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GNU Hurd is the name of a set of servers (or daemons, in Unix-speak) that work on top of GNU Mach; together, they form the kernel of the GNU operating system. It has been under development since 1990 by the GNU Project and is distributed as free software under the GPL. The Hurd aims to surpass Unix kernels in functionality, security, and stability, while remaining largely compatible with them. This is done by having the Hurd track the POSIX specification, while avoiding arbitrary restrictions on the user.
Currently, there are at least three GNU/Hurd distributions in preparation, none of which have been officially released yet outside of interested testers and developers. Richard Stallman, of the GNU project, said in 2002 that it was likely that a production version of the GNU system, using Hurd as its kernel, would be released before the end of the year, but no such release happened.
As of 2004, development of the Hurd has shifted significantly towards the L4 microkernel family. In 2005, Hurd developer Neal Walfield finished the initial memory management framework for the L4/Hurd port and Marcus Brinkmann has ported essential parts of glibc, namely getting the process startup code working and thus actual programs to run. This rather small step is at the end of a giant leap towards the first bootstrapping phase and opens the way for future development.
The Debian project and others have contributed funding to speed up development.
Unlike Unix-like kernels, the Hurd builds on top of a microkernel which is responsible for providing the most basic kernel services – coordinating access to the hardware: the CPU (through multiprocessing), RAM (via memory management), and other various devices for sound, graphics, mass storage, etc. Currently GNU Mach is used as the microkernel, but efforts are underway to port the Hurd to the more modern L4 microkernel.
Other Unix-like systems running on top of the Mach microkernel include Mac OS X, OSF/1, NEXTSTEP, Lites, and MkLinux. These share one detail in common, in that they are implemented as a single so-called server. In effect they replace the monolithic kernel from a traditional Unix system with two parts, the microkernel and Unix server.
The Hurd instead consists of multiple servers working together. Instead of a single large code base which includes everything from handling the clock to handling the networking, in Hurd each of these are handled by a separate server. This makes developing the Hurd much easier (at least in theory) as making changes to one is less likely to have side-effects in others. This explains the mutually recursive acronym: "Hurd" stands for "Hird of Unix-Replacing Daemons", and "Hird" stands for "Hurd of Interfaces Representing Depth".
In the original Mach efforts, this sort of "set of servers" was considered to be one of the main goals of the design, but the Hurd appears to be the first Mach-based system to actually be implemented in this fashion (whereas QNX is similar but based on its own microkernel). It's not entirely clear why this happened, but it appears that groups working on Mach were too busy working on Mach to work on the operating system as a whole. Hurd also aims to be microkernel-independent.
A number of traditional Unix concepts are replaced or extended in the Hurd:
Under Unix every program running has an associated user id, which normally corresponds to the user that started the process. This id largely dictates the actions permitted to the program. No outside process can change the user id of a running program. A Hurd process, on the other hand, runs under a set of user ids, which can contain multiple ids, one, or none. A sufficiently privileged process can add and remove ids to another process. For example there is a password server that will hand out ids in return for a correct login password.
Regarding the file system, a suitable program can be designated as a translator for a single file or a whole directory hierarchy. Every access to the translated file, or files below a hierarchy in the second case, is in fact handled by the program. For example a file translator may simply redirect read and write operations to another file, not unlike a Unix symbolic link. What mounting is to Unix, the Hurd achieves by setting up a filesystem translator (using the "settrans" command). Translators can also be used to provide services to the user. For example, the ftpfs translator allows a user to encapsulate remote FTP sites within a directory. Then, standard tools such as ls, cp, and rm can be used to manipulate files on the remote system. Even more powerful translators are ones such as UnionFS, which allows a user to unify multiple directories into one; so that listing the unified directory reveals the contents of all the unified directories (a feature that is missing in many Unixes, although available in FreeBSD).
Perhaps the most empowering aspect of the Hurd is the ability for any user to start their own system services. Any user can attach any translator to the filesystem for their own personal use. A user could even replace system servers, such as the auth server, with other servers of their own choosing. All this can be done without affecting other users, due to well defined scopes. Indeed, it is even possible for a user to run the Hurd within itself, which is known as a sub-Hurd.
- The GNU Hurd
- Debian GNU/Hurd installation CDs
- Citations from CiteSeer
- The first program executed on Hurd L4
- Interview with Hurd and Hurd-L4 developer Marcus Brinkmann
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