Amazon CloudFront: Origin Access Control

Amazon CloudFront, the AWS Content Delivery Network (CDN) service, has come a long way since I first saw it launch; I recall a slight chortle when it had 53 points of presence (PoPs) account the world, as CloudFront often (normally?) shares edge location facilities with the Amazon Route53 (Hosted DNS) service.

Today it’s over 400 PoPs, and is used for large and small web acceleration workloads.

One common pattern is having CloudFront serve static objects (files) that are stored in AWS’s Simple Storage Service, S3. Those static objects are often HTML files, images, Cascading Style Sheet documents, and more. And while S3 has a native Website serving function, it has long been my strong recommendation to my friends and colleagues to not use it, but use CloudFront in front of S3. There’s many reasons for this, one of which is you can configure the TLS certificate handed out, set the minimally permitted TLS version, and inject the various HTTP Security Headers we’ve come to see as minimal requirements for asking web browsers to help secure workloads.

Indeed, having any CDN sit in front of an origin server is an architecture that’s as old as web 2.0 (or more). One consideration her is that you don’t want end users circumventing the CDN and going direct to your origin server; if that origin gets overloaded, then the CDN (which caches) may not be able to fetch content for it’s viewers.

It’s not uncommon for CDNs to exceed 99.99% caching of objects (files), greatly reducing the origin server(s) that host the content. CDNs can also do conditional GET requests against an origin, to check that a cached version of an object (file) has not changed, which helps ensure the cached object can still be served our to visitors.

Ensuring that origin doesn’t get overloaded then becomes a question of blocking all other requests to the origin except those from the CDN. Amzon CloudFront has evolved its pattern over the years, staring with each edge operating independently. As the number of PoPs grew, this became an issue, so a mid tier cache, called the CloudFront Regional Edge, was introduced to help absorb some of that traffic. It’s a pattern that Akamai was using in the 2000’s when it had hundreds/thousands of PoPs.

For S3, the initial approach was to use a CloudFront Origin Identity (OID), which would cause a CloudFront origin request (from the edge, to the origin) to be authenticated against the S3 endpoint. An S3 Bucket Policy could then be applied that would permit access for this identity, and thus protect the origin from denial of service.

The S3 documentation to restrict access to S3 for this is useful.

Here’s an example S3 Bucket policy from where I serve my web content (from various prefixes therein):

    "Version": "2008-10-17",
    "Statement": [
            "Effect": "Allow",
            "Principal": {
                "AWS": "arn:aws:iam::cloudfront:user/CloudFront Origin Access Identity E6BL78W5XXXXX"
            "Action": "s3:GetObject",
            "Resource": "arn:aws:s3:::xxxxxxxx-my-dev-web-bucket/*"

This has now been revised, an in one release post, labelled as legacy and deprecated. The new approach is called an Origin Access Control (OAC), and will give finer-grained control.

One question I look at is the migration from one to another, trying to reach this with minimal (or no) downtime.

In my case, I am not concerned with restricted access to the S3 object to a specific CloudFront distribution ID; I am happy to have one identity that all my CloudFront distributions share against the same S3 Bucket (with different prefixes). As such, my update is straight forward, in that I am going to start by updating the above Bucket policy:

    "Version": "2008-10-17",
    "Statement": [
            "Effect": "Allow",
            "Principal": {
                "AWS": "arn:aws:iam::cloudfront:user/CloudFront Origin Access Identity E6BL78W5XXXXX",
                "Service": ""
            "Action": "s3:GetObject",
            "Resource": "arn:aws:s3:::xxxxxxxx-my-dev-web-bucket/*"

With this additional Service line, any CloudFront distribution can now grab objects from my account (possibly across account as well). I can add conditions to this policy as well, such as checking the Distribution IDs, but as part of the migration from OID to OAC we’ll come back to that.

