Predicates in EAP 7 Undertow Configuration

Solution Unverified - Updated

Environment

  • Red Hat JBoss Enterprise Application Server (EAP)
    • 7.x
  • Undertow

Issue

  • What are predicates?
  • What are the built-in predicates that can be used?
  • What are the built-in exchange values that can be used in a predicate?

Resolution

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Predicates provide a simple way of making a true/false decision based on an exchange. Many handlers have a requirement that they are applied conditionally, and predicates provide a general way to specify a condition. Predicates can be created programmatically (they are just java classes that implement the Predicate interface), however, there is also a simple language for specifying a predicate. Some examples below:

# regular expression match of the relative URL
regex['/resources/*.\.css'] 

# Matches requests with a text/.* content type
regex[pattern='text/.*', value='%{i,Content-Type}', full-match=true]

# Matches if the content type header is text/xml
equals[{'%{i,Content-Type}', 'text/xml'}] 

# User agent contains MSIE and request URL ends with .js
contains[search='MSIE', value='%{i,User-Agent}'] and path-suffix['.js']

# regex match, with a reference to match group 1 later in the expression
regex['/resources/(*.)\.css'] and equals[{'$1', 'myCssFile'}]

In How to redirect HTTP to HTTPS in JBoss EAP 7 explains how to configure a rewrite rule using a predicate to redirect a non-secure HTTP URL to a secure https URL.

For example:

<filter-ref name="http-to-https" predicate="equals(%p,8080)"/>

The predicate, equals(%p,8080), returns true if the exchange attribute %p is 8080. The exchange attribute, %p, is the short form of %{LOCAL_PORT}. 

<filter-ref name="Access-Control-Allow-Origin" predicate="contains(search='Firefox', value='%{i,User-Agent}')"/>
....
<response-header name="Access-Control-Allow-Origin" header-name="Access-Control-Allow-Origin" header-value="https://www.example.com"/>

The predicate, contains(search='Firefox', value='%{i,User-Agent}'), returns true if the incoming User-Agent header value contains Firefox string, and then only the Access-Control-Allow-Origin header will be added to the outgoing response.

For more information check Undertow documentation to check the table with the predicates:

Root Cause

  • Customers are configuring rewrite rules in the Undertow Subsystem and have inquired about the different predicates available for use in the configuration.
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