dhcp: fix buffer length after adding new option

The size of dhcp option should be o.length + 2 additional bytes of the header.
Incorrect offset results in an extra byte at the end of the packet:
...
0120  04 00 00 00 04 05 04 AC 10 04 01 FF 00
...
RFC2131 says the last should be the 'end' option (ff)

Type: fix

Change-Id: I056d755d29465aab8c1c55a0b930f65ece6fafce
Signed-off-by: Artem Glazychev <glazychev@mts.ru>

src/plugins/dhcp/dhcp4_proxy_node.c

@@ -321,7 +321,8 @@ dhcp_proxy_to_server_input (vlib_main_t * vm,
 		  o->length += id_len + 5;
 		}
 
-	      len = o->length + 3;
+	      /* 2 bytes for option header 82+len */
+	      len = o->length + 2;
 	      b0->current_length += len;
 	      /* Fix IP header length and checksum */
 	      old_l0 = ip0->length;

test/test_dhcp.py

@@ -327,6 +327,9 @@ class TestDHCP(VppTestCase):
                     is_discover = True
         self.assertTrue(is_discover)
 
+        # The last option must always be the 'end' option
+        self.assertEqual(dhcp.options[-1], "end")
+
         data = self.validate_relay_options(
             pkt, src_intf, src_intf.local_ip4, vpn_id, fib_id, oui
         )