ALPN (Application-Layer Protocol Negotiation)

Application-Layer Protocol Negotiation (ALPN) is a TLS extension that lets the client and server choose which protocol will run inside the encrypted connection while the handshake is still in progress. The client offers a list of identifiers and the server replies with one.

Why it matters

ALPN is how browsers learn that a server supports HTTP/2 without an extra round trip. The client offers h2 and http/1.1; if the server selects h2, the first request is already sent over HTTP/2. HTTP/3 over QUIC uses the identifier h3 in the same way.

If ALPN is not configured, or a proxy or TLS terminator in front does not support it, clients quietly fall back to HTTP/1.1. The site still works, but multiplexing and header compression are lost, so the problem often goes unnoticed.

Example

openssl s_client -connect example.com:443 -alpn h2,http/1.1 </dev/null 2>/dev/null | grep "ALPN"

The output ALPN protocol: h2 confirms negotiation. In nginx, listening with listen 443 ssl http2; (or http2 on; in newer versions) enables it.

Common mistakes

Enabling HTTP/2 but terminating TLS on an old proxy that lacks ALPN support.
Checking with a tool that never offers h2.
Expecting ALPN to work for plain-text HTTP.

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