# Τεκμηρίωση του Django - 1.10.7

# Middleware

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 middleware usage guide.

## Available middleware

### Cache middleware

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 `CACHE_MIDDLEWARE_SECONDS` setting defines. See the cache documentation.

### “Common” middleware

class `CommonMiddleware`

Adds a few conveniences for perfectionists:

- Forbids access to user agents in the `DISALLOWED_USER_AGENTS` setting, which should be a list of compiled regular expression objects.

- Performs URL rewriting based on the `APPEND_SLASH` and `PREPEND_WWW` settings.

  If `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 `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.

- Handles ETags based on the `USE_ETAGS` setting. If `USE_ETAGS` is set to `True`, Django will calculate an ETag for each request by MD5-hashing the page content, and it’ll take care of sending `Not Modified` responses, if appropriate.

`CommonMiddleware.``response_redirect_class`

Defaults to `HttpResponsePermanentRedirect`. Subclass `CommonMiddleware` and override the attribute to customize the redirects issued by the middleware.

class `BrokenLinkEmailsMiddleware`

- Sends broken link notification emails to `MANAGERS` (see Αναφορά σφαλμάτων).

### Exception middleware

class `ExceptionMiddleware`

Catches exceptions raised during the request/response cycle and returns the appropriate response.

- `Http404` is processed by `handler404` (or a more friendly debug page if `DEBUG=True`).
- `PermissionDenied` is processed by `handler403`.
- `MultiPartParserError` is processed by `handler400`.
- `SuspiciousOperation` is processed by `handler400` (or a more friendly debug page if `DEBUG=True`).
- Any other exception is processed by `handler500` (or a more friendly debug page if `DEBUG=True`).

Django uses this middleware regardless of whether or not you include it in `MIDDLEWARE`, however, you may want to subclass if your own middleware needs to transform any of these exceptions into the appropriate responses. `LocaleMiddleware` does this, for example.

### GZip middleware

class `GZipMiddleware`

Προειδοποίηση

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.

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`.

You can apply GZip compression to individual views using the `gzip_page()` decorator.

In older versions, Django’s CSRF protection mechanism was vulnerable to BREACH attacks when compression was used. This is no longer the case, but you should still take care not to compromise your own secrets this way.

### Conditional GET middleware

class `ConditionalGetMiddleware`

Handles conditional GET operations. If the response has a `ETag` or `Last-Modified` header, and the request has `If-None-Match` or `If-Modified-Since`, the response is replaced by an `HttpResponseNotModified`.

Also sets the `Date` and `Content-Length` response-headers.

### Locale middleware

class `LocaleMiddleware`

Enables language selection based on data from the request. It customizes content for each user. See the internationalization documentation.

`LocaleMiddleware.``response_redirect_class`

Defaults to `HttpResponseRedirect`. Subclass `LocaleMiddleware` and override the attribute to customize the redirects issued by the middleware.

### Message middleware

class `MessageMiddleware`

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

### Security middleware

Προειδοποίηση

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.

- `SECURE_BROWSER_XSS_FILTER`
- `SECURE_CONTENT_TYPE_NOSNIFF`
- `SECURE_HSTS_INCLUDE_SUBDOMAINS`
- `SECURE_HSTS_SECONDS`
- `SECURE_REDIRECT_EXEMPT`
- `SECURE_SSL_HOST`
- `SECURE_SSL_REDIRECT`

#### 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 `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, `SECURE_HSTS_SECONDS = 3600` 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 `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.

Προειδοποίηση

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!

Σημείωση

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 `SECURE_PROXY_SSL_HEADER` setting.

#### `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 learn more about this header and how the browser treats it, you can read about it on the IE Security Blog.

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 `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 `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.

#### `X-XSS-Protection: 1; mode=block`

Some browsers have the ability to block content that appears to be an XSS attack. They work by looking for JavaScript content in the GET or POST parameters of a page. If the JavaScript is replayed in the server’s response, the page is blocked from rendering and an error page is shown instead.

The X-XSS-Protection header is used to control the operation of the XSS filter.

To enable the XSS filter in the browser, and force it to always block suspected XSS attacks, you can pass the `X-XSS-Protection: 1; mode=block` header. `SecurityMiddleware` will do this for all responses if the `SECURE_BROWSER_XSS_FILTER` setting is `True`.

Προειδοποίηση

The browser XSS filter is a useful defense measure, but must not be relied upon exclusively. It cannot detect all XSS attacks and not all browsers support the header. Ensure you are still validating and sanitizing all input to prevent XSS attacks.

#### 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 `SECURE_SSL_REDIRECT` setting to True, `SecurityMiddleware` will permanently (HTTP 301) redirect all HTTP connections to HTTPS.

Σημείωση

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. `SECURE_SSL_REDIRECT` is intended for the deployment situations where this isn’t an option.

If the `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 `SECURE_REDIRECT_EXEMPT` setting.

Σημείωση

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 `SECURE_PROXY_SSL_HEADER` setting.

### Session middleware

class `SessionMiddleware`

Enables session support. See the session documentation.

### Site middleware

class `CurrentSiteMiddleware`

Adds the `site` attribute representing the current site to every incoming `HttpRequest` object. See the sites documentation.

### Authentication middleware

class `AuthenticationMiddleware`

Adds the `user` attribute, representing the currently-logged-in user, to every incoming `HttpRequest` object. See Authentication in Web requests.

class `RemoteUserMiddleware`

Middleware for utilizing Web server provided authentication. See Authentication using REMOTE_USER for usage details.

class `PersistentRemoteUserMiddleware`

Middleware for utilizing Web server provided authentication when enabled only on the login page. See Χρήση του REMOTE_USER μόνο στις σελίδες login for usage details.

### CSRF protection middleware

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 Cross Site Request Forgery protection documentation.

### `X-Frame-Options` middleware

class `XFrameOptionsMiddleware`

Simple clickjacking protection via the X-Frame-Options header.

## Middleware ordering

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

1. `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.

2. `UpdateCacheMiddleware`

   Before those that modify the `Vary` header (`SessionMiddleware`, `GZipMiddleware`, `LocaleMiddleware`).

3. `GZipMiddleware`

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

   After `UpdateCacheMiddleware`: Modifies `Vary` header.

4. `ConditionalGetMiddleware`

   Before `CommonMiddleware`: uses its `ETag` header when `USE_ETAGS` = `True`.

5. `SessionMiddleware`

   After `UpdateCacheMiddleware`: Modifies `Vary` header.

6. `LocaleMiddleware`

   One of the topmost, after `SessionMiddleware` (uses session data) and `UpdateCacheMiddleware` (modifies `Vary` header).

7. `CommonMiddleware`

   Before any middleware that may change the response (it calculates `ETags`).

   After `GZipMiddleware` so it won’t calculate an `ETag` header on gzipped contents.

   Close to the top: it redirects when `APPEND_SLASH` or `PREPEND_WWW` are set to `True`.

8. `CsrfViewMiddleware`

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

9. `AuthenticationMiddleware`

   After `SessionMiddleware`: uses session storage.

10. `MessageMiddleware`

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

11. `FetchFromCacheMiddleware`

    After any middleware that modifies the `Vary` header: that header is used to pick a value for the cache hash-key.

12. `FlatpageFallbackMiddleware`

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

13. `RedirectFallbackMiddleware`

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