer: http://www.example.com/index.html"

Note

The Referer header is included in requests by many browsers. However, since it is easy to create a request with any desired value in the Referer header, the referrer ACL has very weak security.

Example: Sharing a Container with Another User

Sharing a Container with another user requires the knowledge of few parameters regarding the users.

The sharing user must know:

  • the OpenStack user id of the other user

The sharing user must communicate to the other user:

  • the name of the shared container
  • the OS_STORAGE_URL

Usually the OS_STORAGE_URL is not exposed directly to the user because the swift client by default automatically construct the OS_STORAGE_URL based on the User credential.

We assume that in the current directory there are the two client environment script for the two users sharing.openrc and other.openrc.

The sharing.openrc should be similar to the following:

export OS_USERNAME=sharing
# WARNING: Save the password in clear text only for testing purposes
export OS_PASSWORD=password
export OS_TENANT_NAME=projectName
export OS_AUTH_URL=https://identityHost:portNumber/v2.0
# The following lines can be omitted
export OS_TENANT_ID=tenantIDString
export OS_REGION_NAME=regionName
export OS_CACERT=/path/to/cacertFile

The other.openrc should be similar to the following:

export OS_USERNAME=other
# WARNING: Save the password in clear text only for testing purposes
export OS_PASSWORD=otherPassword
export OS_TENANT_NAME=otherProjectName
export OS_AUTH_URL=https://identityHost:portNumber/v2.0
# The following lines can be omitted
export OS_TENANT_ID=tenantIDString
export OS_REGION_NAME=regionName
export OS_CACERT=/path/to/cacertFile

For more information see using the OpenStack RC file

First we figure out the other user id:

. other.openrc
OUID="$(openstack user show --format json "${OS_USERNAME}" | jq -r .id)"

or alternatively:

. other.openrc
OUID="$(openstack token issue -f json | jq -r .user_id)"

Then we figure out the storage url of the sharing user:

sharing.openrc
SURL="$(swift auth | awk -F = '/OS_STORAGE_URL/ {print $2}')"

Running as the sharing user create a shared container named shared in read-only mode with the other user using the proper acl:

sharing.openrc
swift post --read-acl "*:${OUID}" shared

Running as the sharing user create and upload a test file:

touch void
swift upload shared void

Running as the other user list the files in the shared container:

other.openrc
swift --os-storage-url="${SURL}" list shared

Running as the other user download the shared container in the /tmp directory:

cd /tmp
swift --os-storage-url="${SURL}" download shared

Account ACLs

Note

Account ACLs are not currently supported by Keystone auth

The X-Account-Access-Control header is used to specify account-level ACLs in a format specific to the auth system. These headers are visible and settable only by account owners (those for whom swift_owner is true). Behavior of account ACLs is auth-system-dependent. In the case of TempAuth, if an authenticated user has membership in a group which is listed in the ACL, then the user is allowed the access level of that ACL.

Account ACLs use the “V2” ACL syntax, which is a JSON dictionary with keys named “admin”, “read-write”, and “read-only”. (Note the case sensitivity.) An example value for the X-Account-Access-Control header looks like this, where a, b and c are user names:

{"admin":["a","b"],"read-only":["c"]}

Keys may be absent (as shown in above example).

The recommended way to generate ACL strings is as follows:

from swift.common.middleware.acl import format_acl
acl_data = { 'admin': ['alice'], 'read-write': ['bob', 'carol'] }
acl_string = format_acl(version=2, acl_dict=acl_data)

Using the format_acl() method will ensure that JSON is encoded as ASCII (using e.g. ‘u1234’ for Unicode). While it’s permissible to manually send curl commands containing X-Account-Access-Control headers, you should exercise caution when doing so, due to the potential for human error.

Within the JSON dictionary stored in X-Account-Access-Control, the keys have the following meanings:

Access Level Description
read-only These identities can read everything (except privileged headers) in the account. Specifically, a user with read-only account access can get a list of containers in the account, list the contents of any container, retrieve any object, and see the (non-privileged) headers of the account, any container, or any object.
read-write These identities can read or write (or create) any container. A user with read-write account access can create new containers, set any unprivileged container headers, overwrite objects, delete containers, etc. A read-write user can NOT set account headers (or perform any PUT/POST/DELETE requests on the account).
admin These identities have “swift_owner” privileges. A user with admin account access can do anything the account owner can, including setting account headers and any privileged headers – and thus granting read-only, read-write, or admin access to other users.

For more details, see swift.common.middleware.tempauth. For details on the ACL format, see swift.common.middleware.acl.

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