fix(gateway): ignore self-injected packet frames

AF_PACKET sockets can report packets sent by the host as well as packets
received from the LAN. The gateway writes remote-client frames onto the wired
interface, so treating those outgoing packets as fresh LAN input can reflect
self-injected traffic back to the relay.

Read packet metadata with `recvfrom` and skip `PACKET_OUTGOING` frames before
returning a LAN frame to the bridge loop. This keeps capture scoped to inbound
LAN traffic and is a prerequisite for periodic CAM refresh frames.

Test Plan:
- cargo fmt --check
- cargo test --workspace
- cargo clippy --workspace --all-targets -- -D warnings
- git diff --check

Refs: PLAN.md gateway AF_PACKET bridge

crates/lanparty-gateway/src/packet.rs

@@ -92,21 +92,33 @@ impl PacketSocket {
     }
 
     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());
+        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);
+            }
         }
-
-        Ok(received as usize)
     }
 }
 
@@ -142,6 +154,10 @@ pub fn interface_index(interface: &str) -> io::Result<u32> {
     Ok(index)
 }
 
+fn is_inbound_packet_type(packet_type: u8) -> bool {
+    packet_type != libc::PACKET_OUTGOING
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -164,4 +180,12 @@ mod tests {
 
         assert_ne!(error.kind(), io::ErrorKind::InvalidInput);
     }
+
+    #[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));
+    }
 }