It is possible to test for null but for "".
So:
RewriteCond %{HTTP:proxy} .+
RewriteRule  .* - [F]
Now works lik in mod_rewrite.


git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1753358 13f79535-47bb-0310-9956-ffa450edef68
diff --git a/java/org/apache/catalina/valves/rewrite/ResolverImpl.java b/java/org/apache/catalina/valves/rewrite/ResolverImpl.java
index a3d7447..c074ab8 100644
--- a/java/org/apache/catalina/valves/rewrite/ResolverImpl.java
+++ b/java/org/apache/catalina/valves/rewrite/ResolverImpl.java
@@ -142,7 +142,11 @@
 
     @Override
     public String resolveHttp(String key) {
-        return request.getHeader(key);
+        String header = request.getHeader(key);
+        if (header == null)
+            return ""; 
+        else
+            return header;
     }
 
     @Override