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This class of status code indicates the client must take additional action to complete the request. Many of these status codes are used in URL redirection.[20]
A user agent may carry out the additional action with no user interaction only if the method used in the second request is GET or HEAD. A user agent should not automatically redirect a request more than five times[citation needed], since such redirections usually indicate an infinite loop.[21]
Your Web server thinks that your URL has been temporarily redirected to another URL. The client system is expected to immediately retry the alternate URL.
The 302 response from the Web server should always include an alternative URL to which redirection should occur. If it does, a Web browser will immediately retry the alternative URL. So you never actually see a 302 error in a Web browser, unless perhaps you have a corrupt redirection chain e.g. URL A redirects to URL B which in turn redirects back to URL A. If your client is not a Web browser, it should behave in the same way as a Web browser i.e. immediately retry the alternative URL.
If the Web server does not return an alternative URL with the 302 response, then either the Web server sofware itself is defective or the Webmaster has not set up the URL redirection correctly.
Redirection of URLs may occur for low-level URLs (specific URLs within the Web site such as www.isp.com/products/index.html) when you reorganise the web site, but is relatively uncommon for the top-level URLs (such as www.isp.com) which most CheckUpDown users ask us to check. So this error should be fairly infrequent.
The 302 response from the Web server should always include an alternative URL to which redirection should occur. If it does, CheckUpDown automatically tries the alternative URL. This in turn may possibly lead to another redirection which CheckUpDown then tries. This continues for a maximum of 5 redirections. As soon as 5 redirections have occurred, CheckUpDown gives up and reports the 302 error for your account. So you should only ever see the 302 error if 1) the Web server gives no alternative URL on the 302 response or 2) the number of redirections exceeds 5. This second condition should be fairly unlikely - and may indicate a recursive pattern e.g. URL A redirects to URL B which in turn redirects back to URL A.
You first need to check that the IP name we use to check for your account is accurate. If you or your ISP have configured something so that any access using this name should now be redirected to another name, then you need to update your CheckUpDown account to start using the new name.
If you believe that the IP name we use is exact (should not be redirected), please try accessing the current URL using a Web browser. Note carefully which URL actually gets displayed, because your browser may silently switch to a substitute URL if it receives an 302 message from the Web server. If you see any evidence of a new URL, try accessing that directly from your browser. If this works (you see the Web site as expected), then this new URL is what you may need to update on your CheckUpDown account. If this is a temporary redirection, then you may also need to reinstate the original IP name at a later date when the redirection is no longer effective.
If none of the above help, we can analyse the underlying HTTP data streams we receive from the Web server. These can provide additional information about the new URL(s) which the Web server thinks we should be accessing. Before doing this, we prefer you to identify any deliberate changes on your side, liaising with your ISP if needs be.
302 errors should occur infrequently, because top-level URLs do not change often. If they do change, then this is typically because a redirection URL is being suggested. This pervasive change is unlikely to occur by accident, so most often we can resolve this error by updating our system records for your account following a deliberate change of URL on your part, whether this change is temporary or permanent.
Any client (e.g. your Web browser or our CheckUpDown robot) goes through the following cycle when it communicates with the Web server:
This error occurs in the final step above when the client receives an HTTP status code that it recognises as ‘302‘.
This class of status code indicates that further action needs to be taken by the user agent in order to fulfill the request. The action required MAY be carried out by the user agent without interaction with the user if and only if the method used in the second request is GET or HEAD. A client SHOULD detect infinite redirection loops, since such loops generate network traffic for each redirection.
Note: previous versions of this specification recommended a
maximum of five redirections. Content developers should be aware
that there might be clients that implement such a fixed
limitation.
The requested resource corresponds to any one of a set of representations, each with its own specific location, and agent- driven negotiation information (section 12) is being provided so that the user (or user agent) can select a preferred representation and redirect its request to that location.
Unless it was a HEAD request, the response SHOULD include an entity containing a list of resource characteristics and location(s) from which the user or user agent can choose the one most appropriate. The entity format is specified by the media type given in the Content- Type header field. Depending upon the format and the capabilities of
the user agent, selection of the most appropriate choice MAY be performed automatically. However, this specification does not define any standard for such automatic selection.
If the server has a preferred choice of representation, it SHOULD include the specific URI for that representation in the Location field; user agents MAY use the Location field value for automatic redirection. This response is cacheable unless indicated otherwise.
The requested resource has been assigned a new permanent URI and any future references to this resource SHOULD use one of the returned URIs. Clients with link editing capabilities ought to automatically re-link references to the Request-URI to one or more of the new references returned by the server, where possible. This response is cacheable unless indicated otherwise.
