diff --git a/src/Makefile b/src/Makefile
index 8fba2ae..73570d5 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -1,5 +1,5 @@
 IGMPPROG=igmpgen
-CFLAGS=-Werror -Wall `libnet-config --defines`
+CFLAGS=-Werror -Wall `libnet-config --defines` -D_DEFAULT_SOURCE
 LDFLAGS=`libnet-config --libs`
 
 all: