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AWS CLI S3 FAQ

S3 FAQ

Below are common questions regarding the use of Amazon S3 in the AWS CLI.

Q: Does the AWS CLI validate checksums?

The AWS CLI will perform checksum validation for uploading and downloading files in specific scenarios.

Upload

The AWS CLI will calculate and auto-populate the Content-MD5 header for both standard and multipart uploads. If the checksum that S3 calculates does not match the Content-MD5 provided, S3 will not store the object and instead will return an error message back the AWS CLI. The AWS CLI will retry this error up to 5 times before giving up. On the case that any files fail to transfer successfully to S3, the AWS CLI will exit with a non zero RC. See aws help returncodes for more information.

If the upload request is signed with Signature Version 4, then a Content-MD5 is not calculated. Instead, the AWS CLI uses the x-amz-content-sha256 header as a checksum instead of Content-MD5. The AWS CLI will use Signature Version 4 for S3 in several cases:

  • You're using an AWS region that only supports Signature Version 4. This includes eu-central-1 and ap-northeast-2.
  • You explicitly opt in and set signature_version = s3v4 in your ~/.aws/config file.

Note that the AWS CLI will add a Content-MD5 header for both the high level aws s3 commands that perform uploads (aws s3 cp, aws s3 sync) as well as the low level s3api commands including aws s3api put-object and aws s3api upload-part.

Download

The AWS CLI will attempt to verify the checksum of downloads when possible, based on the ETag header returned from a GetObject request that's performed whenever the AWS CLI downloads objects from S3. If the calculated MD5 checksum does not match the expected checksum, the file is deleted and the download is retried. This process is retried up to 3 times. If a downloads fails, the AWS CLI will exit with a non zero RC. See aws help returncodes for more information.

There are several conditions where the CLI is not able to verify checksums on downloads:

  • If the object was uploaded via multipart uploads
  • If the object was uploaded using server side encryption with KMS
  • If the object was uploaded using a customer provided encryption key
  • If the object is downloaded using range GetObject requests