netdev - Established sockets remain open after iface down or address lost

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Message-ID: <20130926060433.GA9170@cverges-dev-lnx.sentient-energy.com>
Date:	Wed, 25 Sep 2013 23:04:34 -0700
From:	Chris Verges <cverges@...tient-energy.com>
To:	davem@...emloft.net, kuznet@....inr.ac.ru, jmorris@...ei.org,
	yoshfuji@...ux-ipv6.org, kaber@...sh.net, netdev@...r.kernel.org
Subject: Established sockets remain open after iface down or address lost

Hello all,

I've encountered a behavior that appears to be known, but am seeking
some clarity on its rationale.  The scenario is as follows:

  (0) A TCP server socket listens on :: (v4/v6).
  (1) Connect a USB/Ethernet adapter to a Linux system.
  (2) Adapter is brought up as 'eth0' with an IP address.
  (3) A remote TCP client connects to the server socket.
  (4) 'netstat -anp' shows the socket as ESTABLISHED
  (5) The TCP server starts a blocking read waiting for data.
  (6) Physically disconnect the USB/Ethernet adapter from the USB bus.
  (7) Linux removes the 'eth0' interface and associated IP address.

At this point, the socket _still_ shows as ESTABLISHED under netstat.

This is the paradox.  Why is the blocking read not interrupted with a
socket error to indicate that the socket is no longer viable?

Thank you in advance,
Chris

P.S.  I apologize in advance if I missed this answer in the netdev
archives.
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