I wonder if it's possible to choose server-supported ALPN protocols on per-connection basis

[segfault-bilibili]

I'm new to NodeJS. Apologies for my lack of knowledge.

Now we already have SNICallback which is able to specify certificates based on servername. I'm curious whether it's possible to have something similar, like ALPNCallback which (asynchronously) choose ALPN?

I think this might be useful if the server acts like a reverse proxy. If the client supports http/2 while the upstream server doesn't, it seems to help.

[bnoordhuis]

That's already supported, if I understand you correctly. See the tls documentation for the ALPNProtocols option and the tlsSocket.alpnProtocol property.

[segfault-bilibili]

I was talking about the server side. I meant the server should be able to behave differently on per-connection basis. However (maybe I just overlooked or misunderstood something) I didn't find any way to do this.

ALPNProtocols seems to be a global and immutable option for the server. As for tlsSocket.alpnProtocol, it seems to be read-only.

[bnoordhuis]

It sounds like you want to look at the ClientHello during the handshake and have the server customize its advertised ALPN response based on that?

Node could be made to support that but... why? What exactly would you customize?

[segfault-bilibili]

(If I didn't misunderstand the situation) I've seen CDN servers doing that. It replies different ALPN choices according to SNI domain name, like, if the upstream server doesn't suppport HTTP/2, it won't advertise h2 through ALPN.

[bnoordhuis]

Okay, I can see how that would be useful. Node could support that if you know such facts upfront about your upstream.

What wouldn't work is to connect asynchronously to upstream to see if it speaks h2, then report that to the client. openssl doesn't support that, it needs to be synchronous.

[pimterry]

Just a quick update on this, after some more related discussion (#45056) there's now a PR to implement this ALPNCallback idea here: #45190 (but note that it's synchronous only, due to OpenSSL restrictions)