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Configuring
kHTTPd
UNIX-like systems in general, and Linux in
particular, make a clear distinction between two types of processes:kernel space processes and user
space processes. The kernel handles kernel space processes. If some
event that the kernel handles triggers such a process, the process can be
initiated very quickly. User space processes, by contrast, impose an overhead
to start, and to communicate important data back to the kernel. This fact is
not a problem for many processes, because user space processes often do
substantial processing within user space. The
overhead of calling user space programs is also tolerated in the name of
security and stabilitykernel space processes have privileged access to
hardware, filesystems, and so on, so they can wreak havoc if they contain bugs
or if unauthorized individuals gain control of them.Researchers who have looked for ways to
optimize the performance of Web servers have discovered that, although Web
servers like Apache are user space programs, much of the work they do is
performed in kernel space or in calls between the kernel and the server.
Web server. In fact,
results in the kernel performing fairly complex file read operations. Ultimately,
in a simple transfer (the most common type on many sites), Apache does little
more than receive the file from the kernel and then deliver it straight back to
the kernel. This is a huge waste of CPU time, memory, and other resources.
Figure 20.1. User space
Web servers generate a lot of communication between the kernel and the user
space server.
computer's performance, developers have created simple Web servers that run within the kernel. This eliminates the communications
between the Web server and the kernel, thus streamlining the process of serving
Web pages and (it is hoped) improving performance. In fact, the 2.4.x and later kernels include one such kernel-based Web
server: kHTTPd, headquartered at target="_blank">http://www.fenrus.demon.nl . This server is configured by writing data to files in the /proc/sys/net/khttpd directory. To use it, follow these steps:
including it in your Linux kernel configuration. The relevant option is on the
Networking Options menu, and is called Kernel HTTPd Acceleration. You can build
it as a module or directly into the kernel file.
2. Change your Apache configuration
so that it listens on port 8080, or some convenient port other than the usual
port 80.
3. Reboot with a kHTTPd-enabled
kernel, or load the kHTTPd kernel module. It may load automatically, depending
upon your configuration, or you may need to type insmod khttpd .
4. Tell kHTTPd to listen on port 80
for incoming requests. Do this by typing echo 80 > /proc/sys/net/khttpd/serverport .
5. Type echo 8080 > /proc/sys/net/clientport to tell kHTTPd to pass on requests it can't handle to port 8080. (If
you used a port other than 8080 in Step 2, the port you specify here should
match.)
6. Tell kHTTPd where to find
unencrypted static files by typing echo /home/httpd/html > /proc/sys/net/khttpd/documentroot . You can change the directory name ( /home/httpd/html in
this example) as necessary. Be sure you specify the same directory here as you
do with the DocumentRoot directive in httpd.conf .
7. If your site includes PHP3 or
secure HTML documents, repeat Step 6, but echo the directory names in which
these documents reside to the /proc/sys/net/khttpd/dynamic file.
8. Type echo 1 > /proc/sys/net/khttpd/start . This starts the kHTTPd server; it's equivalent to using an Apache
SysV startup script.
You may want to create a custom SysV or local
startup script to handle Steps 4 through 8 automatically when the system boots.
Whether you start kHTTPd manually or through a script, the result is that it
handles simple requeststhose for ordinary files that exist in the specified
directory, aren't CGI scripts, and so on. If a request doesn't meet kHTTPd's
requirements, kHTTPd passes the request on to the user-space Web server via the
port number indicated in Steps 2 and 5. This adds some overhead when dealing
with these file types, so kHTTPd isn't worth using if your site handles mostly
CGI scripts or other nonstatic files. Indeed, kHTTPd may not be worth using
even on a site with moderate amounts of traffic; it's most worth considering if
Apache is having trouble keeping up with requests to your Web site. Also,
kHTTPd is officially experimental, so it might not be as reliable as Apache or
some other fully debugged user-space Web server. Finally, because it runs in
kernel space, the risks if a bug exists in kHTTPd are much greater than are the
risks involved if a bug is present in Apache. For a truly secure Web site,
you're best sticking to a well-tested user-space program such as Apache.Although kHTTPd is the kernel-based Web
server that's most readily accessible, it's not the only one available. Red
Hat's TUX product is one other that's received good reviews, and researchers
are working on several more. In the future, we may see a wide selection of
kernel-based Web servers for Linux.
