Dual NIC Cards

[Rick Stevens]

On Thu, 2006-04-13 at 23:18 -0600, karlp at ourldsfamily.com wrote:
> On Thu, April 13, 2006 10:41 pm, Bret Stern said:
> >
> > I have dual network cards in a new box
> > i'm putting together. Fedora 4
> 
> Me, too.

I have machines with up to 6.

> > Both cards are set to activate on boot, but
> > only one starts on boot. The other will start
> > manually.
> 
> Mine both start on boot.
> 
> >
> > One card has a static ip.
> > One card gets assigned an ip address (dhcp)
> 
> I assigned static IPs to both. That may be why one of yours doesn't start.
> Is the one that starts the static or DHCP assigned nic?
> 
> >
> > Can both cards have the same hostname?
> > (go ahead have fun with that question)
> 
> They have to have the same hostname. I don't see an option in the gui setup
> to give a different hostname for each card.

Ah, fertile ground for misunderstandings.  You must keep in mind that
there are differences between hostnames, nodenames and FQDNs (fully
qualified domain names).

A machine can only have one hostname (as displayed by "hostname" or
"uname -n"), and is the true hostname.  By default, it's "localhost" or
the FQDN "localhost.localdomain".

A machine _may_ have a nodename under NIS/NIS+ and is set and displayed
by the "domainname" command.  Unless you run NIS/NIS+, you needn't worry
about this, and if you do run it, 95 times out of 100 you'll set it to
be the same as the hostname.

Now, each IP address on the machine _may_ have a FQDN associated with
it, but it's not required.  Each FQDN can have a number of DNS aliases
associated with it (called CNAMEs in DNS parlance).

Confused yet?

> > I've heard you can only have a gateway address
> > on one nic card. Is this correct?
> 
> Yes, it's correct.

Uhm, this must be clarified.  There is only one _default_ gateway, and
that is the "route of last resort".  In other words, if you try to send
traffic to a node that is not on a network directly connected to one of
your NICs (as determined by the IP address/netmask combination) AND you
don't have a route forcing traffic for the remote node's network through
one of your NICs, THEN the traffic goes out the default gateway.  If you
do have a route for the network, it goes out the NIC that has that
route.

As an example, assume a machine with two NICs.  eth0 has an IP of
192.168.0.2.  eth1 has an IP of 10.0.0.2.  The default gateway is
192.168.0.1 (obviously on eth1).

Now, let's say you try to ping 10.24.1.1.  The traffic will go out eth0,
since the default gateway (actually, the default route) is on eth0.
However, you really want any traffic for 10.0.0.0/8 to go out on eth1.
You set up a static route:

	route add -net 10.0.0.0 netmask 255.0.0.0 dev eth1

Now, any traffic for the 10.0.0.0/8 network will go out eth1.  You 
can repeat that for as many networks (or hosts, if the netmask is /32)
as you wish on each NIC (well, up to some practical limit).

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- Rick Stevens, Senior Systems Engineer     rstevens at vitalstream.com -
- VitalStream, Inc.                       http://www.vitalstream.com -
-                                                                    -
-         If this is the first day of the rest of my life...         -
-                        I'm in BIG trouble!                         -
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