Configuring Windows 2000 for Etherboot

Index

• Capabilities and Limitations
• Requirements
• Setup
• DNS Considerations
• TFTP
• NFS

Capabilities and Limitations

Windows 2000 Server provides a full featured DHCP server that is quite capable of supporting Etherboot-based thin clients. This document explains how.

The DHCP servers provided with Windows 2000 Server and Windows NT 4.0 Server are functionally equivalent except for some back-end storage differences related to Active Directory. There are substantial interface differences, however - the Windows 2000 DHCP manager interface is much improved and far more intuitive. This document therefore applies generally to Windows NT4 also, but the instructions provided here usually cannot be applied literally to the NT4 interface - you must search for the analogous actions under NT4.

One thing that Windows 2000 DHCP is not capable of is conditional selection of option values based on vendor class identifiers at run-time. It therefore cannot be used to perform a two-step PXE/Etherboot bootstrap. If you need to do this, your only available option at the time of this writing is the Internet Software Consortium version 3 DHCP server.

It should also be noted that the version of DHCP server provided with early distributions of NT 4.0 (circa Service Pack 3) is badly broken - you must apply a recent service pack before it will work with Etherboot.

Requirements

In addition to basic environment information (client IP address, subnet mask, default gateway), open source thin client solutions based on UNIX and Etherboot typically require that the DHCP server:

• Provide the client with a (TFTP) boot server address and a boot file name, and in the case of NFS-based thin clients such as LTSP, NFS root server and path information also.
• Provide the client with its hostname.
• Provide the client with DNS domain name and DNS server information.
• Provide the client with Etherboot-specific options 128 and 129 (possibly also 130), which can be used to pass arguments to the Linux kernel.
• Provide a fixed mapping between MAC (hardware) address and IP address. This is required because variations in hardware capability are typically categorised by hostname in configuration files (for administrative convenience). The client typically has no permanent storage and therefore no way to store a hostname, so dynamic DNS is not useful. A fixed mapping may also be required for accounting or security reasons (eg. billing, monitoring, or firewall access controls). A permanent IP address (either by fixed mapping or permanent lease) is also required if the client cannot adequately reconfigure itself to a new IP address on the fly - difficult to do on a thin client, which is critically network dependent and likely to have established connections even when not actively in use.

This is not the way DHCP is normally used in the Windows universe. Typically the client provides a hostname in its DHCP request and the server issues it with a temporary IP address, which the server then associates with that hostname for the duration of the lease. Windows DHCP can nevertheless be configured to do what we want.

Setup

1. Start the DHCP manager by selecting the DHCP option from the Admin Tools submenu of the Programs menu.
2. If you have not already done so, authorise your DHCP server. This is necessary to allow your DHCP server to update Active Directory (specifically for dynamic DNS). Select the server hostname and then Authorize from the Actions menu.
3. If you have not already done so, create a scope (New Scope.. in the Actions menu). You'll need to specify a start and end address, subnet mask, and any exclusions (excluding your server IP address is probably a good idea!). Otherwise, default values should be OK. You can defer configuring scope options and activating the scope for now, we'll deal with that later.
4. Select the server name and then select Set Predefined Options.. from the right-hand mouse button menu. Click Add.. to add two new entries to DHCP Standard Options set:
   1. Add an option called Etherboot ID with code 128; use data type Byte and check the array box.
   2. Add an option called Etherboot Params with code 129 and data type String.
   3. We now want to preset the contents of the new Etherboot ID option to a special value that cues Etherboot to make use of the 129 and 130 options, if present. To do this, select the Etherboot ID option and click on Edit Array... Select the Hexidecimal radio button and add the hex digits 0x0, 0x68, 0x74, 0x45, 0xe4 (you must include the 0x prefixes) in that order. Do not delete the 0x0 value that is already present. The final display should read "e4 45 74 68 00 00" when done.
5. Now we need to configure some basic options. You could do this at either the global or scope level. If you are using the DHCP level for non-Etherboot purposes, it's probably a better idea to do this at scope level. Click on Scope Options and then select Configure Options from the right-hand mouse button menu. Select the checkboxes corresponding to the desired options and set values appropriately. Minimally you will need the following:
   

