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

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

use lanparty_proto::MacAddr;

const ETH_P_ALL: u16 = libc::ETH_P_ALL as u16;
const SYS_CLASS_NET: &str = "/sys/class/net";
const WIRELESS_INTERFACE_MARKERS: &[&str] = &["wireless", "phy80211"];
const CARRIER_FILE: &str = "carrier";

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

impl PacketSocket {
    pub fn open(interface: &str) -> io::Result<Self> {
        let interface_index = interface_index(interface)?;
        reject_wireless_interface(interface)?;
        reject_disconnected_interface(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());
        }
        enable_promiscuous_membership(fd.as_raw_fd(), interface_index)?;
        let interface_mac = interface_hardware_addr(fd.as_raw_fd(), interface)?;

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

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

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

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

    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> {
        loop {
            let mut address = unsafe {
                // SAFETY: sockaddr_ll is a plain old data kernel ABI struct; zero is a valid
                // base before recvfrom initializes the peer address.
                std::mem::zeroed::<libc::sockaddr_ll>()
            };
            let mut address_len = std::mem::size_of::<libc::sockaddr_ll>() as libc::socklen_t;
            let received = unsafe {
                // SAFETY: buffer.as_mut_ptr() is valid for buffer.len() bytes for the duration
                // of recvfrom, and recvfrom initializes at most that many bytes. address points
                // to a sockaddr_ll-sized output buffer and address_len carries that size.
                libc::recvfrom(
                    self.fd.as_raw_fd(),
                    buffer.as_mut_ptr().cast::<libc::c_void>(),
                    buffer.len(),
                    0,
                    (&mut address as *mut libc::sockaddr_ll).cast::<libc::sockaddr>(),
                    &mut address_len,
                )
            };
            if received < 0 {
                return Err(io::Error::last_os_error());
            }
            if is_inbound_packet_type(address.sll_pkttype) {
                return 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> {
    let name = interface_name(interface)?;

    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)
}

fn enable_promiscuous_membership(fd: RawFd, interface_index: u32) -> io::Result<()> {
    let membership = promiscuous_membership(interface_index);
    let result = unsafe {
        // SAFETY: membership points to a fully initialized packet_mreq and the provided length
        // matches that kernel ABI struct. The socket fd is owned by PacketSocket::open.
        libc::setsockopt(
            fd,
            libc::SOL_PACKET,
            libc::PACKET_ADD_MEMBERSHIP,
            (&membership as *const libc::packet_mreq).cast::<libc::c_void>(),
            std::mem::size_of::<libc::packet_mreq>() as libc::socklen_t,
        )
    };
    if result < 0 {
        return Err(io::Error::last_os_error());
    }

    Ok(())
}

fn promiscuous_membership(interface_index: u32) -> libc::packet_mreq {
    let mut membership = unsafe {
        // SAFETY: packet_mreq is a plain kernel ABI struct; zeroing leaves unused fields empty.
        std::mem::zeroed::<libc::packet_mreq>()
    };
    membership.mr_ifindex = interface_index as libc::c_int;
    membership.mr_type = libc::PACKET_MR_PROMISC as libc::c_ushort;

    membership
}

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

    if interface.contains('/') {
        return Err(io::Error::new(
            io::ErrorKind::InvalidInput,
            "interface name cannot contain path separators",
        ));
    }

    let name = CString::new(interface).map_err(|_| {
        io::Error::new(
            io::ErrorKind::InvalidInput,
            "interface name cannot contain NUL bytes",
        )
    })?;
    if name.as_bytes_with_nul().len() > libc::IFNAMSIZ {
        return Err(io::Error::new(
            io::ErrorKind::InvalidInput,
            "interface name is too long",
        ));
    }

    Ok(name)
}

fn reject_wireless_interface(interface: &str) -> io::Result<()> {
    if interface_is_wireless_in_sysfs(Path::new(SYS_CLASS_NET), interface) {
        return Err(io::Error::new(
            io::ErrorKind::InvalidInput,
            format!("wireless interface {interface} cannot be used as a LAN gateway"),
        ));
    }

    Ok(())
}

fn reject_disconnected_interface(interface: &str) -> io::Result<()> {
    if interface_carrier_state(Path::new(SYS_CLASS_NET), interface)? == Some(false) {
        return Err(io::Error::new(
            io::ErrorKind::InvalidInput,
            format!(
                "wired interface {interface} reports no carrier; plug it into the LAN switch before starting the gateway"
            ),
        ));
    }

    Ok(())
}

fn interface_is_wireless_in_sysfs(sys_class_net: &Path, interface: &str) -> bool {
    WIRELESS_INTERFACE_MARKERS
        .iter()
        .any(|marker| sys_class_net.join(interface).join(marker).exists())
}

fn interface_carrier_state(sys_class_net: &Path, interface: &str) -> io::Result<Option<bool>> {
    let carrier_path = sys_class_net.join(interface).join(CARRIER_FILE);
    match fs::read_to_string(&carrier_path) {
        Ok(value) => match value.trim() {
            "0" => Ok(Some(false)),
            "1" => Ok(Some(true)),
            _ => Ok(None),
        },
        Err(error) if error.kind() == io::ErrorKind::NotFound => Ok(None),
        Err(error) => Err(error),
    }
}

