http.7

.\" Automatically generated by Pandoc 2.9.1.1
.\"
.TH "http" "7" "June 2022" "" ""
.hy
.SH NAME
.PP
http - descripe HTTP status codes
.SH DESCRIPTION
.SS 100 Continue
.PP
The client should continue with its request.
This interim response is used to inform the client that the initial part
of the request has been received and has not yet been rejected by the
server.
The client should continue by sending the remainder of the request or,
if the request has already been completed, ignore this response.
The server must send a final response after the request has been
completed.
.PP
<http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.1.1>
.SS 101 Switching Protocols
.PP
The server understands and is willing to comply with the client\[cq]s
request, via the Upgrade message header field, for a change in the
application protocol being used on this connection.
The server will switch protocols to those defined by the response\[cq]s
Upgrade header field immediately after the empty line which terminates
the 101 response.
.PP
The protocol should be switched only when it is advantageous to do so.
For example, switching to a newer version of HTTP is advantageous over
older versions, and switching to a real-time, synchronous protocol might
be advantageous when delivering resources that use such features.
.PP
<http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.1.2>
.SS 200 OK
.PP
The request has succeeded.
The information returned with the response is dependent on the method
used in the request, for example:
.PP
GET an entity corresponding to the requested resource is sent in the
response;
.PP
POST an entity describing or containing the result of the action;
.PP
HEAD the entity-header fields corresponding to the requested resource
are sent in the response without any message-body
.PP
<http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.2.1>
.SS 201 Created
.PP
The request has been fulfilled and resulted in a new resource being
created.
The newly created resource can be referenced by the URI(s) returned in
the entity of the response, with the most specific URI for the resource
given by a Location header field.
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.
The origin server must create the resource before returning the 201
status code.
If the action cannot be carried out immediately, the server should
respond with 202 (Accepted) response instead.
.PP
A 201 response may contain an ETag response header field indicating the
current value of the entity tag for the requested variant just created.
.PP
<http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.2.2>
.SS 202 Accepted
.PP
The request has been accepted for processing, but the processing has not
been completed.
The request might or might not eventually be acted upon, as it might be
disallowed when processing actually takes place.
There is no facility for re-sending a status code from an asynchronous
operation such as this.
.PP
The 202 response is intentionally non-committal.
Its purpose is to allow a server to accept a request for some other
process (perhaps a batch-oriented process that is only run once per day)
without requiring that the user agent\[cq]s connection to the server
persist until the process is completed.
The entity returned with this response should include an indication of
the request\[cq]s current status and either a pointer to a status
monitor or some estimate of when the user can expect the request to be
fulfilled.
.PP
<http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.2.3>
.SS 203 Non-Authoritative Information
.PP
The returned metainformation in the entity-header is not the definitive
set as available from the origin server, but is gathered from a local or
a third-party copy.
The set presented may be a subset or superset of the original version.
For example, including local annotation information about the resource
might result in a superset of the metainformation known by the origin
server.
Use of this response code is not required and is only appropriate when
the response would otherwise be 200 (OK).
.PP
<http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.2.4>
.SS 204 No Content
.PP
The server has fulfilled the request but does not need to return an
entity-body, and might want to return updated metainformation.
The response may include new or updated metainformation in the form of
entity-headers, which if present should be associated with the requested
variant.
.PP
If the client is a user agent, it should not change its document view
from that which caused the request to be sent.
This response is primarily intended to allow input for actions to take
place without causing a change to the user agent\[cq]s active document
view, although any new or updated metainformation should be applied to
the document currently in the user agent\[cq]s active view.
.PP
The 204 response must not include a message-body, and thus is always
terminated by the first empty line after the header fields.
.PP
<http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.2.5>
.SS 205 Reset Content
.PP
The server has fulfilled the request and the user agent should reset the
document view which caused the request to be sent.
This response is primarily intended to allow input for actions to take
place via user input, followed by a clearing of the form in which the
input is given so that the user can easily initiate another input
action.
The response must not include an entity.
