EpicGames/lore · error · anyhow::Error

Received connection for unsupported protocol

Error message

Received connection for unsupported protocol: {protocol:?}

What it means

A guard in handle_conn: the client negotiated a valid ALPN protocol, but that protocol string is not registered in the stream handler factory, so no service/stream-handler builder exists for it. It fires when a client speaks a protocol this server build does not support (e.g. an outdated or mismatched client), and the connection is dropped with a warning.

Solutions

1. Register a handler for the protocol in the stream handler factory, or update the client to use a protocol the server supports
2. Align protocol version strings between client and server deployments
3. Check server feature flags/config that gate service registration

Example fix

// before
factory.register("lore/1", handler);
// after
factory.register("lore/1", handler).register("lore/2", handler_v2);

// client side: verify protocol is supported before connecting
let supported = advertised_protocols(); // fetched or hardcoded per deployment
assert!(supported.contains("lore/1"), "protocol not supported by server");

if let Err(e) = handle_conn(&connection, &factory).await {
    if e.to_string().contains("unsupported protocol") {
        warn!("client used unsupported protocol; closing gracefully");
        return Ok(());
    }
    return Err(e);
}

When it happens

Trigger: Client negotiates a protocol (via ALPN) that the server's stream_handler_factory has no handler registered for — get_stream_handler_builder(&protocol) returns None.

Common situations: Client updated to a newer protocol version the server hasn't deployed; server started with feature flags that omit a service; typo/divergence in protocol strings between client and server.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

• No alpns provided
• Failed to decode protocol
• No protocol found on request
• handler should succeed even for a miss
• Stream handler factory was not set

AI-assisted analysis of EpicGames/lore@074eb0b0d1 (2026-09-13). Data as JSON: /api/errors/c5293860bc2776a3. Report an issue: GitHub.

Appendix: source

Thrown at lore-server/src/quic/quinn/quinn_server.rs:295

    let protocol = get_protocol(&connection)?;

    let connection_id = connection.stable_id();

    let conn_span = tracing::Span::current();
    conn_span.record("connection_id", connection_id);
    conn_span.record("protocol", &protocol);

    info!(
        remote_address = %connection.remote_address(),
        "Established connection",
    );

    let Some((service_name, service_builder)) =
        stream_handler_factory.get_stream_handler_builder(&protocol)
    else {
        warn!("Protocol {protocol} is not supported by the stream handler factory");
        return Err(anyhow!(
            "Received connection for unsupported protocol: {protocol:?}"
        ));
    };

    let _stats_guard =
        track_connection_stats(service_name, &connection, connection_metrics_interval);

    let context = Arc::new(AttributeMap::default());
    context.insert(conn_span.clone());
    context.insert(ConnectionId(connection_id));

    // Create the stream handler once per connection so per-connection state
    // (e.g. SessionMap in StorageServiceV4) is shared across all streams.
    let connection_handler: Arc<Box<dyn StreamDataHandler>> =
        Arc::new(service_builder(context.clone()));

    // Keeps accepting requests until the connection closes or errors
    loop {

View on GitHub (pinned to 074eb0b0d1)