Diskless Linux Mini Howto: Fundamental ideas

5. Fundamental ideas

The fundamental idea is as follows: the PC will get its IP address from the boot server via the bootp protocol, using 0.0.0.0 as the initial IP address and its kernel via the tftp protocol.

Booting across segments (via router) not a simple question, so either put both the server and the diskless boxes on the same lan segment or configure an UDP helper address in your router to the address of the server. Refer to your router product manual for further info.

For this follow the steps below.

Get the nfsboot package (the package is available from your favourite linux mirror site in the /pub/Linux/system/Linux-boot directory). It contains a booteprom image for the wd8013 card which can be directly burned in.

There are alternative ways to prepare the PC:

if your machine is not quite diskless, then you may use the little DOS program, or
the binary floppy image contained in the same package.

If you choose the latter option you must write the image onto a floppy by the dd command.

These images contain a bootp and tftp client. You need to prepare a linux kernel too, which contains the nfs-root option.

If you are using the latest stable kernel, linux-1.2.13, then you need to patch the kernel with the patchfile included in the nfsboot package
Refer to patch(1)
If you try to use the latest, but unstable kernel from the linux-1.3.x series, then you have to configure in the nfs-root option.

You may or may not configure block device (floppy or hard disk) support, but you must configure tcp/ip support, wd ethernet card support, nfs filesystem support. Then recompile the kernel as usual.

5.2 Setting up a bootpd on the server

It can be found in package bootpd-2.4.tar.gz (which can be found on your favourite linux mirror site in the /pub/Linux/system/Network/boot.net directory). Get the package, compile and install it. If your other Unix box happens to be a Slackware Linux then you may skip this step for the standard distributions contain a bootpd. The daemon can be run either directly by issuing command



        bootpd -s

or by using inetd. In this case you need to edit:

/etc/inetd.conf to remove the hashmark from the start of these lines:



# tftp   dgram   udp     wait    root    /usr/sbin/in.tftpd     tftpd /export
# bootps dgram   udp     wait    root    /usr/sbin/in.bootpd    bootpd

insert or uncomment the following two lines in /etc/services:



bootps          67/tcp          # BOOTP server
tftp            69/udp          # TFTP server

restart inetd by



        kill -HUP <process id of inetd>.

5.3 Configure the bootpd on the server.

First of all, bootpd have a config file called bootptab which usually resides in /etc. You must modify it by inserting the IP addresses of your gateway, dns server, and the ethernet address(es) of your diskless machine(s). An example /etc/bootptab:




global.prof:\
        :sm=255.255.255.0:\
        :ds=192.168.1.5:\
        :gw=192.168.1.19:\
        :ht=ethernet:\
        :bf=linux:
machine1:hd=/export/root/machine1:tc=global.prof:ha=0000c0863d7a:ip=192.168.1.140:
machine2:hd=/export/root/machine2:tc=global.prof:ha=0800110244e1:ip=192.168.1.141:
machine3:hd=/export/root/machine3:tc=global.prof:ha=0800110244de:ip=192.168.1.142:

global.prof is a general template for host entries, where

sm field contains the subnet mask
ds field contains the address of the Domain Name Server
gw field contains the default gateway address
ht field contains the lan media hardware type
bf field contains the name of the boot file

After this, every machine must have a line:

the first field contains the host name,
hd field contains the directory of the bootfile,
the global template can be included with the tc field,
ha field contains the hardvare address of the ethernet card,
ip field contains the assigned ip address.

5.4 Understanding tftp

TFTP (Trivial File Transfer Protocol) is a file transfer protocol, such as ftp, but it's much simpler to help coding it in EPROMs. TFTP can be used in two ways:

simple tftp: means that the client can acces to your whole file system. It's simpler but it's a big security hole (anyone can get your password file via tftp).
secure tftp: the tftp server uses a chroot.2 system call to change it's own root directory. Anything outside the new root directory will be completelly inaccessible. Because of the chroot dir becomes the new root dir, the hd filed in the bootptab must reflect the new situation. For example: when using insecure tftp, the hd field contains the full path to the boot directory: /export/root/machine1. When using secure tftp whith /export as root dir, then /export becomes / and the hd field must be /root/machine1.

Almost every Unix implementation contains tfpt server, probably you don't need to install your own one.

5.5 Setting up a minimal Linux configuration on the remote server.

This may contain packages a, ap, n, and x of the Slackware distribution. To install more is OK; however the above packages suffice for the purposes of a diskless X terminal. For the installation you need a working Linux system. Find some disk space on the remote machine and export it read-write. Mount the exported directory onto somewhere (e.g. /mnt) on the file system of the Linux box. Start Linux setup and change the root option in the setup from / to /mnt. Then setup the above packages as usual. If you want to run no more than one diskless Linux then no changes are needed. On the other hand, if you plan to use more than one diskless machine then the above setup will not work because some files and directories must be private to the machines. The problem can be bypassed by moving the /usr (it contains no private data) and then create a separate subdir for each diskless machine. For example, if /export/linux/machine1 were mounted to /mnt then the directory structure after the initial setup will look like



/export/linux/machine1/bin
/export/linux/machine1/sbin
/export/linux/machine1/lib
/export/linux/machine1/etc
/export/linux/machine1/var
/export/linux/machine1/usr

After the changes you will have



/export/linux/machine1/bin
/export/linux/machine1/sbin
/export/linux/machine1/lib
/export/linux/machine1/etc
/export/linux/machine1/var
/export/linux/usr

Now create the subdirectories for the other machines. Assume for now that your diskless machines are called machine1, machine2, machine3, etc.; then you may use the following bash script to setup the other directories



        
        cd /export/linux
        for x in machine2 machine3 ; do
                mkdir $x; cd $x
                (cd ../machine1; tar cf - *) | tar xvf -
        done

Then do the following export:

/export/linux/usr readonly for everyone.
/export/liunx/machine1 only to machine1 with rw,root rights.
/export/liunx/machine2 only to machine2 with rw,root rights.
/export/liunx/machine3 only to machine3 with rw,root rights.

as follows

the format of this example follows the SunOs 4.1.3 exports file syntax



        
# This file is /etc/export
# for remote linux X terminals by Buci
# this line is only once
/export/root/usr             -access=linuxnet
# these lines once for every host
/export/root/machine1       rw=machine1,root=machine1
/export/root/machine2       rw=machine2,root=machine2
/export/root/machine3       rw=machine3,root=machine3

Don't forget to run exportfs -a.

5.6 Configuring the tftp server

Now it is time to configure the tftp server. If you do not need secure tftp then everything is quite simple for your clients can be booted from the /export directory.

If a secure tftp is used then you can either make a full /export/linux directory structure under /tftpboot (with a single real kernel and symbolic links for the other machines), or let the /export directory be the boot directory of the secure tftpd. Or, if you have a separate tftpboot directory then, similarly, you need only the original directory structure with a single kernel and symbolic links for the others. You can achieve this setup by typing the following:



      mkdir -p /tftpboot/export/linux/machine1
      cd /tftpboot/export/linux/machine1
      cp /export/linux/machine1/<name of the kernel> .

Then type the following:



      mkdir -p /tftpboot/export/linux/machine2
      cd ../machine2 
      ln -s ../machine2/<name of the kernel>

5.7 Final work

Finally, you must insert



   /sbin/mount nfs_server:/export/linux/usr /usr

as the first line of



   /export/linux/<machinex>/etc/rc.d/rc.S

where <machinex> stands for machine1, machine2, etc.