.PP
<http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.2.6>
.SS 206 Partial Content
.PP
The server has fulfilled the partial GET request for the resource.
The request must have included a Range header field indicating the
desired range, and may have included an If-Range header field to make
the request conditional.
.PP
The response must include the following header fields:
.IP \[bu] 2
Either a Content-Range header field indicating the range included with
this response, or a multipart/byteranges Content-Type including
Content-Range fields for each part.
If a Content-Length header field is present in the response, its value
must match the actual number of octets transmitted in the message-body.
.IP \[bu] 2
Date
.IP \[bu] 2
ETag and/or Content-Location, if the header would have been sent in a
200 response to the same request
.IP \[bu] 2
Expires, Cache-Control, and/or Vary, if the field-value might differ
from that sent in any previous response for the same variant
.PP
If the 206 response is the result of an If-Range request that used a
strong cache validator, the response should not include other
entity-headers.
If the response is the result of an If-Range request that used a weak
validator, the response must not include other entity-headers; this
prevents inconsistencies between cached entity-bodies and updated
headers.
Otherwise, the response must include all of the entity-headers that
would have been returned with a 200 (OK) response to the same request.
.PP
A cache must not combine a 206 response with other previously cached
content if the ETag or Last-Modified headers do not match exactly.
.PP
<http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.2.7>
.SS 300 Multiple Choices
.PP
The requested resource corresponds to any one of a set of
representations, each with its own specific location, and agent-driven
negotiation information is being provided so that the user can select a
preferred representation and redirect its request to that location.
.PP
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.
.PP
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.
.PP
<http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.3.1>
.SS 301 Moved Permanently
.PP
The requested resource has been assigned a new permanent URI and any
future references to this resource should use one of the returned URIs.
This response is cacheable unless indicated otherwise.
.PP
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).
.PP
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.
.PP
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.
.PP
301 redirects are recommended when redirecting sites or pages.
This is because 302 redirects do not instruct search engine crawlers
that a page or site has permanently moved.
.PP
<http://moz.com/learn/seo/http-status-codes>
<http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.3.2>
.SS 302 Found
.PP
The requested resource resides temporarily under a different URI.
Since the redirection might be altered on occasion, the client should
continue to use the URI for future requests.
This response is only cacheable if indicated by a Cache-Control or
Expires header field.
.PP
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).
.PP
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.
.PP
This code is similar to a 301 in that for a GET or HEAD request, it
automatically forwards the requestor to a different location, but you
shouldn\[cq]t use it to tell the Googlebot that a page or site has moved
because Googlebot will continue to crawl and index the original
location.
.PP
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.
.PP
<https://support.google.com/webmasters/answer/40132?hl=en>
<http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.3.3>
.SS 303 See Other
.PP
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.
.PP
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).
.PP
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.
.PP
<http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.3.4>
.SS 304 Not Modified
.PP
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.
.PP
The response must include the following header fields:
.IP \[bu] 2
Date, unless its omission is required 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.
.IP \[bu] 2
ETag and/or Content-Location, if the header would have been sent in a
200 response to the same request
.IP \[bu] 2
Expires, Cache-Control, and/or Vary, if the field-value might differ
from that sent in any previous response for the same variant
.PP
If the conditional GET used a strong cache validator 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.
.PP
If a 304 response indicates an entity not currently cached, then the
cache must disregard the response and repeat the request without the
conditional.
.PP
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.
.PP
<http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.3.5>
.SS 305 Use Proxy
.PP
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.
.PP
<http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.3.6>
.SS 307 Temporary Redirect
.PP
The requested resource resides temporarily under a different URI.
Since the redirection may be altered on occasion, the client should
continue to use the URI for future requests.
This response is only cacheable if indicated by a Cache-Control or
Expires header field.
.PP
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.
.PP
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.
.PP
<http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.3.8>
.SS 308 Permanent Redirect
.PP
The request, and all future requests should be repeated using another
URI.
It parallels the behaviour of 301, but doesn\[cq]t allow the HTTP method
to change.
