This is a copy of the documentation available at http://mpm-itk.sesse.net/,
dumped with the Lynx web browser and edited briefly.

                              The Apache 2 ITK MPM

   apache2-mpm-itk (just mpm-itk for short) is an MPM (Multi-Processing
   Module) for the [1]Apache web server. mpm-itk allows you to run each of
   your vhost under a separate uid and gid -- in short, the scripts and
   configuration files for one vhost no longer have to be readable for all
   the other vhosts.

   There are already MPMs available that do this; however, I am not aware
   of any besides mpm-itk that are actively developed. (If you only run
   CGI scripts, suexec will probably solve most of your problems with any
   MPM.)

   mpm-itk is based on the traditional prefork MPM, which means it's
   non-threaded; in short, this means you can run non-thread-aware code
   (like many [2]PHP extensions) without problems. On the other hand, you
   lose out to any performance benefit you'd get with threads, of course;
   you'd have to decide for yourself if that's worth it or not. You will
   also take an additional performance hit over prefork, since there's an
   extra fork per request.

Installation

   If you can't apply a patch, you probably should not be using this. :-)
   However, several distributions now include mpm-itk as a choice
   alongside the other MPMs; in alphabetical order:
     * [3]Arch Linux
     * [4]Debian GNU/Linux
     * [5]FreeBSD ports
     * [6]Gentoo Linux
     * [7]Mandriva
     * [8]Ubuntu

   If you know of any I missed, or if you have included mpm-itk in your
   favourite distribution, please drop me a note (see below). I'd always
   be happy to expand this list :-)

Configuration

   The new configuration settings compared to the prefork MPM are:
     * AssignUserID: Takes two parameters, uid and gid (or really, user
       name and group name); specifies what uid and gid the vhost will run
       as (after parsing the request etc., of course). Note that if you do
       not assign a user ID, the default one from Apache will be used.
     * MaxClientsVHost: A separate MaxClients for the vhost. This can be
       useful if, say, half of your vhosts depend on some NFS server (like
       on our setup); if the NFS server goes down, you do not want the
       children waiting forever on NFS to take the non-NFS-dependent hosts
       down. This can thus act as a safety measure, giving "server too
       busy" on the NFS-dependent vhosts while keeping the other ones
       happily running. (Of course, you could use it to simply keep one
       site from eating way too much resources, but there are probably
       better ways of doing that.)
     * NiceValue: Lets you nice some requests down, to give them less CPU
       time.

   AssignUserID and NiceValue can be set wherever you'd like in the Apache
   configuration, except in .htaccess. MaxClientsVHost can only be set
   inside a VirtualHost directive.

Quirks and warnings

   Since mpm-itk has to be able to setuid(), it runs as root (although
   restricted with POSIX capabilities where possible) until the request is
   parsed and the vhost determined. This means that any security hole
   before the request is parsed will be a root security hole. (The most
   likely place is probably in mod_ssl.) This is not going to change in
   the near future, as the most likely alternative solution (socket
   passing and its variants) is very hard to get to work properly in a
   number of common use cases, like SSL.

   The lack of socket passing also leads to another minor quirk: if you
   connect to httpd, make a request and then make a request on the same
   connection that gets handled by a different uid, mpm-itk simply shuts
   down the connection. This is perfectly legal according to RFC2616, and
   all major clients seem to handle it well; the web server simply
   simulates a timeout, and the client just opens a new connection and
   retries the request. However, there is a small performance hit, and
   thus you should avoid including content from multiple uids in the same
   page.

   Note that mpm-itk is experimental software; and we've done a fair
   amount of stress testing, but it's nowhere as tested as, say, prefork.
   That being said, it's being run in production at several sites in the
   world, both hobbyist and commercial, some as large as ~10 million hits
   a day.

   People have reported issues with mpm-itk and mod_python, mod_ruby and
   FastCGI. I believe the mod_python and FastCGI problems have been
   largely solved by updates to those packages, but as I use neither, I
   can't really guarantee anything. YMMV, test before use.

Licensing

   mpm-itk is licensed under the Apache License, version 2.0, like the
   rest of Apache.

Contact

   mpm-itk is developed by Steinar H. Gunderson; e-mail address is at my
   [9]home page.

   There is a user mailing list at mpm-itk [at] lists.err.no. Visit the
   [10]mailing list page to subscribe, or send a blank e-mail to
   mpm-itk-subscribe [at] lists.err.no.

References

   1. http://www.apache.org/
   2. http://www.php.net/
   3. http://www.archlinux.org/
   4. http://www.debian.org/
   5. http://www.freebsd.org/
   6. http://www.gentoo.org/
   7. http://www.mandriva.com/
   8. http://www.ubuntu.com/
   9. http://www.sesse.net/
  10. http://lists.err.no/mailman/listinfo/mpm-itk
