Apache Module mod_proxy_html

Summary

This module provides an output filter to rewrite HTML links in a proxy situation, to ensure that links work for users outside the proxy. It serves the same purpose as Apache's ProxyPassReverse directive does for HTTP headers, and is an essential component of a reverse proxy.

For example, if a company has an application server at appserver.example.com that is only visible from within the company's internal network, and a public webserver www.example.com, they may wish to provide a gateway to the application server at http://www.example.com/appserver/. When the application server links to itself, those links need to be rewritten to work through the gateway. mod_proxy_html serves to rewrite <a href="http://appserver.example.com/foo/bar.html">foobar</a> to <a href="http://www.example.com/appserver/foo/bar.html">foobar</a> making it accessible from outside.

mod_proxy_html was originally developed at WebÞing, whose extensive documentation may be useful to users.

Directives

• ProxyHTMLBufSize
• ProxyHTMLCharsetOut
• ProxyHTMLDocType
• ProxyHTMLEnable
• ProxyHTMLEvents
• ProxyHTMLExtended
• ProxyHTMLFixups
• ProxyHTMLInterp
• ProxyHTMLLinks
• ProxyHTMLMeta
• ProxyHTMLStripComments
• ProxyHTMLURLMap

Bugfix checklist

• httpd changelog
• Known issues
• Report a bug

ProxyHTMLBufSize Directive ¶

In order to parse non-HTML content (stylesheets and scripts) embedded in HTML documents, mod_proxy_html has to read the entire script or stylesheet into a buffer. This buffer will be expanded as necessary to hold the largest script or stylesheet in a page, in increments of bytes as set by this directive.

The default is 8192, and will work well for almost all pages. However, if you know you're proxying pages containing stylesheets and/or scripts bigger than 8K (that is, for a single script or stylesheet, NOT in total), it will be more efficient to set a larger buffer size and avoid the need to resize the buffer dynamically during a request.

ProxyHTMLCharsetOut Directive ¶

This selects an encoding for mod_proxy_html output. It should not normally be used, as any change from the default UTF-8 (Unicode - as used internally by libxml2) will impose an additional processing overhead. The special token ProxyHTMLCharsetOut * will generate output using the same encoding as the input.

Note that this relies on mod_xml2enc being loaded.

ProxyHTMLDocType Directive ¶

In the first form, documents will be declared as HTML 4.01 or XHTML 1.0 according to the option selected. This option also determines whether HTML or XHTML syntax is used for output. Note that the format of the documents coming from the backend server is immaterial: the parser will deal with it automatically. If the optional second argument is set to Legacy, documents will be declared "Transitional", an option that may be necessary if you are proxying pre-1998 content or working with defective authoring/publishing tools.

In the second form, it will insert your own FPI. The optional second argument determines whether SGML/HTML or XML/XHTML syntax will be used.

The default is changed to omitting any FPI, on the grounds that no FPI is better than a bogus one. If your backend generates decent HTML or XHTML, set it accordingly.

If the first form is used, mod_proxy_html will also clean up the HTML to the specified standard. It cannot fix every error, but it will strip out bogus elements and attributes. It will also optionally log other errors at LogLevel Debug.

ProxyHTMLEnable Directive ¶

A simple switch to enable or disable the proxy_html filter. If mod_xml2enc is loaded it will also automatically set up internationalisation support.

Note that the proxy_html filter will only act on HTML data (Content-Type text/html or application/xhtml+xml) and when the data are proxied. You can override this (at your own risk) by setting the PROXY_HTML_FORCE environment variable.

ProxyHTMLEvents Directive ¶

Specifies one or more attributes to treat as scripting events and apply ProxyHTMLURLMaps to where enabled. You can specify any number of attributes in one or more ProxyHTMLEvents directives.

Normally you'll set this globally. If you set ProxyHTMLEvents in more than one scope so that one overrides the other, you'll need to specify a complete set in each of those scopes.

A default configuration is supplied in proxy-html.conf and defines the events in standard HTML 4 and XHTML 1.

ProxyHTMLExtended Directive ¶

Set to Off, HTML links are rewritten according to the ProxyHTMLURLMap directives, but links appearing in Javascript and CSS are ignored.

Set to On, all scripting events (as determined by ProxyHTMLEvents) and embedded scripts or stylesheets are also processed by the ProxyHTMLURLMap rules, according to the flags set for each rule. Since this requires more parsing, performance will be best if you only enable it when strictly necessary.

You'll also need to take care over patterns matched, since the parser has no knowledge of what is a URL within an embedded script or stylesheet. In particular, extended matching of / is likely to lead to false matches.

ProxyHTMLFixups Directive ¶

This directive takes one to three arguments as follows:

• lowercase Urls are rewritten to lowercase
• dospath Backslashes in URLs are rewritten to forward slashes.
• reset Unset any options set at a higher level in the configuration.

Take care when using these. The fixes will correct certain authoring mistakes, but risk also erroneously fixing links that were correct to start with. Only use them if you know you have a broken backend server.

ProxyHTMLInterp Directive ¶

This enables per-request interpolation in ProxyHTMLURLMap to- and from- patterns.

