Fix BPF header length on OpenBSD.

The actual header length may be larger than the bpf_hdr struct due to aligning:
https://github.com/openbsd/src/blob/37ecb4d066e5566411cc16b362d3960c93b1d0be/sys/net/bpf.c#L1649
https://github.com/apple/darwin-xnu/blob/8f02f2a044b9bb1ad951987ef5bab20ec9486310/bsd/net/bpf.c#L3580

Tests are only valid for 32 and 64 bit architectures. I did not bother
guarding them with additional cfg flags.

src/phy/sys/bpf.rs

@@ -1,8 +1,10 @@
 use std::io;
+use std::mem;
 use std::os::unix::io::{AsRawFd, RawFd};
 
 use libc;
 
+use crate::wire::ETHERNET_HEADER_LEN;
 use super::{ifreq, ifreq_for};
 
 /// set interface
@@ -14,9 +16,19 @@ const BIOCGBLEN: libc::c_ulong = 0x40044266;
 /// set immediate/nonblocking read
 #[cfg(any(target_os = "macos", target_os = "openbsd"))]
 const BIOCIMMEDIATE: libc::c_ulong = 0x80044270;
-// TODO: check if this is same for OSes other than macos
+/// set bpf_hdr struct size
+#[cfg(target_os = "macos")]
+const SIZEOF_BPF_HDR: usize = 18;
+/// set bpf_hdr struct size
+#[cfg(target_os = "openbsd")]
+const SIZEOF_BPF_HDR: usize = 24;
+/// The actual header length may be larger than the bpf_hdr struct due to aligning
+/// see https://github.com/openbsd/src/blob/37ecb4d066e5566411cc16b362d3960c93b1d0be/sys/net/bpf.c#L1649
+/// and https://github.com/apple/darwin-xnu/blob/8f02f2a044b9bb1ad951987ef5bab20ec9486310/bsd/net/bpf.c#L3580
 #[cfg(any(target_os = "macos", target_os = "openbsd"))]
-const BPF_HDRLEN: usize = 18;
+const BPF_HDRLEN: usize = (((SIZEOF_BPF_HDR + ETHERNET_HEADER_LEN) + mem::align_of::<u32>() - 1)
+    & !(mem::align_of::<u32>() - 1))
+    - ETHERNET_HEADER_LEN;
 
 macro_rules! try_ioctl {
     ($fd:expr,$cmd:expr,$req:expr) => {
@@ -145,3 +157,20 @@ impl Drop for BpfDevice {
         }
     }
 }
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    #[test]
+    #[cfg(target_os = "macos")]
+    fn test_aligned_bpf_hdr_len() {
+        assert_eq!(18, BPF_HDRLEN);
+    }
+
+    #[test]
+    #[cfg(target_os = "openbsd")]
+    fn test_aligned_bpf_hdr_len() {
+        assert_eq!(26, BPF_HDRLEN);
+    }
+}

src/wire/ethernet.rs

@@ -91,6 +91,9 @@ mod field {
     pub const PAYLOAD:     Rest  = 14..;
 }
 
+/// The Ethernet header length
+pub const HEADER_LEN: usize = field::PAYLOAD.start;
+
 impl<T: AsRef<[u8]>> Frame<T> {
     /// Imbue a raw octet buffer with Ethernet frame structure.
     pub fn new_unchecked(buffer: T) -> Frame<T> {
@@ -111,7 +114,7 @@ impl<T: AsRef<[u8]>> Frame<T> {
     /// Returns `Err(Error::Truncated)` if the buffer is too short.
     pub fn check_len(&self) -> Result<()> {
         let len = self.buffer.as_ref().len();
-        if len < field::PAYLOAD.start {
+        if len < HEADER_LEN {
             Err(Error::Truncated)
         } else {
             Ok(())
@@ -125,13 +128,13 @@ impl<T: AsRef<[u8]>> Frame<T> {
 
     /// Return the length of a frame header.
     pub fn header_len() -> usize {
-        field::PAYLOAD.start
+        HEADER_LEN
     }
 
     /// Return the length of a buffer required to hold a packet with the payload
     /// of a given length.
     pub fn buffer_len(payload_len: usize) -> usize {
-        field::PAYLOAD.start + payload_len
+        HEADER_LEN + payload_len
     }
 
     /// Return the destination address field.
@@ -262,7 +265,7 @@ impl Repr {
 
     /// Return the length of a header that will be emitted from this high-level representation.
     pub fn buffer_len(&self) -> usize {
-        field::PAYLOAD.start
+        HEADER_LEN
     }
 
     /// Emit a high-level representation into an Ethernet II frame.

src/wire/mod.rs

@@ -119,6 +119,7 @@ pub use self::pretty_print::PrettyPrinter;
 pub use self::ethernet::{EtherType as EthernetProtocol,
                          Address as EthernetAddress,
                          Frame as EthernetFrame,
+                         HEADER_LEN as ETHERNET_HEADER_LEN,
                          Repr as EthernetRepr};
 
 #[cfg(all(feature = "proto-ipv4", feature = "ethernet"))]