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|
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<H1 ALIGN="CENTER">Apache Server Frequently Asked Questions</H1>
<P>
$Revision: 1.9 $ ($Date: 2001/02/28 03:36:00 $)
</P>
<P>
The latest version of this FAQ is always available from the main
Apache web site, at
<<A
HREF="http://www.apache.org/docs/misc/FAQ.html"
REL="Help"
><SAMP>http://www.apache.org/docs/misc/FAQ.html</SAMP></A>>.
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<P>
If you are reading a text-only version of this FAQ, you may find numbers
enclosed in brackets (such as "[12]"). These refer to the list of
reference URLs to be found at the end of the document. These references
do not appear, and are not needed, for the hypertext version.
</P>
<H2>The Questions</H2>
<OL TYPE="A">
<!--#endif -->
<!--#if expr="$TOC || $STANDALONE" -->
<LI VALUE="6"><STRONG>Dynamic Content (CGI and SSI)</STRONG>
<OL>
<LI><A HREF="#CGIoutsideScriptAlias">How do I enable CGI execution
in directories other than the ScriptAlias?</A>
</LI>
<LI><A HREF="#premature-script-headers">What does it mean when my
CGIs fail with "<SAMP>Premature end of script
headers</SAMP>"?</A>
</LI>
<LI><A HREF="#POSTnotallowed">Why do I keep getting "Method Not
Allowed" for form POST requests?</A>
</LI>
<LI><A HREF="#nph-scripts">How can I get my script's output without
Apache buffering it? Why doesn't my server push work?</A>
</LI>
<LI><A HREF="#cgi-spec">Where can I find the "CGI
specification"?</A>
</LI>
<LI><A HREF="#fastcgi">Why isn't FastCGI included with Apache any
more?</A>
</LI>
<LI><A HREF="#ssi-part-i">How do I enable SSI (parsed HTML)?</A>
</LI>
<LI><A HREF="#ssi-part-ii">Why don't my parsed files get cached?</A>
</LI>
<LI><A HREF="#ssi-part-iii">How can I have my script output parsed?</A>
</LI>
<LI><A HREF="#ssi-part-iv">SSIs don't work for VirtualHosts and/or
user home directories</A>
</LI>
<LI><A HREF="#errordocssi">How can I use <CODE>ErrorDocument</CODE>
and SSI to simplify customized error messages?</A>
</LI>
<LI><A HREF="#remote-user-var">Why is the environment variable
<SAMP>REMOTE_USER</SAMP> not set?</A>
</LI>
<LI><A HREF="#user-cgi">How do I allow each of my user directories
to have a cgi-bin directory?</A>
</LI>
</OL>
</LI>
<!--#endif -->
<!--#if expr="$STANDALONE" -->
</OL>
<HR>
<H2>The Answers</H2>
<!--#endif -->
<!--#if expr="! $TOC" -->
<H3>F. Dynamic Content (CGI and SSI)</H3>
<OL>
<LI><A NAME="CGIoutsideScriptAlias">
<STRONG>How do I enable CGI execution in directories other than
the ScriptAlias?</STRONG>
</A>
<P>
Apache recognizes all files in a directory named as a
<A HREF="../mod/mod_alias.html#scriptalias"><SAMP>ScriptAlias</SAMP></A>
as being eligible for execution rather than processing as normal
documents. This applies regardless of the file name, so scripts in a
ScriptAlias directory don't need to be named
"<SAMP>*.cgi</SAMP>" or "<SAMP>*.pl</SAMP>" or
whatever. In other words, <EM>all</EM> files in a ScriptAlias
directory are scripts, as far as Apache is concerned.
</P>
<P>
To persuade Apache to execute scripts in other locations, such as in
directories where normal documents may also live, you must tell it how
to recognize them - and also that it's okay to execute them. For
this, you need to use something like the
<A HREF="../mod/mod_mime.html#addhandler"><SAMP>AddHandler</SAMP></A>
directive.
</P>
<P>
<OL>
<LI>In an appropriate section of your server configuration files, add
a line such as
<P>
<DL>
<DD><CODE>AddHandler cgi-script .cgi</CODE>
</DD>
</DL>
<P></P>
<P>
The server will then recognize that all files in that location (and
its logical descendants) that end in "<SAMP>.cgi</SAMP>"
are script files, not documents.
</P>
</LI>
<LI>Make sure that the directory location is covered by an
<A HREF="../mod/core.html#options"><SAMP>Options</SAMP></A>
declaration that includes the <SAMP>ExecCGI</SAMP> option.
</LI>
</OL>
<P></P>
<P>
In some situations, you might not want to actually
allow all files named "<SAMP>*.cgi</SAMP>" to be executable.
