In this section, I will discuss the non-standard options sent back to the Mac NC client after its request.

It makes the most sense to describe the different options being sent back to the client as if they were read from a dhcpd.conf file. Here is a sample file with an entry for one network booting Mac:

default-lease-time 600;
max-lease-time 7200;
option subnet-mask 255.255.0.0;
option broadcast-address 172.17.255.255;
option routers 172.17.50.15;
option domain-name "sonoma.edu";
option domain-name-servers 130.157.2.5, 130.157.2.3, 130.157.2.2;
allow bootp;

subnet 172.17.0.0 netmask 255.255.0.0

 host mac-nc01 {

     hardware ethernet 0:50:e4:59:2e:43;
     fixed-address 172.17.0.10;
     filename "/private/tftpboot/Mac OS ROM";
     server-name "172.17.0.9";
     next-server 172.17.0.9;
     #Likely Server Version Information: found to be 0:0:0:0 from sniffer.
     option option-230 0:0:0:0;
     #Username used to authenticate against the AFP/IP Server
     option option-232 "Mac NC #1";
     #Machine name to call the client, also would be used in 238 for dir
     # with specialized settings for this client's booting hard disk. 
     option option-237 "Mac NC #1";
     #Password for use with the Username above when authenticating against
     # AFP/IP server for 3 images in 234, 235, and 238
     option option-233 "PassWooD";
     #Used to grab initial BOOT HD Image from server over AFP/IP for mounting
     option option-234 ac:11:0:9:2:24:a:4D:61:63:20:48:44:5F:4E:42:53:0:0:0:0:
2:1b:53:68:61:72:65:64:49:6D:61:67:65:73:0:4E:65:74:42:6F:6F:74:20:48:44:2E:69:6D:
67;
     #Used to grab Applications HD Image from server over AFP/IP for mounting
     option option-235 ac:11:0:9:2:24:a:4D:61:63:20:48:44:5F:4E:42:53:0:0:0:0:
2:20:53:68:61:72:65:64:49:6D:61:67:65:73:0:41:70:70:6C:69:63:61:74:69:6F:6E:73:
20:48:44:2E:69:6D:67;
     #Used to grab Special Client HD Image from server over AFP/IP for
     # mounting
     option option-238 ac:11:0:9:2:24:a:4D:61:63:20:48:44:5F:4E:42:53:0:0:0:0:
2:27:43:6C:69:65:6E:74:49:6D:61:67:65:73:0:4D:61:63:20:4E:43:20:23:31:0:4E:65:74:
42:6F:6F:74:20:48:44:2E:69:6D:67:5F:73;

