How to serve several certificates by hostname (SNI)

Problem

One listener, several hostnames, a different certificate for each. When a client opens a TLS (or QUIC) connection it sends the hostname it is asking for in the ClientHello, as the Server Name Indication (SNI, RFC 6066). You want to look at that name and hand back the matching certificate, instead of pinning the listener to a single cert/key pair.

Solution

Every TLS-bearing adapter lets you install a callback that runs once per connection, receives the SNI, and returns the certificate to present. The hook differs by protocol, because HTTP/1.1 and HTTP/2 run on Erlang's ssl, while HTTP/3 runs on QUIC's own TLS 1.3 stack:

On https the callback is Erlang's standard sni_fun: it returns a list of ssl options (typically certfile/keyfile, or cert/key) that override the defaults for that handshake. On http3 the callback returns {ok, #{cert := Der, key := Key}} (with an optional cert_chain => [Der]), or {error, Reason} to refuse the handshake.

HTTP/1.1 and HTTP/2

ssl_opts is a passthrough to ssl:listen/2: whatever you put there is merged on top of the listener's own TLS defaults, so your options win. Install sni_fun to pick the certificate per hostname.

SniFun = fun(ServerName) ->
    case cert_store:lookup(ServerName) of
        {ok, CertFile, KeyFile} ->
            [{certfile, CertFile}, {keyfile, KeyFile}];
        not_found ->
            %% Falls back to the listener's own cert/key below.
            []
    end
end,

{ok, Pid} = livery:start_service(#{
    https => #{
        port => 8443,
        cert => DefaultCertFile,
        key  => DefaultKeyFile,
        ssl_opts => [{sni_fun, SniFun}]
    },
    router => Router
}).

ServerName is a charlist (for example "api.example.com"), the form Erlang's ssl hands to sni_fun. Keep the listener's cert/key: they are the fallback for clients that send no SNI, or a name your callback does not recognise.

HTTP/3 (QUIC)

QUIC negotiates TLS inside its own transport, so it does not use ssl_opts. Pass sni_callback instead. It is called once per connection with the SNI and returns the certificate map to present.

SniCallback = fun(ServerName) ->
    %% ServerName is a binary here, e.g. <<"api.example.com">>,
    %% or `undefined' when the client sent no SNI.
    case cert_store:lookup_der(ServerName) of
        {ok, CertDer, KeyDer} -> {ok, #{cert => CertDer, key => KeyDer}};
        not_found             -> {error, unknown_host}
    end
end,

{ok, Pid} = livery:start_service(#{
    http3 => #{
        port => 8443,
        cert => DefaultCertDer,
        key  => DefaultKeyDer,
        sni_callback => SniCallback
    },
    router => Router
}).

The callback returns DER-encoded material: cert is the leaf certificate, key the private key term, and the optional cert_chain => [Der] carries intermediates. An {error, _}, a malformed result, or a raised exception fails the handshake with a handshake_failure alert, so a missing host closes the connection rather than serving the wrong certificate.

The same certificates on every protocol

To serve one hostname over H1, H2, and H3, give each TLS adapter its own hook. The two callbacks differ only in their return shape (an ssl option list for https, a {ok, CertMap} for http3), so a small wrapper around one lookup keeps them in step:

Lookup = fun(Name) -> cert_store:lookup(Name) end,

SniFun = fun(ServerName) ->
    case Lookup(list_to_binary(ServerName)) of
        {ok, CertFile, KeyFile, _Der} ->
            [{certfile, CertFile}, {keyfile, KeyFile}];
        not_found ->
            []
    end
end,

SniCallback = fun(ServerName) ->
    case Lookup(ServerName) of
        {ok, _File, _File2, {CertDer, KeyDer}} ->
            {ok, #{cert => CertDer, key => KeyDer}};
        not_found ->
            {error, unknown_host}
    end
end,

{ok, Pid} = livery:start_service(#{
    https => #{
        port => 8443,
        cert => DefaultCertFile, key => DefaultKeyFile,
        ssl_opts => [{sni_fun, SniFun}]
    },
    http3 => #{
        port => 8443,
        cert => DefaultCertDer, key => DefaultKeyDer,
        sni_callback => SniCallback
    },
    alt_svc => advertise,
    router  => Router
}).

https and http3 share the port number because one is TCP and the other UDP. alt_svc => advertise puts an Alt-Svc header on the H1 and H2 responses so a capable client knows it can move up to H3.

Notes

• The ServerName type differs by stack: a charlist on https (what ssl passes sni_fun), a binary on http3. Both receive undefined when the client sends no SNI.
• Keep a static cert/key on the listener. On https it is the fallback whenever sni_fun returns []; on http3 it is the certificate used when no sni_callback is set. HTTP/3 needs quic >= 1.6.5 for sni_callback.
• The callback runs on the listener's connection path, in the handshake. Keep it fast and side-effect-light: look up a cached certificate, do not block on the network. Load and parse certificates ahead of time and keep them in a table your callback reads.
• SNI selects the certificate, not the route. Once the handshake completes the request flows through the same router and middleware as any other; match on the Host/:authority header if you want per-hostname behaviour.

See also

• How-to: Bind to an address or IPv6
• Concept: Adapters
• Reference: livery_service, livery_h2, livery_h3