Apache The Definitive Guide, 3rd Edition

Chapter 14. Server-Side Includes

Contents:

File Size
File Modification Time
Includes
Execute CGI
Echo
Apache v2: SSI Filters

Server-side includes trigger further actions whose output, if any, may then be placed inline into served documents or affect subsequent includes. The same results could be achieved by CGI scripts — either shell scripts or specially written C programs — but server-side includes often achieve these results with a lot less effort. There are, however, some security problems. The range of possible actions is immense, so we will just give basic illustrations of each command in a number of text files in ...site.ssi/htdocs.

The Config file, .../conf/httpd1.conf, is as follows:

User webuser
Group webgroup
ServerName www.butterthlies.com
DocumentRoot /usr/www/APACHE3/site.ssi/htdocs
ScriptAlias /cgi-bin /usr/www/APACHE3/cgi-bin
AddHandler server-parsed shtml
Options +Includes

Run it by executing ./go 1.

shtml is the normal extension for HTML documents with server-side includes in them and is found as the extension to the relevant files in ... /htdocs. We could just as well use brian or dog_run, as long as it appears the same in the file with the relevant command and in the configuration file. Using html can be useful — for instance, you can easily implement site-wide headers and footers — but it does mean that every HTML page gets parsed by the SSI engine. On busy systems, this could reduce performance.

Bear in mind that HTML generated by a CGI script does not get put through the SSI processor, so it's no good including the markup listed in this chapter in a CGI script.

Options Includes turns on processing of SSIs. As usual, look in the error_log if things don't work. The error messages passed to the client are necessarily uninformative since they are probably being read three continents away, where nothing useful can be done about them.

The trick of SSI is to insert special strings into our documents, which then get picked up by Apache on their way through, tested against reference strings using =, !=, <, <=, >, and >=, and then replaced by dynamically written messages. As we will see, the strings have a deliberately unusual form so they won't get confused with more routine stuff. This is the syntax of a command:

<!--#element attribute="value" attribute="value" ... -->

The Apache manual tells us what the elements are:

config

This command controls various aspects of the parsing. The valid attributes are as follows:

errmsg

The value is a message that is sent back to the client if an error occurs during document parsing.

The value sets the format to be used when displaying the size of a file. Valid values are bytes for a count in bytes or abbrev for a count in kilobytes or megabytes, as appropriate.

The value is a string to be used by the strftime( ) library routine when printing dates.

echo

This command prints one of the include variables, defined later in this chapter. If the variable is unset, it is printed as (none). Any dates printed are subject to the currently configured timefmt. This is the only attribute:

var

The value is the name of the variable to print.

exec

The exec command executes a given shell command or CGI script. Options IncludesNOEXEC disables this command completely — a boon to the prudent webmaster. The valid attribute is as follows:

cgi: The value specifies a %-encoded URL relative path to the CGI script. If the path does not begin with a slash, it is taken to be relative to the current document. The document referenced by this path is invoked as a CGI script, even if the server would not normally recognize it as such. However, the directory containing the script must be enabled for CGI scripts (with ScriptAlias or the ExecCGI option). The protective wrapper suEXEC will be applied if it is turned on. The CGI script is given the PATH_INFO and query string (QUERY_STRING) of the original request from the client; these cannot be specified in the URL path. The include variables will be available to the script in addition to the standard CGI environment. If the script returns a Location header instead of output, this is translated into an HTML anchor. If Options IncludesNOEXEC is set in the Config file, this command is turned off. The include virtual element should be used in preference to exec cgi.

cmd

The server executes the given string using /bin/sh. The include variables are available to the command. If Options IncludesNOEXEC is set in the Config file, this is disabled and will cause an error, which will be written to the error log.

fsize

This command prints the size of the specified file, subject to the sizefmt format specification. The attributes are as follows:

file: The value is a path relative to the directory containing the current document being parsed.
virtual: The value is a %-encoded URL path relative to the document root. If it does not begin with a slash, it is taken to be relative to the current document.

This command prints the last modification date of the specified file, subject to the timefmt format specification. The attributes are the same as for the fsize command.

This command includes other files immediately at that point in parsing — right there and then, not later on. Any included file is subject to the usual access control. If the directory containing the parsed file has Options IncludesNOEXEC set and including the document causes a program to be executed, it isn't included: this prevents the execution of CGI scripts. Otherwise, CGI scripts are invoked as normal using the complete URL given in the command, including any query string.

An attribute defines the location of the document; the inclusion is done for each attribute given to the include command. The valid attributes are as follows:

file: The value is a path relative to the directory containing the current document being parsed. It can't contain ../, nor can it be an absolute path. The virtual attribute should always be used in preference to this one.
virtual: The value is a %-encoded URL relative to the document root. The URL cannot contain a scheme or hostname, only a path and an optional query string. If it does not begin with a slash, then it is taken to be relative to the current document. A URL is constructed from the attribute's value, and the server returns the same output it would have if the client had requested that URL. Thus, included files can be nested. A CGI script can still be run by this method even if Options IncludesNOEXEC is set in the Config file. The reasoning is that clients can run the CGI anyway by using its URL as a hot link or simply by typing it into their browser; so no harm is done by using this method (unlike cmd or exec).

14.1. File Size

The fsize command allows you to report the size of a file inside a document. The file size.shtml is as follows:

<!--#config errmsg="Bungled again!"-->
<!--#config sizefmt="bytes"-->
The size of this file is <!--#fsize file="size.shtml"--> bytes.
The size of another_file is <!--#fsize file="another_file"--> bytes.

The first line provides an error message. The second line means that the size of any files is reported in bytes printed as a number, for instance, 89. Changing bytes to abbrev gets the size in kilobytes, printed as 1k. The third line prints the size of size.shtml itself; the fourth line prints the size of another_file. config commands must appear above commands that might want to use them.

You can replace the word file= in this script, and in those which follow, with virtual=, which gives a %-encoded URL path relative to the document root. If it does not begin with a slash, it is taken to be relative to the current document.

If you play with this stuff, you find that Apache is strict about the syntax. For instance, trailing spaces cause an error because valid filenames don't have them:

The size of this file is <!--#fsize file="size.shtml   "--> bytes.
The size of this file is Bungled again! bytes.

If we had not used the errmsg command, we would see the following:

...[an error occurred while processing this directive]...

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