diff --git a/src/bin/auth/main.cc b/src/bin/auth/main.cc
index a19dbb53e2..f7ddd8a797 100644
--- a/src/bin/auth/main.cc
+++ b/src/bin/auth/main.cc
@@ -18,6 +18,7 @@
 #include <sys/socket.h>
 #include <sys/select.h>
 #include <netdb.h>
+#include <netinet/in.h>  // IPPROTO_UDP
 #include <stdlib.h>
 
 #include <set>