Why is an OPTIONS request sent and can I disable it

Question

I am building a web API  I found whenever I use Chrome to POST  GET to my API  there is always an OPTIONS request sent before the real request  which is quite annoying  Currently I get the server to ignore any OPTIONS requests  Now my questions is what s good to send an OPTIONS request to double the server s load  Is there any way to completely stop the browser from sending OPTIONS requests

User · Answer

There is maybe a solution  but i didnt test it    you could use CSP  Content Security Policy  to enable your remote domain and browsers will maybe skip the CORS OPTIONS request verification   I if find some time  I will test that and update this post    CSP   https   developer mozilla org fr docs Web HTTP Headers Content-Security-Policy  CSP Specification   https   www w3 org TR CSP

User · Answer

Have gone through this issue  below is my conclusion to this issue and my solution   According to the CORS strategy  highly recommend you read about it  You can t just force the browser to stop sending OPTIONS request if it thinks it needs to    There are two ways you can work around it    Make sure your request is a  simple request  Set Access-Control-Max-Age for the OPTIONS request   Simple request  A simple cross-site request is one that meets all the following conditions   The only allowed methods are    GET HEAD POST   Apart from the headers set automatically by the user agent  e g  Connection  User-Agent  etc    the only headers which are allowed to be manually set are    Accept Accept-Language Content-Language Content-Type   The only allowed values for the Content-Type header are    application x-www-form-urlencoded multipart form-data text plain   A simple request will not cause a pre-flight OPTIONS request   Set a cache for the OPTIONS check  You can set a Access-Control-Max-Age for the OPTIONS request  so that it will not check the permission again until it is expired      Access-Control-Max-Age gives the value in seconds for how long the response to the preflight request can be cached for without sending another preflight request    Limitation Noted   For Chrome  the maximum seconds for Access-Control-Max-Age is 600 which is 10 minutes  according to chrome source code Access-Control-Max-Age only works for one resource every time  for example  GET requests with same URL path but different queries will be treated as different resources  So the request to the second resource will still trigger a preflight request

User · Answer

You can t but you could avoid CORS using JSONP

User · Answer

One solution I have used in the past - lets say your site is on mydomain com  and you need to make an ajax request to foreigndomain com   Configure an IIS rewrite from your domain to the foreign domain - e g     lt rewrite gt     lt rules gt       lt rule name  ForeignRewrite  stopProcessing  true  gt           lt match url   api v1          gt           lt action type  Rewrite  url  https   foreigndomain com  R 1     gt       lt  rule gt     lt  rules gt   lt  rewrite gt    on your mydomain com site - you can then make a same origin request  and there s no need for any options request

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edit 2018-09-13  added some precisions about this pre-flight request and how to avoid it at the end of this reponse   OPTIONS requests are what we call pre-flight requests in Cross-origin resource sharing  CORS    They are necessary when you re making requests across different origins in specific situations    This pre-flight request is made by some browsers as a safety measure to ensure that the request being done is trusted by the server   Meaning the server understands that the method  origin and headers being sent on the request are safe to act upon    Your server should not ignore but handle these requests whenever you re attempting to do cross origin requests   A good resource can be found here http   enable-cors org   A way to handle these to get comfortable is to ensure that for any path with OPTIONS method the server sends a response with this header   Access-Control-Allow-Origin     This will tell the browser that the server is willing to answer requests from any origin    For more information on how to add CORS support to your server see the following flowchart  http   www html5rocks com static images cors server flowchart png      edit 2018-09-13  CORS OPTIONS request is triggered only in somes cases  as explained in MDN docs      Some requests don   t trigger a CORS preflight  Those are called    simple requests    in this article  though the Fetch spec  which defines CORS  doesn   t use that term  A request that doesn   t trigger a CORS preflight   a so-called    simple request      is one that meets all the following conditions       The only allowed methods are          GET   HEAD   POST         Apart from the headers set automatically by the user agent  for example  Connection  User-Agent  or any of the other headers with names defined in the Fetch spec as a    forbidden header name      the only headers which are allowed to be manually set are those which the Fetch spec defines as being a    CORS-safelisted request-header     which are          Accept   Accept-Language   Content-Language   Content-Type  but note the additional requirements below    DPR   Downlink   Save-Data   Viewport-Width   Width         The only allowed values for the Content-Type header are          application x-www-form-urlencoded   multipart form-data   text plain         No event listeners are registered on any XMLHttpRequestUpload object used in the request  these are accessed using the XMLHttpRequest upload property       No ReadableStream object is used in the request

