Enacting Access Control Lists (ACLs) and Bucket Policies with Linode Object Storage

Updated Thursday, March 5, 2020 by Linode Contributed by Linode

Note

Linode Object Storage is now available to the general public in the Newark and Frankfurt data centers! For billing information on Object Storage, see our Object Storage Pricing and Limitations guide.

Caution

Object Storage is similar to a subscription service. Once enabled, you will be billed at the flat rate regardless of whether or not there are active buckets on your account. You must Cancel Object Storage to stop billing for this service.

Linode Object Storage allows users to share access to objects and buckets with other Object Storage users. There are two mechanisms for setting up sharing: Access Control Lists (ACLs), and bucket policies. These mechanisms perform similar functions: both can be used to restrict and grant access to Object Storage resources.

In this guide you will learn:

• The differences between ACLs and bucket policies
• How to use ACLs
• How to use bucket policies

Before You Begin

• This guide will use the s3cmd command line utility to interact with Object Storage. For s3cmd installation and configuration instructions, visit our How to Use Object Storage guide.

• You’ll also need the canonical ID of every user you wish to grant additional permissions to.

Note

Currently, you can only create a new canonical ID by creating a completely new Linode account. A canonical ID is not assigned to a limited access user on an existing Linode account.

Retrieve a User’s Canonical ID

Follow these steps to determine the canonical ID of the Object Storage users you want to share with:

1. The following command will return the canonical ID of a user, given any of the user’s buckets:

   s3cmd info s3://other-users-bucket
   

   Note

   The bucket referred to in this section is an arbitrary bucket on the target user’s account. It is not related to the bucket on your account that you would like to set ACLs or bucket policies on.

   There are two options for running this command:

   • The users you’re granting or restricting access to can run this command on one of their buckets and share their canonical ID with you, or:

   • You can run this command yourself if you have use of their access tokens (you will need to configure s3cmd to use their access tokens instead of your own).

2. Run the above command, replacing other-users-bucket with the name of the bucket. You’ll see output similar to the following:

     
   s3://other-users-bucket/ (bucket):
   Location:  default
   Payer:     BucketOwner
   Expiration Rule: none
   Policy:    none
   CORS:      none
   ACL:       a0000000-000a-0000-0000-00d0ff0f0000: FULL_CONTROL
   
   

3. The canonical ID of the owner of the bucket is the long string of letters, dashes, and numbers found in the line labeled ACL, which in this case is a0000000-000a-0000-0000-00d0ff0f0000.

4. Alternatively, you may be able to retrieve the canonical ID by curling a bucket and retrieving the Owner ID field from the returned XML. This method is an option when both of these conditions are true:

   • The bucket has objects within it and has already been set to public (with a command like s3cmd setacl s3://other-users-bucket --acl-public).
   • The bucket has not been set to serve static websites.

5. Run the curl command, replacing the bucket name and cluster URL with the relevant values:

   curl other-users-bucket.us-east-1.linodeobjects.com
   

   Note

   A cluster is defined as all buckets hosted by a unique URL; for example: us-east-1.linodeobjects.com, us-east-2.linodeobjects.com, or eu-central-1.linodeobjects.com.

6. This will result in the following output:

   <ListBucketResult xmlns="http://s3.amazonaws.com/doc/2006-03-01/">
       <Name>acl-bucket-example</Name>
       <Prefix/>
       <Marker/>
       <MaxKeys>1000</MaxKeys>
       <IsTruncated>false</IsTruncated>
       <Contents>
       <Key>cpanel_one-click.gif</Key>
       <LastModified>2019-11-20T16:52:49.946Z</LastModified>
       <ETag>"9aeafcb192a8e540e7be5b51f7249e2e"</ETag>
       <Size>961023</Size>
       <StorageClass>STANDARD</StorageClass>
       <Owner>
           <ID>a0000000-000a-0000-0000-00d0ff0f0000</ID>
           <DisplayName>a0000000-000a-0000-0000-00d0ff0f0000</DisplayName>
       </Owner>
       <Type>Normal</Type>
       </Contents>
   </ListBucketResult>
   

   In the above output, the canonical ID is a0000000-000a-0000-0000-00d0ff0f0000.

ACLs vs Bucket Policies

ACLs and bucket policies perform similar functions: both can restrict or grant access to buckets. ACLs can also restrict or grant access to individual objects, but they don’t offer as many fine-grained access modes as bucket policies.

How to Choose Between ACLs and Bucket Policies

If you can organize objects with similar permission needs into their own buckets, then it’s strongly suggested that you use bucket policies. However, if you cannot organize your objects in this fashion, ACLs are still a good option.

