4D's Web server allows you to use HTML template pages containing tags, i.e. a mix of static HTML code and 4D references added by means of transformation tags such as 4DTEXT, 4DIF, or 4DINCLUDE. These tags are usually inserted as HTML type comments (``) into the HTML source code.
When these pages are sent by the HTTP server, they are parsed and the tags they contain are executed and replaced with the resulting data. The pages received by the browsers are thus a combination of static elements and values coming from 4D processing.
For example, if you write in an HTML page:
<P>Welcome to !</P>
The value of the 4D variable vtSiteName will be inserted in the HTML page.
Tags for templates
The following 4D tags are available:
- 4DTEXT, to insert 4D variables and expressions as text,
- 4DHTML, to insert HTML code,
- 4DEVAL, to evaluate any 4D expression,
- 4DSCRIPT, to execute a 4D method,
- 4DINCLUDE, to include a page within another one,
- 4DBASE, to modify the default folder used by the 4DINCLUDE tag,
- 4DCODE, to insert 4D code,
- 4DIF, 4DELSE, 4DELSEIF and 4DENDIF, to insert conditions in the HTML code,
- 4DLOOP and 4DENDLOOP, to make loops in the HTML code,
- 4DEACH and 4DENDEACH, to loop in collections, entity selections, or object properties.
These tags are described in the Transformation Tags page.
It is possible to mix tags. For example, the following HTML code is allowed:
<HTML> ... <BODY> (Method call) (If condition) (Subpage insertion) (End if) (loop on the current selection) (If [TABLE]ValNum>10) (subpage insertion) (Else) <B>Value: </B><BR> (Field display) (End for) </BODY> </HTML>
For optimization reasons, the parsing of the HTML source code is not carried out by the 4D Web server when HTML pages are called using simple URLs that are suffixed with “.HTML” or “.HTM”.
Parsing of the contents of template pages sent by 4D web server takes place when
WEB SEND FILE (.htm, .html, .shtm, .shtml),
WEB SEND BLOB (text/html type BLOB) or
WEB SEND TEXT commands are called, as well as when sending pages called using URLs. In this last case, for reasons of optimization, pages that are suffixed with “.htm” and “.html” are NOT parsed. In order to "force" the parsing of HTML pages in this case, you must add the suffix “.shtm” or “.shtml” (for example,
http://www.server.com/dir/page.shtm). An example of the use of this type of page is given in the description of the
WEB GET STATISTICS command. XML pages (.xml, .xsl) are also supported and always parsed by 4D.
You can also carry out parsing outside of the Web context when you use the
PROCESS 4D TAGS command.
Internally, the parser works with UTF-16 strings, but the data to parse may have been encoded differently. When tags contain text (for example
4DHTML), 4D converts the data when necessary depending on its origin and the information available (charset). Below are the cases where 4D parses the tags contained in the HTML pages, as well as any conversions carried out:
|Action / Command||Content analysis of the sent pages||Support of $ syntax(*)||Character set used for parsing tags|
|Pages called via URLs||X, except for pages with “.htm” or “.html” extensions||X, except for pages with “.htm” or “.html” extensions||Use of charset passed as parameter of the "Content-Type" header of the page. If there is none, search for a META-HTTP EQUIV tag with a charset. Otherwise, use of default character set for the HTTP server|
|X||-||Use of charset passed as parameter of the "Content-Type" header of the page. If there is none, search for a META-HTTP EQUIV tag with a charset. Otherwise, use of default character set for the HTTP server|
|X||-||No conversion necessary|
|X, if BLOB is of the “text/html” type||-||Use of charset set in the "Content-Type" header of the response. Otherwise, use of default character set for the HTTP server|
|Inclusion by the `` tag||X||X||Use of charset passed as parameter of the "Content-Type" header of the page. If there is none, search for a META-HTTP EQUIV tag with a charset. Otherwise, use of default character set for the HTTP server|
|X||X||Text data: no conversion. BLOB data: automatic conversion from the Mac-Roman character set for compatibility|
(*) The alternative $-based syntax is available for 4DHTML, 4DTEXT and 4DEVAL tags.
Accessing 4D methods via the Web
Executing a 4D method with
4DTEXT from a web request is subject to the Available through 4D tags and URLs (4DACTION...) attribute value defined in the properties of the method. If the attribute is not checked for the method, it can not be called from a web request.
Prevention of malicious code insertion
4D tags accept different types of data as parameters: text, variables, methods, command names, etc. When this data is provided by your own code, there is no risk of malicious code insertion since you control the input. However, your database code often works with data that was, at one time or another, introduced through an external source (user input, import, etc.).
In this case, it is advisable to not use tags such as
4DSCRIPT, which evaluate parameters, directly with this sort of data.
In addition, according to the principle of recursion, malicious code may itself include transformation tags. In this case, it is imperative to use the
4DTEXT tag. Imagine, for example, a Web form field named "Name", where users must enter their name. This name is then displayed using a `` tag in the page. If text of the "" type is inserted instead of the name, interpreting this tag will cause the application to be exited. To avoid this risk, you can just use the
4DTEXT tag systematically in this case. Since this tag escapes the special HTML characters, any malicious recursive code that may have been inserted will not be reinterpreted. To refer to the previous example, the "Name" field will contain, in this case, "
<!--#4DEVAL QUIT 4D-->" which will not be transformed.