diff -ur knock-0.3/src/knock.c knock-0.3-fix/src/knock.c
--- knock-0.3/src/knock.c	2004-05-07 00:56:03.000000000 +0200
+++ knock-0.3-fix/src/knock.c	2004-08-04 00:53:25.663567392 +0200
@@ -30,6 +30,7 @@
 #include <string.h>
 #include <resolv.h>
 #include <getopt.h>
+#include <fcntl.h>
 
 static char version[] = "0.3";
 
@@ -79,7 +80,10 @@
 		if(o_udp) {
 			sd = socket(PF_INET, SOCK_DGRAM, 0); 
 		} else {
+			int flags;
 			sd = socket(PF_INET, SOCK_STREAM, 0); 
+			flags = fcntl(sd, F_GETFL, 0);
+			fcntl(sd, F_SETFL, flags | O_NONBLOCK);
 		}
 		memset(&addr, 0, sizeof(addr));
 		addr.sin_family = AF_INET;