How can I share a session across multiple subdomains in ASP.NET?

You tagged this with ASP.NET and IIS, so I will assume that is your environment. Make sure you have this in your web.config:

<httpCookies domain=".usa.com"/>

If your 2 subdomains map to the same application, then you are done. However, if they are different applications you will need to do some additional work, like using a SQL Server based Session storage (and hacking the stored procedures to make sure all applications share the same session data) or with an HttpModule to intercept the application name, since even with shared cookies and the same machine key, 2 applications will still use 2 different stores for their session data.

Track your own sessions and use a cookie with an appropriate domain setting, ie. .usa.com.

Alternatively, if you're using PHP, I believe there's a setting to change the default domain setting of the session cookie it uses, that may be useful too.

The settings you're looking for are:

session.use_cookies = 1
session.use_only_cookies = 1
session.cookie_domain = .usa.com

I recently went thru this and learned the hard way. Localhost is actually considered a TLD. Cookie domains require at least a second level domain - test.com. If you want cookies to work for a domain and all it's sub-domains, prefix with a '.' - .test.com.

When running/debugging locally, setting a domain of localhost will fail, and it will fail even if the domain is set properly because visual studio uses localhost by default.

This default localhost can be changed in the project properties so that the project will actually run at cookie domain test.com. Essentially, if the address in the browser matches , you can get it to work.

My issue is documented here: Setting ServiceStack Cookie Domain in Web.Config Causes Session Id to Change on Every Request

Hope this helps.