The netcat from [extra] didnot connect to any other machine but can accept connection from others. 
I traced the program and found netcat_flag_count() in flagset.c returned error.
I also googled and found a patch from OpenWRT, and it works fine now!
$this->bbcode_second_pass_code('', 'Index: netcat-0.7.1/src/flagset.c
===================================================================
--- netcat-0.7.1.orig/src/flagset.c	2009-02-06 19:56:01.000000000 +0100
+++ netcat-0.7.1/src/flagset.c	2009-02-06 19:56:13.000000000 +0100
@@ -134,7 +134,7 @@
 
 int netcat_flag_count(void)
 {
-  register char c;
+  register unsigned char c;
   register int i;
   int ret = 0;
 
@@ -154,7 +154,7 @@
 	Assumed that the bit number 1 is the sign, and that we will shift the
 	bit 1 (or the bit that takes its place later) until the the most right,
 	WHY it has to keep the wrong sign? */
-      ret -= (c >> 7);
+      ret += (c >> 7);
       c <<= 1;
     }
   }
')
https://dev.openwrt.org/browser/packages/net/netcat/patches/001-netcat_flag_count.patch
			
		
