From mboxrd@z Thu Jan  1 00:00:00 1970
From: Eric Dumazet <eric.dumazet@gmail.com>
Subject: Re: [PATCH net-next] ipv4: accept u8 in IP_TOS ancillary data
Date: Thu, 08 Sep 2016 06:31:14 -0700
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         <20160908111519.62af94bf@redhat.com>
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Cc: David Miller <davem@davemloft.net>,
        netdev <netdev@vger.kernel.org>,
        Francesco Fusco <fusco@ntop.org>,
        Maciej =?UTF-8?Q?=C5=BBenczykowski?= <maze@google.com>
To: Jesper Dangaard Brouer <brouer@redhat.com>
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On Thu, 2016-09-08 at 11:15 +0200, Jesper Dangaard Brouer wrote:
> On Wed, 07 Sep 2016 21:52:56 -0700
> Eric Dumazet <eric.dumazet@gmail.com> wrote:
> 
> > From: Eric Dumazet <edumazet@google.com>
> > 
> > In commit f02db315b8d8 ("ipv4: IP_TOS and IP_TTL can be specified as
> > ancillary data") Francesco added IP_TOS values specified as integer.
> > 
> > However, kernel sends to userspace (at recvmsg() time) an IP_TOS value
> > in a single byte, when IP_RECVTOS is set on the socket.
> > 
> > It can be very useful to reflect all ancillary options as given by the
> > kernel in a subsequent sendmsg(), instead of aborting the sendmsg() with
> > EINVAL after Francesco patch.
> > 
> > So this patch extends IP_TOS ancillary to accept an u8, so that an UDP
> > server can simply reuse same ancillary block without having to mangle
> > it.
> > 
> > Jesper can then augment
> > https://github.com/netoptimizer/network-testing/blob/master/src/udp_example02.c
> > to add TOS reflection ;)
> 
> This is actually your old program ;-)
> Do I need to change anything, as I'm just bouncing the packet back with sendmsg() ?

I guess you want to add an option and if this option is requested by the
user, add :

	setsockopt(fd, SOL_IP, IP_PKTINFO, &on, sizeof(on));
+	if (tos_reflect)
+		setsockopt(fd, SOL_IP, IP_RECVTOS, &on, sizeof(on));

before the loop doing the recvmsg()/sendmsg() calls.

Thanks !