Viewing github.com/django/django-stable-4.1.x/docs/ref/middleware.txt

```
==========
```
```
Middleware
```
```
==========
```
```
.. module:: django.middleware
```

   :synopsis: Django's built-in middleware classes.

This document explains all middleware components that come with Django. For

information on how to use them and how to write your own middleware, see

the :doc:`middleware usage guide </topics/http/middleware>`.

```
Available middleware
```
```
====================
```
```
Cache middleware
```
```
----------------
```
```
.. module:: django.middleware.cache
```

   :synopsis: Middleware for the site-wide cache.

```
.. class:: UpdateCacheMiddleware
```
```
.. class:: FetchFromCacheMiddleware
```

Enable the site-wide cache. If these are enabled, each Django-powered page will

be cached for as long as the :setting:`CACHE_MIDDLEWARE_SECONDS` setting

defines. See the :doc:`cache documentation </topics/cache>`.

```
"Common" middleware
```
```
-------------------
```
```
.. module:: django.middleware.common
```

   :synopsis: Middleware adding "common" conveniences for perfectionists.

```
.. class:: CommonMiddleware
```

Adds a few conveniences for perfectionists:

* Forbids access to user agents in the :setting:`DISALLOWED_USER_AGENTS`

  setting, which should be a list of compiled regular expression objects.

* Performs URL rewriting based on the :setting:`APPEND_SLASH` and

```
  :setting:`PREPEND_WWW` settings.
```

  If :setting:`APPEND_SLASH` is ``True`` and the initial URL doesn't end

  with a slash, and it is not found in the URLconf, then a new URL is

  formed by appending a slash at the end. If this new URL is found in the

  URLconf, then Django redirects the request to this new URL. Otherwise,

  the initial URL is processed as usual.

  For example, ``foo.com/bar`` will be redirected to ``foo.com/bar/`` if

  you don't have a valid URL pattern for ``foo.com/bar`` but *do* have a

```
  valid pattern for ``foo.com/bar/``.
```

  If :setting:`PREPEND_WWW` is ``True``, URLs that lack a leading "www."

  will be redirected to the same URL with a leading "www."

  Both of these options are meant to normalize URLs. The philosophy is that

  each URL should exist in one, and only one, place. Technically a URL

  ``foo.com/bar`` is distinct from ``foo.com/bar/`` -- a search-engine

  indexer would treat them as separate URLs -- so it's best practice to

```
  normalize URLs.
```

  If necessary, individual views may be excluded from the ``APPEND_SLASH``

  behavior using the :func:`~django.views.decorators.common.no_append_slash`

```
  decorator::
```

    from django.views.decorators.common import no_append_slash

```
    @no_append_slash
```

    def sensitive_fbv(request, *args, **kwargs):

        """View to be excluded from APPEND_SLASH."""

```
        return HttpResponse()
```

* Sets the ``Content-Length`` header for non-streaming responses.

.. attribute:: CommonMiddleware.response_redirect_class

Defaults to :class:`~django.http.HttpResponsePermanentRedirect`. Subclass

``CommonMiddleware`` and override the attribute to customize the redirects

```
issued by the middleware.
```
```
.. class:: BrokenLinkEmailsMiddleware
```

* Sends broken link notification emails to :setting:`MANAGERS` (see

```
  :doc:`/howto/error-reporting`).
```
```
GZip middleware
```
```
---------------
```
```
.. module:: django.middleware.gzip
```

   :synopsis: Middleware to serve GZipped content for performance.

```
.. class:: GZipMiddleware
```
```
.. warning::
```

    Security researchers recently revealed that when compression techniques

    (including ``GZipMiddleware``) are used on a website, the site may become

    exposed to a number of possible attacks. Before using ``GZipMiddleware`` on

    your site, you should consider very carefully whether you are subject to

    these attacks. If you're in *any* doubt about whether you're affected, you

    should avoid using ``GZipMiddleware``. For more details, see the `the BREACH

    paper (PDF)`_ and `breachattack.com`_.

