[K12OSN] eth0 and eth1

Moon moon at smbis.com
Mon Sep 8 05:03:41 UTC 2008

Stephen, from all the response I've seen tonight, some of which give very
detailed explanations of how the kernel assigns logical names to physical
devices and so on, I thought I would give you my input based on my personal
experiences with both Ubuntu/Edubuntu and K12LTSP.

>From what I have been able to understand, both use the same default
assignments; eth0 is for the thin client network, and eth1 is for the
internet side. Edubuntu default install assigns eth0 to static ip address, however, eth1 I setup to get an ip address via dhcp, see
/etc/network/interfaces file. As to which interface gets assigned eth0 or
eth1, I think several of the responses addressed that fairly well. Although
I personally found that integrated Ethernet interfaces are generally
assigned to eth0, and the first PCI installed Ethernet interface is assigned
to eth1. However, if you have nic failure in Ubuntu/Edubuntu, and replace
that interface with another, then the network manager will assign a new
logical name like eth2 or eth3. Then you can have a lot of fun reconfiguring
the interface name to the one that originally failed.

One of the quirks with the default Edubuntu setup is that the
/etc/default/dhcp3-server doesn't have eth0 nor eth1 assigned, but uses the
default INTERFACES="", which I assume means either none or all, since no
default setting is configured even though the LTSP default config sets up
dhcp on eth0, and set eth1 to get ip addressing via dhcp.

Another Edubuntu default configuration quirk is in the /etc/ltsp/dhcpd.conf
file where the default settings for eth0 (remember that Edubuntu sets this
as the default thin client interface) are all set to instead of
the statically assigned default ip address automatically
assigned when you install it, see /etc/network/interfaces file. 

It's these kinds of quirks that make me not feel too comfortable with
Edubuntu.  If there was one clear document that explained how it is supposed
to work and gave examples for the various config settings, then I'm sure it
would be a nice platform. But, there are a lot of documents out there that
are not complete, have conflicting information, or missing bits and pieces,
and yes I have googled and been though every piece of information I could
find in forums and online documents.  If the developers would get involved
with a couple actual deployments and walked through the process several
times, I believe they would find areas that need tweaking and others that
need to be put back into "let's rethink this one" category. 

I wish you luck, as I had little to none with Edubuntu.




From: Stephen Crampton [mailto:SteveSings at gmail.com] 
Sent: Sunday, September 07, 2008 10:43 AM
To: k12osn at redhat.com
Subject: [K12OSN] eth0 and eth1


Could someone please explain how an LTSP system decides how to use eth0 and

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

Which interface is serving DHCP and TFTP requests?

/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?

I'm using Edubuntu 8.04 again, because none of the K12LTSP install discs
recognize my SATA hard drives (I tried 5SEL, 6SEL, and a beta live version).

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08:07 PM

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