摘要：近期的項目，前端的是在上跑的，然后向我們后端的開發服務器進行請求。那怎么去掉這個功能呢，我們現在就是要進行跨域請求。這里里面有就直接進入了，沒有下面的檢查了。所以我們只要自己給添加一個這樣的屬性就好了。最直接的方法還是去寫一個啊，哈哈。

近期的項目，前端的js是在localhost上跑的，然后向我們后端的開發服務器進行請求。但是突然前端說所有的post請求都報csrf校驗錯誤了，甚是奇怪，之前為了開發方便已經把django的csrf middleware注釋掉了啊，為什么還會錯誤，由于返回值格式還是django rest的通用格式，肯定問題是出在這里面，于是翻了一下它的源代碼看了看。

from django.middleware.csrf import CsrfViewMiddleware

class CSRFCheck(CsrfViewMiddleware):
    def _reject(self, request, reason):
        # Return the failure reason instead of an HttpResponse
        return reason

class SessionAuthentication(BaseAuthentication):
    """
    Use Django"s session framework for authentication.
    """

    def authenticate(self, request):
        """
        Returns a `User` if the request session currently has a logged in user.
        Otherwise returns `None`.
        """

        # Get the underlying HttpRequest object
        request = request._request
        user = getattr(request, "user", None)

        # Unauthenticated, CSRF validation not required
        if not user or not user.is_active:
            return None

        self.enforce_csrf(request)

        # CSRF passed with authenticated user
        return (user, None)

    def enforce_csrf(self, request):
        """
        Enforce CSRF validation for session based authentication.
        """
        reason = CSRFCheck().process_view(request, None, (), {})
        if reason:
            # CSRF failed, bail with explicit error message
            raise exceptions.PermissionDenied("CSRF Failed: %s" % reason)

原來是這樣，最近給系統增加了用戶登陸功能，使用的就是SessionAuthorization和TokenAuthorization，然后在SessionAuthorization中調用了self.enforce_csrf(request)而這個調用的又是上面的CSRFCheck，這個類是重載了django里面的csrf middleware，而且沒發現有地方可以關掉這個功能，即使在django里面去掉這個middleware，但是這個還是會調用的。

那怎么去掉這個功能呢，我們現在就是要進行跨域請求。

最簡單了，直接注釋掉上面的self.enforce_csrf(request)這一行代碼就行了或者在設置中添加一項，比如改成

GLOBAL_CSRF_CHECK = True
if GLOBAL_CSRF_CHECK：
    self.enforce_csrf(request)

我們繼續看源代碼，到middleware的代碼里面去。

class CsrfViewMiddleware(object):
    """
    Middleware that requires a present and correct csrfmiddlewaretoken
    for POST requests that have a CSRF cookie, and sets an outgoing
    CSRF cookie.

    This middleware should be used in conjunction with the csrf_token template
    tag.
    """
    # The _accept and _reject methods currently only exist for the sake of the
    # requires_csrf_token decorator.
    def _accept(self, request):
        # Avoid checking the request twice by adding a custom attribute to
        # request.  This will be relevant when both decorator and middleware
        # are used.
        request.csrf_processing_done = True
        return None

    def _reject(self, request, reason):
        logger.warning("Forbidden (%s): %s",
                       reason, request.path,
            extra={
                "status_code": 403,
                "request": request,
            }
        )
        return _get_failure_view()(request, reason=reason)

    def process_view(self, request, callback, callback_args, callback_kwargs):

        if getattr(request, "csrf_processing_done", False):
            return None

        try:
            csrf_token = _sanitize_token(
                request.COOKIES[settings.CSRF_COOKIE_NAME])
            # Use same token next time
            request.META["CSRF_COOKIE"] = csrf_token
        except KeyError:
            csrf_token = None
            # Generate token and store it in the request, so it"s
            # available to the view.
            request.META["CSRF_COOKIE"] = _get_new_csrf_key()

        # Wait until request.META["CSRF_COOKIE"] has been manipulated before
        # bailing out, so that get_token still works
        if getattr(callback, "csrf_exempt", False):
            return None

