Grant Taylor wrote:
1. Host 'A' realizes Host 'B' is not on its network
This is what the net mask and subnets are all about in the IP stack
and has nothing to do with a router. (Other than the fact the router
has its own IP stack and (sub)net mask(s) too.)
*dumbly nodding* Ok.
2. Host 'A' contacts Router 'C' and asks it to get the information
out and bring back the response.
(As long as you are not talking about any form of proxying...)
This is what routes are all about, with the /Default Gateway/ route
being special in such as it is the one used when no other routes match.
*dumbly nodding - slight eye glazing* Um....ok.
3. Router 'C', via whatever magical method (DNS/hosts/etc.) figures
out the router responsible for Host 'B's presence on the Internet.
Eh. Now you are sounding more and more like a proxy. Routers only
pass IP packets based on routes. Any DNS operation is the
responsibility of the application that generated the packet that is
now being routed by the router.
*significant eye glazing - jaw slack* Um...er...maybe...
Rephrase - "Router C checks it's list of routes, figures out it's
clueless with regard to the route for Router D, and passes the buck to
Router C's default gateway"
4. Router 'C' contacts Router 'D', sends along the information, and
tells Router 'D' to send any responses to ROUTER C, not Host A
It's not so much that routers ""contact each other as it is that each
router hands off the IP packet to the next router for it to route to
the next and the next ... and you get the idea. There is not really a
request that something be done.
*Excitedly* - That's it! That's the part we need to talk about! Router
"hands off the IP packet to the next router to the next to the next" -
Router C has a route table -
192.168.0.0/24 dev eth0
2.3.4.5 dev eth1
default gateway 2.3.4.4
Router 2.3.4.4 (premise equipment from mysterious ISP)
mysterious routing table
Router 5.4.3.1 (premise equipment from other ISP)
more mysterious routing table
Router D has a route table -
10.0.0.0/8 dev eth0
5.4.3.2 dev eth1
default gateway 5.4.3.1
Does the above communication involve NAT? No "hosts" or private
networks involved - all public IP's between them (unless of course the
packets traverse private IP ranges within the ISPs' networks before
coming back out.
5. D, goes to B, comes back to D, and back to C
You could be talking about IP packets flowing through networks or
proxy requests flowing from clients to the proxy and ultimately to the
destination server and back in reverse.
*Sheepishly* Assume for sake of argument I'm expressing myself poorly -
no proxies involved in this discussion. That would make it too simple.
6. Router C, on receiving a response from D, remembers that Host 'A'
was waiting for this information and sends it on.
This is is probably what you are thinking NAT does, which in some /
most ways is correct. However, the same can be said about a proxy.
*Assertively* No proxy.
In essence, I believe I'm correct in this summary - however the tool
used by Router C for "remembering" that Host A asked for the
information, and that responses from Router D should come back to
Router C, is NAT?
Eh, not really.
I'm going to go out on a limb here and think that you are thinking of
a network like this:
+---+
| B |
+-+-+
:
:
+-+-+ +---+
| C +---+---+ A |
+---+ | +---+
|
| +---+
+---+ D |
+---+
I believe this is the sequence of events you are trying to make happen:
- Client A is trying to contact the server B by way of router C.
- Router C intercepts the request and hands it off to system D.
- System D then initiates the request to B by way of router C (and
many other intermediary routers).
(C does not intercept the request because it is from system D.)
- Server B replies back to D by way of (many other intermediary
routers and) router C.
- System D then replies back to router C.
- Router C then replies back to client A.
Is this close to what you are wanting to happen? (Let me know before
I explain how to make this happen.)
Um...no. Too complicated.
A==>C<==Internet==>D<===B
Two offices on opposite sides of the world linked via Internet.
So does this mean that ANY connection of a private address space to
the Internet MUST be performed via NAT?
Yes. In the scenario above (presuming that my picture above matches
what you have) Router C does NAT to convert the internal IP addresses
used on the internal LAN to that of the internet side of Router C so
that the packets will cross the internet. Refer to the "Simple
Scenario" in my previous reply about NATing.
So the world's most expensive super-duper whatchamacallit (fill in the
blank here with router, firewall, bridge, modem, magic cauldron), placed
between giant corporate's network (using private address space) and the
Internet - will perform NAT? Somewhere somehow NAT (in particular,
source NAT for outbound access from the private and destination NAT to
provide services to Internet) must be performed?
--
Daniel
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