Webmin was Re: Ubuntu network configuration

[Ben Scott]

On Mon, Oct 13, 2008 at 11:13 AM,  <bruce.labitt at autoliv.com> wrote:
> What is a gateway?

  Just a guess, but I suspect Webmin probably means "default gateway",
also called "default route".

  A route is a piece of information that tells your computer how to
reach a given destination.  The system will generally prefer more
specific routes over less specific ones.  A more specific route might
say, "To reach network 192.168.42.0/24, forward the packet to the
router at 10.12.34.56".  A default route matches all packets, and thus
functions as a "catch all" or "last resort".  A default route says,
"If you don't have a more specific route, forward the packet to router
foo".

  Most computers just have a default route, since they're at the end
of a network leaf and don't need to know anything more.
Routers/gateways can have many routes that describe their neighbors on
the network.  Large Internet core routes have tens of thousands of
routes.  The one on our firewall at work has six or so.

> If I have two ethernet interfaces, EXTIF and INTIF what should
> I use for either interface?  For EXTIF it should be the dhcp-served
> gateway, for INTIF what do I use?

  INTIF should not have a default gateway.

  Indeed, default gateways/routes are not actually associated with
individual interfaces.  They exist in the routing table, which is
independent of the interfaces.  Associating the default gateway with
individual interfaces is a bad UI design trend that has caused much
confusion.

  So your system just needs to have a default gateway to the next-hop
router that will get you to the Internet.  If Webmin associates that
with your EXTIF interface, put it there to make Webmin happy.

> For the edit bootup interface on eth1, I have setup a static ip.  What
> should I use for the broadcast box?

  You should be able ignore that.  Try leaving it blank.  (The
computer can normally figure out the broadcast address on its own.
You rarely need to specify it explicitly these days (only on fairly
unusual networks).)

> I am in the active routes page, what goes here?

  Unless you have an unusual routing plan at your place, you can leave
that alone.

> If I ping an address that should be on INTIF, ie it is on 192.168.3.xxx,
> how does my machine know to route to the INTIF?

  When an IP address is configured on a network interface, that
implicitly adds a route to the corresponding IP network.  To use your
example, that route would say, "To reach network 192.168.3.0/24,
forward the packet out local interface INTIF".

  You can review the current routing table by opening up a shell
prompt window and issuing the command:

	route -n

(The "-n" stands for "numeric", and tells the system not to waste time
trying to look up names for network and host addresses.  The names
will just confuse the current discussion.)

  I can't help with TFTP.

-- Ben