[PATCH] Ncat: match traditional and OpenBSD netcat behaviour of terminating on EOF

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

Hi people and apologies for the unwieldy title!

Right now, Ncat keeps running when the remote end closes the connection.  
Only when the client pushes more bytes into Ncat, it fails with "Broken 
pipe." For example, this makes it impossible to wrap Ncat in a 
while-loop to keep reconnecting.

Here is a simple PoC of the problem:
 - run `ncat -l 1234` in one terminal and `nc ::1 1234` in another.
 - ^C the listen-mode ncat.
 - hit return (possibly twice) in the connect-mode ncat.
With the current version, this prints "Ncat: Broken pipe."; with the 
patch applied, it works and the connect-mode ncat exits.

This has been requested multiple times [1] [2] [3] [4], and was accepted 
in 2017 by dmiller [5] and fyodor [6], who for lack of feedback back 
then didn't go ahead with it.

To get Ncat's previous behaviour, I opted for a new command line switch, 
--no-terminate, instead of overloading --keep-open (as Daniel 
suggested), as I felt it is unintuitive to have an option that does two 
different things depending on the context (-l vs no -l). I was 
contemplating overloading --no-shutdown, but didn't, as it is already a 
backwards-compatibility-option for openbsd-nc, so changing it didn't 
feel right.

For your convenience, I have attached the patch to this email, and 
opened https://github.com/nmap/nmap/pull/2246 for it as well.

[1]: https://seclists.org/nmap-dev/2017/q2/67
[2]: https://github.com/nmap/nmap/issues/894
[3]: https://github.com/nmap/nmap/issues/1413
[4]: https://github.com/nmap/nmap/issues/1779
[5]: https://seclists.org/nmap-dev/2017/q2/94
[6]: https://seclists.org/nmap-dev/2017/q2/134

Attachment: no-terminate.patch
Description:

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