    .. _the BREACH paper (PDF): https://www.breachattack.com/resources/BREACH%20-%20SSL,%20gone%20in%2030%20seconds.pdf

    .. _breachattack.com: https://www.breachattack.com/

The ``django.middleware.gzip.GZipMiddleware`` compresses content for browsers

that understand GZip compression (all modern browsers).

This middleware should be placed before any other middleware that need to

read or write the response body so that compression happens afterward.

It will NOT compress content if any of the following are true:

* The content body is less than 200 bytes long.

* The response has already set the ``Content-Encoding`` header.

* The request (the browser) hasn't sent an ``Accept-Encoding`` header

```
  containing ``gzip``.
```

If the response has an ``ETag`` header, the ETag is made weak to comply with

```
:rfc:`7232#section-2.1`.
```

You can apply GZip compression to individual views using the

:func:`~django.views.decorators.gzip.gzip_page()` decorator.

```
Conditional GET middleware
```
```
--------------------------
```
```
.. module:: django.middleware.http
```

   :synopsis: Middleware handling advanced HTTP features.

```
.. class:: ConditionalGetMiddleware
```

Handles conditional GET operations. If the response doesn't have an ``ETag``

header, the middleware adds one if needed. If the response has an ``ETag`` or

``Last-Modified`` header, and the request has ``If-None-Match`` or

``If-Modified-Since``, the response is replaced by an

:class:`~django.http.HttpResponseNotModified`.

```
Locale middleware
```
```
-----------------
```
```
.. module:: django.middleware.locale
```

   :synopsis: Middleware to enable language selection based on the request.

```
.. class:: LocaleMiddleware
```

Enables language selection based on data from the request. It customizes

content for each user. See the :doc:`internationalization documentation

```
</topics/i18n/translation>`.
```

.. attribute:: LocaleMiddleware.response_redirect_class

Defaults to :class:`~django.http.HttpResponseRedirect`. Subclass

``LocaleMiddleware`` and override the attribute to customize the redirects

```
issued by the middleware.
```
```
Message middleware
```
```
------------------
```

.. module:: django.contrib.messages.middleware

```
   :synopsis: Message middleware.
```
```
.. class:: MessageMiddleware
```

Enables cookie- and session-based message support. See the

:doc:`messages documentation </ref/contrib/messages>`.

```
.. _security-middleware:
```
```
Security middleware
```
```
-------------------
```
```
.. module:: django.middleware.security
```
```
    :synopsis: Security middleware.
```
```
.. warning::
```

    If your deployment situation allows, it's usually a good idea to have your

    front-end web server perform the functionality provided by the

    ``SecurityMiddleware``. That way, if there are requests that aren't served

    by Django (such as static media or user-uploaded files), they will have

    the same protections as requests to your Django application.

```
.. class:: SecurityMiddleware
```

The ``django.middleware.security.SecurityMiddleware`` provides several security

enhancements to the request/response cycle. Each one can be independently

```
enabled or disabled with a setting.
```

* :setting:`SECURE_CONTENT_TYPE_NOSNIFF`

* :setting:`SECURE_CROSS_ORIGIN_OPENER_POLICY`

* :setting:`SECURE_HSTS_INCLUDE_SUBDOMAINS`

```
* :setting:`SECURE_HSTS_PRELOAD`
```
```
* :setting:`SECURE_HSTS_SECONDS`
```
```
* :setting:`SECURE_REDIRECT_EXEMPT`
```
```
* :setting:`SECURE_REFERRER_POLICY`
```
```
* :setting:`SECURE_SSL_HOST`
```
```
* :setting:`SECURE_SSL_REDIRECT`
```
```
.. _http-strict-transport-security:
```
```
HTTP Strict Transport Security
```
```
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```

For sites that should only be accessed over HTTPS, you can instruct modern

browsers to refuse to connect to your domain name via an insecure connection

(for a given period of time) by setting the `"Strict-Transport-Security"

header`__. This reduces your exposure to some SSL-stripping man-in-the-middle

