Allow binding an outstation TCP/TLS server to a named network interface

[thomas-rajan]

Server::new_tcp_server / Server::new_tls_server currently accept only a SocketAddr. It will be good to have an option to bind the listening socket to a specific network interface — you can pick an IP, but that breaks down when the interface's address is assigned dynamically (DHCP, PPP, cellular links) or when several interfaces share an address range.

tokio has a bind_device(https://docs.rs/tokio/latest/tokio/net/struct.TcpSocket.html#method.bind_device) API for this. Will it be possible to add this option

[jadamcrain]

Thanks for the request. You can already do this from Rust with the current 1.7.0 release candidates. Server::into_task accepts an externally bound tokio::net::TcpListener, so you can create the socket yourself, call bind_device, and hand the listener to the library:

use tokio::net::TcpSocket;

let socket = TcpSocket::new_v4()?;
socket.set_reuseaddr(true)?;
socket.bind_device(Some(b"eth0"))?;
socket.bind("0.0.0.0:20000".parse()?)?;
let listener = socket.listen(1024)?;

let mut server = Server::new_tcp_server(LinkErrorMode::Close, "0.0.0.0:20000".parse()?);
// add outstations with server.add_outstation(...) as usual
let (handle, task) = server.into_task(listener);
tokio::spawn(task.run());

The same works for new_tls_server. A couple of caveats:

• into_task is behind the unstable cargo feature. It will stay behind that gate until 2.0, where we plan to stabilize it. The signature could still change before then.
• bind_device is only available on Linux, Android, and Fuchsia in tokio, and on Linux kernels before 5.7 SO_BINDTODEVICE requires CAP_NET_RAW.

Since this is Rust only, I'll leave the issue open to track adding a first-class option on the server builder, which would also make it available through the C/C++/Java/.NET bindings.