ALPN in Erlang

1 Mar 2021 11:16 erlang

Per Wikipedia:

Application-Layer Protocol Negotiation (ALPN) is a Transport Layer Security (TLS) extension that allows the application layer to negotiate which protocol should be performed over a secure connection in a manner that avoids additional round trips and which is independent of the application-layer protocols.

In Erlang, the client advertises which protocols it understands by using the alpn_advertised_protocols option:

{ok, Socket} = ssl:connect("localhost", 5555,
        [<<"my-protocol/2">>, <<"my-protocol/1">>]},
     % ...

{ok, Protocol} = ssl:negotiated_protocol(Socket).

The server declares which protocols it prefers by using the alpn_preferred_protocols option, which are specified from most-preferred to least-preferred.

Protocol names must be specified as binaries. The list of protocol names is managed by IANA.

Of note

If the server and client can’t agree on a protocol, the server closes the connection with a TLS alert.

If the client or server uses ALPN, and the other doesn’t, then the connection succeeds, but ssl:negotiated_protocol/1 returns {error, protocol_not_negotiated}.