So, for example, submitting a form to a permanently redirected resource
may continue smoothly.
.PP
<https://tools.ietf.org/html/rfc7538#section-3>
<http://en.wikipedia.org/wiki/List_of_HTTP_status_codes#3xx_Redirection>
.SS 400 Bad Request
.PP
The request could not be understood by the server due to malformed
syntax.
The client should not repeat the request without modifications.
.PP
<http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.1>
.SS 401 Unauthorized
.PP
The request requires user authentication.
The response must include a WWW-Authenticate header field containing a
challenge applicable to the requested resource.
The client may repeat the request with a suitable Authorization header
field.
If the request already included Authorization credentials, then the 401
response indicates that authorization has been refused for those
credentials.
If the 401 response contains the same challenge as the prior response,
and the user agent has already attempted authentication at least once,
then the user should be presented the entity that was given in the
response, since that entity might include relevant diagnostic
information.
.PP
<http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.2>
.SS 402 Payment Required
.PP
This code is reserved for future use.
.PP
<http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.3>
.SS 403 Forbidden
.PP
The server understood the request, but is refusing to fulfill it.
Authorization will not help and the request should not be repeated.
If the request method was not HEAD and the server wishes to make public
why the request has not been fulfilled, it should describe the reason
for the refusal in the entity.
If the server does not wish to make this information available to the
client, the status code 404 (Not Found) can be used instead.
.PP
<http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.4>
.SS 404 Not Found
.PP
The server has not found anything matching the URI.
No indication is given of whether the condition is temporary or
permanent.
The 410 (Gone) status code should be used if the server knows, through
some internally configurable mechanism, that an old resource is
permanently unavailable and has no forwarding address.
This status code is commonly used when the server does not wish to
reveal exactly why the request has been refused, or when no other
response is applicable.
.PP
<http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.5>
.SS 405 Method Not Allowed
.PP
The method specified is not allowed for the resource identified by the
URI.
The response must include an Allow header containing a list of valid
methods for the requested resource.
.PP
<http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.6>
.SS 406 Not Acceptable
.PP
The resource identified by the request is only capable of generating
response entities which have content characteristics not acceptable
according to the accept headers sent in the request.
.PP
Unless it was a HEAD request, the response should include an entity
containing a list of available entity 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.
.PP
Note: HTTP/1.1 servers are allowed to return responses which are not
acceptable according to the accept headers sent in the request.
In some cases, this may even be preferable to sending a 406 response.
User agents are encouraged to inspect the headers of an incoming
response to determine if it is acceptable.
.PP
If the response could be unacceptable, a user agent should temporarily
stop receipt of more data and query the user for a decision on further
actions.
.PP
<http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.7>
.SS 407 Proxy Authentication Required
.PP
This code is similar to 401 (Unauthorized), but indicates that the
client must first authenticate itself with the proxy.
The proxy must return a Proxy-Authenticate header field containing a
challenge applicable to the proxy for the requested resource.
The client may repeat the request with a suitable Proxy-Authorization
header field.
.PP
<http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.8>
.SS 408 Request Timeout
.PP
The client did not produce a request within the time that the server was
prepared to wait.
The client may repeat the request without modifications at any later
time.
.PP
<http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.9>
.SS 409 Conflict
.PP
The request could not be completed due to a conflict with the current
state of the resource.
This code is only allowed in situations where it is expected that the
user might be able to resolve the conflict and resubmit the request.
The response body should include enough information for the user to
recognize the source of the conflict.
Ideally, the response entity would include enough information for the
user or user agent to fix the problem; however, that might not be
possible and is not required.
.PP
Conflicts are most likely to occur in response to a PUT request.
For example, if versioning were being used and the entity being PUT
included changes to a resource which conflict with those made by an
earlier (third-party) request, the server might use the 409 response to
indicate that it can\[cq]t complete the request.
In this case, the response entity would likely contain a list of the
differences between the two versions in a format defined by the response
Content-Type.
