Query Strings

The internal rewriter does not rewrite URL references contained within query strings. Only the hostname portion of the reference is evaluated for rewriting.

HTTP Headers

The internal rewriter rewrites qualified URL references occurring within certain types of HTTP response headers such as Location and Content-Location. The Location header is used to redirect the browser to where the resource can be found. The Content-Location header is used to provide an alternate location where the resource can be found.

JavaScript

Within JavaScript^*, only absolute references are evaluated for rewriting. Relative references and absolute paths are not attempted. Absolute paths (/path/file.html) are evaluated if the file is read from a path-based multi-homing accelerator's origin Web server and the reference follows an HTML tag. For example, the string href=‘/path/file.html' is rewritten to href=‘/accelPath/path/file.html'.

HTML Tags

URL references occurring within the following HTML tags are evaluated for rewriting:

Mime Types

The rewriter parses pages with certain Mime Content-Types regardless of the file extension. By default, the internal rewriter parses pages with the following Mime Content-Types:

If an HTTP or HTTPS response has a Mime Content-Type set to any of the above types, or if the file extension is html, htm, shtml, jhtml, asp, jsp, or NO EXTENSION, the page is parsed for possible rewriting.

Absolute and Relative References

An absolute reference is a reference that has all the information needed to locate a resource, including the hostname, such as http://internal.web.site.com/index.html. The internal rewriter always attempts to rewrite absolute references.

A relative reference is a reference that assumes the host/path and provides only the resource of the URI, such as index.htm. The internal rewriter does not attempt to rewrite a relative reference.

An absolute path is a reference that assumes the host. It provides the complete path, including the resource. The internal rewriter attempts to rewrite an absolute path only when it is defined in a path-based multi-homed accelerator.

Path-Based Multi-Homing

With an accelerator configured for path-based multi-homing, absolute references and absolute paths are evaluated for rewriting. Relative references are not attempted.

[Mime Content-Type]

In addition to files with extensions listed in the [Extension] section below, the rewriter parses pages with certain Mime Content-Types regardless of the file extension. By default, the internal rewriter parses pages with the following Mime Content-Types:

[Mime Content-Type]           text/html
text/css
text/xml
text/javascript
application/javascript
application/x-javascript

Text/plain is not a default content type.

[Extension]

It is unusual for data to come back from a Web server without a content type. If it does happen, the file extension is compared. If a file doesn't have an extension, it always matches the null case, and the rewriter parses the page. The default extensions are as follows:

[Extension]
html, htm
shtml, jhtml
asp, jsp
js
css

Additional file extensions can be added by using the [Extension] section of rewriter.cfg, as shown below:

[Extension]
home, myNewExtension
anotherLongExtension
a,b,c,d,e,f,g

As shown in this example, additional extensions can be specified on individual lines or by using commas to separate multiple extensions specified on a single line. All of the extensions listed are appended to the default list shown above. You cannot remove the default extensions.

[Exclude]

Exclude keeps the entity data from being rewritten if the requesting URL has a match in the exclude list. It checks the requested URL and doesn't check the URL returned from the Web server. For example:

[Exclude]
http://www.a.com/dont_rewrite             ; Match without ending slash
http://www.a.com/dont_rewrite/            ; Match with ending slash
http://www.a.com/dont_rewrite/index.html  ; Match specific file name
http://www.a.com/dont_rewrite/*           ; Includes all files and
                                          ; subfiles and the three 
                                          ; examples above
http://www.a.com/*                        ; Turn off rewriting for 
                                          ; this accelerator and all 
                                          ; path-based children.

[Javascript Variables]

You can add JavaScript variables to look for URL references. This adds to the HTML variable list (href=, src=, onclick=). The word parser edits the variable to the form [w]variable[ow]=[ow], where [w] represents a space, [ow] represents an optional space, and the URL reference follows.

