nc -kle 'cat largefile' doesn't transmit correctly

[Tobias Girstmair <t-nmap () girst at>]

Hi folks,

I'm using ncat as a simple web server, as described in
https://nmap.org/ncat/guide/ncat-tricks.html#ncat-httpserv . The file
I'm serving is relatively large (80kB), and I noticed that it gets
mangled very often: a section of over a kilobyte is missing from the
middle (at packet boundaries; e.g. after 44888 bytes using 1448 sized
packets).

This seems to only happen when using --exec or --sh-exec; not when
piping the same file in through stdin. It also only happens on 'real'
networks (i.e., not loopback).

I've tested 7.50 from centos7, 7.91 from voidlinux and fedora35, as well
as todays' svn/git version, git commit 158c2e4.

Steps to reproduce:

1. generate a large file (`head -c100000 /dev/urandom > largefile`) and
   share it between server and client
2. on the server side, run `ncat -kle '/bin/cat largefile' 1234`
3. on the client side, run `nc --recv-only server.host 1234 | diff -qs - largefile`.
   notice the output (might require multiple tries):
   > Files - and largefile differ
   > write: Broken pipe
4. alternatively, run `nc --recv-only 10.42.1 1234|wc -c` on the client
   and notice that the reported bytes will be less than the filesize
   (e.g.  96688 < 100000).
5. run the following snippets on the server side and repeat; these will
   work correctly:
   - `ncat -l 1234 < largefile`
   - `ncat -klc 'cat largefile; sleep .1' 1234`

I'm a bit confused why this happens. Any ideas?
Thanks,
        tobi
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