Only use first 3 DHCP advertised DNS servers.

The OPT_DOMAIN_NAME_SERVER DHCP option field can advertise an unbounded
DNS servers, but we currently use a fixed length array to store these
servers. This commit ensures only the first 3 (highest priority) servers
are used.

Fixes #305.

src/wire/dhcpv4.rs

@@ -742,7 +742,7 @@ impl<'a> Repr<'a> {
                 }
                 DhcpOption::Other {kind: field::OPT_DOMAIN_NAME_SERVER, data} => {
                     let mut dns_servers_inner = [None; 3];
-                    for i in 0.. {
+                    for i in 0..3 {
                         let offset = 4 * i;
                         let end = offset + 4;
                         if end > data.len() { break }
@@ -840,6 +840,30 @@ mod test {
         0x00, 0x00, 0x37, 0x04, 0x01, 0x03, 0x06, 0x2a, 0xff, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
     ];
 
+    static ACK_BYTES: &[u8] = &[
+        0x02, 0x01, 0x06, 0x00, 0xcc, 0x34, 0x75, 0xab, 0x00, 0x00, 0x80, 0x00, 0x0a, 0xff, 0x06, 0x91,
+        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x0a, 0xff, 0x06, 0xfe, 0x34, 0x17, 0xeb, 0xc9,
+        0xaa, 0x2f, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x63, 0x82, 0x53, 0x63,
+        0x35, 0x01, 0x05, 0x36, 0x04, 0xa3, 0x01, 0x4a, 0x16, 0x01, 0x04, 0xff, 0xff, 0xff, 0x00, 0x2b,
+        0x05, 0xdc, 0x03, 0x4e, 0x41, 0x50, 0x0f, 0x15, 0x6e, 0x61, 0x74, 0x2e, 0x70, 0x68, 0x79, 0x73,
+        0x69, 0x63, 0x73, 0x2e, 0x6f, 0x78, 0x2e, 0x61, 0x63, 0x2e, 0x75, 0x6b, 0x00, 0x03, 0x04, 0x0a,
+        0xff, 0x06, 0xfe, 0x06, 0x10, 0xa3, 0x01, 0x4a, 0x06, 0xa3, 0x01, 0x4a, 0x07, 0xa3, 0x01, 0x4a,
+        0x03, 0xa3, 0x01, 0x4a, 0x04, 0x2c, 0x10, 0xa3, 0x01, 0x4a, 0x03, 0xa3, 0x01, 0x4a, 0x04, 0xa3,
+        0x01, 0x4a, 0x06, 0xa3, 0x01, 0x4a, 0x07, 0x2e, 0x01, 0x08, 0xff
+    ];
+
     const IP_NULL: Ipv4Address = Ipv4Address([0, 0, 0, 0]);
     const CLIENT_MAC: EthernetAddress = EthernetAddress([0x0, 0x0b, 0x82, 0x01, 0xfc, 0x42]);
 
@@ -979,4 +1003,17 @@ mod test {
         assert_eq!(&bytes[0..2], &[field::OPT_PARAMETER_REQUEST_LIST, DATA.len() as u8]);
         assert_eq!(&bytes[2..], DATA);
     }
+
+    #[test]
+    fn test_parse_ack_dns_servers() {
+        let packet = Packet::new_unchecked(ACK_BYTES);
+        let repr = Repr::parse(&packet).unwrap();
+        // The packet described by ACK_BYTES advertises 4 DNS servers
+        // Here we ensure that we correctly parse the first 3 into our fixed
+        // length-3 array (see issue #305)
+        assert_eq!(repr.dns_servers,  Some([
+            Some(Ipv4Address([163, 1, 74, 6])),
+            Some(Ipv4Address([163, 1, 74, 7])),
+            Some(Ipv4Address([163, 1, 74, 3]))]));
+    }
 }