diff --git a/src/common.c b/src/common.c
index a2d23f8..8511498 100644
--- a/src/common.c
+++ b/src/common.c
@@ -130,9 +130,11 @@ open_dns(int localport, in_addr_t listen_ip)
 	setsockopt(fd, IPPROTO_IP, DSTADDR_SOCKOPT, (const void*) &flag, sizeof(flag));
 #endif
 
+#ifdef IP_OPT_DONT_FRAG
 	/* Set dont-fragment ip header flag */
 	flag = DONT_FRAG_VALUE;
 	setsockopt(fd, IPPROTO_IP, IP_OPT_DONT_FRAG, (const void*) &flag, sizeof(flag));
+#endif
 
 	if(bind(fd, (struct sockaddr*)&addr, sizeof(addr)) < 0) 
 		err(1, "bind");
diff --git a/src/common.h b/src/common.h
index a6d1137..f39461a 100644
--- a/src/common.h
+++ b/src/common.h
@@ -44,16 +44,16 @@
 # define dstaddr(x) (&(((struct in_pktinfo *)(CMSG_DATA(x)))->ipi_addr)) 
 #endif
 
-#if defined IP_MTUDISC
-# define IP_OPT_DONT_FRAG IP_MTUDISC
-# define DONT_FRAG_VALUE 1
-#elif defined IP_MTU_DISCOVER
+#if defined IP_MTU_DISCOVER
+  /* Linux */
 # define IP_OPT_DONT_FRAG IP_MTU_DISCOVER
 # define DONT_FRAG_VALUE IP_PMTUDISC_DO
 #elif defined IP_DONTFRAG
+  /* FreeBSD */
 # define IP_OPT_DONT_FRAG IP_DONTFRAG
 # define DONT_FRAG_VALUE 1
 #elif defined IP_DONTFRAGMENT
+  /* Winsock2 */
 # define IP_OPT_DONT_FRAG IP_DONTFRAGMENT
 # define DONT_FRAG_VALUE 1
 #endif