Empty transmit buffers before starting to receive.

src/iface/ethernet.rs

@@ -118,6 +118,12 @@ impl<'a, 'b: 'a,
             Icmpv4(Ipv4Repr, Icmpv4Repr<'a>)
         }
 
+        // First, transmit any outgoing packets.
+        loop {
+            if try!(self.emit()) { break }
+        }
+
+        // Now, receive any incoming packets.
         let rx_buffer = try!(self.device.receive());
         let eth_frame = try!(EthernetFrame::new(&rx_buffer));
 
@@ -229,7 +235,9 @@ impl<'a, 'b: 'a,
                 frame.set_ethertype(EthernetProtocolType::Arp);
 
                 let mut packet = try!(ArpPacket::new(frame.payload_mut()));
-                repr.emit(&mut packet)
+                repr.emit(&mut packet);
+
+                Ok(())
             },
 
             Response::Icmpv4(ip_repr, icmp_repr) => {
@@ -252,17 +260,48 @@ impl<'a, 'b: 'a,
 
                 let mut icmp_packet = try!(Icmpv4Packet::new(ip_packet.payload_mut()));
                 icmp_repr.emit(&mut icmp_packet);
+
+                Ok(())
             }
 
             Response::Nop => {
+                Ok(())
+            }
+        }
+    }
+
+    pub fn emit(&mut self) -> Result<bool, Error> {
         // Borrow checker is being overly careful around closures, so we have
         // to hack around that.
         let src_hardware_addr = self.hardware_addr;
+        let src_protocol_addrs = self.protocol_addrs.borrow();
         let arp_cache = &mut self.arp_cache;
         let device = &mut self.device;
 
+        let mut nothing_to_transmit = true;
         for socket in self.sockets.borrow_mut() {
             let result = socket.dispatch(&mut |src_addr, dst_addr, protocol, payload| {
+                let src_addr =
+                    match src_addr {
+                        &InternetAddress::Ipv4(_) if src_addr.is_unspecified() => {
+                            let mut assigned_addr = None;
+                            for addr in src_protocol_addrs {
+                                match addr {
+                                    addr @ &InternetAddress::Ipv4(_) => {
+                                        assigned_addr = Some(addr);
+                                        break
+                                    }
+                                    _ => ()
+                                }
+                            }
+                            assigned_addr.expect(
+                                "to respond to an UDP packet without a source address,\
+                                 the interface must have an assigned address from \
+                                 the same family")
+                        },
+                        addr => addr
+                    };
+
                 let ip_repr =
                     match (src_addr, dst_addr) {
                         (&InternetAddress::Ipv4(src_addr),
@@ -299,15 +338,16 @@ impl<'a, 'b: 'a,
             });
 
             match result {
-                        Ok(()) => break,
+                Ok(()) => {
+                    nothing_to_transmit = false;
+                    break
+                }
                 Err(Error::Exhausted) => continue,
                 Err(e) => return Err(e)
             }
         }
-            }
-        }
 
-        Ok(())
+        Ok(nothing_to_transmit)
     }
 }