SwarmBuilder::with_websocket hardcodes dns::tokio::Transport::system, bypassing with_dns_config (breaks on Android)

[yexiyue]

Summary

The tokio flavour of SwarmBuilder::with_websocket unconditionally wraps its inner TCP transport in a system-config DNS transport:

rust-libp2p/libp2p/src/builder/phase/websocket.rs

Lines 118 to 127 in 93c5059

	impl_websocket_builder!(
	"tokio",
	super::provider::Tokio,
	// Note this is an unnecessary await for Tokio Websocket (i.e. tokio dns) in order to be
	// consistent with above AsyncStd construction.
	futures::future::ready(libp2p_dns::tokio::Transport::system(
	libp2p_tcp::tokio::Transport::new(libp2p_tcp::Config::default())
	)),
	rw_stream_sink::RwStreamSink<libp2p_websocket::BytesConnection<libp2p_tcp::tokio::TcpStream>>
	);

This ignores any resolver configuration supplied earlier via with_dns_config. On targets where the system resolver is unavailable, the websocket phase fails even though the user explicitly configured a custom resolver one phase earlier.

Concrete case (Android): hickory-resolver 0.26's system_conf reads the system DNS via JNI (ndk-context). In host environments that never initialize the ndk context — e.g. React Native / uniffi mobile apps, where no android-activity-style glue runs — read_system_conf() fails with android context was not initialized. Switching the DNS phase to with_dns_config(custom_cfg, opts) fixes that call site, but with_websocket then constructs a second, system-config resolver and fails with the byte-for-byte identical error. That makes it genuinely confusing to debug: it looks exactly like the bug you just fixed.

Expected behavior

One of:

• with_websocket reuses the resolver configuration established in the DNS phase (or skips DNS-wrapping entirely when the outer transport is already DNS-wrapped);
• a variant like with_websocket_dns_config(cfg, opts, ...) exists so the resolver can be supplied;
• at minimum, the with_websocket docs mention the hidden Transport::system call and its platform requirements.

Actual behavior

with_websocket always calls libp2p_dns::tokio::Transport::system and returns WebsocketError(Dns(...)) on Android RN hosts, regardless of with_dns_config.

Workaround

On Android we skip the websocket transport via the WebsocketPhase::with_relay_client shortcut (which calls without_websocket() internally). Acceptable when websocket has no consumers on that platform, but it forks the builder chain per target and silently removes /ws//wss dial capability.

Version

• master @ 93c5059 (libp2p/src/builder/phase/websocket.rs:123)
• also present in libp2p 0.56.0 (same line in the impl_websocket_builder! expansion)
• hickory-resolver 0.26.1

Would you like to work on fixing this bug?

Happy to send a PR once maintainers agree on the preferred direction.