        # Assume that anything not defined as "safe" by RFC2616 needs protection
        if request.method not in ("GET", "HEAD", "OPTIONS", "TRACE"):
            if getattr(request, "_dont_enforce_csrf_checks", False):
                # Mechanism to turn off CSRF checks for test suite.
                # It comes after the creation of CSRF cookies, so that
                # everything else continues to work exactly the same
                # (e.g. cookies are sent, etc.), but before any
                # branches that call reject().
                return self._accept(request)

            if request.is_secure():
                # Suppose user visits http://example.com/
                # An active network attacker (man-in-the-middle, MITM) sends a
                # POST form that targets https://example.com/detonate-bomb/ and
                # submits it via JavaScript.
                #
                # The attacker will need to provide a CSRF cookie and token, but
                # that"s no problem for a MITM and the session-independent
                # nonce we"re using. So the MITM can circumvent the CSRF
                # protection. This is true for any HTTP connection, but anyone
                # using HTTPS expects better! For this reason, for
                # https://example.com/ we need additional protection that treats
                # http://example.com/ as completely untrusted. Under HTTPS,
                # Barth et al. found that the Referer header is missing for
                # same-domain requests in only about 0.2% of cases or less, so
                # we can use strict Referer checking.
                referer = request.META.get("HTTP_REFERER")
                if referer is None:
                    return self._reject(request, REASON_NO_REFERER)

                # Note that request.get_host() includes the port.
                good_referer = "https://%s/" % request.get_host()
                if not same_origin(referer, good_referer):
                    reason = REASON_BAD_REFERER % (referer, good_referer)
                    return self._reject(request, reason)

            if csrf_token is None:
                # No CSRF cookie. For POST requests, we insist on a CSRF cookie,
                # and in this way we can avoid all CSRF attacks, including login
                # CSRF.
                return self._reject(request, REASON_NO_CSRF_COOKIE)

            # Check non-cookie token for match.
            request_csrf_token = ""
            if request.method == "POST":
                request_csrf_token = request.POST.get("csrfmiddlewaretoken", "")

            if request_csrf_token == "":
                # Fall back to X-CSRFToken, to make things easier for AJAX,
                # and possible for PUT/DELETE.
                request_csrf_token = request.META.get("HTTP_X_CSRFTOKEN", "")

            if not constant_time_compare(request_csrf_token, csrf_token):
                return self._reject(request, REASON_BAD_TOKEN)

        return self._accept(request)

    def process_response(self, request, response):
        if getattr(response, "csrf_processing_done", False):
            return response

        # If CSRF_COOKIE is unset, then CsrfViewMiddleware.process_view was
        # never called, probaby because a request middleware returned a response
        # (for example, contrib.auth redirecting to a login page).
        if request.META.get("CSRF_COOKIE") is None:
            return response

        if not request.META.get("CSRF_COOKIE_USED", False):
            return response

        # Set the CSRF cookie even if it"s already set, so we renew
        # the expiry timer.
        response.set_cookie(settings.CSRF_COOKIE_NAME,
                            request.META["CSRF_COOKIE"],
                            max_age = 60 * 60 * 24 * 7 * 52,
                            domain=settings.CSRF_COOKIE_DOMAIN,
                            path=settings.CSRF_COOKIE_PATH,
                            secure=settings.CSRF_COOKIE_SECURE,
                            httponly=settings.CSRF_COOKIE_HTTPONLY
                            )
        # Content varies with the CSRF cookie, so set the Vary header.
        patch_vary_headers(response, ("Cookie",))
        response.csrf_processing_done = True
        return response

里面主要有兩個函數，一個是process view，另一個是process response。這里就不得不說django middleware的工作原理了。

https://docs.djangoproject.com/en/1.6/topics/http/middleware/

process_request() is called on each request, before Django decides which view to execute.

process_view() is called just before Django calls the view.

process_response() is called on all responses before they’re returned to the browser.

所以這個middleware的process view會在請求到達view函數之前被調用，可以理解為一個過濾器吧。

 if request.method not in ("GET", "HEAD", "OPTIONS", "TRACE"):
            if getattr(request, "_dont_enforce_csrf_checks", False):
                return self._accept(request)

這里request里面有_dont_enforce_csrf_checks就直接進入view了，沒有下面的檢查了。所以我們只要自己給request添加一個這樣的屬性就好了。最直接的方法還是去寫一個middleware啊，哈哈。

代碼很簡單

class DisableCSRFCheck(object):
    def process_request(self, request):
        setattr(request, "_dont_enforce_csrf_checks", True)