You try to access a
web page but all you get is a mysterious code number. What do they mean?
This page attempts to address that topic.
Governing Document
Web standards are governed by documents prepared by standards committees,
approved, and then implemented world-wide. Following are notes extracted
from RFC2068, the draft governing
document for the hypertext protocol.
Message Number Categories
Because you usually only see one or two error numbers it's easy to believe
that's all there are. Actually, there are families of numbers; and, not
all are errors. We'll explore each family and its members below.
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1xx
Codes (Information) |
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There
are a few official codes in the one hundred range. But, if you see
one you have probably stumbled onto some sort of experimental application.
In this case, what you see will be non-standard and could be most
anything. |
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An
interim response telling the browser the initial part of its request
has been received and not rejected by the server. A final response
code should be sent when the remainder of the material has been sent. |
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101 (Switching Protocols) |
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The browser may wish to change protocols it's using.
If such a request is sent and approved by the server this response
is given. |
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The two hundred range is reserved for successful responses.
You probably won't see one of these codes, but your browser will receive
them and know that whatever request was sent by the browser was received,
understood, and accepted. |
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The request was successful and information was returned.
This is, by far, the most common code returned on the web. |
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If a POST command is issued by a browser (usually in
processing a form) then the 201 code is returned if the resource requested
to be created was actually created. If there is a delay in creating
the resource the response should be 202, but may be 201 and contain
a description of when it will be created. |
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If a request for processing was sent and accepted but
not acted upon and the delay in acting is unknown, then this code
should be sent instead of 201. Note that 202 does not commit to processing
the request; it only says the request was accepted. A pointer to some
status monitor for the task is often included with this response so
users can check back later. |
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203 (Non-Authoritative Information) |
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Usually the preliminary information sent from a server
to a browser comes directly from the server. If it does not, then
this code might also be sent to indicate that information did not
come from a known source. |
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The request was accepted and filled but no new information
is being sent back. The browser receiving this response should not
change its screen display (although new, and changed, private header
information may be sent). |
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When you fill in a form and send the data, the server
may send this code telling the browser that the data was received
and the action carried out so the browser should now clear the form
(or reset the display in some manner). |
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This code indicates the server has only filled part
of a specific type of request. |
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The 3xx codes indicate some need for further action
by your browser. User action may or may not be necessary to cause
this further action to take place; often it will just happen automatically.
There are safeguards built into the specification designed to prevent
infinite loops, which can sometimes result from automatic redirection. |
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You should not see 300 standing alone; it serves as
a template for the following specific codes. |
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301 (Moved Permanently) |
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As the name implies, the addressed resource has moved
and all future requests for that resource should be made to a new
URL. Sometimes there is an automatic transfer to the new location. |
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302 (Moved Temporarily) |
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The addresses resource has moved, but future requests
should continue to come to the original URL. Sometimes there is an
automatic transfer to the new location. |
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The response to your browers request can be found elsewhere.
Automatic redirection may take place to the new location. |
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In order to save bandwidth your browser may make a
conditional request for resources. The conditional request contains
an "If-Modified-Since" field and if the resource has not
changed since that date the server will simply return the 304 code
and the browser will use its cached copy of the resource. |
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This is notice that a specific proxy server must be
used to access the resource. The URL of the proxy should be provided. |
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The 4xx codes are the ones you are most likely to actually
see; particularly code 404. These codes indicate some sort of error
has happened. |
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The server did not understand the request. This is
usually cured by resending the request. |
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The request requires some form of authentication (e.g.,
userid and/or password) but did not contain it. Usually, this code
results in a box popping up in your browser asking you for the required
information. Once you supply it the request is sent again. |
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Reserved for future use. [Who says the web is not moving
toward being a commercial medium!] |
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This is a sort of catch-all refusal. If the server
understood the request but, for whatever reason, refuses to fill it,
a code 403 will often be returned. The server may or may not explain
why it is sending a 403 response and there is not much you can do
about it. |
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If you happen to mistype a URL or enter an old one
that no longer exists this is the error you will likely see. The condition
may be temporary or permanent but this information is rarely provided.
