[K12OSN] eth0 and eth1

Les Mikesell lesmikesell at gmail.com
Sun Sep 7 17:21:38 UTC 2008

Stephen Crampton wrote:
> Could someone please explain how an LTSP system decides how to use eth0 and
> eth1?

Physically, recent kernel's can detect and name them in any order.  The 
config files in /etc/sysconfig/network-scripts/ifcfg-eth? store the 
ethernet hardware address and the device name so it doesn't change after 

> For instance, if I pull up firefox and type in cnn.com, over which interface
> does my request go out?

An application can bind to an interface but most just default to 
following the route table.  That is, the source interface and IP address 
will be the one connected to the next hop router for the source address 
or the matching local subnet.

> Which interface is serving DHCP and TFTP requests?

DHCP is sort-of a special case because it does bind to interfaces and 
needs matching subnet addresses in its config file.

> /etc/ltsp/dhcpd.conf doesn't seem to provide the answer.  The DHCP script
> under /etc/init.d refers to a variable $INTERFACES, but I cannot figure out
> where that variable is initialized.
> I'm assuming that, in the default case, firefox sends requests over both
> interfaces, but dhcpd only uses eth0.  But how do I determine whether this
> is true or not for my system?

It's the other way around on k12ltsp: eth1 faces the 'public' side and 
can pick up its address from an existing dhcp server on your network 
while eth0 runs a private dhcp server for your clients - but I don't 
know if Edubuntu uses the same scheme.   One way to be sure you don't 
break the rest of the network by getting this wrong is to not connect 
either interface to the main network until you can boot a client to 
prove you have the dhcp-serving side connected to the right place.

   Les Mikesell

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