   Option No	Option Name	Example Value
   6	DNS Servers	10.0.0.4
   15	DNS Domain Name	cloud.com.au
   17*	Root Path	10.0.0.4:/ltsroot**
   66	Boot Server Host Name	nimbus
   67	Bootfile Name	/tftpboot/bootimg**
   128	Etherboot ID	e4 45 74 68 00 00

   *Only needed if thin client is NFS-based.
   **Must use UNIX filename conventions (forward slashes). The drive prefix (c: or whatever) is implied by the setup of the TFTP/NFS server(s) and must be omitted.
6. For each Etherboot thin client you must now add a reservation. Within the scope, click Reservations and then select New Reservation.. from the right-hand mouse button menu. Important fields are IP address and MAC address; the Reservation name is just a label (you can use the hostname if you like), and Both is a safe default for Supported types. Enter the MAC address as a simple 12-digit hexidecimal string (lower case OK) - no colon or period separators. Click Add, and Close when finished adding reservations.
7. Expand the reservations group, then for each reservation select the reservation and then Configure Options.. from the right-hand mouse button menu. One or both of the following options will be needed:

   Option No	Option Name	Example Value
   12	Host Name	cumulonimbus
   129	Etherboot Params	NIC=ne IO=0x320

DNS Considerations

It isn't strictly necessary to have DNS entries for all of your Etherboot client reservations, but is good practice to do so. At the very least you should ensure that A and PTR records exist for your server name for the interface relevant to your Etherboot scope. Role-specific aliases (CNAMEs) for your server will also be useful.

If you are using Citrix Metaframe XP, you should also add a CNAME alias "ica" for the host providing your ICA XML browser service. The Citrix ICA Client uses this as a hardcoded default in the absence of user-provided browser server information.

TFTP

Windows 2000 provides a TFTP server that can be used to serve Etherboot images as part of the Remote Installation Services package. Although it is not intended to be used outside of a RIS context, it is possible to do so with some difficulty. Unfortunately it is not possible to install the TFTP server in isolation - you must install the entire RIS service, which will subsequently complain incessantly about being left unconfigured until you feed it the Windows XP CDs it wants. Sigh.

To use Windows 2000 TFTP server, you must tweak the registry to tell the server which directory to use as the TFTP root. The value needed is:

HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Tftpd\parameters\Directory

If you have not yet configured RIS, some of the rightmost keys in this path will be missing and you will have to create them. The string value is simply a Windows pathname of your choice, eg. C:\Tftpboot or whatever. Having set this, you need only use the Windows Control Panel to configure the TFTP service to start automatically.

Personally I suggest that you use a better-behaved third party TFTP server instead. I recommend TFTP Server 1.2 for NT by Bud Millwood (Millwood Data A.B., Sweden), a shareware product at an entry price of US$40. This is the best well-behaved no-frills NT service I have found so far. I'm not sure if it is still supported - there doesn't appear to be a home page or authoritative source anymore, so I've made a copy available here. Millwood TFTP Server runs as a service under both Windows NT4 and Windows 2000, and keeps working indefinitely without a license.

NFS

There are no good NFS server implementations for Windows platforms. NFS implements traditional UNIX file ownerships and permissions, which don't map well to FAT (which has no notion of permissions) or NTFS (which has more advanced ownership and permission capabilities) filesystems. Windows also has no notion of a symbolic link or a block/character special file. Consequently the best that one can hope to achieve is a clumsy emulation of a UNIX-like filesystem.

If your NFS-based thin client uses the devfs Linux kernel capability (as most now do), you can sidestep the requirement for block/character special files in your NFS root. However, the requirement for symbolic links is not so easy to dodge. Unfortunately I know of only one NFS server implementation for Windows that supports symbolic links, and it is not free: Microsoft Windows Services For UNIX (SFU) 2.0, at a cost of approximately US$150. I don't know of any Windows NFS servers that support block/character special files.

