Here is how I setup duplicity to use S3 as a backend while giving duplicity the minimum set of permissions to my Amazon Web Services account.

AWS Security Settings

First of all, I enabled the following general security settings in my AWS account:

• MFA with a U2F device
• no root user access keys

Then I set a password policy in the IAM Account Settings and turned off all public access in the S3 Account Settings.

Creating an S3 bucket

As a destination for the backups, I created a new backup-foobar S3 bucket keeping all of the default options except for the region which I set to ca-central-1 to ensure that my data would stay in Canada.

The bucket name can be anything you want as long as:

• it's not already taken by another AWS user
• it's a valid hostname (i.e. alphanumeric characters or dashes)

Note that I did not enable S3 server-side encryption since I will be encrypting the backups client-side using the support built into duplicity instead.

Creating a restricted user account

Then I went back into the Identity and Access Managment console and created a new DuplicityBackup policy:

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "VisualEditor0",
            "Effect": "Allow",
            "Action": [
                "s3:PutObject",
                "s3:GetObject",
                "s3:ListBucketMultipartUploads",
                "s3:AbortMultipartUpload",
                "s3:CreateBucket",
                "s3:ListBucket",
                "s3:DeleteObject",
                "s3:ListMultipartUploadParts"
            ],
            "Resource": [
                "arn:aws:s3:::backup-foobar",
                "arn:aws:s3:::backup-foobar/*"
            ]
        },
        {
            "Sid": "VisualEditor1",
            "Effect": "Allow",
            "Action": "s3:ListAllMyBuckets",
            "Resource": "*"
        }
    ]
}

It's unfortunate that the unrestricted s3:ListAllMyBuckets permission has to be granted, but in my testing, duplicity would error out without it. No other permissions were needed.

The next step was to create a new DuplicityBackupHosts IAM group to which I attached the DuplicityBackup policy.

Finally, I created a new machinename IAM user:

• Access: programmatic only
• Group: DuplicityBackupHosts
• Tags: duplicity=1

and wrote down the access key and the access key secret.

Duplicity settings

I installed duplicity like this:

apt install duplicity python3-boto3

then used the following options:

• --s3-use-new-style: apparently required on non-US regions
• --s3-use-ia: recommended pricing structure for backups
• --s3-use-multiprocessing: speeds up uploading of backup chunks

and the following remote URL:

boto3+s3://backup-foobar/machinename

I ended up with the following command:

http_proxy= AWS_ACCESS_KEY_ID=<access_key> AWS_SECRET_ACCESS_KEY=<access_key_secret> PASSPHRASE=<password> duplicity --s3-use-new-style --s3-use-ia --s3-use-multiprocessing --no-print-statistics --verbosity 1 --exclude-device-files --exclude-filelist <exclude_file> --include-filelist <include_file> --exclude '**' / <remote_url>

where <exclude_file> is a file which contains the list of paths to keep out of my backup:

/etc/.git
/home/francois/.cache

<include_file> is a file which contains the list of paths to include in the backup:

/etc
/home/francois
/usr/local/bin
/usr/local/sbin
/var/log/apache2
/var/www

and <password> is a long random string (pwgen -s 64) used to encrypt the backups.

Backup script

Here are two other things I included in my backup script prior to the actual backup line listed in the previous section.

The first one deletes files related to failed backups:

http_proxy= AWS_ACCESS_KEY_ID=<access_key> AWS_SECRET_ACCESS_KEY=<access_key_secret> PASSPHRASE=<password> duplicity cleanup --verbosity 1 --force <remote_url>

and the second deletes old backups (older than 12 days in this example):

http_proxy= AWS_ACCESS_KEY_ID=<access_key> AWS_SECRET_ACCESS_KEY=<access_key_secret> PASSPHRASE=<password> duplicity remove-older-than 12D --verbosity 1 --force <remote_url>

Feel free to leave a comment if I forgot anything that might be useful!