Re: Running seperate eth0 & ppp0 networks
>On Wed, Dec 16, 1998 at 01:19:41PM -0800, Ian Eure wrote:
>> Ok. Let's say I have a linux box on an ethernet. It's ip is
>> 192.168.111.55. The router on the ethernet is 192.168.111.1. Let's say I
>> have a web server on 192.168.111.55, accessible from outside my local
>> network.
I'm assuming that you know that 192.168.*.* isn't supposed to be accessible
from outside of anything. Hopefully, you're just using these addresses for
the sake of illustration.
>> Let's further say that I set up ppp the way you described, with...
192.168.111.0/24 -> eth0
0.0.0.0 -> ppp0
okay... gotcha...
>> What happens when a request
>> for a web page comes in? Does the page get sent back from the ethernet
>> interface, with the ip 192.168.111.55, or through the ppp0 interface
>> with the dynamic address of the dialup?
I'm assuming you're asking about the request coming in from over the ppp
link, right?
>From what I have been able to figure out, Linux cares very little about what
interface a packet came in through or what the IP's of that interface are.
It seems to only care that it now has a packet that it has to do something
with. If the destination address matches an address of one of its own
interfaces, it seems to keep it. (In other words, if you have two interfaces
like eth0 and eth1... and you get a packet through eth0 that is addressed to
eth1's IP address, Linux will keep it anyway. What this means is that you
could *probably* do the same thing as IP aliasing by creating a bunch of
loopback interfaces... and then just do a little trickery with the arp
command.... but I digress...). If Linux does *not* have an interface with an
IP that matches the destination of the packet (and probably also depending
upon whether you have some kernel options installed... like IP forwarding or
something), Linux will look through the routing tables to see where to send
it.
When looking through the tables to see where to send it, it will go with the
closest match. Usually, "closest match" is pronounced "longest subnet mask".
:) The default route is the worst match.
In the case you mention, you've got 192.168.111.0/24 routing to your eth0,
and the default goes to your ppp0. If the linux box sees a packet come in on
ppp0 destined for, say, 192.168.111.55, the closest match is the one for
your eth0, so that's where it's going to go. The only time your Linux box
should bounce stuff right back through the ppp0 is if it gets a packet
addressed to something other than 192.168.111.*. In that case, packets would
be being sent to and fro across the ppp link for no reason... just eating
bandwidth... which could be a fun prank to pull on a friend if you could
figure out how to pull it off. :)
- Joe
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