2. Remote boot from the AS/400

Most of the informations following are not limited to remotely booting the IPCS from the AS/400. You may remotely boot any PC having a boot-capable network interface card the same way.

2.1 The NFS/400 Support

V4R3M0 of the Operating System OS/400 supports exporting files using the NFS (Network File System). However as an NFS-Server the AS/400 is somewhat limited and rather slow, especially when communicating with clients running a linux kernel before 2.4.

The Linux kernel needs access to some special block devices, usually to be found in the /dev file system. While unix NFS servers are able to provide those special devices using NFS, NFS/400 does not support that function and for that reason the AS/400 can't be the NFS root server for a linux machine.

There are two workarounds available:

• Putting /dev (and usually the / filesystem as well) into an initial ramdisk. Other mountpoints such as /usr, /lib etc. may be mounted later.
• Using a Loopback root device. A loopback device is a block device (usually called /dev/loop0) associated to a file residing on another filesystem. From Linux kernel 2.4 on it is possible to use a file mounted from a remote NFS filesystem as a loopback device. It is also possible to use such a file as the root filesystem.

Using a loopback root device has several advantages:

• It allows a much better performance since the AS/400 will only have to access a single (large) object. Our Model /170 supports transfer rates up to about 5 MByte per second using this method.
• There are no compatibility issues involved since the Linux Server works on it's normal ext2 file system. Options like block or character special files, FIFOs and quotas will work as usual.
• The user profiles on the linux box and the AS/400 are completely separated. Normally the AS/400 would map files from unknown UIDs on the linux box to a single anonymous ID, so you'd need to register an AS/400 user profile for all the Linux daemon profiles listed in /etc/passwd.
• The access to the filesystem does not need to be performed on the QSECOFR level. Since the UID 0 from linux matches to UID QSECOFR on the AS/400, changing files with root perimissions means that the export needs to be performed with root permissions.

  Using a loopback root device allows accessing files with another user id.

A remote loopback root device requieres Linux kernel 2.4, but kernel 2.2.x seems to be no good choice for NFS communication with NFS/400 anyway, since it is slow and has compatibility problems.

2.2 Boot sequence

Remotely booting linux implies the following steps to be done:

• the 3Com network card retrieves IP addresses and boot file images by DHCP from the AS/400 DHCP server.
• The 3Com network card retrieves the file pxelinux.bin by using the TFTP (Trivial File Transfer Protocol) and starts execution of pxelinux.
• pxelinux retrieves further configuration information associated to the IP address from the directly pxelinux.cfg
• pxelinux retrieves the configured kernel image and initial RAM disk image file.
• pxelinux starts the kernel with a command line telling the kernel to use the initial ram disk (initrd), defining IP-Configuration and root device.
• The linux kernel mounts the initial RAM disk and starts execution of the /linuxrc script.
• /linuxrc mounts a directory from the OS/400 IFS containing it's root image file and associates that file with the loopback device /dev/loop0.
• The Linux kernel moves the initial ramdisk to the mountpoint /initrd and mounts /dev/loop0 as a read-only root filesystem.
• The regular linux boot procedure continues using /dev/loop0 the way usually known from a harddisk. This includes filesystem integrity checks, remounting read/write and so on.