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From: David Miller <davem@davemloft.net>
Subject: Re: Add a SOCK_DESTROY operation to close sockets from userspace
Date: Thu, 19 Nov 2015 19:55:04 -0500 (EST)
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From: Lorenzo Colitti <lorenzo@google.com>
Date: Fri, 20 Nov 2015 09:19:25 +0900

> In this case, userspace knows that that app's connections are now
> unusable because it configured an iptables rule to block them. The
> kernel doesn't really know until it the time comes to send a packet,
> and maybe not even then.

Netfilter could perform signalling on skb->sk when it drops packets.

Your example is actually a argument _for_ doing this in the kernel.

:-)