User · Answer

As mentioned in previous posts already  OPTIONS requests are there for a reason  If you have an issue with large response times from your server  e g  overseas connection  you can also have your browser cache the preflight requests   Have your server reply with the Access-Control-Max-Age header and for requests that go to the same endpoint the preflight request will have been cached and not occur anymore

User · Answer

It can be solved in case of use of a proxy that intercept the request and write the appropriate headers   In the particular case of Varnish these would be the rules   if  req http host     CUSTOM URL      set resp http Access-Control-Allow-Origin        if  req method     OPTIONS        set resp http Access-Control-Max-Age    1728000      set resp http Access-Control-Allow-Methods    GET  POST  PUT  DELETE  PATCH  OPTIONS      set resp http Access-Control-Allow-Headers    Authorization Content-Type Accept Origin User-Agent DNT Cache-Control X-Mx-ReqToken Keep-Alive X-Requested-With If-Modified-Since      set resp http Content-Length    0      set resp http Content-Type    text plain charset UTF-8      set resp status   204

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I have solved this problem like   if   SERVER  REQUEST METHOD       OPTIONS   amp  amp  ENV     devel         header  Access-Control-Allow-Origin           header  Access-Control-Allow-Headers  X-Requested-With        header  HTTP 1 1 200 OK        die        It is only for development  With this I am waiting 9ms and 500ms and not 8s and 500ms  I can do that because production JS app will be on the same machine as production so there will be no OPTIONS but development is my local

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Please refer this answer on the actual need for pre-flighted OPTIONS request  CORS - What is the motivation behind introducing preflight requests   To disable the OPTIONS request  below conditions must be satisfied for ajax request    Request does not set custom HTTP headers like  application xml  or  application json  etc The request method has to be one of GET  HEAD or POST  If POST  content type should be one of application x-www-form-urlencoded  multipart form-data  or text plain   Reference  https   developer mozilla org en-US docs Web HTTP Access control CORS

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What worked for me was to import  github com gorilla handlers  and then use it this way   router    mux NewRouter   router HandleFunc   config   getConfig  Methods  GET   router HandleFunc   config emcServer   createEmcServers  Methods  POST    headersOk    handlers AllowedHeaders   string  X-Requested-With    Content-Type    originsOk    handlers AllowedOrigins   string       methodsOk    handlers AllowedMethods   string  GET    HEAD    POST    PUT    OPTIONS     log Fatal http ListenAndServe       webServicePort  handlers CORS originsOk  headersOk  methodsOk  router      As soon as I executed an Ajax POST request and attaching JSON data to it  Chrome would always add the Content-Type header which was not in my previous AllowedHeaders config

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For a developer who understands the reason it exists but needs to access an API that doesn t handle OPTIONS calls without auth  I need a temporary answer so I can develop locally until the API owner adds proper SPA CORS support or I get a proxy API up and running    I found you can disable CORS in Safari and Chrome on a Mac   Disable same origin policy in Chrome  Chrome  Quit Chrome  open an terminal and paste this command  open  Applications Google  Chrome app --args --disable-web-security --user-data-dir  Safari  Disabling same-origin policy in Safari      If you want to disable the same-origin policy on Safari  I have 9 1 1   then you only need to enable the developer menu  and select  Disable Cross-Origin Restrictions  from the develop menu

User · Answer

Yes it s possible to avoid options request  Options request  is a preflight request when you send  post  any data to another domain  It s a browser security issue  But we can use another technology  iframe transport layer  I strongly recommend you forget about any CORS configuration and use readymade solution and it will work anywhere    Take a look here  https   github com jpillora xdomain  And working example  http   jpillora com xdomain

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When you have the debug console open and the Disable Cache option turned on  preflight requests will always be sent  i e  before each and every request   if you don t disable the cache  a pre-flight request will be sent only once  per server

User · Answer

After spending a whole day and a half trying to work through a similar problem I found it had to do with IIS    My Web API project was set up as follows      WebApiConfig cs public static void Register HttpConfiguration config        var cors   new EnableCorsAttribute                     config EnableCors cors                 I did not have CORS specific config options in the web config   system webServer node like I have seen in so many posts  No CORS specific code in the global asax or in the controller as a decorator  The problem was the app pool settings    The managed pipeline mode was set to classic  changed it to integrated  and the Identity was set to Network Service  changed it to ApplicationPoolIdentity   Changing those settings  and refreshing the app pool  fixed it for me

[http] Why is an OPTIONS request sent and can I disable it?

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