The new permanent URI SHOULD be given by the Location field in the response. Unless the request method was HEAD, the entity of the response SHOULD contain a short hypertext note with a hyperlink to the new URI(s).
If the 301 status code is received in response to a request other than GET or HEAD, the user agent MUST NOT automatically redirect the request unless it can be confirmed by the user, since this might change the conditions under which the request was issued.
Note: When automatically redirecting a POST request after
receiving a 301 status code, some existing HTTP/1.0 user agents
will erroneously change it into a GET request.
The requested resource resides temporarily under a different URI. Since the redirection might be altered on occasion, the client SHOULD continue to use the Request-URI for future requests. This response is only cacheable if indicated by a Cache-Control or Expires header field.
The temporary URI SHOULD be given by the Location field in the response. Unless the request method was HEAD, the entity of the response SHOULD contain a short hypertext note with a hyperlink to the new URI(s).
If the 302 status code is received in response to a request other than GET or HEAD, the user agent MUST NOT automatically redirect the request unless it can be confirmed by the user, since this might change the conditions under which the request was issued.
Note: RFC 1945 and RFC 2068 specify that the client is not allowed
to change the method on the redirected request. However, most
existing user agent implementations treat 302 as if it were a 303
response, performing a GET on the Location field-value regardless
of the original request method. The status codes 303 and 307 have
been added for servers that wish to make unambiguously clear which
kind of reaction is expected of the client.
The response to the request can be found under a different URI and SHOULD be retrieved using a GET method on that resource. This method exists primarily to allow the output of a POST-activated script to redirect the user agent to a selected resource. The new URI is not a substitute reference for the originally requested resource. The 303 response MUST NOT be cached, but the response to the second (redirected) request might be cacheable.
The different URI SHOULD be given by the Location field in the response. Unless the request method was HEAD, the entity of the response SHOULD contain a short hypertext note with a hyperlink to the new URI(s).
Note: Many pre-HTTP/1.1 user agents do not understand the 303
status. When interoperability with such clients is a concern, the
302 status code may be used instead, since most user agents react
to a 302 response as described here for 303.
If the client has performed a conditional GET request and access is allowed, but the document has not been modified, the server SHOULD respond with this status code. The 304 response MUST NOT contain a message-body, and thus is always terminated by the first empty line after the header fields.
The response MUST include the following header fields:
- Date, unless its omission is required by section 14.18.1
If a clockless origin server obeys these rules, and proxies and clients add their own Date to any response received without one (as already specified by [RFC 2068], section 14.19), caches will operate correctly.
- ETag and/or Content-Location, if the header would have been sent
in a 200 response to the same request
- Expires, Cache-Control, and/or Vary, if the field-value might
differ from that sent in any previous response for the same
variant
If the conditional GET used a strong cache validator (see section 13.3.3), the response SHOULD NOT include other entity-headers. Otherwise (i.e., the conditional GET used a weak validator), the response MUST NOT include other entity-headers; this prevents inconsistencies between cached entity-bodies and updated headers.
If a 304 response indicates an entity not currently cached, then the cache MUST disregard the response and repeat the request without the conditional.
If a cache uses a received 304 response to update a cache entry, the cache MUST update the entry to reflect any new field values given in the response.
The requested resource MUST be accessed through the proxy given by the Location field. The Location field gives the URI of the proxy. The recipient is expected to repeat this single request via the proxy. 305 responses MUST only be generated by origin servers.
Note: RFC 2068 was not clear that 305 was intended to redirect a
single request, and to be generated by origin servers only. Not
observing these limitations has significant security consequences.
The 306 status code was used in a previous version of the specification, is no longer used, and the code is reserved.
The requested resource resides temporarily under a different URI. Since the redirection MAY be altered on occasion, the client SHOULD continue to use the Request-URI for future requests. This response is only cacheable if indicated by a Cache-Control or Expires header field.
The temporary URI SHOULD be given by the Location field in the response. Unless the request method was HEAD, the entity of the response SHOULD contain a short hypertext note with a hyperlink to the new URI(s) , since many pre-HTTP/1.1 user agents do not understand the 307 status. Therefore, the note SHOULD contain the information necessary for a user to repeat the original request on the new URI.
If the 307 status code is received in response to a request other than GET or HEAD, the user agent MUST NOT automatically redirect the request unless it can be confirmed by the user, since this might change the conditions under which the request was issued.
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原文地址：http://www.cnblogs.com/timdes/p/5157815.html
