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+[[meta copyright="Copyright © 2008, 2009 Free Software Foundation, Inc."]]
+
+[[meta license="""[[toggle id="license" text="GFDL 1.2+"]][[toggleable
+id="license" text="Permission is granted to copy, distribute and/or modify this
+document under the terms of the GNU Free Documentation License, Version 1.2 or
+any later version published by the Free Software Foundation; with no Invariant
+Sections, no Front-Cover Texts, and no Back-Cover Texts.  A copy of the license
+is included in the section entitled
+[[GNU_Free_Documentation_License|/fdl]]."]]"""]]
+
+[[meta title="Server Overriding Mechanism"]]
+
+The main idea of the Hurd is that every user can influence almost all system
+functionality ([[extensible_system|extensibility]]), by running private Hurd
+servers that replace or proxy the global default implementations.
+
+However, running such a cumstomized subenvironment presently is not easy,
+because there is no standard mechanism to easily replace an individual standard
+server, keeping everything else.  (Presently there is only the [[hurd/subhurd]]
+method, which creates a completely new system instance with a completely
+independent set of servers.)
+
+The goal of this project is to provide a simple method for overriding
+individual standard servers, using environment variables, or a special
+subshell, or something like that.
+
+Various approaches for such a mechanism has been discussed before.
+Probably the easiest (1) would be to modify the Hurd-specific parts of [[hurd/glibc]],
+which are contacting various standard servers to implement certain system
+calls, so that instead of always looking for the servers in default locations,
+they first check for overrides in environment variables, and use these instead
+if present.
+
+A somewhat more generic solution (2) could use some mechanism for arbitrary
+client-side namespace overrides. The client-side part of the filename lookup
+mechanism would have to check an override table on each lookup, and apply the
+desired replacement whenever a match is found.
+
+Another approach would be server-side overrides. Again there are various
+variants. The actual servers themself could provide a mechanism to redirect to
+other servers on request. (3) Or we could use some more generic server-side
+namespace overrides: Either all filesystem servers could provide a mechanism to
+modify the namespace they export to certain clients (4), or proxies could be
+used that mirror the default namespace but override certain locations. (5)
+
+Variants (4) and (5) are the most powerful. They are intimately related to
+chroots: (4) is like the current chroot implementation works in the Hurd, and
+(5) has been proposed as an alternative. The generic overriding mechanism could
+be implemented on top of chroot, or chroot could be implemented on top of the
+generic overriding mechanism. But this is out of scope for this project...
+
+In practice, probably a mix of the different approaches would prove most useful
+for various servers and use cases. It is strongly recommended that the student
+starts with (1) as the simplest approach, perhaps augmenting it with (3) for
+certain servers that don't work with (1) because of indirect invocation.
+
+This tasks requires some understanding of the Hurd internals, especially a good
+understanding of the file name lookup mechanism. It's probably not too heavy on
+the coding side.
+
+This is [[GNU_Savannah_task 6612]].  Also there are quite a bit of emails
+discussing this topic, from a last year's GSoC application -- see
+<http://lists.gnu.org/archive/html/bug-hurd/2007-03/msg00050.html>,
+<http://lists.gnu.org/archive/html/bug-hurd/2007-03/msg00114.html>,
+<http://lists.gnu.org/archive/html/bug-hurd/2007-06/msg00082.html>,
+<http://lists.gnu.org/archive/html/bug-hurd/2008-03/msg00039.html>.
+
+Possible mentors: Olaf Buddenhagen (antrik)
+
+Exercise: Come up with a glibc patch that allows overriding one specific
+standard server using method (1).
+
+*Status*: Overriding of socket servers through environment variables has been
+implemented by Zheng Da for GSoC 2008, as part of his
+[[network_virtualization|zhengda]] project.