softlan-vpn/crates/lanparty-gateway/src/packet.rs

use std::{
    ffi::CString,
    io,
    os::fd::{AsRawFd, FromRawFd, OwnedFd, RawFd},
};

const ETH_P_ALL: u16 = libc::ETH_P_ALL as u16;

#[derive(Debug)]
pub struct PacketSocket {
    fd: OwnedFd,
    interface: String,
    interface_index: u32,
}

impl PacketSocket {
    pub fn open(interface: &str) -> io::Result<Self> {
        let interface_index = interface_index(interface)?;
        let protocol = i32::from(ETH_P_ALL.to_be());
        let raw_fd = unsafe {
            // SAFETY: socket is called with constant domain/type/protocol values and returns
            // a new file descriptor or -1 without aliasing Rust-owned memory.
            libc::socket(
                libc::AF_PACKET,
                libc::SOCK_RAW | libc::SOCK_CLOEXEC | libc::SOCK_NONBLOCK,
                protocol,
            )
        };
        if raw_fd < 0 {
            return Err(io::Error::last_os_error());
        }
        let fd = unsafe {
            // SAFETY: raw_fd was just returned by socket and is owned by this function.
            OwnedFd::from_raw_fd(raw_fd)
        };

        let mut address = unsafe {
            // SAFETY: sockaddr_ll is a plain old data kernel ABI struct; zero is a valid base
            // before filling the fields required by bind.
            std::mem::zeroed::<libc::sockaddr_ll>()
        };
        address.sll_family = libc::AF_PACKET as u16;
        address.sll_protocol = ETH_P_ALL.to_be();
        address.sll_ifindex = interface_index as i32;

        let result = unsafe {
            // SAFETY: address points to a properly initialized sockaddr_ll and the length
            // matches that struct. fd remains owned by this function across the call.
            libc::bind(
                fd.as_raw_fd(),
                (&address as *const libc::sockaddr_ll).cast::<libc::sockaddr>(),
                std::mem::size_of::<libc::sockaddr_ll>() as libc::socklen_t,
            )
        };
        if result < 0 {
            return Err(io::Error::last_os_error());
        }

        Ok(Self {
            fd,
            interface: interface.to_owned(),
            interface_index,
        })
    }

    #[must_use]
    pub fn interface(&self) -> &str {
        &self.interface
    }

    #[must_use]
    pub const fn interface_index(&self) -> u32 {
        self.interface_index
    }

    pub fn send_frame(&self, frame: &[u8]) -> io::Result<usize> {
        let sent = unsafe {
            // SAFETY: frame.as_ptr() is valid for frame.len() bytes for the duration of send,
            // and send does not retain the pointer after returning.
            libc::send(
                self.fd.as_raw_fd(),
                frame.as_ptr().cast::<libc::c_void>(),
                frame.len(),
                0,
            )
        };
        if sent < 0 {
            return Err(io::Error::last_os_error());
        }

        Ok(sent as usize)
    }

    pub fn recv_frame(&self, buffer: &mut [u8]) -> io::Result<usize> {
        let received = unsafe {
            // SAFETY: buffer.as_mut_ptr() is valid for buffer.len() bytes for the duration of
            // recv, and recv initializes at most that many bytes.
            libc::recv(
                self.fd.as_raw_fd(),
                buffer.as_mut_ptr().cast::<libc::c_void>(),
                buffer.len(),
                0,
            )
        };
        if received < 0 {
            return Err(io::Error::last_os_error());
        }

        Ok(received as usize)
    }
}

impl AsRawFd for PacketSocket {
    fn as_raw_fd(&self) -> RawFd {
        self.fd.as_raw_fd()
    }
}

pub fn interface_index(interface: &str) -> io::Result<u32> {
    if interface.trim().is_empty() {
        return Err(io::Error::new(
            io::ErrorKind::InvalidInput,
            "interface name cannot be empty",
        ));
    }

    let name = CString::new(interface).map_err(|_| {
        io::Error::new(
            io::ErrorKind::InvalidInput,
            "interface name cannot contain NUL bytes",
        )
    })?;
    let index = unsafe {
        // SAFETY: name is a valid NUL-terminated C string and if_nametoindex does not retain it.
        libc::if_nametoindex(name.as_ptr())
    };

    if index == 0 {
        return Err(io::Error::last_os_error());
    }

    Ok(index)
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn rejects_invalid_interface_names() {
        assert_eq!(
            interface_index("").unwrap_err().kind(),
            io::ErrorKind::InvalidInput
        );
        assert_eq!(
            interface_index("eth0\0bad").unwrap_err().kind(),
            io::ErrorKind::InvalidInput
        );
    }

    #[test]
    fn reports_missing_interface() {
        let error = interface_index("lanparty-definitely-missing0").unwrap_err();

        assert_ne!(error.kind(), io::ErrorKind::InvalidInput);
    }
}