   }

1. default-lease-time 600; specifies that a client shall have its IP address for 600 seconds (10 minutes) by default before assuming it has timed out. See the man pages for dhcpd.conf for more information.
2. max-lease-time 7200; specifies that a client shall have its IP address for a maximum of 7200 seconds (120 minutes/2 hours) before assuming it has timed out. See the man pages for dhcpd.conf for more information.
3. option subnet-mask 255.255.0.0; specifies the subnet mask that the client will be using when it configures its own TCP/IP information. In this case, this specifies that the network will be a class B type of network (ignoring CIDR for now) where I control the last 2 octets of any IP address for any machine on my subnet. Any IP address reference on the network that has any part of the first two octets that is different than the IP address I am assigned will cause me to contact the router on my network to continue delivery.
4. option broadcast-address 172.17.255.255; specifies the broadcast address for my subnet. In this case I will be using the 172.17.0.0 reserved network as specified by rfc1918.txt. Since my subnet mask is 255.255.0.0, and this states I am controlling the last two octets, the first two octets are static, and remain. The max IP value for my subnet is equal to my broadcast address. (I am using 172.17.0.0 to 172.17.255.255 where the very first, and very last addresses are reserved for special broadcasts for all network devices on the same subnet.)
5. option routers 172.17.50.15; specifies what machine (by IP address) will deal with IP packets that have destinations that are not on my subnet. In this case, and packet with a destination that starts as "172.17." will be on my subnet. If it starts with anything different, then the router will be needed to deal with routing the packet to the proper destination.
6. option domain-name "sonoma.edu"; specifies the domain name to be appended for looking up host names. This is used to complete the name for hosts that are being looked up. An example for its use: while in Netscape, if I type "www" as my location, my machine will first look for www, then www.sonoma.edu, then Netscape would do its own thing and convert www into http://www.www.com/. If any one of those lookups succeed, then the remaining checks are short circuited, and not performed.
7. option domain-name-servers 130.157.2.5, 130.157.2.3, 130.157.2.2; tells the client what DNS are willing to offer it Domain Name to IP address translation. These servers convert names like, "www.yahoo.com" into IP addresses used by TCP/IP like, "204.71.200.75" for you to browse their web site.
8. allow bootp; is normally set by default. This just means that the server will offer responses to clients that send bootp requests.
9. host mac-nc01 is a unique identifier used by dhcpd to maintain unique settings for each client. It may be the DNS value for the IP address being given, or something you make up on your own, just so long as it is unique, and does not use characters that are unacceptable. (For example unprintable characters, or tabs and spaces...)
10. hardware ethernet 0:50:e4:59:2e:43; is the hardware address that the DHCP server is looking for so that it can offer it a DHCP response. These are supposed to be unique, and burned into the NICs at the factories. To find one on a Mac OS 7.6.1 System or later, go to the Control Panels folder and choose the TCP/IP control panel. After it opens, do a Get Info which may be in the File pull down menu. There should be 6 hexadecimal values for an Ethernet hardware address, and for dhcpd.conf, they should be colon delimited.
11. fixed-address 172.17.0.10; tells the client that it will be using 172.17.0.10 as its IP address on this subnet.
12. filename "/private/tftpboot/Mac OS ROM"; tells the client it will need to connect up to a TFTP (Trivial File Transfer Protocol) server to get a file called "Mac OS ROM" located in a directory and subdirectory of "/private/tftpboot/". This file contains the Boot ROM information necessary for the mac NC to mount up images over AFPFS/IP.
13. server-name "172.17.0.9"; tells the client the IP address of the server from which it is booting.
14. next-server 172.17.0.9; tells the client what the TFTP server's IP address is so that it can act upon the "filename" specified above.
15. option option-230 0:0:0:0; The purpose of this is unknown to me as of present. I have a theory that it may be a server version number, and may be returned to the client in the options described below immediately after the volume name.
16. option option-232 "Mac NC #1"; specifies the username to use for authentication with the AFPFS/IP file server to get its images for mounting.
17. option option-237 "Mac NC #1"; specifies the directory name from which to grab its own writable image file for keeping track of things like preferences that may change.
18. option option-233 "PassWooD"; specifies the password to use (in plain text) when connecting to the AFPFS/IP file server and using the above username for authentication. (Mac OS X appears to use a One Time Password.)
19. option option-234 contains many things to tell the client where to get an initial boot network disk image for booting.
    1. ac:11:0:9 IP Address of the AFPFS/IP file server that holds the images that this option needs. In this case, the decimal values would offer and IP address of "172.17.0.9"
    2. 2:24 Harry Johnston found the purpose and meaning of this. (I incorrectly thought it might have been an escape sequence or field separator, but his suggestion seem much more logical. :-) It is the Hexadecimal value of the port for AppleShare over IP. (0x0224: 2*512 + 2*16 + 4 = 548d)
    3. a appears to be the total number of characters to be read by the client that follow this value to use as the volume name from which to find images from the AFPFS/IP file server. In this case, a value of 10 in decimal,which is exactly how many bytes long the next item is shown to be:
    4. 4D:61:63:20:48:44:5F:4E:42:53 specifies the volume name being shared on the AFPFS/IP file server that holds images required by the client for net booting. In this case, the ASCII string would become "Mac HD_NBS"
    5. 0:0:0:0 has an unknown purpose. Guesses: This string is returned to the client based on the option-220 field it offers. If it sends a "0:0:0:0" during it's boot, then maybe the server is supposed to send this back too. I am accepting suggestions for what this is, and what purpose it serves. If it is the 220 field, then it may be a versioning scheme.
    6. 1b or 20 or 27 appears to be the number of bytes left after this byte to the end of the option. For 234 it is hex 1b which is decimal 27. For 235 it is hex 20 which is decimal 32. For 238 it is 27 which is decimal 39.
    7. 1b:53:68:61:72:65:64:49:6D:61:67:65:73 specifies the folder that contains an image that is desired. In this case, this maps out the the ASCII value of "SharedImages"
    8. 0 The null is a directory/filename separator. For each null, a "/" may be substituted on the *NIX file system. It is a delimiter to tell the client where this part ends, so that it can be prepared for reading in the next part.
    9. 4E:65:74:42:6F:6F:74:20:48:44:2E:69:6D:67 appears to be the name of the very first image to be mounted that will allow the mac to boot itself up. This image is on the server as a read-only file. If it is indeed mounted initially, then it is replaced with the image in option 238 listed below so that the user may say preferences. This maps out to an ASCII string of "NetBoot HD.img"
20. option option-235 contains many things to tell the client where to get an Application network disk image for booting.
    (Only listing the values that are different from above.)
    • 41:70:70:6C:69:63:61:74:69:6F:6E:73:20:48:44:2E:69:6D:67 appears to be the name of the second image to be mounted that will allow the mac to have a volume just for applications. This image is on the server as a read-only file. This maps out to an ASCII string of "Applications HD.img"
21. option option-238 contains many things to tell the client where to get a writable (by this client) boot disk image for keeping preferences between boots.
    (Only listing the values that are different from above.)
    • 43:6C:69:65:6E:74:49:6D:61:67:65:73 specifies the folder that contains a folder that is named after this client. In this case, this maps out the the ASCII value of "ClientImages"
    • 4D:61:63:20:4E:43:20:23:31 specifies the folder that contains an image that is desired specifically for this client. In this case, this maps out the the ASCII value of "Mac NC #1"
    • 4E:65:74:42:6F:6F:74:20:48:44:2E:69:6D:67:5F:73 I think this image is mounted by the client after the first system net-boot image so that the user may save preferences. This maps out to an ASCII string of "NetBoot HD.img_s"

Comments and/or suggestions?: Email me at: dugan@passwall.com(Realize that I am very busy, and may not have time to respond to all E-mail messages. If you include NETBOOT MACNC (all Caps) as beginning it is more likely to get my attention... ;-)

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