Next up, in the CloudFront Console (or in a Cloud Formation update) we create a new OAC entry, with the v4sig being enabled for origin requests. Here’s the CloudFormation snippet:

    Type: AWS::CloudFront::OriginAccessControl
        DisplayName: S3Access
        Description: "Access to S3"
        OriginType: s3
        SigningBehavior: always
        SigningProtocol: sigv4

Now we have an Origin Access Control, which in the console looks like this:

With this in place, then we need to update the CloudFront distributions to use this for each behaviour’s origin.

Give it a few minutes, check the content is still being delivered, and then its time to now back out the old CloudFront origin Access identity from the S3 Bucket Policy:

    "Version": "2008-10-17",
    "Statement": [
            "Effect": "Allow",
            "Principal": {
                "Service": ""
            "Action": "s3:GetObject",
            "Resource": "arn:aws:s3:::xxxxxxxx-my-dev-web-bucket/*"

Then pop back to the CloudFront world and remove the old Origin Access Id (again, but either Cloud Formation update if that’s how you created it, or via the console or API).

This is also a good time to look at the Condition options in that policy, and see if you want to place further restrictions on access to your S3 Bucket, possibly like:

        "Condition": {
            "StringEquals": {
                "AWS:SourceArn": "arn:aws:cloudfront::111122223333:distribution/*"

(where the 1111… number in red is your AWS account number).

AWS has been key to say that:

Any distributions using Origin Access Identity will continue to work and you can continue to use Origin Access Identity for new distributions.


However, that position may change in future, and given this has already marked the existing OID approach as “legacy“, it’s time to start evaluating your configuration changes.

AWS Reservation Coverage: Perhaps 100% is too much

I was reading a post the other day advising AWS customers to consider why they aren’t reaching 100% RI coverage. This triggered me, as 100% coverage is often not a good thing. And yes, now we have Savings Plans for Some Things in AWS, but some places remain with Reservations as the way to get consumption discounts by trading on flexibility.

And it’s that trade on flexibility that is critical.

1 year versus 3 years

First off, 3 years versus 1 year; the difference in percentage discount is often negligible, sometimes as low as 1% – 2%. Whereas, over the difference (years 2 and 3), there is the distinct possibility that a new instance type may come out, offering better power, performance, or price. That price improvement point has historically been seen as around 15%, which makes for an ideal time to “roll forward”, if you can. Reservations don’t technically STOP you from doing this, but if you’re not using the capacity you reserved then you may find your still paying for what you no longer use.

Rolling forward on services like RDS is not a problem; as the customer, you’re not managing the OS in the Virtual Machine or Container that its running in.

But in the EC2 world, you may find that your Linux or Windows OS needs an update to support the newer instance family. This was the case ones with RedHat 7.x and the change from m3 to m4; an updated Linux kernel was required. You were fortunate if you were on RedHat Enterprise Linux >=7, as this was when in-place upgrades were introduced — not that this is the recommended DevOps path (rip and replace the instance is my preference).

In-place upgrades made this something that could get you out of a lot of re-engineering if a workload was not already designed with rolling updates and instance replacement in mind. Revolutionary as this was for Redhat 7 GA in 2014, but (as a Debian Developer) Debian’s been doing that since 1996.

Reservations in Waves

The next thing to look at is slicing your reservations into waves, to give you future flexibility.

Typically a partial or full up front payment for your reservation is going to give you the biggest discount, but at the cost of hitting your cash flow now. If you had 20 Reservations required, then you’d be tempted to acquire all of them immediately.

But wait, what happens if you change your mind for some of that workload now, or in 3 months time. And sinking all that capital now may be undesirable.

I’d strongly suggest slicing this into quarterly reservations (each at one year’s duration, as above), picking up (at most) a quarter of your fleet each time. This will, in future, give you a quarterly opportunity to adjust your coverage mix.

And while I say at most a quarter of the fleet; you may still want some flexibility to scale down a little, so perhaps your target is not continual 100% coverage, but continual 80% coverage.

This discussion is then a risk conversation, of making commitments you may want to adjust. And knowing the way you may want to adjust is something that is learnt through experience.

At each quarter, there is a smaller bump for the upfront or partial up front payment, but each of those bumps is now (and in future) a decision point.

