Bucket policies are a mechanism for managing permissions and access to Object Storage. When compared to ACLs, bucket policies can only be applied across an entire bucket (not to individual objects), though they offer finer control over the types of permissions you can grant to a user.

Components of a policy

Bucket policies are formatted using JSON with the following structure:

{
 "Version": "2012-10-17",
 "Statement": [{
   "Effect": ...,
   "Principal": ...,
   "Action": ...,
   "Resource": ...
 }]
}

This file consists of a Version string (set to 2012-10-17, which is the current version) and one or more Statement arrays, which define the actual policies you wish to use. Within each statement array are the Effect, Principal, Action, Resource, and optional Condition elements. Each of these are discussed below.

Effect

The Effect section defines if access is allowed (Allow) or denied (Deny) to the specified resource. See IAM JSON policy elements: Effect.

"Effect":"Allow"

Principal

The Principal section defines the user or entity to which the policy applies. See Amazon S3 principals.

Specific user: Specify an Object Storage canonical ID to have the policy apply to that user. For help finding the canonical ID, see Find canonical user ID.
```
"Principal": {
  "AWS": [
    "arn:aws:iam:::user/a0000000-000a-0000-0000-00d0ff0f0000"
  ]
}
```
Public/anonymous access: Use a wildcard to grant access to everyone. This is commonly used for hosting a website through Object Storage.
```
"Principal":"*"
```

Action

Action are the permissions granted (or removed) by the policy. These actions include the ability to list buckets, view objects, upload objects, and more:

s3:PutObject: Upload objects
s3:GetObject: Retrieve objects
s3:ListBucket: List the contents of a bucket

For a full list of actions, see Ceph > Bucket Policies. You can also reference the Amazon S3 actions guide.

Resource

A policy is applied to Object Storage resources, such as buckets and objects. Bucket resources are formatted as "arn:aws:s3:::[bucket]". To apply a policy to some or all objects within a bucket, use "arn:aws:s3:::[bucket]/[object]". In both cases, replace [bucket] with the label for the bucket and [object] with either the wildcard value (*) that designates all objects or the path and name of the object. See Amazon S3 resources.

All objects: Apply the policy to all objects within the bucket labeled example-bucket.
```
"Resource": [
  "arn:aws:s3:::example-bucket/*"
]
```
All objects in specific directory: Apply the policy to all objects in the assets folder within the bucket labeled example-bucket.
```
"Resource": [
  "arn:aws:s3:::example-bucket/folder/*"
]
```
Specific object: Apply the policy to the object example-file.ext within the bucket labeled example-bucket.
```
"Resource": [
  "arn:aws:s3:::example-bucket/example-file.ext"
]
```

❗️
While a resource can target the bucket itself (by removing the /* in the first example), this can cause the bucket to become inaccessible to Cloud Manager, API, and CLI.

Bucket policy examples

Allow public read access

If you wish to allow anyone to view and download objects within a bucket, use the following policy:

{
 "Version": "2012-10-17",
 "Statement": [{
   "Effect": "Allow",
   "Principal": "*",
   "Action": [
     "s3:GetObject",
     "s3:ListBucket"
   ],
   "Resource": [
     "arn:aws:s3:::bucket-example",
     "arn:aws:s3:::bucket-example/*"
   ]
 }]
}

Grant an account limited access to a directory

This policy file allows a user to list the bucket called example-bucket and view/download objects within the test directory. They are not able to perform any other actions.

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

Allow or deny access from a specific IP address

By using the Condition section and the IpAddress and NotIpAddress conditions, you can choose to allow or deny traffic from the specified IP address or range.

If the Effect is set to Allow, use the IpAddress condition to specify that just traffic from that IP address is allowed and use NotIpAddress to allow all traffic except from that IP.

If the Effect is set to Deny, use the IpAddress condition to deny traffic from that IP address and use NotIpAddress to deny all traffic except from that IP.

The example below allows all traffic from only the specified IP address:

{
 "Version": "2012-10-17",
 "Statement": [
   {
     "Effect": "Allow",
     "Principal": "*",
     "Action": "s3:*",
     "Resource": "arn:aws:s3:::example-bucket/*",
     "Condition": {
       "IpAddress": {
         "aws:SourceIp": "192.0.2.1/32"
       }
     }
   }
 ]
}

Apply bucket policies

After creating your bucket policy file and defining your policies, you need to use a third party tool to apply those policies to a bucket.

S3cmd

Command: s3cmd setpolicy [policy-file] s3://[bucket-label], replacing [bucket-label] with the label for your bucket and [policy-file] with the filename and path of your bucket policy file.

Example: Apply the bucket policies defined within the file "policy.json" to the bucket called "example-bucket":

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

See S3cmd > Apply a Bucket Policy for more details.