```
(MITM) attacks.
```

``SecurityMiddleware`` will set this header for you on all HTTPS responses if

you set the :setting:`SECURE_HSTS_SECONDS` setting to a non-zero integer value.

When enabling HSTS, it's a good idea to first use a small value for testing,

for example, :setting:`SECURE_HSTS_SECONDS = 3600<SECURE_HSTS_SECONDS>` for one

hour. Each time a web browser sees the HSTS header from your site, it will

refuse to communicate non-securely (using HTTP) with your domain for the given

period of time. Once you confirm that all assets are served securely on your

site (i.e. HSTS didn't break anything), it's a good idea to increase this value

so that infrequent visitors will be protected (31536000 seconds, i.e. 1 year,

```
is common).
```

Additionally, if you set the :setting:`SECURE_HSTS_INCLUDE_SUBDOMAINS` setting

to ``True``, ``SecurityMiddleware`` will add the ``includeSubDomains`` directive

to the ``Strict-Transport-Security`` header. This is recommended (assuming all

subdomains are served exclusively using HTTPS), otherwise your site may still

be vulnerable via an insecure connection to a subdomain.

If you wish to submit your site to the `browser preload list`_, set the

:setting:`SECURE_HSTS_PRELOAD` setting to ``True``. That appends the

``preload`` directive to the ``Strict-Transport-Security`` header.

```
.. warning::
```

    The HSTS policy applies to your entire domain, not just the URL of the

    response that you set the header on. Therefore, you should only use it if

    your entire domain is served via HTTPS only.

    Browsers properly respecting the HSTS header will refuse to allow users to

    bypass warnings and connect to a site with an expired, self-signed, or

    otherwise invalid SSL certificate. If you use HSTS, make sure your

    certificates are in good shape and stay that way!

```
.. note::
```

    If you are deployed behind a load-balancer or reverse-proxy server, and the

    ``Strict-Transport-Security`` header is not being added to your responses,

    it may be because Django doesn't realize that it's on a secure connection;

    you may need to set the :setting:`SECURE_PROXY_SSL_HEADER` setting.

__ https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Strict-Transport-Security

.. _browser preload list: https://hstspreload.org/

```
.. _referrer-policy:
```
```
Referrer Policy
```
```
~~~~~~~~~~~~~~~
```

Browsers use `the Referer header`__ as a way to send information to a site

about how users got there. When a user clicks a link, the browser will send the

full URL of the linking page as the referrer. While this can be useful for some

purposes -- like figuring out who's linking to your site -- it also can cause

privacy concerns by informing one site that a user was visiting another site.

__ https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Referer

Some browsers have the ability to accept hints about whether they should send

the HTTP ``Referer`` header when a user clicks a link; this hint is provided

via `the Referrer-Policy header`__. This header can suggest any of three

```
behaviors to browsers:
```

__ https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Referrer-Policy

* Full URL: send the entire URL in the ``Referer`` header. For example, if the

  user is visiting ``https://example.com/page.html``, the ``Referer`` header

  would contain ``"https://example.com/page.html"``.

* Origin only: send only the "origin" in the referrer. The origin consists of

  the scheme, host and (optionally) port number. For example, if the user is

  visiting ``https://example.com/page.html``, the origin would be

```
  ``https://example.com/``.
```

* No referrer: do not send a ``Referer`` header at all.