.PP
<http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.10>
.SS 410 Gone
.PP
The requested resource is no longer available at the server and no
forwarding address is known.
This condition is expected to be considered permanent.
Clients with link editing capabilities should delete references to the
URI after user approval.
If the server does not know, or has no facility to determine, whether or
not the condition is permanent, the status code 404 (Not Found) should
be used instead.
This response is cacheable unless indicated otherwise.
.PP
The 410 response is primarily intended to assist the task of web
maintenance by notifying the recipient that the resource is
intentionally unavailable and that the server owners desire that remote
links to that resource be removed.
Such an event is common for limited-time, promotional services and for
resources belonging to individuals no longer working at the server\[cq]s
site.
It is not necessary to mark all permanently unavailable resources as
\[lq]gone\[rq] or to keep the mark for any length of time \[en] that is
left to the discretion of the server owner.
.PP
<http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.11>
.SS 411 Length Required
.PP
The server refuses to accept the request without a defined Content-
Length.
The client may repeat the request if it adds a valid Content-Length
header field containing the length of the message-body in the request
message.
.PP
<http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.12>
.SS 412 Precondition Failed
.PP
The precondition given in one or more of the request-header fields
evaluated to false when it was tested on the server.
This response code allows the client to place preconditions on the
current resource metainformation (header field data) and thus prevent
the requested method from being applied to a resource other than the one
intended.
.PP
<http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.13>
.SS 413 Request Entity Too Large
.PP
The server is refusing to process a request because the request entity
is larger than the server is willing or able to process.
The server may close the connection to prevent the client from
continuing the request.
.PP
If the condition is temporary, the server should include a Retry- After
header field to indicate that it is temporary and after what time the
client may try again.
.PP
<http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.14>
.SS 414 Request-URI Too Long
.PP
The server is refusing to service the request because the URI is longer
than the server is willing to interpret.
This rare condition is only likely to occur when a client has improperly
converted a POST request to a GET request with long query information,
when the client has descended into a URI \[lq]black hole\[rq] of
redirection (e.g., a redirected URI prefix that points to a suffix of
itself), or when the server is under attack by a client attempting to
exploit security holes present in some servers using fixed-length
buffers for reading or manipulating the URI.
.PP
<http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.15>
.SS 415 Unsupported Media Type
.PP
The server is refusing to service the request because the entity of the
request is in a format not supported by the requested resource for the
requested method.
.PP
<http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.16>
.SS 416 Requested Range Not Satisfiable
.PP
A server should return a response with this status code if a request
included a Range request-header field, and none of the range-specifier
values in this field overlap the current extent of the selected
resource, and the request did not include an If-Range request-header
field.
(For byte-ranges, this means that the first- byte-pos of all of the
byte-range-spec values were greater than the current length of the
selected resource.)
.PP
When this status code is returned for a byte-range request, the response
should include a Content-Range entity-header field specifying the
current length of the selected resource (see section 14.16).
This response must not use the multipart/byteranges content- type.
.PP
<http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.17>
.SS 417 Expectation Failed
.PP
The expectation given in an Expect request-header field could not be met
by this server, or, if the server is a proxy, the server has unambiguous
evidence that the request could not be met by the next-hop server.
.PP
<http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.18>
.SS 418 I\[cq]m a teapot
.PP
This code was defined in 1998 as one of the traditional IETF April
Fools\[cq] jokes, in RFC 2324, and is not expected to be implemented by
actual HTTP servers.
The RFC specifies this code should be returned by tea pots requested to
brew coffee.
.PP
<https://www.ietf.org/rfc/rfc2324.txt>
<http://en.wikipedia.org/wiki/List_of_HTTP_status_codes#4xx_Client_Error>
.SS 419 Authentication Timeout
.PP
Not a part of the HTTP standard, 419 Authentication Timeout denotes that
previously valid authentication has expired.
It is used as an alternative to 401 Unauthorized in order to
differentiate from otherwise authenticated clients being denied access
to specific server resources.
