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A SCCM PXE-DHCP Network boot process


Architecture detection using wdsnbp

A SCCM PXE-Handshake with architecture detection uses an additional step. Before sending the final bootstrap file, the SCCM server sends out a bootstrap file which has an included architecture detection. The file is called „wdsnbp.com“. This is necessary, because the architecture information provided by the smbios is not very reliable. The wdsnbp bootstrap file sends back a DHCP request reply which contains an additional DHCP option 250. This high option is normally not used by other tools and contains information concerning the architecture of the client system.

 

Client pre-boot check and bootfile check

Right after the first contact of the client, die SCCM server checks for the necessary bootfiles. If the bootfiles are not present in the boot-directory (normally Remoteinstall\SMSboot), the server will not send out a DHCP offer. Since the bootfiles are extracted of the WinPE wim files during the startup of the WDS service, you should make sure, that only WinPE WIM files are stored within the SMSPXEIMAGES$ share and not full-os WIM files.

Secondarily the SCCM server checks whether the client exists within the SCCM database. If the client is unknown, the SCCM server will not send out a DHCP offer.

Note, that there has been added an unknown computer support in SCCM 2007 R2:

About Unknown Computer Support for Operating System Deployment

http://technet.microsoft.com/en-us/library/cc431374.aspx

The check of the client can be surveilled within the smspxe.log. Important are the stored procedures „lookupdevice“ and „getbootaction“ The first one checks, whether the client is known while the second one checks, what boot-file should be sent:

· Pxeboot.com (user has to press F12 to proceed – optional Advertisement)
· Pxeboot.n12 (TFTP download starts instantly – mandatory Advertisement)
· Abortpxe.com (stops the PXE Boot)

 

A SCCM PXE-DHCP handshake looks like that:

DHCP and PXE service run on different machines)

DHCP Discover
· Client broadcast asking for IP address and PXE-capable DHCP service
· DHCP discover package contains option 60
· DHCP discover uses port 67 UDP

SCCM site check
· SCCM server checks, whether client is known (lookupdevice)

DHCP Offer
· Reply of the DHCP server containing IP address offering on port 68 UDP
· Reply of the DHCP or PXE server containing option 60

DHCP Request
· Client broadcast, ready to acknowledge IP address and boot server (port 67 UDP)

DHCP Ack
· Server broadcast, acknowledge clients IP address and lease (port 68 UDP)

DHCP Request
· Client unicast request for options 66 (boot server) and option 67 (boot file) (port 4011 UDP)
· Unicast addresses server which offered option 60

DHCP Ack
· Server ack from PXE server including options 66 and 67 (send WDSNBP)
Start of the bootstrap file “wdsnbp” download using TFTP (UDP 69 and UDP highlevel ports)

DHCP Request
· Client unicast request for options 66 (boot server) and option 67 (boot file) (port 4011 UDP)
· Unicast addresses server which offered option 60
· Contains option 250 with architecture

SCCM site server check
· SCCM server checks for current boot action (getbootaction)

DHCP Ack
· Server ack from PXE server including options 66 and 67 (send pxeboot.n12 for example)

TFTP download of bootfile
TFTP download of WinPE

Screenshot of a sample boot:
image

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