User manual

Etherboot User Manual

<ken_yap AT users PERIOD sourceforge PERIOD net> and Gero Kuhlmann <gero AT gkminix

PERIOD han PERIOD de>. I will happily include any of your extensions, but I would like to avoid the

proliferation of different incompatible revisions of this document. If these conditions are a problem to

you, then feel free to contact me.

RFC1533 allows for vendor-speciﬁc extensions to the BOOTP protocol. It deﬁnes that all tags in the

range 128 thru 254 are set aside for site-speciﬁc extensions. Common implementations of DHCP

daemons can assign arbitrary null-terminated character strings to these tags.

This is a list of the tags that are currently used by Etherboot. Tag 128 and 129 are handled by Etherboot.

The other tags are handled by an external menu program, generated using mkelf-menu. This is changed

behaviour from 5.0, where the menuing code was part of Etherboot. As a general rule, you should never

ﬁll out any of the other tags, unless you positively know that your BOOT-Prom supports the extension or

ignores this particular tag:

TAG 128

this is a six-byte hexadecimal entry. The ﬁrst four bytes have to be the magic number E4 45 74 68;

if this magic cannot be found, then none of the other vendor tags are valid! The ﬁfth byte is the

major version number and the sixth byte is the minor version number of this protocol extension. The

current version is 0.0; the BOOT-Prom can assume that incompatible changes increase the major

version number. Mere extensions to the existing protocol, increase the minor version number. If the

BOOT-Prom code has been written in a way that anticipates future extensions, then it is acceptable

to honor the vendor tags even though, the minor version number does not match exactly. Before

making a change, that requires updating the major version number, you should contact all of the

persons that are listed at the top of this document.

TAG 129

the BOOT-Prom uses this tag for passing a user-provided command line to Linux. Anything in this

tag will be appended to the kernel parameters already in the boot image created by mkelf.

TAG 160

a string that contains a colon separated list of "parameter=value" pairs. Currently, these parameters

are only used to control the behavior of an interactive menu for selecting different boot images;

future extensions are quite likely:

timeout

after this many seconds, the default image will be loaded. If no ’timeout’ has been set, then the

program will wait indeﬁnitely.

default

either an integer ’n’ in the range 0 thru 15 or 192 thru 207, selecting the default image. If ’n’ is

in the ﬁrst range, it refers to the ’n’th menu entry; if it is in the later range, it refers to the entry

with the tag number ’n’. This distinction is important, if the list of images contains gaps. If no

value has been set, then the image with the lowest tag number will be the default image.