ch05.htm

Web_Programming_with_Perl5,一个不错的Perl语言教程。

REMOTE_ADDR       : 123.255.123.1

AUTH_TYPE         :

REMOTE_USER       :

REMOTE_IDENT      :

CONTENT_TYPE      :

CONTENT_LENGTH    :

HTTP_ACCEPT       : image/gif, image/jpeg, */*

HTTP_USER_AGENT   : Mozilla/2.0S (X11; I; IRIX 6.2 IP22)

HTTP_REFERER      : http://server.sgi.com/html/cgi.html

ANNOTATION_SERVER :

</FONT></PRE>

<CENTER>

<H4><A NAME="Heading11"></A><FONT COLOR="#000077">The Response</FONT></H4>

</CENTER>

<P>Now that your CGI script is running, a response must immediately be sent back

to the browser. The response follows a specific format, which will be discussed later,

called an HTTP Response Header. <B><TT>HTTP Response Headers</TT></B>



<DL>

	<DT></DT>

</DL>



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<H3>

<HR WIDTH="83%">

<BR>

<FONT COLOR="#000077">NOTE:</FONT></H3>

</CENTER>





<BLOCKQUOTE>

	<P>An HTTP Response Header is sent by the server in response to any HTTP request,

	not just requests for CGI scripts. If the client requests an HTML page, a response

	header is generated by the server and is sent to the browser before the HTML page.

	HTTP response headers are not typically generated for requests for GIF and JPEG images.<BR>

	

<HR>





</BLOCKQUOTE>



<P>Here is an example of an HTTP response header:</P>

<PRE><FONT COLOR="#0066FF">HTTP/1.0 200 OK

Server: Netscape-Communications/1.12

Date: Monday, 21-Oct-96 01:42:01 GMT

Last-modified: Thursday, 10-Oct-96 17:17:20 GMT

Content-length: 1048

Content-type: text/html

CrLf

</FONT></PRE>

<P>The contents (or body) of the message would follow after the Content-type and

a Carriage-<BR>

Return Line-Feed (<TT>CrLf</TT>) sequence. <B><TT>Parsed Header Output</TT></B> There

are two ways the response can be handled. By default, the Web server reads and parses

the output (STDOUT) of your CGI script before sending the output on to the browser.

The Web server will then automatically generate all necessary HTTP response headers.

This is called parsed header output. The output of your CGI script is actually appended

to the response headers generated by the Web server. If your script contains no output

(HTML, or text, or whatever), it is expected to output either a <TT>Location</TT>

or <TT>Status</TT> header (discussed below in the &quot;Status Response Header&quot;

and &quot;Location Response Header&quot; sections). Thus, in summary, the Web server

sends to the client in this order: a response header, a blank line (CrLf), then any

output from your CGI script. This presents some problems:



<UL>

	<LI>All output from your CGI script must be processed by the Web server, which must

	then generate response headers and append the output of your script to them. This

	is very inefficient and significantly slows down your program.

</UL>





<UL>

	<LI>The Web server is slowed down because of the additional processing that must

	be done prior to sending the output to the client. The Web server is an unnecessary

	middleman.

</UL>





<UL>

	<LI>The Web server still parses and processes the output of your script, even if

	you want your script to pass headers directly to the client.

</UL>



<P><B><TT>Non-Parsed Header Output</TT></B> Fortunately, there is an easy way around

this problem. By simply beginning the name of your scripts with <TT>nph-</TT>, the

Web server will not add any headers or interfere in any way with the output of your

script. So, if you were working on a script called mycgi.pl, just call it nph-mycgi.pl,

and the Web server won't interfere with its headers. This is called a non-parsed-header

script, or NPH script. The output of a NPH script is not parsed and is sent directly

to the client. This is highly recommended. On most popular Macintosh based Web servers,

all CGI scripts are treated as NPH scripts. Figure 5.2 depicts an HTTP transaction

with a NPH CGI script. <TT><BR>

<BR>

</TT><A HREF="06wpp02.jpg" tppabs="http://210.32.137.15/ebook/Web%20Programming%20with%20Perl%205/06wpp02.jpg"><TT><B>Figure 5.2.</B></TT></A> The HTTP transaction

with a NPH CGI.



