Thursday, 6 August 2020

Web standards never sleep

As we wait for ratification/adoption of  HTTP3 news arrives of a new image format promising a 50% reduction in size compared with JPEG; AV1.

There's nothing more I can add to Daniel Aleksandersen's analysis published on but to say that support is coming soon in Firefox and already available in Chrome.

When it will be available in Edge and Safari still seems a bi of a mystery - they took a lot longer to adopt webp, but since Microsoft's browser now uses a lot of Chrome under the hood, it might appear a lot quicker there. has a test page with links to additional resources.

AV1 also does video.

Saturday, 13 June 2020

HTTP2 and 421 errors

I am in the process of migrating an online B2B enterprise built on design patterns from a 1990's dial-up ISP to a more modern infrastructure. It has taken over 20 years of effort to build a network of services which are difficult to manage to and insecure. Replacing this, with no downtime, is something of a challenge. A really important stepping stone to the target architecture is getting every exposed service routed via a proxy. This makes it much, MUCH simpler for us to:

  • (re)route services within our network
  • Upgrade services to HTTP2
  • Provision certificates / manage encryption
  • Configure browser-side caching
  • Analyse traffic
  • ...and more 

Running a handful of BigIp F5s is unfortunately not an option, so my switchboard is a stack of Ubuntu + nginx.

Up till recently the onboarding exercise had gone really well - but then I started encountering 421 errors. My initial reading kept leading me to old bugs in Chrome and other issues with HTTP2. These are neatly summarized by Kevin as follows:

This is caused by the following sequence of events:
  1. The server and client both support and use HTTP/2.
  2. The client requests a page at
  3. During TLS negotiation, the server presents a certificate which is valid for both and (and the client accepts it). This could be done with a wildcard certificate or a SAN certificate.
  4. The client reuses the connection to make a request for
  5. The server is unable or unwilling to support cross-domain connection reuse (for example because you configured their SSL differently and Apache wants to force a TLS renegotiation), and serves HTTP 421.
  6. The client does not automatically retry with a new connection (see for example Chrome bug #546991, now fixed). The relevant RfC says that the client MAY retry, not that it SHOULD or MUST. Failing to retry is not particularly user-friendly, but might be desirable for a debugging tool or HTTP library.
However, I was able to reproduce the bug in the first request from a browser instance I had just started. Also I was able to see the issue across sites using distinct certificates. I was confused.

Eventually I looked at the logs for the origin server (Apache 2.4.26). I hadn't considered that before as I knew it did not support HTTP2. But low and behold, there in the logs, a 421 error against my request.

[Fri Jun 12 18:44:47.945706 2020] [ssl:error] [pid 21423:tid 140096556701440] AH02032: Hostname provided via SNI and hostname provided via HTTP have no compatible SSL setup

So disabling SSL session re-use on the connection from the nginx proxy to the origin resolved the issue:

proxy_ssl_session_reuse off;

This does mean slightly more overhead between the proxy and the origin, but since both are on the same LAN, its not really noticeable.

Although its something of a gray area, I don't think this is a bug with nginx - I had multiple sites in nginx pointing at the same backend URL. It would be interesting to check if the issue occurs when I have multiple unique DNS names for the origin - one for each nginx front end - if it still occurs there, then that is probably a bug.