Perhaps all you want is to enable a particular file in a normal directory to
be executable. This can be alternatively accomplished
<EM>via</EM> <A HREF="../mod/mod_rewrite.html"><SAMP>mod_rewrite</SAMP></A>
and the following steps:
</P>
<P>
<OL>
<LI>Locally add to the corresponding <SAMP>.htaccess</SAMP> file a ruleset
similar to this one:
<P>
<DL>
<DD><CODE>RewriteEngine on
<BR>
RewriteBase /~foo/bar/
<BR>
RewriteRule ^quux\.cgi$ - [T=application/x-httpd-cgi]</CODE>
</DD>
</DL>
<P></P>
</LI>
<LI>Make sure that the directory location is covered by an
<A HREF="../mod/core.html#options"><SAMP>Options</SAMP></A>
declaration that includes the <SAMP>ExecCGI</SAMP> and
<SAMP>FollowSymLinks</SAMP> option.
</LI>
</OL>
<P></P>
<HR>
</LI>
<LI><A NAME="premature-script-headers">
<STRONG>What does it mean when my CGIs fail with
"<SAMP>Premature end of script headers</SAMP>"?</STRONG>
</A>
<P>
It means just what it says: the server was expecting a complete set of
HTTP headers (one or more followed by a blank line), and didn't get
them.
</P>
<P>
The most common cause of this problem is the script dying before
sending the complete set of headers, or possibly any at all, to the
server. To see if this is the case, try running the script standalone
from an interactive session, rather than as a script under the server.
If you get error messages, this is almost certainly the cause of the
"premature end of script headers" message. Even if the CGI
runs fine from the command line, remember that the environment and
permissions may be different when running under the web server. The
CGI can only access resources allowed for the <A
HREF="../mod/mpm_common.html#user"><CODE>User</CODE></A> and
<A HREF="../mod/mpm_common.html#group"><CODE>Group</CODE></A> specified in
your Apache configuration. In addition, the environment will not be
the same as the one provided on the command line, but it can be
adjusted using the directives provided by <A
HREF="../mod/mod_env.html">mod_env</A>.
</P>
<P>
The second most common cause of this (aside from people not
outputting the required headers at all) is a result of an interaction
with Perl's output buffering. To make Perl flush its buffers
after each output statement, insert the following statements around
the <CODE>print</CODE> or <CODE>write</CODE> statements that send your
HTTP headers:
</P>
<P>
<DL>
<DD><CODE>{<BR>
local ($oldbar) = $|;<BR>
$cfh = select (STDOUT);<BR>
$| = 1;<BR>
#<BR>
# print your HTTP headers here<BR>
#<BR>
$| = $oldbar;<BR>
select ($cfh);<BR>
}</CODE>
</DD>
</DL>
<P></P>
<P>
This is generally only necessary when you are calling external
programs from your script that send output to stdout, or if there will
be a long delay between the time the headers are sent and the actual
content starts being emitted. To maximize performance, you should
turn buffer-flushing back <EM>off</EM> (with <CODE>$| = 0</CODE> or the
equivalent) after the statements that send the headers, as displayed
above.
</P>
<P>
If your script isn't written in Perl, do the equivalent thing for
whatever language you <EM>are</EM> using (<EM>e.g.</EM>, for C, call
<CODE>fflush()</CODE> after writing the headers).
</P>
<P>
Another cause for the "premature end of script headers"
message are the RLimitCPU and RLimitMEM directives. You may
get the message if the CGI script was killed due to a
resource limit.
</P>
<P>
In addition, a configuration problem in <A
HREF="../suexec.html">suEXEC</A>, mod_perl, or another third party
module can often interfere with the execution of your CGI and cause
the "premature end of script headers" message.
</P>
<HR>
</LI>
<LI><A NAME="POSTnotallowed">
<STRONG>Why do I keep getting "Method Not Allowed" for
form POST requests?</STRONG>
</A>
<P>
This is almost always due to Apache not being configured to treat the
file you are trying to POST to as a CGI script. You can not POST
to a normal HTML file; the operation has no meaning. See the FAQ
entry on <A HREF="#CGIoutsideScriptAlias">CGIs outside ScriptAliased
directories</A> for details on how to configure Apache to treat the
file in question as a CGI.
</P>
<HR>
</LI>
<LI><A NAME="nph-scripts">
<STRONG>How can I get my script's output without Apache buffering
it? Why doesn't my server push work?</STRONG>
</A>
<P>
As of Apache 1.3, CGI scripts are essentially not buffered. Every time
your script does a "flush" to output data, that data gets relayed on to
the client. Some scripting languages, for example Perl, have their own
buffering for output - this can be disabled by setting the <CODE>$|</CODE>
special variable to 1. Of course this does increase the overall number
of packets being transmitted, which can result in a sense of slowness for
the end user.
