netdev - Re: TCP using IPv4-mapped IPv6 address as source

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Date:   Wed, 11 Jan 2017 14:59:09 -0500
From:   Sowmini Varadhan <sowmini.varadhan@...cle.com>
To:     "Jonathan T. Leighton" <jtleight@...l.edu>
Cc:     Eric Dumazet <eric.dumazet@...il.com>, netdev@...r.kernel.org,
        edumazet@...gle.com, Yuchung Cheng <ycheng@...gle.com>,
        Neal Cardwell <ncardwell@...gle.com>
Subject: Re: TCP using IPv4-mapped IPv6 address as source

On (01/11/17 14:48), Jonathan T. Leighton wrote:
> 
> I would say that an IPv6 socket binds to an IPv4-mapped IPv6
> address, but yes - using it to connect to an IPv6 address would be
> an application bug. It also think that having connect() spend 2
> minutes sending packets containing an illegal source address is a
> bug. (And unfriendly!) I'll look into writing a patch for this, and
> let you know whether or not I think I'm up to it.
> 

BTW, linux probably has a number of bugs in this space. 

you can do a number of "interesting" things, where the
v4-mapped address ends up being treated as a global address.
E.g., you can configure a v4 mapped address on an interface
using /sbin/ip, then you can add an onlink route, like
 # ip -6 route add ::ffff:13.0.0.1 nexthop onlink dev eth1
and you can ping6 ::ffff:13.0.0.1, and (if you do this
on both sides) watch a merry little packet exchange on
the wire, where we send out an NS for ::ffff:13.0.0.1
to the solmcast of the v4 mapped address, the peer politely
responds, and the 2 nodes then have a nice chat. 

I think the RFC states somewhere that you should never ever
send out a v4 mapped address on the wire.

--Sowmini