Sometimes code 403 is sent in place of 404. |
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405 (Method Not Allowed) |
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Your browser has requested a resource using a procedure
not allowed to obtain that resource. The response should contain allowed
procedures. |
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Your browser said only certain response types will
be accepted and the server says the content requested does not fit
those response types. (This is one way content monitoring can be implemented.) |
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407 (Proxy Authentication Required) |
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This code is similar to 401, except that the browser
must first authenticate itself. |
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Your browser waited too long and the server timed out.
A new request must be sent. |
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If a site allows users to change resources and two
users attempt to change the same resource there is a conflict. In
this, and other such situations, the server may return the 409 code
and should also return information necessary to help the user (or
browser) resolve the conflict. |
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Code 410 is more specific than 404 when a resource
can't be found. If the server knows, for a fact, that the resource
is no longer available and no forwarding address is known, then 410
should be returned. If the server does not have specific information
about the resource, then 404 is returned. |
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For some processes a server needs to know exactly how
long the content is. If the browser does not supply the proper length
code 411 may result. |
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412 (Precondition Failed) |
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A browser can put conditions on a request. If the server
evaluates those conditions and comes up with a false answer, the 412
code may be returned. |
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413 (Request Entity Too Large) |
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If your browser makes a request that is longer than
the server can process code 413 may be returned. Additionally, the
server may even close the connection to prevent the request from being
resubmitted (this does not mean a phone connection will hang up; just
that the browser's link to the site may be terminated and have to
be started over again). |
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414 (Request-URI Too Long) |
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You will likely not see this one as it is rare. But,
if the resource address you've sent to the browser is too long this
code will result. One of the reasons this code exists is to give the
server a response when the server is under attack by someone trying
to exploit fixed-length buffers by causing them to overflow. |
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415 (Unsupported Media Type) |
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If your browser makes a request using the wrong format,
this code may result. |
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The 5xx series of codes indicate cases where the server
knows it has made an error or is not capable of answering the request.
In most cases the server should include some information explaining
the error and say if the situation is temporary or permanent. |
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500 (Internal Server Error) |
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An unexpected condition prevented the server from filling
the request. |
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The server is not designed (or does not have the software)
to fill the request. |
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When a server acts as a go-between it may receive an
invalid request. This code is returned when that happens. |
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503 (Service Unavailable) |
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This code is returned when the server cannot respond
due to temporary overloading or maintenance. Some users, for example,
have limited accounts which can only handle so many requests per day
or bytes send per period of time. When the limits are exceeded a 503
code may be returned. |
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A gateway or proxy server timed out without responding. |
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505 (HTTP Version Not Supported) |
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The browser has requested a specific transfer protocol
version that is not supported by the server. The server should return
what protocols are supported. |
What Can Webmasters Do?
Users get frustrated by error messages that don't really tell them anything.
Even the descriptions above for the various return codes don't say what
you, the user, can do.
Webmasters can help. By analyzing their logs a webmaster can determine which
error codes are being returned to users. For the most common, more descriptive
error messages can be generated and the system told to use them. This latter
is done using a file named ".htaccess" placed in the main directory
for the web site.
The .htaccess file can control many things, but to help with error messages
the webmaster has only to insert line(s) of the form (each of these should
be on a line by itself starting with "ErrorDocument" but they
may be wrapped in this display):
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ErrorDocument 402 "<H1>You have to <B>BUY</B>
a <A href="/subscribe.html">subscription!</A></H1> |
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ErrorDocument 403 /forbidden.html |
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ErrorDocument 404 http://cknow.com/notfound.html |
Note that the ErrorDocument command can have raw HTML code (note the leading
quote), file references, or URL references. Use whichever is appropriate
to help users when they encounter errors at your site. If nothing else,
include a 404 ErrorDocument command to help those who mistype something.
If you don't they may not come back! |