lsnet 0.7.0

//! Host discovery over ARP.
//!
//! With raw-socket access (root; CAP_NET_RAW on Linux; BPF access on macOS) we broadcast ARP requests
//! ourselves and listen for replies: fast, finds devices that ignore every
//! port, and gives us MAC addresses. Either way we also read the kernel's
//! ARP cache, which is all we have without it. Windows sends ARP requests
//! for anyone who asks, so there the sweep never needs privileges.

use crate::iface::Iface;
use crate::platform;
use pnet_base::MacAddr;
use std::collections::HashMap;
use std::io;
use std::net::Ipv4Addr;
use std::thread;
use std::time::Duration;

pub type Found = HashMap<Ipv4Addr, MacAddr>;

/// Active ARP sweep. Fails with PermissionDenied when we can't open BPF.
#[cfg(unix)]
pub fn sweep(ifc: &Iface, targets: &[Ipv4Addr], wait: Duration) -> io::Result<Found> {
    use pnet_datalink::{Channel, Config};
    use pnet_packet::Packet;
    use pnet_packet::arp::ArpPacket;
    use pnet_packet::ethernet::{EtherTypes, EthernetPacket};
    use std::sync::atomic::{AtomicBool, Ordering};
    use std::sync::{Arc, Mutex};

    // Replies go to the source MAC we put in the frame, so we must know our real one.
    let Some(own_mac) = ifc.mac else {
        return Err(io::Error::new(
            io::ErrorKind::PermissionDenied,
            "own MAC address is hidden",
        ));
    };
    let iface = pnet_datalink::interfaces()
        .into_iter()
        .find(|i| i.name == ifc.iface.name)
        .ok_or_else(|| io::Error::other(format!("{} disappeared", ifc.iface.name)))?;
    // pnet's default 4 KB capture buffer holds about 50 frames. We see our
    // own broadcasts as well as every other frame on the link, so on a busy
    // network a sweep of a /22 overflows it and replies are dropped. 256 KB
    // is within macOS's BPF limit (512 KB).
    let cfg = Config {
        read_timeout: Some(Duration::from_millis(50)),
        read_buffer_size: 256 * 1024,
        ..Default::default()
    };
    // pnet walks /dev/bpf0..N and reports whatever the last failure was
    // (usually ENOENT), so any open failure here means "no raw access".
    let channel = pnet_datalink::channel(&iface, cfg)
        .map_err(|e| io::Error::new(io::ErrorKind::PermissionDenied, e))?;
    let (mut tx, mut rx) = match channel {
        Channel::Ethernet(tx, rx) => (tx, rx),
        _ => return Err(io::Error::other("unsupported datalink channel")),
    };

    let found = Arc::new(Mutex::new(Found::new()));
    let stop = Arc::new(AtomicBool::new(false));
    {
        let (found, stop, net) = (found.clone(), stop.clone(), ifc.net);
        let own_ip = ifc.ip;
        thread::spawn(move || {
            while !stop.load(Ordering::Relaxed) {
                let Ok(frame) = rx.next() else { continue };
                let Some(eth) = EthernetPacket::new(frame) else {
                    continue;
                };
                if eth.get_ethertype() != EtherTypes::Arp {
                    continue;
                }
                let Some(arp) = ArpPacket::new(eth.payload()) else {
                    continue;
                };
                // Any ARP traffic (replies, and other hosts' requests) proves the sender is alive.
                let ip = arp.get_sender_proto_addr();
                if net.contains(ip) && ip != own_ip && !ip.is_unspecified() {
                    found.lock().unwrap().insert(ip, arp.get_sender_hw_addr());
                }
            }
        });
    }

    // Two rounds: Wi-Fi clients in power-save often miss the first request.
    for round in 0..2 {
        let pending: Vec<_> = {
            let f = found.lock().unwrap();
            targets
                .iter()
                .filter(|ip| !f.contains_key(ip))
                .copied()
                .collect()
        };
        for (i, &target) in pending.iter().enumerate() {
            let frame = arp_request(own_mac, ifc.ip, target);
            if let Some(Err(e)) = tx.send_to(&frame, None)
                && round == 0
                && i == 0
            {
                return Err(e);
            }
            if i % 64 == 63 {
                thread::sleep(Duration::from_millis(2)); // don't overrun the send buffer
            }
        }
        thread::sleep(wait / 2);
    }
    stop.store(true, Ordering::Relaxed);

    let found = found.lock().unwrap().clone();
    Ok(found)
}

/// Active ARP sweep through `SendARP`, which needs no privileges. Each
/// request blocks until the address answers or Windows gives up, which takes
/// seconds for an empty address, so every address gets its own thread and
/// we stop listening at the deadline. That's too many threads past a /22, so
/// larger sweeps fail with Unsupported and fall back to the ARP cache.
#[cfg(windows)]
pub fn sweep(ifc: &Iface, targets: &[Ipv4Addr], wait: Duration) -> io::Result<Found> {
    use std::sync::mpsc;
    use std::time::Instant;

    if targets.len() > 1024 {
        return Err(io::Error::new(
            io::ErrorKind::Unsupported,
            "too many addresses for SendARP",
        ));
    }

    let deadline = Instant::now() + wait;
    let (tx, rx) = mpsc::channel();
    for &ip in targets {
        let (tx, source) = (tx.clone(), ifc.ip);
        thread::spawn(move || {
            if let Some(mac) = platform::send_arp(ip, source) {
                let _ = tx.send((ip, mac));
            }
        });
    }
    drop(tx);
    let mut found = Found::new();
    while let Some(left) = deadline.checked_duration_since(Instant::now()) {
        let Ok((ip, mac)) = rx.recv_timeout(left) else {
            break;
        };
        found.insert(ip, mac);
    }
    Ok(found)
}

#[cfg(unix)]
fn arp_request(src_mac: MacAddr, src_ip: Ipv4Addr, target: Ipv4Addr) -> [u8; 42] {
    use pnet_packet::arp::{ArpHardwareTypes, ArpOperations, MutableArpPacket};
    use pnet_packet::ethernet::{EtherTypes, MutableEthernetPacket};

    let mut buf = [0u8; 42];
    {
        let mut eth = MutableEthernetPacket::new(&mut buf).unwrap();
        eth.set_destination(MacAddr::broadcast());
        eth.set_source(src_mac);
        eth.set_ethertype(EtherTypes::Arp);
    }
    let mut arp = MutableArpPacket::new(&mut buf[14..]).unwrap();
    arp.set_hardware_type(ArpHardwareTypes::Ethernet);
    arp.set_protocol_type(EtherTypes::Ipv4);
    arp.set_hw_addr_len(6);
    arp.set_proto_addr_len(4);
    arp.set_operation(ArpOperations::Request);
    arp.set_sender_hw_addr(src_mac);
    arp.set_sender_proto_addr(src_ip);
    arp.set_target_hw_addr(MacAddr::zero());
    arp.set_target_proto_addr(target);
    buf
}

/// Whatever the kernel's ARP cache knows about this network. Linux shares
/// it freely; recent macOS returns nothing to a binary that isn't signed
/// with a Developer ID, even through `arp`.
pub fn read_cache(ifc: &Iface) -> Found {
    platform::arp_cache(&ifc.iface)
        .into_iter()
        .filter(|(ip, mac)| {
            ifc.net.contains(*ip)
                && *ip != ifc.ip
                && *ip != ifc.net.broadcast()
                && *mac != MacAddr::broadcast()
        })
        .collect()
}