There are two types of conditions this header can tell a browser to watch out

```
for:
```

* Same-origin versus cross-origin: a link from ``https://example.com/1.html``

  to ``https://example.com/2.html`` is same-origin. A link from

  ``https://example.com/page.html`` to ``https://not.example.com/page.html`` is

```
  cross-origin.
```

* Protocol downgrade: a downgrade occurs if the page containing the link is

  served via HTTPS, but the page being linked to is not served via HTTPS.

```
.. warning::
```

    When your site is served via HTTPS, :ref:`Django's CSRF protection system

    <how-csrf-works>` requires the ``Referer`` header to be present, so

    completely disabling the ``Referer`` header will interfere with CSRF

    protection. To gain most of the benefits of disabling ``Referer`` headers

    while also keeping CSRF protection, consider enabling only same-origin

```
    referrers.
```

``SecurityMiddleware`` can set the ``Referrer-Policy`` header for you, based on

the :setting:`SECURE_REFERRER_POLICY` setting (note spelling: browsers send a

``Referer`` header when a user clicks a link, but the header instructing a

browser whether to do so is spelled ``Referrer-Policy``). The valid values for

```
this setting are:
```
```
``no-referrer``
```

    Instructs the browser to send no referrer for links clicked on this site.

```
``no-referrer-when-downgrade``
```

    Instructs the browser to send a full URL as the referrer, but only when no

```
    protocol downgrade occurs.
```
```
``origin``
```

    Instructs the browser to send only the origin, not the full URL, as the

```
    referrer.
```
```
``origin-when-cross-origin``
```

    Instructs the browser to send the full URL as the referrer for same-origin

    links, and only the origin for cross-origin links.

```
``same-origin``
```

    Instructs the browser to send a full URL, but only for same-origin links. No

    referrer will be sent for cross-origin links.

```
``strict-origin``
```

    Instructs the browser to send only the origin, not the full URL, and to send

    no referrer when a protocol downgrade occurs.

```
``strict-origin-when-cross-origin``
```

    Instructs the browser to send the full URL when the link is same-origin and

    no protocol downgrade occurs; send only the origin when the link is

    cross-origin and no protocol downgrade occurs; and no referrer when a

```
    protocol downgrade occurs.
```
```
``unsafe-url``
```

    Instructs the browser to always send the full URL as the referrer.

```
.. admonition:: Unknown Policy Values
```

    Where a policy value is `unknown`__ by a user agent, it is possible to

    specify multiple policy values to provide a fallback. The last specified

    value that is understood takes precedence. To support this, an iterable or

    comma-separated string can be used with :setting:`SECURE_REFERRER_POLICY`.

    __ https://w3c.github.io/webappsec-referrer-policy/#unknown-policy-values

```
.. _cross-origin-opener-policy:
```
```
Cross-Origin Opener Policy
```
```
~~~~~~~~~~~~~~~~~~~~~~~~~~
```
```
.. versionadded:: 4.0
```

Some browsers have the ability to isolate top-level windows from other

documents by putting them in a separate browsing context group based on the

value of the `Cross-Origin Opener Policy`__ (COOP) header. If a document that

is isolated in this way opens a cross-origin popup window, the popup’s

``window.opener`` property will be ``null``. Isolating windows using COOP is a

defense-in-depth protection against cross-origin attacks, especially those like

Spectre which allowed exfiltration of data loaded into a shared browsing

```
context.
```

__ https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Cross-Origin-Opener-Policy

``SecurityMiddleware`` can set the ``Cross-Origin-Opener-Policy`` header for

you, based on the :setting:`SECURE_CROSS_ORIGIN_OPENER_POLICY` setting. The

```
valid values for this setting are:
```
```
``same-origin``
```

    Isolates the browsing context exclusively to same-origin documents.

    Cross-origin documents are not loaded in the same browsing context. This

    is the default and most secure option.

```
``same-origin-allow-popups``
```

    Isolates the browsing context to same-origin documents or those which

    either don't set COOP or which opt out of isolation by setting a COOP of

```
    ``unsafe-none``.
```
```
``unsafe-none``
```

    Allows the document to be added to its opener's browsing context group

    unless the opener itself has a COOP of ``same-origin`` or

```
    ``same-origin-allow-popups``.
```
```
.. _x-content-type-options:
```
```
``X-Content-Type-Options: nosniff``
```
```
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```

Some browsers will try to guess the content types of the assets that they

fetch, overriding the ``Content-Type`` header. While this can help display

sites with improperly configured servers, it can also pose a security

```
risk.
```

If your site serves user-uploaded files, a malicious user could upload a

specially-crafted file that would be interpreted as HTML or JavaScript by

the browser when you expected it to be something harmless.

