Bug 1724416

This is not a bug in ncat.
Ncat adds stdin to select() and wait for ready FDs. In your case select() returns with [1a]
while "normal" execution doesn't return STDIN as ready [1b]

Thus ncat sees 0 input and returns [2]
This was done to properly handle protocols which don't support half-closed sockets like sctp [3]

I'd suggest to adjust your script not to proxy ncat input or redirect /dev/full to stdin
[1a]

[pid  3252] select(5, [0<pipe:[39940975]> 4<socket:[39941357]>], [], NULL, NULL) = 1 (in [0])
[pid  3252] read(0<pipe:[39940975]>, "", 8192) = 0
[1b]
select(5, [0 4], [], NULL, NULL)        = 1 (in [4])
recvfrom(4, "abc\n", 8192, 0, NULL, NULL) = 4
write(1, "abc\n", 4abc
)                    = 4
select(5, [0 4], [], NULL, NULL)        = 1 (in [4])
recvfrom(4, "", 8192, 0, NULL, NULL)    = 0
close(4)                                = 0
exit_group(0)                           = ?
+++ exited with 0 +++

[2]             } else if (i == STDIN_FILENO) {
                    /* Read from stdin and write to all clients. */
                    rc = read_stdin();
                    if (rc == 0) {
                        if (o.proto != IPPROTO_TCP || (o.proto == IPPROTO_TCP && o.sendonly)) {
                            /* There will be nothing more to send. If we're not
                               receiving anything, we can quit here. */
                            return 0;
                        }
                        if (!o.noshutdown) shutdown_sockets(SHUT_WR);
                   

[3]
https://github.com/nmap/nmap/commit/811d6096e1e7f2380c7ab3a4ae098a8f997c81b7

I thought about the issue a bit more and looks there's room for improvement at least
Will send patch upstream for review

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This bug has been reviewed by Support and Engineering representative and does not meet the inclusion criteria for Maintenance Support 1 phase. If this issue still exists in newer major version of Red Hat Enterprise Linux, it has been cloned there and work will continue in the cloned bug.

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