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I have an ARM system that has been pre-loaded with some variant of Linux. I don't know the distribution; I can only see the kernel number in dmesg.

In /etc/network/interfaces, I set eth0 to 

auto eth0
iface eth0 inet dhcp

After I save and reboot (or run ifdown eth0 followed by ifup eth0), I can see that the networking system is searching for a DHCP server and actually obtains a lease on a valid IP address, but when I run ifconfig, the interface has not been assigned the address that it pulled down from DHCP. It has been assigned a 192.168.. address.

Where can this possibly be coming from?

I noticed in dmesg that a variable "ip" is passed to the kernel at boot with the same address that is overriding my DHCP address. How can I disable this overriding behavior? I noticed a dynamic environment variable in u-boot called ip. I set it dhcp and saved it to nv storage, but the problem persisted. I tried to set the u-boot environment variable ipaddr to dhcp, but was informed that this wasn't a valid value for the variable.

How can I get the interface to be configured through DHCP?

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The ip parameter is describe here:

http://www.mjmwired.net/kernel/Documentation/filesystems/nfs/nfsroot.txt#81

The parameters that are passed to the kernel are stored in a variable called bootargs in u-boot.

Do a printenv to check what the value of bootargs is. If you see a parameter "ip=" being passed to the kernel, then you can delete that from bootargs.

There's another possibility through which kernel parameters can be passed.

That is CONFIG_CMDLINE option in kernel configuration.

If your kernel has been build with CONFIG_IKCONFIG support, then there should be a /proc/config.gz file.

Copy that to /tmp and gunzip it and check if the CONFIG_CMDLINE option is present and if it is, whether the ip parameter is being passed or not.

If it is, then you can't do anything about it, only a kernel recompilation can change that situation. I'm not sure if its possible to fetch ip using dhcp again in /etc/rc.local and have it overwrite the ip obtained from the kernel parameter.

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  • Thank you for your ideas. I did not mention in the original post, but bootargs is not a defined variable. I did your suggestion with config.gz, but there is nothing in the file that sets the variable ip or has the 192.168 ip address in it. I will try setting the client-ip to dhcp in the ip agrument as suggested in your link and will get back to you. Thanks.
    – user20926
    Commented May 19, 2011 at 12:22
  • It appears as though u-boot had disabled dhcp on the interface. Oddly, as I said before, a valid address was leased, but it was never applied to the interface. By setting dhcp to on in u-boot, the problem was solved. Thanks.
    – user20926
    Commented May 19, 2011 at 12:31

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