Debian 8 network installation failed

I'm trying to install Debian 8 on my iBook G4. As suggested in another thread I chose the netboot installation method.

I donwloaded and compiled dnsmasq (2.72) and configured both DHCP and TFTP with it. The dnsmasq.conf file looks like this:

# for easy trouble-shooting

    no-daemon
    keep-in-foreground

        # disable DNS
    port=0

    dhcp-range=192.168.0.201,192.168.0.210
        # 192.168.0.1 is my wifi router. I've turned off DHCP on it.
    dhcp-option=option:router,192.168.0.1
    dhcp-option=option:dns-server,192.168.0.1
        # 192.168.0.10 is my macbook where dnsmasq is running
    dhcp-boot=yaboot,macbook,192.168.0.10
    dhcp-leasefile=/Users/clarkw/utils/dnsmasq/dhcp-leases

    enable-tftp
    tftp-root=/Users/clarkw/utils/dnsmasq/tftp

I've downloaded the Debian 8 netboot images from here and put them under the tftp root dir:

    initrd.gz
    vmlinux
    yaboot

yaboot.conf

From the iBook's Open Firmware prompt ("0 >") I inputed

boot enet:0,yaboot

From the dnsmasq's output I can see that the iBook has requested an IP address and downloaded the yaboot file but did not download the vmlinux and initrd.gz files. Then the system panics. The final panic message is like this:

Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0)

(The screenshot can be found here.)

So what may be wrong with my configuration? Do I need to update the downloaded yaboot.conf file?

Thanks.

-clark