Chainloading Windows Deployment Services

Windows Deployment Services (WDS) is a set of services and APIs to facilitate Windows operating system installation by using PXE, DHCP and TFTP to bootstrap WinPE, the Windows Preinstallation Environment. You can think of it as providing similar functionality to iPXE with server-side scripting, where clients are served boot configuration and images based on various criteria, such as hardware architecture.

This appnote is about chainloading WDS from iPXE in a straight-forward manner.

Configure the WDS TFTP service

The WDS TFTP service relies on a registry key HKEY_LOCAL_MACHINE/SYSTEM/CurrentControlSet/Services/WDSServer/Providers/WDSTFTP/ReadFilter that controls which TFTP paths are mapped to the directory containing the various installation files defined by the RootFolder registry key in the same location.

RegEdit WDSTFTP Depending on the existing configuration, it is likely that additional patterns must be added to make iPXE and the WDS boot programs work correctly.

Make sure the following patterns are included in the ReadFilter list:

  • boot/*
  • boot\*
  • /boot/*
  • \boot\*
  • /boot\*

This allows all relavant combinations of forward and backward slashes, with or without a prefix slash. The last entry is particularly odd, but is actually used by the initial WDS network boot program once it has been chainloaded by iPXE, as seen in the example below.

After changing the ReadFilter list, reload the TFTP service to ensure it accepts the full list of patterns.

Chainload wdsnbp.com

Loading the initial WDS network boot program wdsnbp.com can be done either directly from the iPXE shell or via a menu entry. The only thing that needs to be modified is the DHCP next-server parameter, which will let wdsnbp.com know which server to communicate with.

Be sure to set DHCP options so that they are made available to the PXE NBP. For example, the next-server parameter can be set via netX/next-server.

Example iPXE commands to chainload wdsnbp.com:

  set netX/next-server ip.of.wds.server
  imgexec tftp://${netX/next-server}/boot/x86/wdsnbp.com

The WDS boot process

Once wdsnbp.com starts, it initiates a session with the WDS server that was specified in the DHCP next-server parameter. The session protocol uses a combination of DHCP requests and responses and TFTP to provide the client with the appropriate boot loader (such as bootmgr.exe or bootmgfw.efi) and boot configuration data (BCD).

The wdsnbp.com program performs client architecture detection and reports it back to the server via the WDS session. This session protocol uses DHCP as an RPC service endpoint, and the data passed back and forth (such as architecture information) is encoded in DHCP option 250. Together with option 252 (used by WDS to indicate BCD file name) and the DHCP file field (pointing the client to the next network boot program), the DHCP+TFTP negotiation completes the WDS session.

Example boot process

  1. Client: iPXE requests TFTP /boot/x86/wdsnbp.com from ip.of.wds.server.
  2. Client: wdsnbp.com uses a direct DHCP request to ip.of.wds.server, with no option 250 nor 252 defined.
  3. Server: DHCP response:
    • filename: boot\x86\wdsnbp.com
    • option 250: 0b0101100400000001ff
    • option 252: \Tmp\x86{<GUID>}.bcd
  4. Client: wdsnbp.com uses a direct DHCP request to ip.of.wds.server:
    • option 250: 0d0208000e010001020006ff
  5. Server: DHCP response:
    • filename: boot\x86\wdsnbp.com
    • option 250: 0b0101100400000001ff
    • option 252: \Tmp\x86x64{GUID}.bcd
  6. Client: wdsnbp.com requests TFTP /boot\x86\wdsnbp.com from ip.of.wds.server. Note the use of slashes.
  7. Client: wdsnbp.com uses a direct DHCP request to ip.of.wds.server:
    • option 250: 0c01010d0208000e010001020006ff
  8. Server: DHCP response:
    • filename: boot\x64\pxeboot.n12.
    • option 250: 0b0101100400000001ff
    • option 252: \Tmp\x86x64{GUID}.bcd
  9. Client: wdsnbp.com requests TFTP /boot\x64\pxeboot.n12.
  10. Client: pxeboot requests TFTP /boot\x64\bootmgr.exe.
  11. Client: pxeboot requests TFTP \Tmp\x86x64{GUID}.bcd.
  12. Client: bootmgr.exe executes and reads the BCD.

Troubleshooting

WDS in Wireshark Due to the poor error reporting in the WDS boot programs, the go-to troubleshooting tool should be a network packet analyzer like Wireshark. Packets can be dumped using other tools like tcpdump or tshark before being loaded into Wireshark for analysis.

Here's a Wireshark filter that shows only DHCP and useful TFTP packets by omitting TFTP opcode 3 (data packet) and 4 (acknowledgement):

  bootp || !(tftp.opcode==3 || tftp.opcode==4)

Things to pay particular attention to:

  • The destination IP for TFTP and DHCP requests.
  • The TFTP request filename.
  • TFTP error codes from the WDS server.
  • DHCP options in the client requests and server responses.
  • The WDS event log in Windows.

TFTP download errors

Ensure that the DHCP next-server parameter is specified correctly in the iPXE commands. If it is not configured correctly, it should be evident from the network capture that either the first DHCP or TFTP request from wdsnbp.com is directed towards the wrong IP.

Make sure the WDS TFTP server's read filter has mapped the relevant path names. Inspect the use of slashes in the DHCP packets to see how they are used, and check that the ReadFilter registry setting is configured appropriately.

TFTP loops

If either wdsnbp.com or pxeboot.n12 gets stuck in a request loop, it is likely that the initial TFTP download worked, but subsequent ones are failing. This can happen because iPXE uses forward slashes in the TFTP file path, while the WDS boot programs are prone to using a mix of forward and backward slashes.

Make sure the WDS TFTP server's read filter has mapped the relevant path names. Inspect the use of slashes in the DHCP packets to see how they are used, and check that the ReadFilter registry setting is configured appropriately.

Alternatives

The upside of chainloading WDS from iPXE is that it provides a fully working and isolated WDS setup, while iPXE remains in control as the initial boot program with WDS supplied as just another choice amongst the variety of alternate boot options. WDS remains in full control of its own configuration.

A drawback with WDS is that it uses TFTP (although with extensions that allow larger receive windows), which is not nearly as fast as HTTP, especially if there is more than a few ms round-trip time between the client and server. This results in somewhat long loading times for large WIM images, particularly on links with medium to high latency.

A good alternative to the PXE mechanism of WDS is wimboot, a boot loader that takes over the roles of wdsnbp.com and pxeboot.n12. It lets you fetch all the relevant files over HTTP and hand over execution to bootmgr.exe. There is a drawback, however: Control over the dynamic BCD boot menu is taken away from WDS, as a static BCD file is served instead.

References

 
appnote/chainload_wds.txt · Last modified: 2015/11/05 14:58 by svenx
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