diff --git a/bfdd/bfd_packet.c b/bfdd/bfd_packet.c
index 1ec761e3b8..12e508c601 100644
--- a/bfdd/bfd_packet.c
+++ b/bfdd/bfd_packet.c
@@ -779,7 +779,7 @@ int bp_udp_send(int sd, uint8_t ttl, uint8_t *data, size_t datalen,
 			cmsg->cmsg_level = IPPROTO_IPV6;
 			cmsg->cmsg_type = IPV6_HOPLIMIT;
 		} else {
-#if BFD_LINUX
+#ifdef BFD_LINUX
 			cmsg->cmsg_level = IPPROTO_IP;
 			cmsg->cmsg_type = IP_TTL;
 #else
diff --git a/configure.ac b/configure.ac
index fe389ebb35..628e0c8afc 100755
--- a/configure.ac
+++ b/configure.ac
@@ -295,6 +295,7 @@ AC_C_FLAG([-Wmissing-declarations])
 AC_C_FLAG([-Wpointer-arith])
 AC_C_FLAG([-Wbad-function-cast])
 AC_C_FLAG([-Wwrite-strings])
+AC_C_FLAG([-Wundef])
 if test "$enable_gcc_ultra_verbose" = "yes" ; then
   AC_C_FLAG([-Wcast-qual])
   AC_C_FLAG([-Wstrict-prototypes])