netdev - Re: [PATCH] net/icmp: restore source address if packet is NATed

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Date:   Mon, 26 Jun 2017 00:52:09 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     David Miller <davem@...emloft.net>
Cc:     Netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] net/icmp: restore source address if packet is NATed

Hi David,

On Sun, Jun 25, 2017 at 5:49 PM, David Miller <davem@...emloft.net> wrote:
> This violates things on so many levels.

Yes, indeed.

> I think this kind of thing need to be hidden inside of netfilter,
> it can do the rate limiting and stuff like that in the spot
> where it makes the transformation and knows all of the original
> addressing and ports.

Indeed I'd prefer that, and I'll look again into trying to make that
work. But when I tried last, it seemed like there were some
insurmountable challenges. With the ratelimiting, the limit has
already been applied to one IP -- the masqueraded one -- before
netfilter even has a chance to act -- so that IP will already hit the
ratelimits well before any additional one inside netfilter would. Then
the issue of transformation: last I looked it seemed like icmp_send
threw away a bit too much information to do the transformation
entirely correctly, but I could be wrong, so I'll give it another
look. Hopefully it winds up being as easy as just reverse-transforming
ICMP's payload IP header.

>
> You definitely can't just rewrite header fields here either.  The
> SKB could be shared, for example.

I was afraid of that. It's easy to rework this particular patch,
though, if you're still interested in the crufty bolt on approach...
But I think I should investigate the netfilter-only approach instead,
as you suggested.

Jason