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kHTTPd  -  Kernel httpd accelerator

(C) 1999 by Arjan van de Ven
Licensed under the terms of the GNU General Public License

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1. Introduction
---------------
   kHTTPd is a http-daemon (webserver) for Linux. kHTTPd is different from 
   other webservers in that it runs from within the Linux-kernel as a module 
   (device-driver).

   kHTTPd handles only static (file based) web-pages, and passes all requests 
   for non-static information to a regular userspace-webserver such as Apache 
   or Zeus. The userspace-daemon doesn't have to be altered in any way.

   Static web-pages are not a very complex thing to serve, but these are very
   important nevertheless, since virtually all images are static, and a large
   portion of the html-pages are static also. A "regular" webserver has little
   added value for static pages, it is simply a "copy file to network"
   operation.
   This can be done very efficiently from within the Linux-kernel, for example 
   the nfs (network file system) daemon performs a similar task and also runs 
   in the kernel.

   By "accelerating" the simple case within the kernel, userspace daemons can
   do what they are very good at: Generating user-specific, dynamic content.

   Note: This document sometimes uses "Apache" instead of "any webserver you
   ever might want to use", just for reasons of readability.
   

2. Quick Start  
--------------

   1) compile and load the module
   2) configure the module in /proc/sys/net/khttpd if needed
   3) echo 1 > /proc/sys/net/khttpd/start

   unloading:
 
   echo 1 > /proc/sys/net/khttpd/stop
   echo 1 > /proc/sys/net/khttpd/unload 
   sleep 2
   rmmod khttpd
   


3. Configuration 
----------------

   Modes of operation
   ==================


   There are two recommended modes of operation:

   1) "Apache" is main webserver, kHTTPd is assistant
        clientport   -> 80
        serverport   -> 8080 (or whatever)

   2) kHTTPd is main webserver, "Apache" is assistant
        clientport   -> 8080 (or whatever)
        serverport   -> 80

   
   Configuring kHTTPd
   ==================

   Before you can start using kHTTPd, you have to configure it. This
   is done through the /proc filesystem, and can thus be done from inside
   a script. Most parameters can only be set when kHTTPd is stopped.

   The following things need configuration:

   1) The port where kHTTPd should listen for requests
   2) The port (on "localhost") where "Apache" is listening
   3) The location of the documents (documentroot)
   4) The strings that indicate dynamic content (optional)
      [  "cgi-bin" is added by default ]

   It is very important that the documentroot for kHTTPd matches the
   documentroot for the userspace-daemon, as kHTTPd might "redirect"
   any request to this userspace-daemon.

   A typical script (for the first mode of operation) to do this would 
   look like:

#!/bin/sh
modprobe khttpd
echo 80 > /proc/sys/net/khttpd/clientport
echo 8080 > /proc/sys/net/khttpd/serverport
echo /var/www > /proc/sys/net/khttpd/documentroot
echo php3 > /proc/sys/net/khttpd/dynamic
echo shtml > /proc/sys/net/khttpd/dynamic
echo 1 > /proc/sys/net/khttpd/start

   For the second mode of operation, this would be:

#!/bin/sh
modprobe khttpd
echo 8080 > /proc/sys/net/khttpd/clientport
echo 80 > /proc/sys/net/khttpd/serverport
echo /var/www > /proc/sys/net/khttpd/documentroot
echo php3 > /proc/sys/net/khttpd/dynamic
echo shtml > /proc/sys/net/khttpd/dynamic
echo 1 > /proc/sys/net/khttpd/start

   In this case, you also have to change the configuration of the 
   userspace-daemon. For Apache, you do this by changing

   Port 80

   to 

   Port 8080

   Starting kHTTPd
   ===============
   Once you have set up the configuration, start kHTTPD by running
   echo 1 > /proc/sys/net/khttpd/start
   It may take a jiffie or two to start.

   Stopping kHTTPd
   ===============
   To stop kHTTPd, do
   echo 1 > /proc/sys/net/khttpd/stop
   It should stop in a jiffy or two.

   Unloading kHTTPd
   ===============
   To unload the module, do
   echo 1 > /proc/sys/net/khttpd/stop
   echo 1 > /proc/sys/net/khttpd/unload
   #killall -HUP khttpd
   sleep 2
   rmmod khttpd

   If this doesn't work fast enough for you (unloading can wait for 
   a remote connection to close down), you can send the daemons a "HUP"
   signal after you told them to stop. This will cause the daemon-threads to
   stop immediately. 
   

4. Permissions
--------------
   The security model of kHTTPd is very strict. It can be, since there is a 
   userspace daemon that can handle the complex exceptions. 

   kHTTPd only serves a file if

        1)  There is no "?" in the URL
        2)  The URL starts with a "/"
        3)  The file indicated by the URL exists
        4)  The file is world-readable (*)
        5)  The file is not a directory, executable or has the Sticky-bit
            set (*)
        6)  The URL doesn't contain any "forbidden" substrings such as ".."
            and "cgi-bin" (*)
        7)  The mime-type is known (*)

   The items marked with a (*) are configurable through the
   sysctl-parameters in /proc/sys/net/khttpd.


   In all cases where any of the above conditions isn't met, the
   userspace-daemon is handed the request.



5. Parameters
-------------
   The following parameters are settable through /proc/sys/net/khttpd:
 
        Name            Default         Description

        serverport      8080            The port where kHTTPd listens on

        clientport      80              The port of the userspace
                                        http-daemon

        threads         2               The number of server-threads. Should
                                        be 1 per CPU for small websites, 2
                                        per CPU for big (the active files
                                        do not fit in the RAM) websites.

        documentroot    /var/www        the directory where the
                                        document-files are

        start           0                Set to 1 to start kHTTPd 
                                        (this also resets "stop" to 0)

        stop            0                Set to 1 to stop kHTTPd
                                        (this also resets "start" to 0)

        unload          0                Set to 1 to prepare kHTTPd for
                                        unloading of the module

        sloppymime      0                If set to 1, unknown mime-types are
                                        set to text/html. If set to 0,
                                        files with unknown mime-types are
                                        handled by the userspace daemon

        perm_required   S_IROTH         Minimum permissions required
                                        (for values see "man 2 stat")
        
        perm_forbid     dir+sticky+     Permission-mask with "forbidden"
                        execute         permissions.
                                        (for values see "man 2 stat")

        dynamic         cgi-bin ..      Strings that, if they are a subset
                                        of the URL, indicate "dynamic
                                        content"

        maxconnect      1000            Maximum number of concurrent
                                        connections

6. Known Issues
   kHTTPd is *not* currently compatible with tmpfs.  Trying to serve
   files stored on a tmpfs partition is known to cause kernel oopses
   as of 2.4.18.  This is due to the same problem that prevents sendfile()
   from being usable with tmpfs.  A tmpfs patch is floating around that seems
   to fix this, but has not been released as of 27 May 2002.
   kHTTPD does work fine with ramfs, though.

   There is debate about whether to remove kHTTPd from the main
   kernel sources.  This will probably happen in the 2.5 kernel series,
   after which khttpd will still be available as a patch.

   The kHTTPd source code could use a good spring cleaning.

7. More information
-------------------
   More information about the architecture of kHTTPd, the mailinglist and
   configuration-examples can be found at the kHTTPd homepage:

        http://www.fenrus.demon.nl

   Bugreports, patches, etc can be send to the mailinglist
   (khttpd-users@zgp.org) or to khttpd@fenrus.demon.nl
   Mailing list archives are at 
      http://lists.alt.org/mailman/listinfo/khttpd-users