From owner-freebsd-net@FreeBSD.ORG  Mon Dec  1 12:23:56 2014
Return-Path: <owner-freebsd-net@FreeBSD.ORG>
Delivered-To: freebsd-net@freebsd.org
Received: from mx1.freebsd.org (mx1.freebsd.org [8.8.178.115])
 (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits))
 (No client certificate requested)
 by hub.freebsd.org (Postfix) with ESMTPS id 82558751
 for <freebsd-net@freebsd.org>; Mon,  1 Dec 2014 12:23:56 +0000 (UTC)
Received: from shell1.rawbw.com (shell1.rawbw.com [198.144.192.42])
 by mx1.freebsd.org (Postfix) with ESMTP id 6D84A7B7
 for <freebsd-net@freebsd.org>; Mon,  1 Dec 2014 12:23:55 +0000 (UTC)
Received: from eagle.yuri.org (c-73-162-232-245.hsd1.ca.comcast.net
 [73.162.232.245]) (authenticated bits=0)
 by shell1.rawbw.com (8.14.9/8.14.9) with ESMTP id sB1CNm6H089752
 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES128-SHA bits=128 verify=NO)
 for <freebsd-net@freebsd.org>; Mon, 1 Dec 2014 04:23:49 -0800 (PST)
 (envelope-from yuri@rawbw.com)
X-Authentication-Warning: shell1.rawbw.com: Host
 c-73-162-232-245.hsd1.ca.comcast.net [73.162.232.245] claimed to be
 eagle.yuri.org
Message-ID: <547C5DD3.90604@rawbw.com>
Date: Mon, 01 Dec 2014 04:23:47 -0800
From: Yuri <yuri@rawbw.com>
User-Agent: Mozilla/5.0 (X11; FreeBSD amd64;
 rv:31.0) Gecko/20100101 Thunderbird/31.2.0
MIME-Version: 1.0
To: freebsd-net@freebsd.org
Subject: Can multiple apps listen for TCP on the same port?
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Transfer-Encoding: 7bit
X-BeenThere: freebsd-net@freebsd.org
X-Mailman-Version: 2.1.18-1
Precedence: list
List-Id: Networking and TCP/IP with FreeBSD <freebsd-net.freebsd.org>
List-Unsubscribe: <http://lists.freebsd.org/mailman/options/freebsd-net>,
 <mailto:freebsd-net-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/freebsd-net/>
List-Post: <mailto:freebsd-net@freebsd.org>
List-Help: <mailto:freebsd-net-request@freebsd.org?subject=help>
List-Subscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-net>,
 <mailto:freebsd-net-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Mon, 01 Dec 2014 12:23:56 -0000

I have a simple 'nc' based TCP server with shell script serving http 
protocol. But when I run the second instance, it never gets any 
connections and never fails.

'nc -l PORT' first calls listen with backlog=1 and socket option 
SO_REUSEADDR, and then calls accept. Once client is accepted, it closes 
the listening socket and only works with this connection.

Why the second nc instance still never accepts any connections, once the 
first connection is accepted, and first listening socket is closed?

When one listening socket is closed, shouldn't other listening sockets 
on the same port be accepting connections? It looks like the whole port 
is blocked without even having the listening socket on it by one alive 
connection (which after it was accepted doesn't have much to do with the 
original port).

Yuri