For example, suppose you use the following JavaScript variables:

[Javascript Variables]
headingGif
plusGifUrl
minusGifUrl
stylesheetUrl

A JavaScript example would be:

var headingGif = “/path/path/file.gif”;

[Javascript Calls]

A JavaScript call can be made within JavaScript code or in HTML code, such as:

onclick='open(“/path/file.html”)'

By default, all parameters of a JavaScript call are looked at for rewriting. The first JavaScript call within an HTML tag (href, src, onclick, etc.) has all of its parameters parsed for rewriting.

This adds to the word parser the call to parse the parameters within a JavaScript call that appears in JavaScript code, not within the HTML URL tags.

The word parser edits the variable to the form “[w]call([ow][url]...)” where the URL references are parameters within the calls.

For example:

[Javascript Calls]
openWindow

[Alias Host Names]

Sometimes a URL reference specifies a hostname that does not meet default criteria for being rewritten; that is, it does not match the accelerator's alternate hostname or any value in the Web server address list. For example, assume that a URL reference contains the hostname of home (http://home/index.html), and home is not included in the Web server address list because it is not resolvable, nor is it the value of the accelerator’s Alternate Host Name field. By default, rewriting of the URL reference http://home/index.html would not occur. The [Alias Host Names] section of rewriter.cfg can be used to specify additional hostnames to be rewritten. Using alias hostnames only applies for absolute URLs. You cannot use alias hostnames for relative URLs.

The following is an example of how to use the [Alias Host Names] section. It has the following syntax:

[Alias Host Names]
AcceleratorName=aliasName

where AcceleratorName is the value specified in the accelerator's Name field, and aliasName is the string that is rewritten with the value specified in the accelerator's DNS name field.

For the home example used above, if the accelerator name is accel2 and the URL reference to be rewritten is the hostname home, the correct syntax is:

[Alias Host Names]
accel2=home

You can use [Alias Host Names] to add and remove the set of hostnames, schemes, ports, and paths that are used to represent the identity of an accelerator's Web server.

The following example illustrates the syntax to use to rewrite these items and then provides examples:

#add an alias hostname
acceleratorName=aliasHostName

#add a full host reference
# http://alias
acceleratorName=scheme://aliasHostName
# http://alias:80
accleratorName=scheme://aliasHostName

#add an addition subpath for a path-based multi-homed accelerator
# that does NOT have the remove subpath option checked
acceleratorName=/additionalPath  

#Remove a hostname from being used to rewriting to this accelerator.
# Use this option when there are multiple accelerators that contain
# the same alternate hostname and Web server port.
acceleratorName!=alternateHostName

[Alias Host Names]
novell=HOME
novell=https://www.backend.com:444
download=/fileDownloads
novell!=testserver

Disabling Per Accelerator

By default, the internal rewriter is enabled for all accelerators. The internal rewriter can slow performance because of the parsing overhead. In some cases, a Web site might not have content with URL references that need to be rewritten. The internal rewriter can be disabled on a per-accelerator basis using the SET command on the command line of the iChain machine. The following is an example of how you would use this command:

SET ACCELERATOR <name> DisableRewriter=Yes

where <name> is the name of the accelerator for which you want to disable rewriting. This action is permanent upon reboot and is exported to the .nas file.

Disabling Per URL

The rewriter.cfg file also allows you to specify a list of URLs which are to be excluded by the rewriter. For example:

[exclude]
http://www.abc.com/xyz/*
http://www.abc.com/donotrewrite.html

As shown in this example, the exclusion causes all pages in the xyz subdirectory and the donotrewrite.html file to be left untouched. The syntax of the URLs requires them to be prefixed by http, and the domain name of the accelerator also must be defined.

Disabling In a Page

In some circumstances, you might find that you need more granularity. There are cases when only part of a page cannot or should not be rewritten. Although this deviates from the premise of iChain that you shouldn’t have to modify the origin server, you might encounter circumstances where it cannot be avoided.

In these cases, you can use the following tags in your origin pages.

For example:

<!--NOVELL_REWRITER_OFF-->
.
.
HTML data not to be rewritten
.
.
<!--NOVELL_REWRITER_ON-->

These tags are seen by browsers as a comment mark, and do show up on the screen (except possibly on older browser versions). Also, the last tag is optional, and if omitted, it prevents the rest of the page from being rewritten after the initial tag is encountered.