ACLs offer permissions with less fine-grained control than the permissions available through bucket policies. If you are looking for more granular permissions beyond read and write access, choose bucket policies over ACLs.

Additionally, bucket policies are created by applying a written bucket policy file to the bucket. This file cannot exceed 20KB in size. If you have a policy with a lengthy list of policy rules, you may want to look into ACLs instead.

Note

ACLs and bucket policies can be used at the same time. When this happens, any rule that limits access to an Object Storage resource will override a rule that grants access. For instance, if an ACL allows a user access to a bucket, but a bucket policy denies that user access, the user will not be able to access that bucket.

ACLs

Access Control Lists (ACLs) are a legacy method of defining access to Object Storage resources. You can apply an ACL to a bucket or to a specific object. There are two generalized modes of access: setting buckets and/or objects to be private or public. A few other more granular settings are also available.

With s3cmd, you can set a bucket to be public with the setacl command and the --acl-public flag:

s3cmd setacl s3://acl-example --acl-public

This will cause the bucket and its contents to be downloadable over the general Internet.

To set an object or bucket to private, you can use the setacl command and the --acl-private flag:

s3cmd setacl s3://acl-example --acl-private

This will prevent users from accessing the bucket’ contents over the general Internet.

Other ACL Permissions

The more granular permissions are:

Permission	Description
read	Users with can list objects within a bucket
write	Users can upload objects to a bucket and delete objects from a bucket.
read_acp	Users can read the ACL currently applied to a bucket.
write_acp	Users can change the ACL applied to the bucket.
full_control	Users have read and write access over both objects and ACLs.

• Setting a permission: To apply these more granular permissions for a specific user with s3cmd, use the following setacl command with the --acl-grant flag:

  s3cmd setacl s3://acl-example --acl-grant=PERMISSION:CANONICAL_ID
  

  Substitute acl-example with the name of the bucket (and the object, if necessary), PERMISSION with a permission from the above table, and CANONICAL_ID with the canonical ID of the user to which you would like to grant permissions.

• Revoking a permission: To revoke a specific permission, you can use the setacl command with the acl-revoke flag:

  s3cmd setacl s3://acl-example --acl-revoke=PERMISSION:CANONICAL_ID
  

  Substitute the bucket name (and optional object), PERMISSION, and CANONICAL_ID with your relevant values.

• View current ACLs: To view the current ACLs applied to a bucket or object, use the info command, replacing acl-example with the name of your bucket (and object, if necessary):

  s3cmd info s3://acl-example
  

  You should see output like the following:

    
  s3://acl-bucket-example/ (bucket):
     Location:  default
     Payer:     BucketOwner
     Expiration Rule: none
     Policy:    none
     CORS:      b'<CORSConfiguration xmlns="http://s3.amazonaws.com/doc/2006-03-01/"><CORSRule><AllowedMethod>GET</AllowedMethod><AllowedMethod>PUT</AllowedMethod><AllowedMethod>DELETE</AllowedMethod><AllowedMethod>HEAD</AllowedMethod><AllowedMethod>POST</AllowedMethod><AllowedOrigin>*</AllowedOrigin><AllowedHeader>*</AllowedHeader></CORSRule></CORSConfiguration>'
     ACL:       *anon*: READ
     ACL:       a0000000-000a-0000-0000-00d0ff0f0000: FULL_CONTROL
     URL:       http://us-east-1.linodeobjects.com/acl-example/
  
  

  Note

  The owner of the bucket will always have the full_control permission.

Bucket Policies

Bucket policies can offer finer control over the types of permissions you can grant to a user.

Basic Access Policy

Below is an example bucket policy written in JSON:

bucket_policy_example.json

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{ "Version": "2012-10-17", "Statement": [{ "Effect": "Allow", "Principal": { "AWS": [ "arn:aws:iam:::a0000000-000a-0000-0000-00d0ff0f0000" ] }, "Action": [ "s3:PutObject", "s3:GetObject", "s3:ListBucket" ], "Resource": [ "arn:aws:s3:::bucket-policy-example/*" ] }] }

This policy allows the user with the canonical ID a0000000-000a-0000-0000-00d0ff0f0000, known here as the “principal”, to interact with the bucket, known as the “resource”. The “resource” that is listed (bucket-policy-example) is the only bucket the user will have access to.

Note

The principal (a.k.a. the user) must have the prefix of arn:aws:iam:::, and the resource (a.k.a. the bucket) must have the prefix of arn:aws:s3:::.

