Websocket transport (with tokio) doesn't work on Android

[jsamol]

Summary

Since the websocket transport relies on the DNS transport, it's affected by the same issue as its dependency (#2064) and fails to work on Android. For DNS, the issue can be mitigated by providing custom libp2p_dns::ResolverConfig and libp2p_dns::ResolverOpts instead of trying to read the system configuration, but this is not currently possible for websockets, since the websocket builder (for tokio) creates a DNS transport instance internally and doesn't allow any customization:

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

Lines 126 to 135 in 4192fc3

	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>>
	);

Expected behavior

SwarmBuilder::with_websocket successfully configures the websocket transport on Android.

Actual behavior

SwarmBuilder::with_websocket returns an error due to an internal DNS issue where /etc/resolv.conf can't be found.

Relevant log output

No response

Possible Solution

I'm not very familiar with the library’s internals, but the most straightforward solution appears to be to allow custom configuration for the internal DNS transport so the builder doesn’t attempt to read the system config. This could be achieved by introducing a libp2p_websocket::Config structure, similar to those used by other transports (e.g., TCP), with optional DNS configuration for creating the DNS transport instance. However, the DNS configuration should probably be only included when applicable, such as when using the tokio runtime, to avoid cluttering the interface.

Version

0.54.1

Would you like to work on fixing this bug ?

Maybe

[dariusc93]

Thank you for the report. A workaround would probably be to use Swarm::new and supply a transport that way with your option. Long term, we should probably pass a configuration option from the builder or maybe just use TokioDnsConfig::custom by default.