NIC Selection

[Timothy J. Miller]

When a system has 2 network cards that are on the same network,
how does FC1 select which NIC to use?  By that I mean, Mozilla starts up,
what causes it to use one card, say eth1 over eth0?  Anyone know?

Thanks

- Tim

[Martin Stone]

That's not taken care of by Mozilla, rather by your routing setup.  Check out 
the output of:

netstat -nr

That's your routing table.  By default, when an interface comes up on an IP, an 
interface route is added through that interface for that IP network - so if your 
eth0 came up on 10.0.1.2/24, you'd see a route in there for 10.0.1.0/24 through 
eth0 - then there is the default (0.0.0.0) route. That's usually the route that 
matches any network that is not directly connected.  So if you had a default 
route through say 10.0.1.1, your internet traffic would want to be routed thru 
10.0.1.1 - to get to 10.0.1.1, IP knows that it has to use eth0, so the traffic 
goes out eth0 destined for 10.0.1.1.

Or maybe you're just asking about where the NIC's get configured?  That info is 
in /etc/sysconfig/network-scripts - check out ifcfg-* in that directory.  Also, 
your default route is set by the GATEWAY entry in /etc/sysconfig/network.

Hope this helps!
Martin


Timothy J. Miller wrote:
>  When a system has 2 network cards that are on the same network,
>  how does FC1 select which NIC to use?  By that I mean, Mozilla starts up,
>  what causes it to use one card, say eth1 over eth0?  Anyone know?
>  
>  Thanks
>  
>  - Tim
>  
>

[Martin Stone]

Whoa, I just realized that I totally misread your question!  Sorry if i insulted 
you with newbie info.  I have no idea how the kernel would select an interface 
if both were on the same network.

Sorry, *duh*, I'll go drink some more coffee now...

Martin


Martin Stone wrote:
>  That's not taken care of by Mozilla, rather by your routing
> setup.  
>  Check out the output of:
>  
>  netstat -nr
>  
>  That's your routing table.  By default, when an interface comes up on an 
>  IP, an interface route is added through that interface for that IP 
>  network - so if your eth0 came up on 10.0.1.2/24, you'd see a route in 
>  there for 10.0.1.0/24 through eth0 - then there is the default (0.0.0.0) 
>  route. That's usually the route that matches any network that is not 
>  directly connected.  So if you had a default route through say 10.0.1.1, 
>  your internet traffic would want to be routed thru 10.0.1.1 - to get to 
>  10.0.1.1, IP knows that it has to use eth0, so the traffic goes out eth0 
>  destined for 10.0.1.1.
>  
>  Or maybe you're just asking about where the NIC's get configured?  That 
>  info is in /etc/sysconfig/network-scripts - check out ifcfg-* in that 
>  directory.  Also, your default route is set by the GATEWAY entry in 
>  /etc/sysconfig/network.
>  
>  Hope this helps!
>  Martin
>  
>  
>  Timothy J. Miller wrote:
>  
> > When a system has 2 network cards that are on the same network,
> > how does FC1 select which NIC to use?  By that I mean, Mozilla starts up,
> > what causes it to use one card, say eth1 over eth0?  Anyone know?
> >
> > Thanks
> >
> > - Tim
> >
> >
>  
>  
>

[Alexander Dalloz]

Am Fr, den 30.04.2004 schrieb Timothy J. Miller um 17:05:

>  When a system has 2 network cards that are on the same network,
>  how does FC1 select which NIC to use?  By that I mean, Mozilla starts up,
>  what causes it to use one card, say eth1 over eth0?  Anyone know?
>  
>  Thanks
>  
>  Tim 
Given that you did not have created any specific routes for each device
or created iptables source routing, then the kernel chooses the card
with the lowest IP.

Alexander


-- 
Alexander Dalloz | Enger, Germany | GPG key 1024D/ED695653 1999-07-13
Fedora GNU/Linux Core 1 (Yarrow) on Athlon CPU kernel 2.4.22-1.2188.nptl
Sirendipity 17:14:55 up 3 days, 16:03, load average: 0.42, 0.33, 0.38 
                   [ Γνωθι σ'αυτον - gnothi seauton ]
             my life is a planetarium - and you are the stars

[Timothy J. Miller]

Ok, then lets assume that eth0 has the lowest ip.  Does that mean that
eth1 would never be used?


>  Given that you did not have created any specific routes for each device
>  or created iptables source routing, then the kernel chooses the card
>  with the lowest IP.
>
>  Alexander
>

[Alexander Dalloz]

Am Fr, den 30.04.2004 schrieb Timothy J. Miller um 18:07:

>  Ok, then lets assume that eth0 has the lowest ip.  Does that mean
> that
>  eth1 would never be used? 
IMHO right.