.PP
<http://en.wikipedia.org/wiki/List_of_HTTP_status_codes#4xx_Client_Error>
.SS 420 Method Failure
.PP
Not part of the HTTP standard, but defined by Spring in the HttpStatus
class to be used when a method failed.
This status code is deprecated by Spring.
.PP
It is also returned by version 1 of the Twitter Search and Trends API
when the client is being rate limited.
Other services may wish to implement the 429 Too Many Requests response
code instead.
.PP
<http://en.wikipedia.org/wiki/List_of_HTTP_status_codes#4xx_Client_Error>
.SS 422 Unprocessable Entity
.PP
The request was well-formed but was unable to be followed due to
semantic errors.
(extension for WebDAV)
.PP
<https://tools.ietf.org/html/rfc4918#section-11.2>
<http://en.wikipedia.org/wiki/List_of_HTTP_status_codes#4xx_Client_Error>
.SS 423 Locked
.PP
The 423 (Locked) status code means the source or destination resource of
a method is locked.
This response should contain an appropriate precondition or
postcondition code, such as `lock-token-submitted' or
`no-conflicting-lock'.
(extension for WebDAV)
.PP
<https://tools.ietf.org/html/rfc4918#section-11.3>
.SS 426 Upgrade Required
.PP
The client should switch to a different protocol such as TLS/1.0, given
in the Upgrade header field.
.PP
<http://en.wikipedia.org/wiki/List_of_HTTP_status_codes#4xx_Client_Error>
.SS 428 Precondition Required
.PP
The 428 status code indicates that the origin server requires the
request to be conditional.
.PP
Its typical use is to avoid the \[lq]lost update\[rq] problem, where a
client GETs a resource\[cq]s state, modifies it, and PUTs it back to the
server, when meanwhile a third party has modified the state on the
server, leading to a conflict.
By requiring requests to be conditional, the server can assure that
clients are working with the correct copies.
.PP
Responses using this status code should explain how to resubmit the
request successfully.
For example:
.PP
HTTP/1.1 428 Precondition Required Content-Type: text/html
Precondition Required
Precondition Required
This request is required to be conditional; try using
\[lq]If-Match\[rq].
.PP
<http://tools.ietf.org/html/rfc6585#section-3>
.SS 500 Internal Server Error
.PP
The server encountered an unexpected condition which prevented it from
fulfilling the request.
.PP
<http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.5.1>
.SS 501 Not Implemented
.PP
The server does not support the functionality required to fulfill the
request.
This is the appropriate response when the server does not recognize the
request method and is not capable of supporting it for any resource.
.PP
<http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.5.2>
.SS 502 Bad Gateway
.PP
The server, while acting as a gateway or proxy, received an invalid
response from the upstream server it accessed in attempting to fulfill
the request.
.PP
<http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.5.3>
.SS 503 Service Unavailable
.PP
The server is currently unable to handle the request due to a temporary
overloading or maintenance of the server.
The implication is that this is a temporary condition which will be
alleviated after some delay.
If known, the length of the delay may be indicated in a Retry-After
header.
If no Retry-After is given, the client should handle the response as it
would for a 500 response.
.PP
Note: The existence of the 503 status code does not imply that a server
must use it when becoming overloaded.
Some servers may wish to simply refuse the connection.
.PP
<http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.5.4>
.SS 504 Gateway Timeout
.PP
The server, while acting as a gateway or proxy, did not receive a timely
response from the upstream server specified by the URI (e.g.\ HTTP, FTP,
LDAP) or some other auxiliary server (e.g.\ DNS) it needed to access in
attempting to complete the request.
.PP
Note: Note to implementors: some deployed proxies are known to return
400 or 500 when DNS lookups time out.
.PP
<http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.5.5>
.SS 505 HTTP Version Not Supported
.PP
The server does not support, or refuses to support, the HTTP protocol
version that was used in the request message.
The server is indicating that it is unable or unwilling to complete the
request using the same major version as the client, other than with this
error message.
The response should contain an entity describing why that version is not
supported and what other protocols are supported by that server.
.PP
<http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.5.6>