AWS S3 DNS load balancer

Tue Jun 15 15:14:58 UTC 2021

On Tue, 2021-06-15 at 10:33 -0400, Christopher Morrow wrote:
> Maybe Deepak means:
>   "When I ask for an S3 endpoint I get 1 answer, which is 1 of a set
> of N.
> Why would
>    the 'loadbalancer' send me all N?"

I've just taken a squiz at an S3-based website we have, and via the S3
URL it is a CNAME with a 60-secod TTL pointing at a set of A records
with 5-second TTLs.

Any one dig returns the CNAME and a single IP address:

dig our-domain.s3-website-ap-southeast-2.amazonaws.com.
our-domain.s3-website-ap-southeast-2.amazonaws.com.	14 IN CNAME s3-
website-ap-southeast-2.amazonaws.com.
s3-website-ap-southeast-2.amazonaws.com. 5 IN A	52.95.134.145

If the query is multiply repeated, the returned IP address changes,
roughly every five seconds.

What's interesting is the name attached to the A records, which does
not include "our-domain". It seems to be a record pointing to ALL S3
websites in the region. And all of the addresses I saw reverse-resolve
to that one name. So there is definitely some under-the-bonnet magic
discrimination going on.

In Route53 the picture is very different, with the published website
host name (think "our-domain.com.au") resolving to four IP addresses
that are all returned in the response to a single dig query. There is
an A-ALIAS (a non-standard AWS record type) that points to a CloudFront
distribution that has the relevant S3 bucket as its origin.

Using the CNAME bypasses the CloudFront distribution unless steps are
taken to forbid direct access to the bucket. It would be usual to use
(and enforce) access via CloudFront, if for no other reason than to
provide for HTTPS access. 

Regards, K.

-- 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Karl Auer (kauer at biplane.com.au)
http://www.biplane.com.au/kauer