Re: [Spud] SPUD magic number

> On 10 Apr 2015, at 04:35, Tom Herbert <tom@herbertland.com> wrote:
> 
> Hi,
> 
> The SPUD magic number seems like a good idea and in fact I anticipate
> that we'll want to adopt this technique in other UDP encapsulation
> protocols.
> 
+1

STUN (RFC5389) also uses this trick in section 6 (https://tools.ietf.org/html/rfc5389#section-6)

> I believe the magic number could be applied in two different ways:
> 
> 1) A middlebox must match both the UDP destination port and the magic
> number before accepting the packet is SPUD protocol. In this case ,
> the magic number is used to distinguish SPUD from other arbitrary uses
> of a SPUD assigned port.
> 2) A middlebox matches the magic number in any UDP packet regardless
> of destination port and declares it to be SPUD. This is nice because
> we would not need to configure SPUD ports on middleboxes, but
> increases the chances of misinterpreting something which is not SPUD
> at all (a larger magic number size might be warranted).
> 
> What is the intended use for this in the SPUD prototype protocol?
> 
This was actually the first bet we implemented (https://github.com/iptube/SPUDlib). Mostly due to your 2) bullet point. 

It allowed us to quickly get a linux router with netfiletr/iptables up and running with the following rule:

-A PREROUTING -p udp —match u32 --u32 "0>>22&0x3C@8=0xd80000d8" -j NFQUEUE --queue-num 0

See https://github.com/iptube/TubeNode for a quick and dirty implementation.

From a client perspective lessons learned from implementing ICE and STUN where RTP, STUN and vide variety of other packets might be multiplexed on the same port was that having a simple isSPUD function is really useful..

In the spud prototype it is defined like this:

bool spud_is_spud(const uint8_t *payload, size_t length)
{
    if (length < sizeof(spud_header)) {
        return false;
    }
    return (memcmp(payload, (void *)SpudMagicCookie, SPUD_MAGIC_COOKIE_SIZE) == 0);
}

.-.
Pål-Erik

> Thanks,
> Tom
> 
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