Re: decode MPLS-contained packets?

[Gert Doering via tcpdump-workers <tcpdump-workers () lists tcpdump org>]

> --- Begin Message ---
>
>
> From: Gert Doering <gert () greenie muc de>
>
> Date: Thu, 7 May 2020 09:13:56 +0200
>
> Hi,
>
> On Thu, May 07, 2020 at 08:20:40AM +0200, Francois-Xavier Le Bail wrote:
> > Proposed patch attached.
> >
> > With new '-T mplsethnocw' option to force 'Ethernet without Control Word' decode.
> > (from Francesco Fondelli comment)
>
> There's one bug here:
>
> > +                     case 0x00:      /* RFC 4448 PW Ethernet Control Word */
> > +                             if (ndo->ndo_vflag) {
> > +                                     ND_PRINT("\n\tPW Ethernet Control Word");
> > +                                     p += 2;
> > +                                     ND_PRINT(", Sequence Number %u", GET_BE_U_2(p));
> > +                                     p += 2;
> > +                             } else
> > +                                     ND_PRINT(" PWETHCW");
> > +                                     p += 4;
> > +                             length -= 4;
>
> Due to missing {}, the "p += 4" will always be executed, skipping the
> control word twice if "-v" is set.
>
> I'd totally leave off printing the "PWETHCW", though.  If it's decoding the
> payload, that is all information I need - if I want more, I can always
> do "-v".
>
> So the code would become:
>
> +                       case 0x00:      /* RFC 4448 PW Ethernet Control Word */
> +                               if (ndo->ndo_vflag) {
> +                                       ND_PRINT("\n\tPW Ethernet Control Word");
> +                                       ND_PRINT(", Sequence Number %u", GET_BE_U_2(p+2));
> +                               }
> +                               p += 4;
> +                               length -= 4;
> +                               pt = PT_ETHER;
> +                               break;
>
> There's something else:
>
> +       case PT_ETHER:
> +               ether_print(ndo, p, length, ND_BYTES_AVAILABLE_AFTER(bp), NULL, NULL);
>
> This might actually be a bug in my original patch, ND_BYTES_AVAILABLE_AFTER()
> should possibly call "(p)", not "(bp)".  Copy-pasted that line from
> print-gre.c, and only changed the first "bp" to "p.
>
> gert
>
> -- 
> "If was one thing all people took for granted, was conviction that if you 
>  feed honest figures into a computer, honest figures come out. Never doubted 
>  it myself till I met a computer with a sense of humor."
>                              Robert A. Heinlein, The Moon is a Harsh Mistress
>
> Gert Doering - Munich, Germany                             gert () greenie muc de
>
> --- End Message ---
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