rawlink/
packet.rs

1//! Linux backend of [`Link`]: an `AF_PACKET`/`SOCK_RAW` socket bound to one
2//! interface. Needs `CAP_NET_RAW` (and `CAP_NET_ADMIN` for promiscuous mode).
3
4// std has no raw packet sockets; the unsafety is confined to the libc calls.
5#![allow(unsafe_code)]
6
7use anyhow::{bail, Context, Result};
8use std::ffi::CString;
9use std::os::fd::{AsRawFd, FromRawFd, OwnedFd};
10use std::time::Duration;
11
12/// Largest frame one `recv` can return; more than any Ethernet MTU.
13const RECV_BUF_LEN: usize = 65536;
14
15#[derive(Debug)]
16pub struct Link {
17    fd: OwnedFd,
18    /// Receive buffer, reused across `recv` calls.
19    buf: Vec<u8>,
20}
21
22impl Link {
23    /// Open a raw packet socket bound to `iface`, promiscuous: the card
24    /// replies to `SENDER_MAC`, not to the host's own address.
25    ///
26    /// # Errors
27    /// Fails without `CAP_NET_RAW`, or if the interface does not exist.
28    pub fn open(iface: &str, read_timeout: Duration) -> Result<Self> {
29        let proto = (libc::ETH_P_ALL as u16).to_be();
30        let raw =
31            unsafe { libc::socket(libc::AF_PACKET, libc::SOCK_RAW, libc::c_int::from(proto)) };
32        if raw < 0 {
33            let e = std::io::Error::last_os_error();
34            if e.raw_os_error() == Some(libc::EPERM) || e.raw_os_error() == Some(libc::EACCES) {
35                bail!(
36                    "socket(AF_PACKET): {e} (try: sudo setcap cap_net_raw,cap_net_admin+ep \
37                     $(command -v rxp), or run as root)"
38                );
39            }
40            return Err(e).context("socket(AF_PACKET)");
41        }
42        let fd = unsafe { OwnedFd::from_raw_fd(raw) };
43
44        let name = CString::new(iface).with_context(|| format!("interface name: {iface}"))?;
45        let ifindex = unsafe { libc::if_nametoindex(name.as_ptr()) };
46        if ifindex == 0 {
47            bail!("no such interface: {iface}");
48        }
49
50        let mut sa: libc::sockaddr_ll = unsafe { std::mem::zeroed() };
51        sa.sll_family = libc::AF_PACKET as u16;
52        sa.sll_protocol = proto;
53        sa.sll_ifindex = ifindex as libc::c_int;
54        let rc = unsafe {
55            libc::bind(
56                fd.as_raw_fd(),
57                std::ptr::from_ref(&sa).cast(),
58                std::mem::size_of::<libc::sockaddr_ll>() as libc::socklen_t,
59            )
60        };
61        if rc < 0 {
62            return Err(std::io::Error::last_os_error()).with_context(|| format!("bind {iface}"));
63        }
64
65        let mut mr: libc::packet_mreq = unsafe { std::mem::zeroed() };
66        mr.mr_ifindex = ifindex as libc::c_int;
67        mr.mr_type = libc::PACKET_MR_PROMISC as u16;
68        setsockopt(&fd, libc::SOL_PACKET, libc::PACKET_ADD_MEMBERSHIP, &mr)
69            .context("PACKET_ADD_MEMBERSHIP promisc")?;
70
71        // `suseconds_t` is i32 on 32-bit targets, so `From<u32>` would not port.
72        #[allow(clippy::cast_lossless)]
73        let tv = libc::timeval {
74            tv_sec: read_timeout.as_secs() as libc::time_t,
75            tv_usec: read_timeout.subsec_micros() as libc::suseconds_t,
76        };
77        setsockopt(&fd, libc::SOL_SOCKET, libc::SO_RCVTIMEO, &tv).context("SO_RCVTIMEO")?;
78
79        Ok(Self {
80            fd,
81            buf: vec![0u8; RECV_BUF_LEN],
82        })
83    }
84
85    /// Send one whole Ethernet frame.
86    ///
87    /// # Errors
88    /// Fails if the kernel rejects or truncates the write.
89    pub fn send(&mut self, frame: &[u8]) -> Result<()> {
90        let n = unsafe { libc::send(self.fd.as_raw_fd(), frame.as_ptr().cast(), frame.len(), 0) };
91        if n < 0 {
92            return Err(std::io::Error::last_os_error()).context("send on packet socket");
93        }
94        if n as usize != frame.len() {
95            bail!("short write: {n} of {} bytes", frame.len());
96        }
97        Ok(())
98    }
99
100    /// At most one frame received within the read timeout, borrowed from the
101    /// socket buffer until the next call. A timeout or EINTR yields no frames.
102    ///
103    /// # Errors
104    /// Fails on any other receive error.
105    pub fn recv(&mut self) -> Result<Frames<'_>> {
106        let n = unsafe {
107            libc::recv(
108                self.fd.as_raw_fd(),
109                self.buf.as_mut_ptr().cast(),
110                self.buf.len(),
111                0,
112            )
113        };
114        if n < 0 {
115            let e = std::io::Error::last_os_error();
116            return match e.raw_os_error() {
117                Some(libc::EAGAIN | libc::EINTR) => Ok(Frames(None)),
118                _ => Err(e).context("recv on packet socket"),
119            };
120        }
121        Ok(Frames(Some(&self.buf[..n as usize])))
122    }
123}
124
125/// The frames from one `recv`: zero or one on a packet socket.
126#[derive(Clone, Debug)]
127pub struct Frames<'a>(pub Option<&'a [u8]>);
128
129impl<'a> Iterator for Frames<'a> {
130    type Item = &'a [u8];
131
132    fn next(&mut self) -> Option<&'a [u8]> {
133        self.0.take()
134    }
135}
136
137fn setsockopt<T>(fd: &OwnedFd, level: libc::c_int, name: libc::c_int, value: &T) -> Result<()> {
138    let rc = unsafe {
139        libc::setsockopt(
140            fd.as_raw_fd(),
141            level,
142            name,
143            std::ptr::from_ref(value).cast(),
144            std::mem::size_of::<T>() as libc::socklen_t,
145        )
146    };
147    if rc < 0 {
148        return Err(std::io::Error::last_os_error().into());
149    }
150    Ok(())
151}
152
153#[cfg(test)]
154mod tests {
155    use super::*;
156
157    #[test]
158    fn frames_yield_at_most_one_frame() {
159        assert!(Frames(None).next().is_none());
160        let mut f = Frames(Some(&[1, 2, 3][..]));
161        assert_eq!(f.next(), Some(&[1, 2, 3][..]));
162        assert!(f.next().is_none());
163    }
164}
rawlink/packet.rs

rawlink/
packet.rs
