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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
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 not active.

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



Stopping kHTTPd
===============
In order to change the configuration, you should stop kHTTPd by typing
echo 1 > /proc/sys/net/khttpd/stop
on a command-prompt.

If you want to unload the module, you should type
echo 1 > /proc/sys/net/khttpd/unload
after stopping kHTTPd first.

If this doesn't work fast enough for you (the commands above 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.

Note that the daemons will restart immediately if they are not told to
stop.



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. 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