The permissions are specified in the Action array. For the current example, these are:

• s3:PutObject: The ability to upload objects to a bucket
• s3:GetObject: The ability to retrieve objects from a bucket

• s3:ListBucket: The ability to list the contents of the bucket

  Note

  For a full list of of available actions, visit the Ceph bucket policy documentation.

The Action and Principal.AWS fields of the bucket policy are arrays, so you can easily add additional users and permissions to the bucket policy, separating them by a comma. To grant permissions to all users, you can supply a wildcard (*) to the Principal.AWS field.

Subdirectory Access Policy

You can also define a finer level of control over the level of access to your bucket’s directory structure using policy rules.

bucket-policy-directories.json

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{ "Version": "2012-10-17", "Statement": [ { "Effect": "Allow", "Principal": { "AWS": "arn:aws:iam::a0000000-000a-0000-0000-00d0ff0f0000" }, "Action": [ "s3:ListBucket" ], "Resource": [ "arn:aws:s3:::*" ] }, { "Effect": "Allow", "Principal": { "AWS": "arn:aws:iam::a0000000-000a-0000-0000-00d0ff0f0000" }, "Action": [ "s3:GetObject" ], "Resource": [ "arn:aws:s3:::bucket-policy-example/test/*" ] } ] }

This example shows how you can grant read-only access to a user by allowing them to list buckets and get objects from the bucket only from the test directory. However, they will not be able to perform any other actions.

Denying Access by IP Address

If you wanted to deny access to a user by IP address, you can change the Effect field from Allow to Deny and supply an IP address in a condition.

bucket-policy-deny.json

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{ "Version": "2012-10-17", "Statement": [{ "Effect": "Deny", "Principal": "*", "Action": "s3:*", "Resource": "arn:aws:s3:::bucket-policy-example/*", "Condition": { "NotIpAddress": { "aws:SourceIp": "172.104.2.4" } } }] }

Combining Rules

Only one policy file can be enabled at a time. Therefore, if you wanted to enact several of the above rules together, instead of enabling them one at a time, you would need to combine them into a single file with each rule listed as items in the Statements array.

bucket-policy-combo.json

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{ "Version": "2012-10-17", "Statement": [{ "Effect": "Allow", "Principal": { "AWS": [ "arn:aws:iam:::a0000000-000a-0000-0000-00d0ff0f0000" ] }, "Action": [ "s3:PutObject", "s3:GetObject", "s3:ListBucket" ], "Resource": [ "arn:aws:s3:::bucket-policy-example/*" ] }, { "Effect": "Allow", "Principal": { "AWS": "arn:aws:iam::a0000000-000a-0000-0000-00d0ff0f0000" }, "Action": [ "s3:ListBucket" ], "Resource": [ "arn:aws:s3:::*" ] }, { "Effect": "Allow", "Principal": { "AWS": "arn:aws:iam::a0000000-000a-0000-0000-00d0ff0f0000" }, "Action": [ "s3:GetObject" ], "Resource": [ "arn:aws:s3:::bucket-policy-example/test/*" ] }, { "Effect": "Deny", "Principal": "*", "Action": "s3:*", "Resource": "arn:aws:s3:::bucket-policy-example/*", "Condition": { "NotIpAddress": { "aws:SourceIp": "172.104.2.4" } } }] }

Enable a Bucket Policy

To enable the bucket policy, use the setpolicy s3cmd command, supplying the file name of the bucket policy as the first argument, and the S3 bucket address as the second argument:

s3cmd setpolicy bucket_policy_example.json s3://bucket-policy-example

To ensure that it has been applied correctly, you can use the info command:

s3cmd info s3://bucket-policy-example

You should see output like the following:

  
s3://bucket-policy-example/ (bucket):
   Location:  default
   Payer:     BucketOwner
   Expiration Rule: none
   Policy:    b'{\n  "Version": "2012-10-17",\n  "Statement": [{\n    "Effect": "Allow",\n    "Principal": {"AWS": ["arn:aws:iam:::a0000000-000a-0000-0000-00d0ff0f0000"]},\n    "Action": ["s3:PutObject","s3:GetObject","s3:ListBucket"],\n    "Resource": [\n      "arn:aws:s3:::bucket-policy-example/*"\n    ]\n  }]\n}'
   CORS:      none
   ACL:       a0000000-000a-0000-0000-00d0ff0f0000: FULL_CONTROL

Note

The policy is visible in the output.

More Information

You may wish to consult the following resources for additional information on this topic. While these are provided in the hope that they will be useful, please note that we cannot vouch for the accuracy or timeliness of externally hosted materials.

• Ceph Bucket Policy Supported Actions

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This guide is published under a CC BY-ND 4.0 license.