Backstory
I recently found myself writing some OpenID/SSO code and realized that for some reason Azure AD Proxy doesn’t rewrite the header value for replyurl. This means that while you connect to Azure AD Proxy to access your app, when your internal app then attempts to authenticate to Azure AD via OpenID (or SAML), once it is successful it returns you to the internal URL not the proxied url.
Manually Setting the RedirectUri / Reply URL
First you must understand, you can not set this value on the Azure side, it MUST be set on the app. In our case we wrote our own app so to fix it we wrote code to trap the event for OnRedirectToIdentityProvider and then set our own hardcoded Azure AD Proxy external URL. We cleaned this up by making the external URL a parameter in the configuration file instead of code itself.
options.Events.OnRedirectToIdentityProvider = (context) => {
context.ProtocolMessage.RedirectUri = <Azure AD External URL/SSOpath>;
await Task.FromResult(0);
}Determining an Azure AD Proxy Client Request from a Normal one
Next up we didn’t want to just hard code the azure external url. This would mean we could never use the internal URL for testing. So we also added in a check of the following request header value:
Name: HTTP_X_MS_PROXY
Value: AzureAD-Application-Proxy
We now check to see if HTTP_X_MS_PROXY is present and if so change the RedirectUri to the Azure AD Proxy External URL. Otherwise, we let it return the internal URL.