</P>
<P>Prior to 1.3, you needed to use "nph-" scripts to accomplish
non-buffering. Today, the only difference between nph scripts and
normal scripts is that nph scripts require the full HTTP headers to
be sent.
</P>
<HR>
</LI>
<LI><A NAME="cgi-spec">
<STRONG>Where can I find the "CGI specification"?</STRONG>
</A>
<P>
The Common Gateway Interface (CGI) specification can be found at
the original NCSA site
<<A HREF="http://hoohoo.ncsa.uiuc.edu/cgi/interface.html">
<SAMP>http://hoohoo.ncsa.uiuc.edu/cgi/interface.html</SAMP></A>>.
This version hasn't been updated since 1995, and there have been
some efforts to update it.
</P>
<P>
A new draft is being worked on with the intent of making it an informational
RFC; you can find out more about this project at
<<A HREF="http://web.golux.com/coar/cgi/"
><SAMP>http://web.golux.com/coar/cgi/</SAMP></A>>.
</P>
<HR>
</LI>
<LI><A NAME="fastcgi">
<STRONG>Why isn't FastCGI included with Apache any more?</STRONG>
</A>
<P>
The simple answer is that it was becoming too difficult to keep the
version being included with Apache synchronized with the master copy
at the
<A HREF="http://www.fastcgi.com/"
>FastCGI web site</A>. When a new version of Apache was released, the
version of the FastCGI module included with it would soon be out of date.
</P>
<P>
You can still obtain the FastCGI module for Apache from the master
FastCGI web site.
</P>
<HR>
</LI>
<LI><A NAME="ssi-part-i">
<STRONG>How do I enable SSI (parsed HTML)?</STRONG>
</A>
<P>
SSI (an acronym for Server-Side Include) directives allow static HTML
documents to be enhanced at run-time (<EM>e.g.</EM>, when delivered to
a client by Apache). The format of SSI directives is covered
in the <A HREF="../mod/mod_include.html">mod_include manual</A>;
suffice it to say that Apache supports not only SSI but
xSSI (eXtended SSI) directives.
</P>
<P>
Processing a document at run-time is called <EM>parsing</EM> it;
hence the term "parsed HTML" sometimes used for documents
that contain SSI instructions. Parsing tends to be
resource-consumptive compared to serving static files, and is not
enabled by default. It can also interfere with the cachability of
your documents, which can put a further load on your server. (See
the <A HREF="#ssi-part-ii">next question</A> for more information
about this.)
</P>
<P>
To enable SSI processing, you need to
</P>
<UL>
<LI>Build your server with the
<A HREF="../mod/mod_include.html"><SAMP>mod_include</SAMP></A>
module. This is normally compiled in by default.
</LI>
<LI>Make sure your server configuration files have an
<A HREF="../mod/core.html#options"><SAMP>Options</SAMP></A>
directive which permits <SAMP>Includes</SAMP>.
</LI>
<LI>Make sure that the directory where you want the SSI documents to
live is covered by the "INCLUDES" output filter,
either explicitly or in some ancestral location. That can be done
with the following
<A HREF="../mod/core.html#setoutputfilter"><SAMP>SetOutputFilter</SAMP></A>
directive:
<P>
<blockquote><code>
<FilesMatch "\.shtml[.$]"><br>
SetOutputFilter INCLUDES<br>
</FilesMatch>
</code></blockquote>
<P></P>
<P>
This indicates that all files with the extension
".shtml" in that location (or its descendants) should be
parsed. Note that using ".html" will cause all normal
HTML files to be parsed, which may put an inordinate load on your
server.
</P>
</LI>
</UL>
<P>
For additional information, see the <CITE>Apache Week</CITE> article on
<A HREF="http://www.apacheweek.com/features/ssi" REL="Help"
><CITE>Using Server Side Includes</CITE></A>.
</P>
<HR>
</LI>
<LI><A NAME="ssi-part-ii">
<STRONG>Why don't my parsed files get cached?</STRONG>
</A>
<P>
Since the server is performing run-time processing of your SSI
directives, which may change the content shipped to the client, it
can't know at the time it starts parsing what the final size of the
result will be, or whether the parsed result will always be the same.
This means that it can't generate <SAMP>Content-Length</SAMP> or
<SAMP>Last-Modified</SAMP> headers. Caches commonly work by comparing
the <SAMP>Last-Modified</SAMP> of what's in the cache with that being
delivered by the server. Since the server isn't sending that header
for a parsed document, whatever's doing the caching can't tell whether
the document has changed or not - and so fetches it again to be on the
safe side.
