bsd.rd breakdown

Contributed by Janne Johansson on 2026-02-17 from the ram-a-llama-ding-dong dept.

Every OpenBSD admin has booted bsd.rd at least once — to install, upgrade, or rescue a broken system. But few people stop to look at what’s actually inside that file.

In this article over at the OpenBSD Jumpstart site, they take a look at just that.

The contents and format of the bsd.rd ramdisk kernel is shown, so you can understand and customize it for your own needs.

Enjoy!

Comments

By Yoshihiro Kawamata (ykaw) kaw@on.rim.or.jp on 2026-02-18 16:07 https://fuguita.org

An important mechanism that should be further explained is the crunched binary.
This is generated by crunchgen(8).
crunchgen combines the source code of multiple commands into a single executable, but, like install.sub, its behavior changes depending on the command name it was invoked with.
You can see how it is actually used in files such as /usr/src/distrib/amd64/ramdisk_cd/Makefile.

Reply

Latest Articles

Thu, Mar 12
- 18:56 Delayed hibernation comes to OpenBSD/amd64 laptops (4)
Wed, Mar 11
- 06:29 OpenBSD -current moves to 7.9-beta (0)
Tue, Mar 10
- 10:29 Major update to drm(4) code in OpenBSD-current (to linux 6.18.16) (0)
Fri, Mar 06
- 13:11 The Book of PF, 4th Edition Spotted in the Wild (0)
Thu, Mar 05
- 14:39 OpenBSD on SGI: a rollercoaster story, as told by miod@ (2)
Thu, Feb 26
- 16:40 tmppath promise removed from pledge(2) in -current (0)
- 11:06 Another subprocess for vmd(8) (0)
- 05:45 Game of Trees 0.123 released (0)
Wed, Feb 18
- 06:16 bsd.rd breakdown (1)

Credits

Copyright © 2004-2008 Daniel Hartmeier. All rights reserved. Articles and comments are copyright their respective authors, submission implies license to publish on this web site. Contents of the archive prior to April 2nd 2004 as well as images and HTML templates were copied from the fabulous original deadly.org with Jose's and Jim's kind permission. This journal runs as CGI with httpd(8) on OpenBSD, the source code is BSD licensed. undeadly \Un*dead"ly\, a. Not subject to death; immortal. [Obs.]