To prevent the browser from guessing the content type and force it to

always use the type provided in the ``Content-Type`` header, you can pass

the `X-Content-Type-Options: nosniff`__ header.  ``SecurityMiddleware`` will

do this for all responses if the :setting:`SECURE_CONTENT_TYPE_NOSNIFF` setting

```
is ``True``.
```

Note that in most deployment situations where Django isn't involved in serving

user-uploaded files, this setting won't help you. For example, if your

:setting:`MEDIA_URL` is served directly by your front-end web server (nginx,

Apache, etc.) then you'd want to set this header there. On the other hand, if

you are using Django to do something like require authorization in order to

download files and you cannot set the header using your web server, this

```
setting will be useful.
```

__ https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-Content-Type-Options

```
.. _ssl-redirect:
```
```
SSL Redirect
```
```
~~~~~~~~~~~~
```

If your site offers both HTTP and HTTPS connections, most users will end up

with an unsecured connection by default. For best security, you should redirect

```
all HTTP connections to HTTPS.
```

If you set the :setting:`SECURE_SSL_REDIRECT` setting to True,

``SecurityMiddleware`` will permanently (HTTP 301) redirect all HTTP

```
connections to HTTPS.
```
```
.. note::
```

    For performance reasons, it's preferable to do these redirects outside of

    Django, in a front-end load balancer or reverse-proxy server such as

    `nginx`_. :setting:`SECURE_SSL_REDIRECT` is intended for the deployment

    situations where this isn't an option.

If the :setting:`SECURE_SSL_HOST` setting has a value, all redirects will be

sent to that host instead of the originally-requested host.

If there are a few pages on your site that should be available over HTTP, and

not redirected to HTTPS, you can list regular expressions to match those URLs

in the :setting:`SECURE_REDIRECT_EXEMPT` setting.

```
.. note::
```

    If you are deployed behind a load-balancer or reverse-proxy server and

    Django can't seem to tell when a request actually is already secure, you

    may need to set the :setting:`SECURE_PROXY_SSL_HEADER` setting.

```
.. _nginx: https://nginx.org/
```
```
Session middleware
```
```
------------------
```

.. module:: django.contrib.sessions.middleware

```
   :synopsis: Session middleware.
```
```
.. class:: SessionMiddleware
```

Enables session support. See the :doc:`session documentation

```
</topics/http/sessions>`.
```
```
Site middleware
```
```
---------------
```

.. module:: django.contrib.sites.middleware

```
  :synopsis: Site middleware.
```
```
.. class:: CurrentSiteMiddleware
```

Adds the ``site`` attribute representing the current site to every incoming

``HttpRequest`` object. See the :ref:`sites documentation <site-middleware>`.

```
Authentication middleware
```
```
-------------------------
```

.. module:: django.contrib.auth.middleware

  :synopsis: Authentication middleware.

```
.. class:: AuthenticationMiddleware
```

Adds the ``user`` attribute, representing the currently-logged-in user, to

every incoming ``HttpRequest`` object. See :ref:`Authentication in web requests

```
<auth-web-requests>`.
```
```
.. class:: RemoteUserMiddleware
```

Middleware for utilizing web server provided authentication. See

:doc:`/howto/auth-remote-user` for usage details.

.. class:: PersistentRemoteUserMiddleware

Middleware for utilizing web server provided authentication when enabled only

on the login page. See :ref:`persistent-remote-user-middleware-howto` for usage

```
details.
```
```
CSRF protection middleware
```
```
--------------------------
```

.. currentmodule:: django.middleware.csrf

```
.. class:: CsrfViewMiddleware
```

Adds protection against Cross Site Request Forgeries by adding hidden form

fields to POST forms and checking requests for the correct value. See the

:doc:`Cross Site Request Forgery protection documentation </ref/csrf>`.

