Netcat sure is a handy utility, but there's one minor problem I've come
across, for which I'm offering the following simple fix. First, the
explanation
why I think the fix is a good idea:

Netcat doesn't drop the connection until the server closes it so that you
don't ever lose the final output from the server, which is what may happen
if
the client simply closes the connection when the client reads EOF from
stdin.
This is good, and I'm not suggesting that it change. However, a deadlock
occurs
if the server doesn't close its end until it reads EOF on the connection.
For
example, if you netcat to the "echo" service (on port 7), sending EOF (by
hitting CTRL-D or whatever, say) won't ever cause netcat to exit; it will
simply stop reading its stdin, and the server will keep waiting for more
input,
which will never arrive. Some protocols (e.g. http) avoid this problem by
signalling EOF in-band, but I don't think netcat should make this
assumption.

The solution is easy: use shutdown to tell the server that the client has
finished writing, but can keep on reading. Servers like "echo", which wait
for
EOF from the client will then gracefully close their end of the connection,
causing netcat to exit with no loss of data. I'd be interested in anyone's
feedback; I reckon this simple patch makes netcat an even more useful
program.


*** netcat-1.10.orig/netcat.c   Thu Mar 21 11:38:04 1996
--- netcat-1.10.greyham/netcat.c        Mon Sep 13 10:38:14 1999
***************
*** 1216,1221 ****
--- 1216,1222 ----
        if (rr <= 0) {                  /* at end, or fukt, or ... */
          FD_CLR (0, ding1);            /* disable and close stdin */
          close (0);
+         shutdown(fd, 1);              /* no more sending from us */
        } else {
          rzleft = rr;
          zp = bigbuf_in;

To test it:
% vi /etc/inetd.conf    # enable 'echo' service
% nc localhost echo
hello
hello
<CTRL-D>
%

Without the patch, netcat-1.10 never exits. With the patch, it exits
cleanly.

Thanks,
Graham