diff --git a/contrib/tcpdump/print-esp.c b/contrib/tcpdump/print-esp.c
index 1bdf76a8697d..33edf8a1e478 100644
--- a/contrib/tcpdump/print-esp.c
+++ b/contrib/tcpdump/print-esp.c
@@ -50,6 +50,7 @@ static const char rcsid[] =
 #endif
 
 #include <stdio.h>
+#include <unistd.h>
 
 #include "ip.h"
 #include "esp.h"