Question

When a user is not logged in and tries to access a page that requires login, what is the correct HTTP status code for a redirect to the login page?

I am asking because none of the 3xx response codes set out by the W3C seem to fit the requirements:

10.3.1 300 Multiple Choices

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.

10.3.2 301 Moved Permanently

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.

10.3.3 302 Found

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.

10.3.4 303 See Other

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.

10.3.5 304 Not Modified

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.

10.3.6 305 Use Proxy

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.

10.3.7 306 (Unused)

The 306 status code was used in a
previous version of the specification,
is no longer used, and the code is
reserved.

10.3.8 307 Temporary Redirect

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.

I’m using 302 for now, until I find the correct answer.

Update & conclusion:

HTTP 302 is better since its known to have best compatibility with clients/browsers.

Answer 1

I’d say 303 see other 302 Found:

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.

fits a login page most closely in my opinion. I initially considered 303 see other which would work just as well. After some thought, I’d say 302 Found is more fitting because the requested resource was found, there just is another page to go through before it can be accessed. The response doesn’t get cached by default which is fine as well.