is a NAT device/'home router'

is a NAT device/'home router' - a router?

I see that they receive a frame, and then forward it on to a local
computer. This isn't routing. Infact, I've heard that NAT is really a
firewall feature, and these devices do have built in firewalls.

And I can't see that these NAT devices have a routing table either.
When they send a frame out, they just send it down the wire, to the
ISP's router.

A 'home router' with its 2 arms and apparently no knowledge of teh
outside world, doesn't seem like a router to me.

But I've also heard that it uses RIP and us a router, it's hard to see
how or where. Or what is right

Posted by Patrick Schaaf on September 28, 2005, 7:13 am

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jameshanley39@yahoo.co.uk writes:

>is a NAT device/'home router' - a router?

It is.

>I see that they receive a frame, and then forward it on to a local
>computer. This isn't routing.

It is routing when it has two interfaces. It could even be routing
if there were only one interface. The essence of routing, is to
look at the L3 header, and decide where the packet has to go to.
Even if the decision appears to always be the same.

What do you think a NAT device/'home router' is doing when,
from the internet, a packet arrives with a destination IP
which is not known on the LAN side? Leaving aside firewall
rules, I'd guess the packet would take the default route
straigt out the link it came in on.

What about the (not so uncommon) boxen with an additional WLAN
interface? Do they become a router when the WLAN is configured?
Or when the first station really connects to the WLAN? Do they
then stop being a router when somebody pulls the LAN cable?

>And I can't see that these NAT devices have a routing table either.

Many of them run Linux with a normal Linux IP stack. You bet there's
a routing table, somewhere!

Don't be blinded by the devices-for-dummies totally-dumbed-down
web interface those boxen present. That's just pretty packaging.

>A 'home router' with its 2 arms and apparently no knowledge of teh
>outside world, doesn't seem like a router to me.

You are entitled to use terminology all the way you like. You are also
entitled - guessing here - to play word definition games with your friends.

Even with a single physical arm, a thing can be a router. Think about
multiple VLANs on a single ethernet cable.

My take: if it forwards IP frames, it _is_ a router.

BTW, words are irrelevant. The box works without them.

best regards
Patrick

Posted by on September 28, 2005, 3:55 am

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Patrick Schaaf wrote:

> jameshanley39@yahoo.co.uk writes:
>

> >is a NAT device/'home router' - a router?

>
> It is.
>

> >I see that they receive a frame, and then forward it on to a local
> >computer. This isn't routing.

>
> It is routing when it has two interfaces. It could even be routing
> if there were only one interface. The essence of routing, is to
> look at the L3 header, and decide where the packet has to go to.
> Even if the decision appears to always be the same.

I think the essence of routing is
a)look at the dest ip
b)use the dest ip to consult a routing table
c)decide where the packet should go

In this case - for incoming packets, the Dest IP is always that of the
router itself. The router doesn't look at the Dest IP to see where the
frame should go. It looks at the TCP Port in the packet, and forwards
the packet accordingly.

less importantly, but furthermore, as I said, i've heard that NAT is a
firewall function rather than a router function. and the 'home routers'
do have built in firewalls.

<snip>

> >And I can't see that these NAT devices have a routing table either.

>
> Many of them run Linux with a normal Linux IP stack. You bet there's
> a routing table, somewhere!

If there's a routing table, what is in it? (I will speculate)

As far as I know, Port Forwarding has nothing to do with a routing
table. As far as I know, Routing tables don't mention the TCP Port.
They mention

Subnet, Next Hop, Router Interface

So are you saying that they have a routing table with a single entry
and the next Hop is the ISP's router?

This is all very well for outgoing frames. But incoming frames are not
routed. AFAIK NAT and port forwarding, have nothing to do with a
routing table.

> My take: if it forwards IP frames, it _is_ a router.

This is your attitude speaking. You think that whether by port
forwarding or not, it is routing. But you don't consider words
important. You may be right about the forwarding being routing, or you
may be wrong. But you don't mind inventing words as you go along. I
clearly value correct terminology more than you do.

> BTW, words are irrelevant. The box works without them.

There are many like you. Most often people in marketting have tha
attitude to terminology.

Perhaps somebody that values terminology can respond to this post
regarding correct terminology!!!

Posted by Patrick Schaaf on September 28, 2005, 11:08 am

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jameshanley39@yahoo.co.uk writes:

>I think the essence of routing is
>a)look at the dest ip
>b)use the dest ip to consult a routing table
>c)decide where the packet should go

>In this case - for incoming packets, the Dest IP is always that of the
>router itself.

When it's configured to do NAT, yes. Otherwise, no. So in general, no.

>[...] i've heard that NAT is a firewall function rather than a router
>function.

NAT is a function by itself. It is implemented and/or configured in
otherwise pure routers, in otherwise pure firewalls, or in any
combination thereof. No understanding is gained by calling it
'a router function' or 'a firewall function'. NAT is NAT.

>and the 'home routers' do have built in firewalls.

Part of the software and configuration can be called 'firewall'.
Just as other parts can be called 'router'.
And other parts can be called 'address translation'.

><snip>

>> >And I can't see that these NAT devices have a routing table either.

>>
>> Many of them run Linux with a normal Linux IP stack. You bet there's
>> a routing table, somewhere!

>If there's a routing table, what is in it? (I will speculate)

It will be a default route out the WAN interface, and one or more
connected routes towards internal networks. Depending on the feature
set of the configuration interface, it could also contain whatever
routes the local administrator desired.

>As far as I know, Port Forwarding has nothing to do with a routing
>table.

No dispute. But, after port forwarding or other forms of NAT have done
their packet manipulation, the resulting packet is usually routed as if
it were just arrived from the same interface as the original, unmangled
packet.

best regards
Patrick

Posted by Rick Jones on September 28, 2005, 6:32 pm

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> NAT is NAT.

I thought it went 'NAT is evil' :)

As I recall it:

*) devices that operate at the physical layer (eg electrical/optical)
are repeaters (a "hub" being a multi-port repeater :)

*) devices that operate at the data-link layer (eg MAC) are bridges
(a "switch" simply a multi-port bridge :)

*) decices that operate at the network layer (eg IP) are routers

*) devices that operate at the transport layer and higher are gateways

Now, when you create eierlegendwolmilchsau (*), layer-blurring devices
such as firewalls and NATs you basically toss a grenade into the works
and knuth only knows what to call it besides "bletch."

rick jones

(*) I've probably butchered the german spelling of egg-laying, wolly,
milk-pig

--
denial, anger, bargaining, depression, acceptance, rebirth...
where do you want to be today?
these opinions are mine, all mine; HP might not want them anyway... :)
feel free to post, OR email to rick.jones2 in hp.com but NOT BOTH...

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