I therefore recommend that where possible you avoid NFS-based thin client solutions (such as LTSP) if you must serve them from a Windows platform. Use a self-booting solution (CDROM or flash disk) or a solution that only requires TFTP (such as DIET-PC or Netstation) instead. If you do want to use NFS, here's how to do it using Windows SFU 2.0:

1. Create a directory to serve as your NFS root directory, eg. c:\ltsroot, and a subdirectory named "etc" within this directory. You must use an NTFS filesystem.
2. Using a Windows TFTP or FTP client program (e.g. tftp.exe or ftp.exe), transfer the /etc/passwd and /etc/group files from the thin client root filesystem on your development box into the newly created etc directory. If you are using LTSP, you will find these files in /opt/ltsp/i386/etc.
3. Install Windows Services for UNIX 2.0. SFU provides a wide range of services; the only components of interest to us in this instance are Server for NFS, User Name Mapping and Server for NFS Authentication.
4. Select Services for UNIX Administration from the menu. Select Server for NFS and then the User Mapping tab. Enter "localhost" in the text box.
5. Select User Name Mapping, and then the Configuration tab. Click the PCNFS radio button, and enter the local pathnames of the passwd and group files your transferred earlier, e.g. c:\ltsroot\etc\passwd etc. Don't be alarmed by the term "PCNFS", it really just means flat files, as opposed to NIS.

6. Select the Maps tab, and click the Advanced maps radio button. We must now create advanced maps for both passwd and group. Click on Show User/Group Maps to edit, then click List Windows Users/Groups and List UNIX Users/Groups to expand the lists. You should select Set Primary for each mapping you create. Suggested mappings for LTSP are as follows:

   Type	Windows Name	UNIX Name
   passwd	YOURDOMAIN/Administrator	root
   passwd	YOURDOMAIN/Guest	nobody
   passwd	YOURDOMAIN/krbtgt	bin
   group	YOURDOMAIN/Domain Admins	root
   group	YOURDOMAIN/Guests	nobody

7. Click Apply (at top) and close the Services for UNIX Administration snap-in.
8. To activate NFS sharing, start Windows NT Explorer from the menu, right click on on the NFS root directory you created earlier, select Sharing... and then select the NFS Sharing tab in the window which appears. Use the Permissions button to set sharing to Read-Only for the default group ALL_MACHINES, and then add an explicit Root permission for the development box from which you will be transferring your thin client root filesystem.
9. Recursively change onwership of the NFS root directory to the Windows user which you mapped to "root" above. Under NT4, you can't do this - although NTFS will support it, NT4 only gives you the option to "Take Ownership", not to assign it to an arbitrary user, and the implied owner is the Administrators group, not the Administrator user. As a workaround, you will have to recursively assign Full Control privileges to the Everyone group for the time being.
10. Before copying files from your development platform, check and correct permissions on the files you are about to copy. SFU 2.0 NFS server doesn't interpret some permission combinations properly, because of the way it maps permissions onto NTFS. SFU approximately equates the implicit NTFS owner with the UNIX "user" permission set, explicit group permissions (not including the Everyone group) with the "group" permission set, and explicit Everyone permissions with the "other" permission set. In some cases SFU uses explicit denials to ensure no access, which has some odd side effects because the denials take precedence over the allows, e.g. you must ensure that all files are readable by group and other, because Administrator is a member of Everyone and Everyone would be denied access. Don't use --x in any column, it isn't understood. Directories must be owner-writable or else you won't be able to read them. In general, it's safest to stick to rwxr-xr-x, rw-r--r-- and r--r--r-- only.
11. On your development platform, mount the NFS root directory from your Windows server (use forward slashes instead of backslashes and omit the drive letter prefix), and transfer files using a method that preserves ownership and permissions (e.g. tar cf - . | (cd /mnt; tar xf -)).