Re: [Sheflug] How NAT Works

[Robin Wood <robin@xxxxxxxxxxxxxxxxxxxxx>]

I've got as far as the birthday problem so I might just need to read some
more.

Does Picky NAT not care what IP the traffic is coming from? The port is
just open for a short period of time for any connections? That seems quite
dangerous.

Robin

On Wed, 8 Jan 2025, 22:13 John Southern, <linuxtarragon@xxxxxxxxx> wrote:

> Hi Robin,
>
> My understanding is you send out to the STUN server and your NATTing router
> keeps that port linked back to your client1 for a short period.
> In that time client 2 can send info to it and get through.
>
> In your scenario you are using the Picky NAT where the port it gives when
> talking to the STUN server is different to the port it would want to use
> from client2.
> That is where the article then jumps down to the Birthday problem if one of
> the NAT devices is an Easy NAT or having to use one of the three protocols
> (UPnP-IGD, NAT-PMP or PCP) to find the port number if they are both HardNAT
> devices and so not have to go via the TURN relay for all traffic.
>
> I think a TURN server is more likely to enable a connection but then has to
> be able to handle all the traffic thrown at it. I think in Tailscales
> situation they are saying that by using all the tricks, they can avoid
> having TURN servers in most cases.
>
> There are also TURNS servers for TCP traffic.
>
> Regards
> John
>
> On Wed, 8 Jan 2025 at 21:34, Robin Wood <robin@xxxxxxxxxxxxxxxxxxxxx>
> wrote:
>
> > Hi
> > I've read through most of this and I'm stuck on how STUN works. I think I
> > must be missing something but this is where I'm having problems.
> >
> > A NAT device handles connections by quads of source IP and port, and
> > destination IP and port. So the client on the inside of my network
> > (client1) makes a call out to the STUN server, that records the external
> IP
> > and port the connection is coming from and is then able to pass it on to
> > the other side of the connection (client2).
> >
> > But, if client2 tries to connect to client1 using that IP and port the
> NAT
> > box will see a different source IP, one that doesn't match any that it
> > knows, so it would just drop the traffic.
> >
> > I know the idea is that once client1 has punched out of the NAT, the hole
> > is open so the other side is able to send packets back, but I can only
> see
> > that working when the other side is using the same IP as client1 started
> > talking to. If client2 tries to talk to the external IP and port client1
> > used to talk to the STUN server it shouldn't work.
> >
> > Is this the failing that TURN is used to handle? If so, then isn't STUN
> > dead in most situations? I'd imagine a lot of clients, especially VOIP,
> are
> > behind at least one layer of NAT.
> >
> > To have written such a big article on STUN, it feels like I've missed
> > something important that means it will work in a lot more situations,
> but I
> > can't see what it is. Can anyone explain?
> >
> > Robin
> >
> > On Sun, 5 Jan 2025 at 11:56, Richard Ibbotson <richard@xxxxxxxxxxxxxx>
> > wrote:
> >
> > > Hi
> > >
> > > https://tailscale.com/blog/how-nat-traversal-works
> > >
> > >
> > > Might interest someone out there. How NAT works.
> > >
> > > --
> > > Richard
> > >
> > >
> > > _______________________________________________
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> >
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> >
> _______________________________________________
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