get original dst addr

main.rs

use std::io::{Error, Result};
use std::mem::{MaybeUninit, size_of};
use std::net::{Ipv4Addr, Ipv6Addr, SocketAddr, SocketAddrV4, SocketAddrV6};
use std::os::unix::io::AsRawFd;

use async_std::net::TcpListener;
use futures::StreamExt;

#[async_std::main]
async fn main() {
    let listener = TcpListener::bind("[::]:9876").await.unwrap();

    let mut incoming = listener.incoming();

    while let Some(stream) = incoming.next().await {
        let stream = stream.unwrap();

        let addr = get_original_addr(&stream).unwrap();

        println!("{}", addr);
    }
}

fn get_original_addr<T: AsRawFd>(fd: &T) -> Result<SocketAddr> {
    let mut ipv6_addr = MaybeUninit::<libc::sockaddr_in6>::zeroed();
    let mut size = size_of::<libc::sockaddr_in6>() as u32;

    unsafe {
        if libc::getsockopt(fd.as_raw_fd(), libc::SOL_IPV6, libc::IP6T_SO_ORIGINAL_DST, ipv6_addr.as_mut_ptr() as _, &mut size as _) == 0 {
            let ipv6_addr = ipv6_addr.assume_init();

            return Ok(SocketAddr::V6(SocketAddrV6::new(Ipv6Addr::from(ipv6_addr.sin6_addr.s6_addr), ipv6_addr.sin6_port.to_be(), ipv6_addr.sin6_flowinfo, ipv6_addr.sin6_scope_id)));
        }
    }

    let mut ipv4_addr = MaybeUninit::<libc::sockaddr_in>::zeroed();
    let mut size = size_of::<libc::sockaddr_in>() as u32;

    unsafe {
        if libc::getsockopt(fd.as_raw_fd(), libc::SOL_IP, libc::SO_ORIGINAL_DST, ipv4_addr.as_mut_ptr() as _, &mut size as _) < 0 {
            Err(Error::last_os_error())
        } else {
            let ipv4_addr = ipv4_addr.assume_init();

            Ok(SocketAddr::V4(SocketAddrV4::new(Ipv4Addr::from(ipv4_addr.sin_addr.s_addr.to_be()), ipv4_addr.sin_port.to_be())))
        }
    }
}