just initialize with an address and add a note that the function for …

…setting the ip address mutates an entry

examples/dhcp_client.rs

@@ -38,6 +38,13 @@ fn main() {
     config.random_seed = rand::random();
     let mut iface = Interface::new(config, &mut device, Instant::now());
 
+    // Initialize with an address so that there is something to mutate later.
+    // This also allows sockets made before a real address is negotiated
+    // via DHCP to update properly afterward.
+    iface.update_ip_addrs(|addrs| {
+        addrs.push(IpCidr::Ipv4(Ipv4Cidr::new(Ipv4Address::UNSPECIFIED, 0))).unwrap();
+    });
+
     // Create sockets
     let mut dhcp_socket = dhcpv4::Socket::new();
 
@@ -77,7 +84,6 @@ fn main() {
             }
             Some(dhcpv4::Event::Deconfigured) => {
                 debug!("DHCP lost config!");
-                iface.update_ip_addrs(|addrs| addrs.clear());
                 set_ipv4_addr(&mut iface, Ipv4Cidr::new(Ipv4Address::UNSPECIFIED, 0));
                 iface.routes_mut().remove_default_ipv4_route();
             }
@@ -87,8 +93,10 @@ fn main() {
     }
 }
 
+/// Mutate the first IP address to match the one supplied
 fn set_ipv4_addr(iface: &mut Interface, cidr: Ipv4Cidr) {
     iface.update_ip_addrs(|addrs| {
-        addrs.push(IpCidr::Ipv4(cidr)).unwrap();
+        let dest = addrs.iter_mut().next().unwrap();
+        *dest = IpCidr::Ipv4(cidr);
     });
 }