RIs applied over time.

This financial operating model may not fit your risk/reward requirements, but its worth considering your approach to long term discounts, and the flexibility you may want in future.

Amazon CloudFront and HTTP/3

Today was the day that Amzon CloudFront, the global Content Delivery Network (CDN) service, made HTTP/3 optionally available. This is something we’ve been anticipating for some time, and the result, thus far, is a seamless acceleration of delivery.

HTTP has been around for almost 30 years. It works with other protocols such as encryption (Transport Layer Security, or TLS), and network (Internet Protocol, or IP) to request Objects (documents, images, data) from a Server.

With HTTP 1.0 (and 0.9) clients would connect to a server, request a single object and then disconnect. They would then read the content they had, and realise they needed a second object (like an image), and then repeat the process. When this was unencrypted HTTP, this was typically done by connecting using the Transmission Control Protocol (TCP) over the Internet Protocol (IP), using a hostname and port number. The convention was to use TCP port 80 for your web server, and you could make a plain-text (unencrypted) connection using telnet and just type your request: “GET / HTTP/1.0\n\n”. Note the backslash-n is actually carriage return, two in a row, which indicated the end of the request.

Over time, additional options were added to the request (before the double carriage-return), such as cookies, client User Agent strings, and so on.

Then, with the dawn of Netscape Navigator came public key encryption in the browser, and the opportunity to encrypt data over the untrusted Internet with some degree of privacy and identity. However, the encryption was negotiated first, before the client had made any HTTP request, so a new endpoint had to be deployed that specifically was ready to start encryption, before eventually talking the HTTP protocol. A new TCP port was needed, and 443 was assigned.

Now we have two conversations happening: the negotiation of encryption, and then separately, the request and response of web content. Both conversations have changed over time.

Encryption Improvements

The encryption conversation started as Secure Socket Layer (SSL) version 2 (v1 never saw the light of day). This was replaced with SSLv3, and then standardised and renamed to Transport Layer Security (TLS) 1.0; TLS 1.0 was improved upon for version 1.1, and then version 1.2, and today we have TLS version 1.3 – and those older versions have mostly been deemed to be no longer reliable/safe to use. TLS 1.2 and 1.3 are all that seen today, and we’ll likely see 1.2 disappear at some stage. TLS 1.3 is slightly faster, in that there can be one (sometimes two) less round-trips to establish the encrypted connection. And less found trips = faster.

HTTP Improvements

So the first improvement here was the slight bump to HTTP 1.1, where by the client could request an Object, but ask the server to hold the link open, and after sending the requested Object, be ready for another request. This was called HTTP keep-alive, and it was good. Well, better than shutting down a perfectly good connection only open another one up.

But then came resources which would potentially block. If I request 6 items in series, and item number 2 takes some time to be processed and returned, then it may block items 3, 4, 5 and 6 (they’re all in a single line of request and response).

HTTP/2 fixed this, by permitting the client to ask for multiple Objects simultaneously, not one after the other. It also headers in the request to be compressed (as they were getting to be quite large), and it switched up the conversation form plain text, to binary.

The response from the server was binary simultaneous streams of Objects in parallel.

This fixed the speed issue of blocking caused by slow objects, and while it was faster, but it started to uncover a the next bottle neck; packet loss and retransmission of packets over the Internet. This was done by the TCP layer – it implements buffers and handles retransmissions, but in the strict OSI layer mode of networking, those retransmissions were transparent to the higher level protocols.

In order to handle this better, the QUIC protocol had to abandon the safety-net of TCP’s implementation of packet loss, and revert to the more basic UDP approach, and implement more intelligent retransmissions that could understand which concurrent stream(s) were impacted by a packet drop.

This fusing of QUIC and TLS with HTTP gives us HTTP/3. QUIC itself has its own IETF Working Group, and in future we could see other uses for QUIC.

