S3 permission for hosting public images for a web app - amazon-s3

Question

I've started using S3 to host images connected to Rails models. Images are not uploaded by users so I just use aws_sdk gem to store images to S3 buckets.
So far I've succeeded in storing the image, but I am confused about the permission. Maybe I'm wrong but it seems most of the documents talking about S3 permission are outdated and I can't find what they are referring to.
What I want to do is pretty basic. I just want to host images and the image themselves are public so anyone can view. However I don't want anyone to just access my bucket and see everything else that's hosted there. So basically it's just a normal web app that hosts images on S3. How and where can I change the permission settings so it works that way? Currently the access is only granted to myself and images are not viewable by typing the url in a browser.

Answer 1

For people that are looking for more specific information on how to write a policy that allows anyone to access an s3 bucket this may be helpful.
{
"Version": "2012-10-17",
"Statement": [
{
"Sid": "PublicRead",
"Effect": "Allow",
"Principal": "*",
"Action": [
"s3:GetObject",
"s3:GetObjectVersion"
],
"Resource": [
"arn:aws:s3:::DOC-EXAMPLE-BUCKET/*"
]
}
]
}
This, and other examples at, the link below.
https://docs.aws.amazon.com/AmazonS3/latest/userguide/example-bucket-policies.html#example-bucket-policies-use-case-2
If your are still having issues, check your block settings.

Answer 2

Answering my own question:
It seems that it's not as simple as setting something as "public" and expect everything to work. Basically you need to write a policy and apply it to a bucket if you want your bucket to be viewable by public.
Here's the page where you can build your policy: http://awspolicygen.s3.amazonaws.com/policygen.html

Answer 3

Take a look at the docs, more specific at the S3Objects write method: Class: AWS::S3::S3Object, which allows you to set a bunch of options for the uploaded file.
When uploading to your S3 bucket you have so set the proper :acl permission, because its default is :private and no public access is granted. Here's a modified snippet I grabbed from github:
# get an instance of the S3 interface using the default configuration
s3 = AWS::S3.new
# create a bucket
b = s3.buckets.create('example')
# upload a file
basename = File.basename('image.png')
o = b.objects[basename]
o.write(:file => file_name, :acl => :public_read)
# grab public url
image_public_url = o.public_url
...