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Multi-OS Installation: Configuring the Windows Image – Part 1

It takes a bit of work to prepare the contents of a windows CD image for use with the installation system. There is nothing terribly complicated about the process but it is a somewhat tedious, and after the 20th or so image that is being prepared, you may find yourself wishing for a script or two to simplify things. This is not to worry, I will provide those as well.

First a bit of background on the windows boot process. As a windows machine starts the boot process, the following things happen approximately in this order (1):

  1. The MBR is queried for the boot sector, and Ntldr is read.
  2. Ntldr is loaded into memory. Its reads the Boot.ini file to present the menu and loads the kernel.
    1. Ntldr will eventually load Ntoskernl.exe, Bootvid.dll Hal.dll and the boot-start device drivers.
  3. Ntdetect.com then runs and performs a basic hardware detection for Ntldr.
  4. Ntbootdd.sys is loaded for I/O.
  5. Ntoskrnl.exe is initialized and starts the system-start device drivers then kicks off smss.exe
  6. Hall.dll is loaded and is used to speak to the hardware in the machine.
  7. Smss starts the windows subsystem.
  8. Winlogon is started.
  9. SCM (service control manager) is started for windows services/drivers.

The boot process across the network is somewhat similar:

  1. The pxeclient on the network interface in computer drops a boot request onto the wire.
  2. The dhcp server catches this request and gives the pxeclient an IP address, the IP address of the next server in the process, and the name of the file to fetch. .
    1. pxelinux was used for the windows installs.
  3. The pxeclient then fetches pxlinux via tftp from the bootserver and presents the menu to the user and waits for entry.

Up until this point the pxe boot procedure is the same regardless of what OS is being loaded.

  1. The user selects the desired windows installation and pxelinux tftps back to the server and asks for the appropriate file, typically startrom.n12, the network bootloader for the MS boot process.
  2. startrom.n12 then calls for setupldr.exe
  3. setupldr.exe, actually named ntldr on the filesystem, is the pre-installation setup loader for windows.
  4. as before ntldr calls ntdetect.com for hardware detection,
  5. similarly, ntdetect.com handles hardware detection for the ntldr process as for the disk based boot
  6. finally the file winnt.sif is loaded which contains the instructions for the setup/RIS based installation.

Now, in order to make this all work a bit of filename manipulation magic needs to be undertaken. This is accomplished in the tftpd.map file which is loaded by the tftp server and additionally by making a few changes to both the names of the binaries and to the binaries themselves.

The naming convention chosen at the site was based on the fact that NTLDR is five characters. A method to encode OS names/types into the five characters was devised such that any OS/SP level could be encoded. The first letter was either W for 32 bit or X for 64 bit. The next two characters were the Microsoft OS version number, W50 for Windows 2000 32bit, X51 for Windows XP Pro 64 Bit. Finally the last two characters were used to denote patch level and any other desired information. For example, at the client site, X51AA was an unpatched XP Pro 64 Bit. While X51BA was SP2.

The path used on the filesystem to contain the filesystem images was /export/install/images. The tftp root is /export/install. The mapfile contained:

# windows to unix pathing
rgi \\ /

# anything asking for /IMAGES is broken and likely a Microsoft client.
rge ^/IMAGES/ /images/

# rehome 32 and 64 2k and 2k3 to /images
rgi ^/X5.* /images\0
rgi ^/W5.* /images\0
# Windows 2000 32 bit – W50AA

rgi ^/boot/pxelinux.0ntdw50aa.com /images/W50AA/boot/ntdw50aa.com
rgi ^boot/pxelinux.0ntdw50aa.com /images/W50AA/boot/ntdw50aa.com
rgi ^/boot/pxelinux.0w50aa.sif /images/W50AA/boot/w50aa.sif
rgi ^boot/pxelinux.0w50aa.sif /images/W50AA/boot/w50aa.sif
rgi ^w50aa /images/W50AA/boot/w50aa
rgi ^/images/W50AA/i386/W50AA/sys/(.*) /images/W50AA/sys/\1

# Windows XP Professional 64bit – X51AA
rgi ^x51aa /images/X51AA/boot/x51aa
rgi ^ntdx51aa.com /images/X51AA/boot/\0
rgi ^x51aa.sif /images/X51AA/boot/\0
rgi X51AA/i386 X51AA/amd64
rgi ^/images/X51AA/amd64/X51AA/sys/(.*) /images/X51AA/sys/\1
rgi ^/images/X51AA/i386/X51AA/sys/(.*) /images/X51AA/sys/\1

First off we need to rewrite dos paths to unix paths. Next we need to send anything asking for just /[WX]5 to /images. That allows the client to find the files its going to request next. The next section addresses some bugs in windows 2000 in which it tacks on the file its looking for onto the file it loaded previously, ie the pxelinux bootloader. Thankfully XP lacks this bug. You’ll notice that the filenames are not those mentioned in the process flow earlier, but are instead files based on the new naming convention. Any additional OSes based on 2000 need to simply copy the 2000 stanza and adjust the w50aa to the new 5 digit code. The same holds true for XP. These mappings allow the pxeclient to find the contents of the appropriate boot directory.

Configuring the directories in the W51AA tree will be covered in the next post.

>>> Karl

(1. This disk based bootup process was adapted from “Table 5-1, Microsoft Windows Internals, Fourth Edition, p252”. Any typos are mine. I’d recommend the original version of this from the text if possible. Its one of the few windows reference books on my shelf.)