Finding and using HTTP/3 over UDP

Now we know why the move from TCP to UDP, we look at how this works. With previous versions of HTTP, the location was easy; a scheme, hostname and a port number: https,, and 443. Because we said https, we can assume that if no port number is specified, then its probably the assigned default 443. But QUIC and HTTP/3 doesn’t have an assigned UDP port (at this time). Its any available port the administrator wishes to use.

So how does a browser know what to connect to?

Turns out this is an header that the current HTTP/2 (over TCP) service has configured called “alt-svc“. So any current browser that makes its default https over TCP to a web server asks for its first object, and in response, it gets told something like:

h3=":443"; ma=86400

Indeed, I’ve just lifted this from CloudFront today. It’s telling the client that for h3 (HTTP/3) endpoint, to connect to the same hostname on UDP port 443 (what a coincidence!). It’s also saying the Maximum Age (MA) to remember this Alternate Service endpoint is one day, or 86,400 seconds.

Using Google Chrome browser, with the Developer tools open to the Network tab, when I visit an H3 capable website for the first time I see this:

Network traffic in Google Chrome Developer tools

We saw the initial protocol was an HTTP/2, but the request for the style.css object was done over HTTP/3.

The Max Age is currently not configurable by CloudFront customers, and may indeed change over time.

Network Firewalls and Proxies

Many enterprises have network firewalls that only permit certain traffic and protocols. Some organisations deploy internal proxies that intercept their staff web traffic and inspect it to remove malware and viruses from being downloaded. In both these scenarios you may hit restrictions that inhibit HTTP 3, but luckily, browsers are smart enough to silently revert to the existing HTTP/2 protocol on trusty TCP.

As UDP is not commonly used in organisations over the Internet, the chances are this is already blocked. As UDP doesn’t have a standardised port number, there’s no easy fix: its not as easy as saying “just unblock egress UDP port 443”. Even then you may want some introspection then to ensure the traffic going in/out is QUIC/HTTP 3 and is encrypted. But this could be any UDP port. I hope that 443 becomes a pseudo standard.

Network proxies, which could also benefit from the speed improvements, will need to be updated. But this was already an issue – any intercepting proxy that doesn’t support HTTP/2 is already out of date and slowing you down.

Configuring CloudFront

You’ll find a slight tweak to the CloudFront distribution console, which now can show this:

Hitting the Edit button will now show:

You’ll also find support for this in the CloudFormation for a CloudFront Distribution.

HTTP/3 (and indeed 2) are not enabled by default at this time, but there’s few reasons not to enable them. There’s nothing else to set, just tick the box.


Thus far I haven’t seen any issues from clients trying to browse a number of CloudFront Distributions that I help administrate. I recommend testing this in your non-production environments and see what issues you see. You’ll also want to check in with to see if you can use HTTP/3 for some of your modern clients.

Some other tools may not yet be ready to do inspection of HTTP/3 endpoints, like various web page speed tests, or my other favourites,,, and

If you see websites are suddenly faster, you may find its because QUIC just became available to them. If you have use cases where milliseconds of speed improvements are critical, then this may be for you.

To be fair, Fastly (hi Arthur!) and CloudFlare has supported HTTP/3 for some time. Even Akamai has HTTP/3 available in beta to some customers.

This blog is using CloudFront, and there’s a chance that some of your requests you just did here are over HTTP/3. The origin server supports HTTP/2 over TCP, but that’s separate to the connection that your browser made to the CloudFront edge closest to you.

Congratulations to the CloudFront service team on this release. It appears this has been smooth, seamless, doesn’t cost customers any more for optimal delivery. There’s nothing not to like!

Postscript: IPv6 on CloudFront

HTTP/3 also works over IPv6, but check you have taken the TWO steps to enable it; in the CloudFront distribution, enable IPv6 (either via the API, in your CloudFormation template, or in the Web Console), AND ensure you have a DNS record of type AAAA that lets clients find the IPv6 endpoint for your distribution. If you use Route53 as your DNS service, look for the Alias record option for AAAA, with the name of the record equal to the desired hostname (and check you have a corresponding TLS Certificate with that hostname, possibly from Amazon Certificate Manager for free!).