Boot On Lan - Cannot VFS: Cannot open root device "<NULL>"

neilneil2000 · 06-29-2008, 01:24 PM

I am currently trying to get one of my PCs to boot on Lan. I am pretty close now. The PC gets into PXE and gets the config file via TFTP from the server. But I get this error message when it tries to boot:

VFS: Cannot open root device "<NULL>" or unknown-block(8,3)
Please append a correct "root=" boot option
Kernelpanic - not syncing: VFS: Unable to mount root fs on unknown-block(8,3)

And it just hangs.

I'm not sure what to do and google & co haven't come up with any answers.

If anyone has any ideas, I'll post whatever output is needed.

Thanks in advance.

chort · 06-29-2008, 01:37 PM

If you're trying to boot "diskless", you need to export the root file system from the PXE server over NFS, and the kernel has to be aware it's looking for the root file system on NFS instead of local disk (on most OSs this is a kernel compile option, not sure on Linux).

neilneil2000 · 06-29-2008, 02:23 PM

Hi, thanks for the quick reply. I followed this guide:

https://help.ubuntu.com/community/DisklessUbuntuHowto

I believe the step where the boot flag is changed in /nfsroot/etc/initramfs-tools/initramfs.conf addresses this issue but I could be wrong!

chort · 06-29-2008, 04:42 PM

Did you remember this step?

Quote:

create a new config file as /tftpboot/pxelinux.cfg/default
NOTE: There are more options available such as MAC or IP identification for multiple config files see syslinux/pxelinux documentation for help.

Code:

LABEL linux
KERNEL vmlinuz-2.6.15-23-686
APPEND root=/dev/nfs initrd=initrd.img-2.6.15-23-686 nfsroot=192.168.2.2:/nfsroot ip=dhcp rw

neilneil2000 · 06-30-2008, 01:02 PM

I did indeed, and I've checked it twice...just like Santa!

format C: · 06-30-2008, 01:53 PM

I've set up 20+ computers for network boot over the past few weeks
It seems you have your sever working right. The problem is most likely in the kernel. You need to compile a custom kernel for this. Most distro's kernels won't work.

Make sure your kernel is compiled with the following options (NOT MODULES!!)
-->NFS client
CONFIG_NETWORK_FILESYSTEMS=y
CONFIG_NFS_FS=y
CONFIG_NFS_V3=y

-->NFS root
CONFIG_ROOT_NFS=y

-->Kernel level network autoconfiguration
CONFIG_IP_PNP=y
CONFIG_IP_PNP_DHCP=y

and of course, your network card's driver must be included.

Hope that helps.

format C: · 06-30-2008, 01:56 PM

Oh yeah, you can see if your running kernel has these by looking in /proc/config.gz.