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6.3.1 Error Documents using none toreceive none on web,windows application EAN/UCC-13 In 5, we me none none ntioned that if our handler returned an HTTP status code (or, indeed, any value other than OK or DECLINED), this would divert the entire request processing into an error path. Any function implementing an earlier hook in the request cycle may likewise return an HTTP status code. At that point, Apache sets up an internal redirection to the error document for the HTTP status code in question.

An error document is, by default, a predefined document that presents the user with a brief explanation of the error. A server administrator can change this document. 6.4 Gathering Information: Subrequests by using the none none ErrorDocument directive. Because an error document is treated internally as a different request, it can be served by any handler (such as a CGI or PHP script). To avoid going into an error loop, this functionality is not recursive: An error document will not divert the path to another error document by returning an HTTP status code itself.

If that happens, it generates a predefined server error. A special case involves error documents for HTTP 3xx status codes. These codes are not errors, but rather redirections and similar messages.

Thus, in addition to an internal redirection, they generate an HTTP redirection. This operation is straightforward and perfectly normal, as illustrated by our earlier example in which mod_alias handles the Redirect directive..

6.3.2 Dealing with Malformed and Malicious Requests A fundamenta none for none l principle of security on the Web is always to exercise caution in what you accept from any unknown source. That includes HTTP requests coming from anywhere on the Web. Most of these requests will be legitimate, being generated by human-driven browsers, spiders such as Googlebot, proxy cache agents, QA tools such as Site Valet, and so on.

Unfortunately, a significant number of HTTP requests represent attempts to exploit security vulnerabilities. Traces of some rather old IIS worms (e.g.

, Nimda, Code Red) are routinely seen in Apache logs, in which automated attacks attempt to use IIS bugs to take control of Windows servers. Although Apache has not suffered a comparable attack, it is every module developer s business to keep Apache clean! The basic rule is to determine which inputs, or pattern of inputs, an application will accept, and then to reject any request that fails to match an acceptable pattern. Apache offers a ready-made solution that allows any module to deal with bad requests: Simply abort by returning HTTP status code 400 (Bad Request) or, where applicable, a more specific HTTP 4xx status code, as soon as you encounter the bad inputs.

Don t even try to deal with the bad request directly that way complexity and security vulnerabilities lie.. 6.4 Gathering Information: Subrequests A second for none none m of diversion from normal request processing is the subrequest. A subrequest is a diversion to a new request. Unlike with internal redirection, however, processing returns to the original request after the subrequest completes.

. 6 Request Processing Cycle and Metadata Handlers Subrequests none for none constituted an important tool in Apache 1.x, where they could be used to improvise a primitive form of filtering in which a module sets up a handler to run another handler in a subrequest, and intercepts incoming and/or outgoing data. In Apache 2.

x, this kind of hack is no longer necessary. The main role of the subrequest now is to run a fast partial request, to gather information: What would happen if we ran this request For example, mod_autoindex runs a subrequest to each file in a directory, producing a list of only those files that are accessible to the server. Of course, at the system level, we could achieve the same goal with a simple stat, but running a subrequest means that we can also ascertain whether the server configuration permits access.

The subrequest API in Apache 2 comprises four methods to create a subrequest from a request ap_sub_req_lookup_uri ap_sub_req_lookup_file ap_sub_req_lookup_dirent ap_sub_req_method_uri together with a method to run it ap_run_sub_req and a method to dispose of it when done ap_destroy_sub_req When we create a subrequest using one of the first four methods, Apache goes through the request preparation phase (up to the fixups hook). This may be sufficient if the purpose of the subrequest is to gather information on What would happen if we request this URL Running a subrequest is optional. Destroying the subrequest can be a more complex issue.

It is always required, whether or not the request was run. Modules can either run ap_destroy_sub_req explicitly or leave it to the pool cleanup when the parent request is destroyed. Take care when destroying a subrequest, as anything allocated on the subrequest s pool will die along with it!.

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