An elementary question on LANs

Timothy Murphy gayleard at eircom.net
Sat Jul 16 18:04:41 UTC 2011


g wrote:

> so, to op, Timothy Murphy,
> 
> do you wish to know;
> 
>  1- what determines mapping of nic [network interface card/chipset] to an
>   ip address?

I assume that this is set in ifcfg-ethX .

>  2- what determines mapping of nic [network interface card/chipset] to an
>   ethx position?

That is an interesting question.
I see that in CentOS-6 there is a file 
/etc/udev/rules.d/70-persistent-net.rules
which matches interface (ethX) to MAC address.
But this file does not seem to be present in CentOS-5.6 .

I assume that in CentOS-5.6 the match is set in ifcfg-ethX ;
but I'm not sure which of the two is dominant in CentOS-6.

>  3- what determines order of mapping of nic [network interface
>  card/chipset]
>   to an ip address?

I'm not sure what you mean by the _order_ of the mapping?
My guess is that in the absence of any action by the user
which NIC is assigned to eth0 and which to eth1 (etc)
is determined by the position of the NICs in the PCI device table?
But I'd be interested to know the answer to this.

>  4- what determines order of mapping of nic [network interface
>  card/chipset]
>   to an ethx position?

I don't see how this differs from the previous question.

>  5- none of above?

I think my question was slightly different to any of the above.
I was asking how the connection between an interface (ethX)
and a subnet (192.168.2.0) was, or could be, established,
since that seems to be what dhcpd wants before it starts.

My problem basically is the error message
------------------------------------
alfred dhcpd: No subnet declaration for eth1 (no IPv4 addresses).
alfred dhcpd: ** Ignoring requests on eth1.  If this is not what
alfred dhcpd:    you want, please write a subnet declaration
alfred dhcpd:    in your dhcpd.conf file for the network segment
alfred dhcpd:    to which interface eth1 is attached. **
alfred dhcpd:
alfred dhcpd:
alfred dhcpd: Not configured to listen on any interfaces!
------------------------------------

It seems that dhcpd requires that a connection should have been established
between eth1 and the subnet 192.168.2.0 _before_ dhcpd starts.

I have a subnet declaration for the subnet 192.168.2.0
in /etc/dhcpd.conf :
------------------------------------
subnet 192.168.2.0 netmask 255.255.255.0 {

        authoritative;

# --- default gateway
        option routers                  192.168.2.1;
        option subnet-mask              255.255.255.0;

#       option nis-domain               "gayleard.com";
        option domain-name              "gayleard.com";
        option domain-name-servers      208.67.222.222,208.67.220.220;

        range 192.168.2.100 192.168.2.250;
        range dynamic-bootp 192.168.2.128 192.168.2.254;

        host alfred {
                hardware 00:1B:21:AB:C9:4C;
                fixed-address 192.168.2.2;
        }
...
}
------------------------------------

Also ifconfig contains

eth1      Link encap:Ethernet  HWaddr 00:1B:21:AB:C9:4C  
          inet addr:192.168.2.2  Bcast:192.168.2.255  Mask:255.255.255.0

But it seems this is not sufficient to establish 
"a subnet declaration for eth1".

I tried adding an empty declaration for 192.168.1.0 ,
and also (as was suggested) sandwiching the subset declartion(s) in
shared-network opt {
...
}
but neither seemed to make any difference.

If anyone can tell me what I am missing I should be very grateful.
  
http://wiki.alteeve.com/index.php/Changing_the_ethX_to_Ethernet_Device_Mapping_in_EL6_and_Fedora_12%2B
> 
> is a  good read.

I did look at this, but it did not seem relevant to my problem.
 

-- 
Timothy Murphy  
e-mail: gayleard /at/ eircom.net
tel: +353-86-2336090, +353-1-2842366
s-mail: School of Mathematics, Trinity College, Dublin 2, Ireland



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