If you are looking for something like solutions for multiple uplinks or
load balancing and such advanced routing stuff, then you might be higly
interested in

http://www.lartc.org/howto/

Be warned: it's not trivial :)

Alexander


-- 
Alexander Dalloz | Enger, Germany | GPG key 1024D/ED695653 1999-07-13
Fedora GNU/Linux Core 1 (Yarrow) on Athlon CPU kernel 2.4.22-1.2188.nptl
Sirendipity 18:17:27 up 3 days, 17:06, load average: 0.93, 0.83, 0.73 
                   [ Γνωθι σ'αυτον - gnothi seauton ]
             my life is a planetarium - and you are the stars

[Bryan Hepworth]

>  Ok, then lets assume that eth0 has the lowest ip.  Does that mean
> that
>  eth1 would never be used?
>
>
>  >
>  > Given that you did not have created any specific routes for each device
>  > or created iptables source routing, then the kernel chooses the card
>  > with the lowest IP. 
I always thought it dished out eth0 eth1 and so on with the PCI slot it was
plugged into. The higher up in the slots it was the lower the number. I have
to admit I've not checked too carefully before but it stuck in me nog that
that was how it worked.

Bry

[jludwig]

On Fri, 2004-04-30 at 12:19, bryan(a)redfedora.co.uk wrote:
>  > Ok, then lets assume that eth0 has the lowest ip.  Does that
> mean that
>  > eth1 would never be used?
>  >
>  >
>  > >
>  > > Given that you did not have created any specific routes for each device
>  > > or created iptables source routing, then the kernel chooses the card
>  > > with the lowest IP.
>  
>  I always thought it dished out eth0 eth1 and so on with the PCI slot it was
>  plugged into. The higher up in the slots it was the lower the number. I have
>  to admit I've not checked too carefully before but it stuck in me nog that
>  that was how it worked.
>  
>  Bry 
Not only Will it use the lowest IP, but if two cards are on the same
physical net it does the same act.  I know -- having this happen testing
hubs and switches. 
jludwig <wralphie(a)comcast.net&gt;

[Alan]

On Fri, 30 Apr 2004, Timothy J. Miller wrote:

>  When a system has 2 network cards that are on the same network,
>  how does FC1 select which NIC to use?  By that I mean, Mozilla starts up,
>  what causes it to use one card, say eth1 over eth0?  Anyone know? 
The routing table.

Question:  Why are you using two nics on the same network?  You can 
generate some very odd network problems doing that.  Routing loops can get 
created that will send packets around and around your network until they 
expire from ttl.

[Timothy J. Miller]

The machine use to be on two networks and was reconfigured.  Don't know
why it was reconfigured with two cards on the same network but when I
realized that it was configured this way I figured there maybe a reason.
Not sure what, but need to ask before I took the interface down.

Thanks

>  The routing table.
>
>  Question:  Why are you using two nics on the same network?  You can
>  generate some very odd network problems doing that.  Routing loops can get
>  created that will send packets around and around your network until they
>  expire from ttl.
>
>
>
>  --
>  fedora-list mailing list
>  fedora-list(a)redhat.com
>  To unsubscribe: http://www.redhat.com/mailman/listinfo/fedora-list
>

[Jeff Vian]

Timothy J. Miller wrote:

> The machine use to be on two networks and was reconfigured.  Don't
> know
> why it was reconfigured with two cards on the same network but when I
> realized that it was configured this way I figured there maybe a reason.
> Not sure what, but need to ask before I took the interface down.
>
> Thanks
>   
>  
In my experience and has been mentioned before, 2 nics on the same 
*subnet* are often a problem.  The system has a problem deciding which 
is the actual gate to that subnet and usually causes a lot of hangs and 
slowdown in network communication (if it works at all).

I do not ever recommend that config.

However, I have not seen multiple NICs on the same physical network 
cause a problem as long as the IPs do not cause a subnet conflict.

>   
>
> >The routing table.
> >
> >Question:  Why are you using two nics on the same network?  You can
> >generate some very odd network problems doing that.  Routing loops can get
> >created that will send packets around and around your network until they
> >expire from ttl.
> >
> >
> >
> >--
> >fedora-list mailing list
> >fedora-list(a)redhat.com
> >To unsubscribe: http://www.redhat.com/mailman/listinfo/fedora-list
> >
> >    
> >
>
>
>   
>

[Jay Daniels]

On Fri, Apr 30, 2004 at 11:04:36AM -0700, alan wrote:
>  On Fri, 30 Apr 2004, Timothy J. Miller wrote:
>  
>  > When a system has 2 network cards that are on the same network,
>  > how does FC1 select which NIC to use?  By that I mean, Mozilla starts up,
>  > what causes it to use one card, say eth1 over eth0?  Anyone know?
>  
>  The routing table.
>  
>  Question:  Why are you using two nics on the same network?  You can 
>  generate some very odd network problems doing that.  Routing loops can get 
>  created that will send packets around and around your network until they 
>  expire from ttl. 
Not the same network but...
That's what I thought, but after doing some research it seems you can
even have two default routes or multiple default routes.

http://lartc.org/howto/lartc.rpdb.multiple-links.html


jay