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Date:   Wed, 20 Jul 2022 16:20:02 +0200
From:   Florian Westphal <fw@strlen.de>
To:     Erik Skultety <eskultet@redhat.com>
Cc:     netfilter-devel@vger.kernel.org
Subject: Re: [iptables PATCH] iptables: xshared: Ouptut '--' in the opt field
 in ipv6's fake mode
Message-ID: <20220720142002.GA22790@breakpoint.cc>
References: <bb391c763171f0c5511f73e383e1b2e6a53e2014.1658322396.git.eskultet@redhat.com>
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Erik Skultety <eskultet@redhat.com> wrote:
> The fact that the 'opt' table field reports spaces instead of '--' for
> IPv6 as it would have been the case with IPv4 has a bit of an
> unfortunate side effect that it completely confuses the 'jc' JSON
> formatter tool (which has an iptables formatter module).
> Consider:
>     # ip6tables -L test
>     Chain test (0 references)
>     target     prot opt source   destination
>     ACCEPT     all      a:b:c::  anywhere    MAC01:02:03:04:05:06
> 
> Then:
>     # ip6tables -L test | jc --iptables
>     [{"chain":"test",
>       "rules":[
>           {"target":"ACCEPT",
>            "prot":"all",
>            "opt":"a:b:c::",
>            "source":"anywhere",
>            "destination":"MAC01:02:03:04:05:06"
>           }]
>     }]
> 
> which as you can see is wrong simply because whitespaces are considered
> as a column delimiter.

Looks like ip6tables and iptables had this behaviour since day 1.
original iptables:

       if (format & FMT_OPTIONS) {
               if (format & FMT_NOTABLE)
                       fputs("opt ", stdout);
               fputc(fw->ip.invflags & IPT_INV_FRAG ? '!' :
       		'-', stdout);
               fputc(flags & IPT_F_FRAG ? 'f' : '-', stdout);
               fputc(' ', stdout);
       }

original ip6tables (5eed48af2516ebce0412121713d285bc30edb10d, June 2000):
       if (format & FMT_OPTIONS) {
               if (format & FMT_NOTABLE)
                       fputs("opt ", stdout);
               fputc(' ', stdout);
               fputc(' ', stdout);
               fputc(' ', stdout);
       }

While I like the idea of making those two identical I'm not sure its
worh the risk, we've hit bugs for a myriad of other reasons when making 
seemingly innocent changes like this.

What do others think?