</P>
<P>
You can work around this in some cases by causing an
<SAMP>Expires</SAMP> header to be generated. (See the
<A HREF="../mod/mod_expires.html" REL="Help"><SAMP>mod_expires</SAMP></A>
documentation for more details.) Another possibility is to use the
<A HREF="../mod/mod_include.html#xbithack" REL="Help"
><SAMP>XBitHack Full</SAMP></A>
mechanism, which tells Apache to send (under certain circumstances
detailed in the XBitHack directive description) a
<SAMP>Last-Modified</SAMP> header based upon the last modification
time of the file being parsed. Note that this may actually be lying
to the client if the parsed file doesn't change but the SSI-inserted
content does; if the included content changes often, this can result
in stale copies being cached.
</P>
<HR>
</LI>
<LI><A NAME="ssi-part-iii">
<STRONG>How can I have my script output parsed?</STRONG>
</A>
<P>
So you want to include SSI directives in the output from your CGI
script, but can't figure out how to do it?
The short answer is "you can't." This is potentially
a security liability and, more importantly, it can not be cleanly
implemented under the current server API. The best workaround
is for your script itself to do what the SSIs would be doing.
After all, it's generating the rest of the content.
</P>
<P>
This is a feature The Apache Group hopes to add in the next major
release after 1.3.
</P>
<HR>
</LI>
<LI><A NAME="ssi-part-iv">
<STRONG>SSIs don't work for VirtualHosts and/or
user home directories.</STRONG>
</A>
<P>
This is almost always due to having some setting in your config file that
sets "Options Includes" or some other setting for your DocumentRoot
but not for other directories. If you set it inside a Directory
section, then that setting will only apply to that directory.
</P>
<HR>
</LI>
<LI><A NAME="errordocssi">
<STRONG>How can I use <CODE>ErrorDocument</CODE>
and SSI to simplify customized error messages?</STRONG>
</A>
<P>
Have a look at <A HREF="custom_errordocs.html">this document</A>.
It shows in example form how you can a combination of XSSI and
negotiation to tailor a set of <CODE>ErrorDocument</CODE>s to your
personal taste, and returning different internationalized error
responses based on the client's native language.
</P>
<HR>
</LI>
<LI><A NAME="remote-user-var">
<STRONG>Why is the environment variable
<SAMP>REMOTE_USER</SAMP> not set?</STRONG>
</A>
<P>
This variable is set and thus available in SSI or CGI scripts <STRONG>if and
only if</STRONG> the requested document was protected by access
authentication. For an explanation on how to implement these restrictions,
see
<A HREF="http://www.apacheweek.com/"><CITE>Apache Week</CITE></A>'s
articles on
<A HREF="http://www.apacheweek.com/features/userauth"
><CITE>Using User Authentication</CITE></A>
or
<A HREF="http://www.apacheweek.com/features/dbmauth"
><CITE>DBM User Authentication</CITE></A>.
</P>
<P>
Hint: When using a CGI script to receive the data of a HTML <SAMP>FORM</SAMP>
notice that protecting the document containing the <SAMP>FORM</SAMP> is not
sufficient to provide <SAMP>REMOTE_USER</SAMP> to the CGI script. You have
to protect the CGI script, too. Or alternatively only the CGI script (then
authentication happens only after filling out the form).
</P>
<HR>
</LI>
<LI><A NAME="user-cgi"><STRONG>How do I allow each of my user directories
to have a cgi-bin directory?</STRONG></A>
<P>
Remember that CGI execution does not need to be restricted only to
cgi-bin directories. You can <A HREF="#CGIoutsideScriptAlias">allow
CGI script execution in arbitrary parts of your filesystem</A>.
</P>
<P>
There are many ways to give each user directory a cgi-bin directory
such that anything requested as
<SAMP>http://example.com/~user/cgi-bin/program</SAMP> will be
executed as a CGI script.
Two alternatives are:
<OL>
<LI>Place the cgi-bin directory next to the public_html directory:
<DL>
<DD><CODE>ScriptAliasMatch ^/~([^/]*)/cgi-bin/(.*) /home/$1/cgi-bin/$2</CODE>
</DD>
</DL>
</LI>
<LI>Place the cgi-bin directory underneath the public_html directory:
<DL>
<DD><CODE><Directory /home/*/public_html/cgi-bin><BR>
Options ExecCGI<BR>
SetHandler cgi-script<BR>
</Directory></CODE>
</DD>
</DL>
</LI>
</OL>
<HR>
</LI>
</OL>
<!--#endif -->
<!--#if expr="$STANDALONE" -->
<!-- Don't forget to add HR tags at the end of each list item.. -->
<!--#include virtual="footer.html" -->
</BODY>
</HTML>
<!--#endif -->
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