Change in SharePoint DNS breaking remote authentication code through office 365 login

Question

I have a website that connects to a SharePoint online site in order to access content from there. The authentication is done through the Office 365 login. In order to do that, I composed the following URL:

https://login.microsoftonline.com/login.srf?wa=wsignin1.0&rpsnv=3&rver=6.1.6206.0&wp=MBI&wreply=https://www10226.sharepoint.com/_layouts/15/landing.aspx?Source=" + window.location

so that the window.location is returned to after the authentication is done. It worked so far, but today I encountered the following problem: https://www10226.sharepoint.com now says page cannot be found, DNS lookup failed. Apparently the correct address is now https://www10501.sharepoint.com . Does anybody know about this sort of change? Is it a one time thing or it happens on a regular basis? How can I get the right DNS dynamically so my code won't be affected by changes like this one in the future?

Any help is highly appreciated.

Answer 1

If anyone ever has the same problem, I found a solution to this: instead of hardcoding the wreply parameter, I first access the SharePoint site + "/_forms/default.aspx" with a request that does not follow redirects and has the user agent set. The response has a Location header that I get and which contains a wreply parameter. Only the left part (in the Uri left part - authority sense) is of interest. After you get that you should add the suffix "/_layouts/15/landing.aspx" and that is the value of the wreply parameter, regardless of the DNS changes in CNAME that occur regularly to some Office 365 tenants.

This is a solution meant to work with multiple SharePoint online addresses, migrated or older versions, default CNAME set or not.