Patch to nc

[Dave Edwards]

I've written a small patch for nc. It adds a switch that causes nc to 
return after the first read from network (instead of establishing a 
connection).

Why this is useful for me (and potentially others): Some network protocols 
(such as Tivo's home networking protocols) use a periodic broadcast UDP 
message as a service locator (a "beacon"). The existing 'nc' will block 
after the first such packet, expecting a UDP session to be established. 
This patch adds a switch to tell it to write out the contents of the 
buffer and return immediately.

I used the '-O' ('Oh', not 'zero') flag, for 'Only one packet', which is 
(as far as I can tell) currently unused by both the BusyBox 'nc' and the 
BSD 'nc'. It's enabled as part of the "USE_NC_EXTRA" ifdef.

Example:
$ ./busybox nc -l -u -v -p 2190
listening on [::]:2190 ...
connect to 192.168.1.255:2190 from TiVo:2190 ([::ffff:192.168.1.31]:2190)
tivoconnect=1
swversion=9.1-01-2-140
method=broadcast
identity=240000080XXXXXX
machine=Living Room
platform=tcd/Series2
services=TiVoMediaServer:80/http
[ ... time passes ... ]
punt! [ I ^C busybox ]


$ ./busybox nc -l -u -O -v -p 2190
listening on [::]:2190 ...
connect to 192.168.1.255:2190 from TiVo:2190 ([::ffff:192.168.1.31]:2190)
tivoconnect=1
swversion=9.1-01-2-140
method=broadcast
identity=240000080XXXXXX
machine=Living Room
platform=tcd/Series2
services=TiVoMediaServer:80/http
$
-------------- next part --------------
Index: networking/nc_bloaty.c
===================================================================
--- networking/nc_bloaty.c      (revision 20365)
+++ networking/nc_bloaty.c      (working copy)
@@ -1,3 +1,4 @@
+/* vi: set sw=4 ts=4: */
 /* Based on netcat 1.10 RELEASE 960320 written by hobbit at avian.org.
  * Released into public domain by the author.
  *
@@ -129,6 +130,7 @@
        OPT_i = (1 << (7+ENABLE_NC_SERVER)) * ENABLE_NC_EXTRA,
        OPT_o = (1 << (8+ENABLE_NC_SERVER)) * ENABLE_NC_EXTRA,
        OPT_z = (1 << (9+ENABLE_NC_SERVER)) * ENABLE_NC_EXTRA,
+       OPT_O = (1 << (10+ENABLE_NC_SERVER)) * ENABLE_NC_EXTRA,
 };
 
 #define o_nflag   (option_mask32 & OPT_n)
@@ -141,9 +143,11 @@
 #if ENABLE_NC_EXTRA
 #define o_ofile   (option_mask32 & OPT_o)
 #define o_zero    (option_mask32 & OPT_z)
+#define o_onlyone (option_mask32 & OPT_O)
 #else
 #define o_ofile   0
 #define o_zero    0
+#define o_onlyone 0
 #endif
 
 /* Debug: squirt whatever message and sleep a bit so we can see it go by. */
@@ -584,6 +588,16 @@
                        } else {
                                rnleft = rr;
                                np = bigbuf_net;
+#if ENABLE_NC_EXTRA
+                               /* The following is a special case intended mostly for UDP listeners, 
+                                * where the contents of the first packet get printed and then the 
+                                * connection gets closed. Useful for tracking broadcast UDP beacons. */
+                               if(o_onlyone) {         /* Special case where we just want the first packet */
+                                       rr = write(1, np, rnleft); /* don't care what write returns */
+                                       close(netfd);
+                                       return 0;
+                               }
+#endif
                        }
 Debug("got %d from the net, errno %d", rr, errno);
                } /* net:ding */
@@ -707,7 +721,7 @@
        // -g -G -t -r deleted, unimplemented -a deleted too
        opt_complementary = "?2:vv"; /* max 2 params, -v is a counter */
        getopt32(argv, "hnp:s:uvw:" USE_NC_SERVER("l")
-                       USE_NC_EXTRA("i:o:z"),
+                       USE_NC_EXTRA("i:o:zO"),
                        &str_p, &str_s, &str_w
                        USE_NC_EXTRA(, &str_i, &str_o, &o_verbose));
        argv += optind;
Index: include/usage.h
===================================================================
--- include/usage.h     (revision 20365)
+++ include/usage.h     (working copy)
@@ -2494,6 +2494,7 @@
        USE_NC_EXTRA( \
        "\n             (use -l twice with -e for persistent server)") \
        "\n     -p PORT Local port number" \
+       "\n     -O      Only print the first packet, then exit (use with -l)"\
        )
 
 #define nc_notes_usage "" \