```
``X-Frame-Options`` middleware
```
```
------------------------------
```

.. currentmodule:: django.middleware.clickjacking

```
.. class:: XFrameOptionsMiddleware
```

Simple :doc:`clickjacking protection via the X-Frame-Options header </ref/clickjacking/>`.

```
.. _middleware-ordering:
```
```
Middleware ordering
```
```
===================
```

Here are some hints about the ordering of various Django middleware classes:

#. :class:`~django.middleware.security.SecurityMiddleware`

   It should go near the top of the list if you're going to turn on the SSL

   redirect as that avoids running through a bunch of other unnecessary

```
   middleware.
```

#. :class:`~django.middleware.cache.UpdateCacheMiddleware`

   Before those that modify the ``Vary`` header (``SessionMiddleware``,

   ``GZipMiddleware``, ``LocaleMiddleware``).

#. :class:`~django.middleware.gzip.GZipMiddleware`

   Before any middleware that may change or use the response body.

   After ``UpdateCacheMiddleware``: Modifies ``Vary`` header.

#. :class:`~django.contrib.sessions.middleware.SessionMiddleware`

   Before any middleware that may raise an exception to trigger an error

   view (such as :exc:`~django.core.exceptions.PermissionDenied`) if you're

```
   using :setting:`CSRF_USE_SESSIONS`.
```

   After ``UpdateCacheMiddleware``: Modifies ``Vary`` header.

#. :class:`~django.middleware.http.ConditionalGetMiddleware`

   Before any middleware that may change the response (it sets the ``ETag``

```
   header).
```

   After ``GZipMiddleware`` so it won't calculate an ``ETag`` header on gzipped

```
   contents.
```

#. :class:`~django.middleware.locale.LocaleMiddleware`

   One of the topmost, after ``SessionMiddleware`` (uses session data) and

   ``UpdateCacheMiddleware`` (modifies ``Vary`` header).

#. :class:`~django.middleware.common.CommonMiddleware`

   Before any middleware that may change the response (it sets the

   ``Content-Length`` header). A middleware that appears before

   ``CommonMiddleware`` and changes the response must reset ``Content-Length``.

   Close to the top: it redirects when :setting:`APPEND_SLASH` or

   :setting:`PREPEND_WWW` are set to ``True``.

   After ``SessionMiddleware`` if you're using :setting:`CSRF_USE_SESSIONS`.

#. :class:`~django.middleware.csrf.CsrfViewMiddleware`

   Before any view middleware that assumes that CSRF attacks have been dealt

```
   with.
```

   Before :class:`~django.contrib.auth.middleware.RemoteUserMiddleware`, or any

   other authentication middleware that may perform a login, and hence rotate

   the CSRF token, before calling down the middleware chain.

   After ``SessionMiddleware`` if you're using :setting:`CSRF_USE_SESSIONS`.

#. :class:`~django.contrib.auth.middleware.AuthenticationMiddleware`

   After ``SessionMiddleware``: uses session storage.

#. :class:`~django.contrib.messages.middleware.MessageMiddleware`

   After ``SessionMiddleware``: can use session-based storage.

#. :class:`~django.middleware.cache.FetchFromCacheMiddleware`

   After any middleware that modifies the ``Vary`` header: that header is used

   to pick a value for the cache hash-key.

#. :class:`~django.contrib.flatpages.middleware.FlatpageFallbackMiddleware`

   Should be near the bottom as it's a last-resort type of middleware.

#. :class:`~django.contrib.redirects.middleware.RedirectFallbackMiddleware`

   Should be near the bottom as it's a last-resort type of middleware.