fn interface_hardware_addr(fd: RawFd, interface: &str) -> io::Result<MacAddr> {
    let name = interface_name(interface)?;
    let mut request = unsafe {
        // SAFETY: ifreq is a kernel ABI struct; zeroing it gives a valid base before filling the
        // interface name and asking ioctl to populate the hardware-address union field.
        std::mem::zeroed::<libc::ifreq>()
    };
    for (destination, source) in request
        .ifr_name
        .iter_mut()
        .zip(name.as_bytes_with_nul().iter())
    {
        *destination = *source as libc::c_char;
    }

    let result = unsafe {
        // SAFETY: request points to an initialized ifreq with a NUL-terminated interface name.
        // ioctl writes the hardware address into the ifreq union and does not retain pointers.
        libc::ioctl(fd, libc::SIOCGIFHWADDR, &mut request)
    };
    if result < 0 {
        return Err(io::Error::last_os_error());
    }

    let hardware_addr = unsafe {
        // SAFETY: SIOCGIFHWADDR succeeded, so reading the hardware-address union field is valid.
        request.ifr_ifru.ifru_hwaddr
    };
    if hardware_addr.sa_family != libc::ARPHRD_ETHER {
        return Err(io::Error::new(
            io::ErrorKind::InvalidData,
            "interface is not an Ethernet device",
        ));
    }

    let data = hardware_addr.sa_data;
    let mac = MacAddr::new([
        data[0] as u8,
        data[1] as u8,
        data[2] as u8,
        data[3] as u8,
        data[4] as u8,
        data[5] as u8,
    ]);
    if !mac.is_valid_unicast() {
        return Err(io::Error::new(
            io::ErrorKind::InvalidData,
            format!("interface has invalid Ethernet MAC address {mac}"),
        ));
    }

    Ok(mac)
}

fn is_inbound_packet_type(packet_type: u8) -> bool {
    packet_type != libc::PACKET_OUTGOING
}

#[cfg(test)]
mod tests {
    use std::{
        fs,
        time::{SystemTime, UNIX_EPOCH},
    };

    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
        );
        assert_eq!(
            interface_index("eth0/bad").unwrap_err().kind(),
            io::ErrorKind::InvalidInput
        );
    }

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

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

    #[test]
    fn builds_promiscuous_packet_membership_request() {
        let membership = promiscuous_membership(12);

        assert_eq!(membership.mr_ifindex, 12);
        assert_eq!(membership.mr_type, libc::PACKET_MR_PROMISC as u16);
        assert_eq!(membership.mr_alen, 0);
        assert_eq!(membership.mr_address, [0; 8]);
    }

    #[test]
    fn classifies_inbound_packet_types() {
        assert!(!is_inbound_packet_type(libc::PACKET_OUTGOING));
        assert!(is_inbound_packet_type(libc::PACKET_HOST));
        assert!(is_inbound_packet_type(libc::PACKET_BROADCAST));
        assert!(is_inbound_packet_type(libc::PACKET_MULTICAST));
    }

    #[test]
    fn detects_wireless_interfaces_from_sysfs_markers() {
        let sys_class_net = unique_temp_dir("lanparty-gateway-sysfs");
        let wired = sys_class_net.join("eth0");
        let wireless = sys_class_net.join("wlan0");
        let phy_wireless = sys_class_net.join("wlp1s0");
        fs::create_dir_all(&wired).unwrap();
        fs::create_dir_all(wireless.join("wireless")).unwrap();
        fs::create_dir_all(phy_wireless.join("phy80211")).unwrap();

        assert!(!interface_is_wireless_in_sysfs(&sys_class_net, "eth0"));
        assert!(interface_is_wireless_in_sysfs(&sys_class_net, "wlan0"));
        assert!(interface_is_wireless_in_sysfs(&sys_class_net, "wlp1s0"));

        fs::remove_dir_all(sys_class_net).unwrap();
    }

    #[test]
    fn detects_disconnected_interfaces_from_sysfs_carrier() {
        let sys_class_net = unique_temp_dir("lanparty-gateway-carrier");
        let connected = sys_class_net.join("eth0");
        let disconnected = sys_class_net.join("eth1");
        let unknown = sys_class_net.join("eth2");
        fs::create_dir_all(&connected).unwrap();
        fs::create_dir_all(&disconnected).unwrap();
        fs::create_dir_all(&unknown).unwrap();
        fs::write(connected.join(CARRIER_FILE), "1\n").unwrap();
        fs::write(disconnected.join(CARRIER_FILE), "0\n").unwrap();
        fs::write(unknown.join(CARRIER_FILE), "unknown\n").unwrap();

        assert_eq!(
            interface_carrier_state(&sys_class_net, "eth0").unwrap(),
            Some(true)
        );
        assert_eq!(
            interface_carrier_state(&sys_class_net, "eth1").unwrap(),
            Some(false)
        );
        assert_eq!(
            interface_carrier_state(&sys_class_net, "eth2").unwrap(),
            None
        );
        assert_eq!(
            interface_carrier_state(&sys_class_net, "eth-missing").unwrap(),
            None
        );

        fs::remove_dir_all(sys_class_net).unwrap();
    }

    fn unique_temp_dir(prefix: &str) -> std::path::PathBuf {
        let nanos = SystemTime::now()
            .duration_since(UNIX_EPOCH)
            .unwrap()
            .as_nanos();

        std::env::temp_dir().join(format!("{prefix}-{}-{nanos}", std::process::id()))
    }
}