Bug 1784751

+++ This bug was initially created as a clone of Bug #1724416 +++

Description of problem:
In this format:  
echo ncat --sctp -l 127.0.0.1 | bash
ncat no output for sctp, but tcp/udp has

Version-Release number of selected component (if applicable):
nmap-ncat-7.70-4.el8.x86_64
nmap-ncat-6.40-19.el7.x86_64
Both exists on RHEL7 and RHEL8

How reproducible:
always

Steps to Reproduce:
1.
# echo ncat -l 127.0.0.1 2000 |bash
abc
# echo ncat -u -l 127.0.0.1 2000 |bash
abc
# echo ncat --sctp -l 127.0.0.1 2000 |bash
#
Get nothing here!

2.
At another terminal
# ncat  127.0.0.1 2000 <<<'abc'
# ncat -u 127.0.0.1 2000 <<<'abc'
# ncat --sctp 127.0.0.1 2000 <<<'abc'

3.

Actual results:
# echo ncat --sctp -l 127.0.0.1 2000 |bash
This command returned but has no ouput.

Expected results:
ncat --sctp work as expected.

Additional info:
This issue affect our test script. Please fix it.

--- Additional comment from RHEL Program Management on 2019-06-27 05:54:18 CEST ---

Since this bug report was entered in Red Hat Bugzilla, the release flag has been set to ? to ensure that it is properly evaluated for this release.

--- Additional comment from Pavel Zhukov on 2019-06-27 10:44:45 CEST ---

** SELF NOTE **
SIGPIPE should be ignored in sctp mode as well as in dgram and stream modes

--- Additional comment from Pavel Zhukov on 2019-07-31 10:54:20 CEST ---

https://github.com/nmap/nmap/commit/811d6096e1e7f2380c7ab3a4ae098a8f997c81b7

--- Additional comment from Pavel Zhukov on 2019-07-31 13:54:24 CEST ---

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

--- Additional comment from Pavel Zhukov on 2019-07-31 15:27:23 CEST ---

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

--- Additional comment from Pavel Zhukov on 2019-07-31 15:47:58 CEST ---

https://github.com/nmap/nmap/pull/1673/commits/63a10182c48ecdf46332ffbaea6aa6030097e37c

--- Additional comment from yiche on 2019-08-01 04:39:23 CEST ---

Thank you very much for investigate!