recvmsg / sendmsg with Rust

main.rs

// in Cargo.toml
// [dependencies]
// libc = "0.2.83"
// nix = "0.19.0"

use std::os::unix::io::AsRawFd;
use std::net::UdpSocket;
use std::net::SocketAddr;

use nix::cmsg_space;
use nix::sys::uio::IoVec;
use nix::sys::socket::recvmsg;
use nix::sys::socket::sendmsg;
use nix::sys::socket::setsockopt;
use nix::sys::socket::ControlMessage;
use nix::sys::socket::ControlMessageOwned;
use nix::sys::socket::MsgFlags;
use nix::sys::socket::sockopt::Ipv4PacketInfo;

use libc::in_addr;
use libc::in_pktinfo;

fn main() {
    let listen_addr = "0.0.0.0:8080".parse::<SocketAddr>().unwrap();
    let sock = UdpSocket::bind(listen_addr).unwrap();
    let fd = sock.as_raw_fd();
    let mut cmsg = cmsg_space!(in_pktinfo);

    // 1. setsockopt IP_PKTINFO
    setsockopt(fd, Ipv4PacketInfo, &true).unwrap();

    loop {
        // 2. recvmsg from socket
        let buf: &mut [u8] = &mut [0; 2048];
        let iov = &[IoVec::from_mut_slice(buf)];

        let res = recvmsg(fd, iov, Some(&mut cmsg), MsgFlags::empty()).unwrap();
        match res.cmsgs().next().unwrap() {
            ControlMessageOwned::Ipv4PacketInfo(info) => {
                let iov = &[IoVec::from_slice(buf)];

                let info = in_pktinfo {
                    ipi_ifindex: info.ipi_ifindex,
                    ipi_spec_dst: in_addr { s_addr: 0 },
                    ipi_addr: in_addr { s_addr: 0 },
                };

                sendmsg(fd, iov, &[ControlMessage::Ipv4PacketInfo(&info)], MsgFlags::empty(), res.address.as_ref()).unwrap();
            },
            _ => break,
        };
    }
}