<DL>

	<DT></DT>

</DL>



<CENTER>

<H3>

<HR WIDTH="83%">

<BR>

<FONT COLOR="#000077">NOTE:</FONT></H3>

</CENTER>





<BLOCKQUOTE>

	<P>This feature must be enabled explicitly in the config files for some servers.<BR>

	

<HR>





</BLOCKQUOTE>



<P>The CGI must respond with a valid and appropriate response header that tells the

browser what to expect or do next. If you use NPH scripts, you must be sure to output

the proper headers; otherwise, the client will see a <TT>500</TT> error message.</P>

<P>There are three basic Response headers: <TT>Status</TT>, <TT>Content-Type</TT>,

and <TT>Location</TT>. All responses must contain either a <TT>Status</TT> header

or a <TT>Location</TT> header that redirects the browser to another URI. If the script

will be outputting HTML, text, or some other MIME compliant data object, the <TT><BR>

Status</TT> header must be followed by a <TT>Content-Type</TT> header that specifies

the MIME type of the object that follows. Other RFC-822 compliant headers such as

<TT>Date</TT> and <TT>Server</TT> can also be <BR>

specified in the response header. <B><TT>Status Response Header</TT></B> The <TT>Status</TT>

response header (or status line) indicates which version of HTTP the server is running,

together with a result code and an optional message. It should not be sent if the

script returns a <TT>Location</TT> header.</P>

<P>The first line of the response from the server typically looks something like

this:</P>

<PRE><FONT COLOR="#0066FF">HTTP/1.0 200 OK

</FONT></PRE>

<P>where <TT>HTTP/1.0</TT> is the version of HTTP the server is running, <TT>200</TT>

is the result code, and <TT>OK</TT> is the message that describes the result code.

There are many other result codes besides <TT>200</TT> that can be sent. The following

<TT>Status</TT> response headers can be sent to the browser by your CGI in NPH mode.

<B><TT>Success 2xx</TT></B> Result codes in the <TT>200</TT>s indicate success. The

body section if present is the object returned by the request. The body must be in

MIME format and may be only in text/plain, text/html, or one of the formats specified

as acceptable in the request in the <TT>HTTP/ACCEPT</TT> header. 

<TABLE BORDER="0">

	<TR ALIGN="LEFT" rowspan="1">

		<TD WIDTH="31" ALIGN="LEFT" VALIGN="TOP"><TT>200</TT></TD>

		<TD ALIGN="LEFT" VALIGN="TOP"><TT>OK</TT></TD>

		<TD ALIGN="LEFT">The request was successful.</TD>

	</TR>

	<TR ALIGN="LEFT" rowspan="1">

		<TD WIDTH="31" ALIGN="LEFT" VALIGN="TOP"><TT>201</TT></TD>

		<TD ALIGN="LEFT" VALIGN="TOP"><TT>CREATED</TT></TD>

		<TD ALIGN="LEFT">Following a <TT>POST</TT> command, indicates success, but the textual part of the

			response indicates the URI by which the newly created document should be known.</TD>

	</TR>

	<TR ALIGN="LEFT" rowspan="1">

		<TD WIDTH="31" ALIGN="LEFT" VALIGN="TOP"><TT>202</TT></TD>

		<TD ALIGN="LEFT" VALIGN="TOP"><TT>Accepted</TT></TD>

		<TD ALIGN="LEFT">The request has been accepted for processing, but the processing has not been completed.</TD>

	</TR>

	<TR ALIGN="LEFT" rowspan="1">

		<TD WIDTH="31" ALIGN="LEFT" VALIGN="TOP"><TT>203</TT></TD>

		<TD ALIGN="LEFT" VALIGN="TOP"><TT>Partial Information</TT></TD>

		<TD ALIGN="LEFT">When received in the response to a <TT>GET</TT> command, this indicates that the

			returned meta-information is not a definitive set of the object from a server with

			a copy of the object but is from a private overlaid Web. This may include annotation

			information about the object, for example.</TD>

	</TR>

	<TR ALIGN="LEFT" rowspan="1">

		<TD WIDTH="31" ALIGN="LEFT" VALIGN="TOP"><FONT COLOR="#000000">204</FONT></TD>

		<TD ALIGN="LEFT" VALIGN="TOP"><FONT COLOR="#000000">No Response</FONT></TD>

		<TD ALIGN="LEFT"><FONT COLOR="#000000">Server has received the request, but there is no information

			to send back, and the client should stay in the same document view. This is mainly

			to allow input for scripts without changing the document at the same time.</FONT></TD>

	</TR>

</TABLE>

</P>

<P><B><TT>Redirection 3xx</TT></B> Result codes in the <TT>300</TT>s indicate the

action to be taken (normally automatically) by the client in order to fulfill the

request. 

<TABLE BORDER="0">

	<TR ALIGN="LEFT" rowspan="1">

		<TD ALIGN="LEFT" VALIGN="TOP"><TT>301</TT></TD>

		<TD ALIGN="LEFT" VALIGN="TOP"><TT>Moved</TT></TD>

		<TD ALIGN="LEFT">The data requested has been assigned a new URI; the change is permanent. The response

			contains a header line of the form: <TT>URI: &lt;url&gt; CrLf</TT>, which specifies

			an alternative address for the object in question.</TD>

	</TR>

	<TR ALIGN="LEFT" rowspan="1">

		<TD ALIGN="LEFT" VALIGN="TOP"><TT>302</TT></TD>

		<TD ALIGN="LEFT" VALIGN="TOP"><TT>Temporarily Moved</TT></TD>

		<TD ALIGN="LEFT">The data requested actually resides under a different URL; however, this is not permanent.

