bin/60598: nc(1) UDP port scans are not useful.

>Number:         60598
>Category:       bin
>Synopsis:       nc(1) UDP port scans are not useful.
>Confidential:   no
>Severity:       non-critical
>Priority:       low
>Responsible:    bin-bug-people
>State:          open
>Class:          change-request
>Submitter-Id:   net
>Arrival-Date:   Sat Aug 15 09:05:01 +0000 2026
>Originator:     RVP
>Release:        11.99.7
>Organization:
>Environment:
NetBSD CoreBook.local 11.99.7 NetBSD 11.99.7 (COREBOOK_DIAG) #0: Sat Aug 15 04:21:15 UTC 2026  [email protected]:/tmp/obj/usr/src/sys/arch/amd64/compile/COREBOOK_DIAG amd64
>Description:
Doing a UDP port scans using nc(1) right now is kinda pointless:

```
$ nc -uvz 192.168.68.1 50-60
Connection to 192.168.68.1 50 port [udp/re-mail-ck] succeeded!
Connection to 192.168.68.1 51 port [udp/*] succeeded!
Connection to 192.168.68.1 52 port [udp/xns-time] succeeded!
Connection to 192.168.68.1 53 port [udp/domain] succeeded!
Connection to 192.168.68.1 54 port [udp/xns-ch] succeeded!
Connection to 192.168.68.1 55 port [udp/isi-gl] succeeded!
Connection to 192.168.68.1 56 port [udp/xns-auth] succeeded!
Connection to 192.168.68.1 57 port [udp/*] succeeded!
Connection to 192.168.68.1 58 port [udp/xns-mail] succeeded!
Connection to 192.168.68.1 59 port [udp/*] succeeded!
Connection to 192.168.68.1 60 port [udp/*] succeeded!
$
```
All ports show up because nc(1) blasts through all the ports, and
expects the write to immediately return an error if a UDP port is
unreachable.

>How-To-Repeat:
As shown above.
>Fix:
Wait a bit for the ICMP port unreachable message to reach us.
Patch is almost the same as in Debian's `netcat-openbsd' package.

```
diff -urN usr.bin/nc.orig/netcat.c usr.bin/nc/netcat.c
--- usr.bin/nc.orig/netcat.c	2026-03-03 03:54:49.716087313 +0000
+++ usr.bin/nc/netcat.c	2026-06-04 11:37:51.676518886 +0000
@@ -1518,6 +1518,8 @@
 	}
 }
 
+#define UDP_SCAN_TIMEOUT	3	/* seconds */
+
 /*
  * udptest()
  * Do a few writes to see if the UDP port is there.
@@ -1526,15 +1528,23 @@
 int
 udptest(int s)
 {
-	int i, ret;
+	int i, t = (timeout == -1) ? UDP_SCAN_TIMEOUT : (timeout / 1000);
+ 
+ 	/* Only write to the socket in scan mode or interactive mode. */
+ 	if (!zflag && !isatty(STDIN_FILENO))
+ 		return 0;
+ 
+	if ((write(s, "X", 1) != 1) ||
+	    ((write(s, "X", 1) != 1) && (errno == ECONNREFUSED)))
+		return -1;
 
-	for (i = 0; i <= 3; i++) {
-		if (write(s, "X", 1) == 1)
-			ret = 1;
-		else
-			ret = -1;
-	}
-	return (ret);
+	/* Give the remote host some time to reply. */
+	for (i = 0; i < t; i++) {
+		sleep(1);
+		if ((write(s, "X", 1) != 1) && (errno == ECONNREFUSED))
+			return -1;
+ 	}
+	return 1;
 }
 
 void
```