Home / ubuntu / Questions / 121376
In Process
user43816
Asked: 2012-04-13 07:27:04 +0800 CST

Why does '~$ nc -z T43 1-1024' not produce any output although '~$ ping T43' is successful?

  • 772

Why does '~$ nc -z T43 1-1024' not produce any output although '~$ ping T43' is successful?

'~$ nc -z -v T43 1-1024' prints 'Connection to T43 22 port [tcp/ssh] succeeded!' and 'nc: connect to T43 port 1000 (tcp) failed: Connection refused.'

'man nc': "PORT SCANNING: It may be useful to know which ports are open and running services on a target machine. The -z flag can be used to tell nc to report open ports, rather than initiate a connection. For example: '$ nc -z host.example.com 20-30'.

internet

1 Answers

  1. I've checked the source code, first the original (netcat-openbsd_1.89.orig.tar.gz). The relevant lines in netcat.c (367-388) (edited):

    if (vflag || zflag) {
    /* ... */

    /* Don't look up port if -n. */
    if (nflag)
        sv = NULL;
    else {
        sv = getservbyport(
            ntohs(atoi(portlist[i])),
            uflag ? "udp" : "tcp");
    }

    printf("Connection to %s %s port [%s/%s] succeeded!\n",
        host, portlist[i], uflag ? "udp" : "tcp",
        sv ? sv->s_name : "*");
}

    But the diff package (netcat-openbsd_1.89-4ubuntu1.diff.gz) contains a patch, removing the verbose display functionality:

    --- netcat-openbsd-1.89.orig/debian/patches/silence-z.patch
+++ netcat-openbsd-1.89/debian/patches/silence-z.patch
@@ -0,0 +1,13 @@
+Index: netcat-openbsd-1.89/netcat.c
+===================================================================
+--- netcat-openbsd-1.89.orig/netcat.c  2008-06-19 16:33:52.000000000 -0400
++++ netcat-openbsd-1.89/netcat.c   2008-06-19 16:34:58.000000000 -0400
+@@ -364,7 +364,7 @@
+               continue;
+ 
+           ret = 0;
+-          if (vflag || zflag) {
++          if (vflag) {
+               /* For UDP, make sure we are connected. */
+               if (uflag) {
+                   if (udptest(s) == -1) {

    So it seems that without the v flag, the z flag branch never „runs” and does not display the message. (Although I'm not an expert C programmer, so please take this with a bit of skepticism.)

    Also the changelog refers to this:

    netcat-openbsd (1.89-3) unstable; urgency=low

    • Silence -z flag, for compatibility with netcat-traditional (Closes: #464564)

    The Debian bug report for #464564 contains the following:

    The -z flag produces a "Connected...successfully." message, even when -v is not selected. This is a significant deviation from the original, and conflates the function of the -z and -v flags. Patch attached.

    So I think it is a bug in the manual page: the port scanning section have not followed the changed functionality.

    • 3