If interpolation is not enabled, all rules are pre-compiled at startup. With interpolation, they must be re-compiled for every request, which implies an extra processing overhead. It should therefore be enabled only when necessary.

ProxyHTMLLinks Directive ¶

Specifies elements that have URL attributes that should be rewritten using standard ProxyHTMLURLMaps. You will need one ProxyHTMLLinks directive per element, but it can have any number of attributes.

Normally you'll set this globally. If you set ProxyHTMLLinks in more than one scope so that one overrides the other, you'll need to specify a complete set in each of those scopes.

A default configuration is supplied in proxy-html.conf and defines the HTML links for standard HTML 4 and XHTML 1.

ProxyHTMLLinks  a          href
ProxyHTMLLinks  area       href
ProxyHTMLLinks  link       href
ProxyHTMLLinks  img        src longdesc usemap
ProxyHTMLLinks  object     classid codebase data usemap
ProxyHTMLLinks  q          cite
ProxyHTMLLinks  blockquote cite
ProxyHTMLLinks  ins        cite
ProxyHTMLLinks  del        cite
ProxyHTMLLinks  form       action
ProxyHTMLLinks  input      src usemap
ProxyHTMLLinks  head       profile
ProxyHTMLLinks  base       href
ProxyHTMLLinks  script     src for

ProxyHTMLMeta Directive ¶

This turns on or off pre-parsing of metadata in HTML <head> sections.

If not required, turning ProxyHTMLMeta Off will give a small performance boost by skipping this parse step. However, it is sometimes necessary for internationalisation to work correctly.

ProxyHTMLMeta has two effects. Firstly and most importantly it enables detection of character encodings declared in the form

<meta http-equiv="Content-Type" content="text/html;charset=foo">

or, in the case of an XHTML document, an XML declaration. It is NOT required if the charset is declared in a real HTTP header (which is always preferable) from the backend server, nor if the document is utf-8 (unicode) or a subset such as ASCII. You may also be able to dispense with it where documents use a default declared using xml2EncDefault, but that risks propagating an incorrect declaration. A ProxyHTMLCharsetOut can remove that risk, but is likely to be a bigger processing overhead than enabling ProxyHTMLMeta.

The other effect of enabling ProxyHTMLMeta is to parse all <meta http-equiv=...> declarations and convert them to real HTTP headers, in keeping with the original purpose of this form of the HTML <meta> element.

ProxyHTMLStripComments Directive ¶

This directive will cause mod_proxy_html to strip HTML comments. Note that this will also kill off any scripts or styles embedded in comments (a bogosity introduced in 1995/6 with Netscape 2 for the benefit of then-older browsers, but still in use today). It may also interfere with comment-based processors such as SSI or ESI: be sure to run any of those before mod_proxy_html in the filter chain if stripping comments!

ProxyHTMLURLMap Directive ¶

This is the key directive for rewriting HTML links. When parsing a document, whenever a link target matches from-pattern, the matching portion will be rewritten to to-pattern, as modified by any flags supplied and by the ProxyHTMLExtended directive. Only the elements specified using the ProxyHTMLLinks directive will be considered as HTML links.

The optional third argument may define any of the following Flags. Flags are case-sensitive.

h

Ignore HTML links (pass through unchanged)

e

Ignore scripting events (pass through unchanged)

c

Pass embedded script and style sections through untouched.

L

Last-match. If this rule matches, no more rules are applied (note that this happens automatically for HTML links).

l

Opposite to L. Overrides the one-change-only default behavior with HTML links.

R

Use Regular Expression matching-and-replace. from-pattern is a regexp, and to-pattern a replacement string that may be based on the regexp. Regexp memory is supported: you can use brackets () in the from-pattern and retrieve the matches with $1 to $9 in the to-pattern.

If R is not set, it will use string-literal search-and-replace. The logic is starts-with in HTML links, but contains in scripting events and embedded script and style sections.

x

Use POSIX extended Regular Expressions. Only applicable with R.

i

Case-insensitive matching. Only applicable with R.

n

Disable regexp memory (for speed). Only applicable with R.

s

Line-based regexp matching. Only applicable with R.

^

Match at start only. This applies only to string matching (not regexps) and is irrelevant to HTML links.

$

Match at end only. This applies only to string matching (not regexps) and is irrelevant to HTML links.

V

Interpolate environment variables in to-pattern. A string of the form ${varname|default} will be replaced by the value of environment variable varname. If that is unset, it is replaced by default. The |default is optional.

NOTE: interpolation will only be enabled if ProxyHTMLInterp is On.

v

Interpolate environment variables in from-pattern. Patterns supported are as above.

NOTE: interpolation will only be enabled if ProxyHTMLInterp is On.

The optional fourth cond argument defines a condition that will be evaluated per Request, provided ProxyHTMLInterp is On. If the condition evaluates FALSE the map will not be applied in this request. If TRUE, or if no condition is defined, the map is applied.

A cond is evaluated by the Expression Parser. In addition, the simpler syntax of conditions in mod_proxy_html 3.x for HTTPD 2.0 and 2.2 is also supported.