			The response format is the same as for <TT>Moved</TT>.</TD>

	</TR>

	<TR ALIGN="LEFT" rowspan="1">

		<TD ALIGN="LEFT" VALIGN="TOP"><TT>303</TT></TD>

		<TD ALIGN="LEFT" VALIGN="TOP"><TT>Method</TT></TD>

		<TD ALIGN="LEFT">Like the found response, this suggests that the client go try another network address.

			In this case, a different method may be used, too, rather than <TT>GET</TT>. Method:

			<TT>&lt;method&gt; &lt;url&gt;body-section</TT>. The <TT>body-section</TT> contains

			the parameters to be used for the method. This allows a document to be a pointer

			to a complex query operation.</TD>

	</TR>

	<TR ALIGN="LEFT" rowspan="1">

		<TD ALIGN="LEFT" VALIGN="TOP"><FONT COLOR="#000000">304</FONT></TD>

		<TD ALIGN="LEFT" VALIGN="TOP"><FONT COLOR="#000000">Not Modified</FONT></TD>

		<TD ALIGN="LEFT"><FONT COLOR="#000000">If the client performs a conditional GET request, but the document

			has not been modified as specified in the If-Modified-Since variable, this response

			is generated.</FONT></TD>

	</TR>

</TABLE>

</P>

<PRE></PRE>

<P><B><TT>Client ERROR 4xx</TT></B> The 4xx codes are intended for cases in which

the client seems to have erred. The body section may contain a document describing

the error in human readable form. The document is in MIME format and may only be

in text/plain, text/html, or one of the formats specified as acceptable in the request.



<TABLE BORDER="0">

	<TR ALIGN="LEFT" rowspan="1">

		<TD ALIGN="LEFT" VALIGN="TOP"><TT>400</TT></TD>

		<TD ALIGN="LEFT" VALIGN="TOP"><TT>Bad request</TT></TD>

		<TD ALIGN="LEFT">The request had bad syntax or was impossible to be satisfied.</TD>

	</TR>

	<TR ALIGN="LEFT" rowspan="1">

		<TD ALIGN="LEFT" VALIGN="TOP"><TT>401</TT></TD>

		<TD ALIGN="LEFT" VALIGN="TOP"><TT>Unauthorized</TT></TD>

		<TD ALIGN="LEFT">The parameter to this message gives a specification of authorization schemes that

			are acceptable. The client should retry the request with a suitable Authorization

			header.</TD>

	</TR>

	<TR ALIGN="LEFT" rowspan="1">

		<TD ALIGN="LEFT" VALIGN="TOP"><TT>402</TT></TD>

		<TD ALIGN="LEFT" VALIGN="TOP"><TT>Payment Required</TT></TD>

		<TD ALIGN="LEFT">The parameter to this message gives a specification of charging schemes acceptable.

			This code is not currently supported.</TD>

	</TR>

	<TR ALIGN="LEFT" rowspan="1">

		<TD ALIGN="LEFT" VALIGN="TOP"><TT>403</TT></TD>

		<TD ALIGN="LEFT" VALIGN="TOP"><TT>Forbidden</TT></TD>

		<TD ALIGN="LEFT">The request is for something forbidden. Authorization will not help.</TD>

	</TR>

	<TR ALIGN="LEFT" rowspan="1">

		<TD ALIGN="LEFT" VALIGN="TOP"><FONT COLOR="#000000">404</FONT></TD>

		<TD ALIGN="LEFT" VALIGN="TOP"><FONT COLOR="#000000">Not found</FONT></TD>

		<TD ALIGN="LEFT"><FONT COLOR="#000000">The server has not found anything matching the URI given.</FONT></TD>

	</TR>

</TABLE>

</P>

<P><B><TT>Server ERROR 5xx</TT></B> The 5xx codes are for the cases in which the

server has erred, or all indications point to the server as being the cause of the

error. Like the 400 codes, the body section may contain a document describing the

error in human readable form. 

<TABLE BORDER="0">

	<TR ALIGN="LEFT" rowspan="1">

		<TD ALIGN="LEFT" VALIGN="TOP"><TT>500</TT></TD>

		<TD WIDTH="104" ALIGN="LEFT" VALIGN="TOP"><TT>Internal Error</TT></TD>

		<TD ALIGN="LEFT">The server encountered an unexpected condition that prevented it from fulfilling

			the request.</TD>

	</TR>

	<TR ALIGN="LEFT" rowspan="1">

		<TD ALIGN="LEFT" VALIGN="TOP"><TT>501</TT></TD>

		<TD WIDTH="104" ALIGN="LEFT" VALIGN="TOP"><TT>Not Implemented</TT></TD>

		<TD ALIGN="LEFT">The server does not support the ability to satisfy this request.</TD>

	</TR>