How to log user in to multiple drupal sites? I have multiple drupal sites on the same server (VPS). Thise sites has separated d7 core and db. All sites has own domains: domain1.com, domain2.com, domain3.com..

When user logs in to one site I need him to log also in to the another site.
This should be done in single sign on so user does not have to log first to one site and then to another.

How to automatically log user in to the both/multiple sites?
You are looking for Multisite Login module

Multisite Login allows a user to login to all sites in a multisite
configuration, even though the multisites do not share domain names.
NOTE: if you are using several sites on the same domain (ex.
politicker.com, nj.politicker.com, ny.politicker.com, etc.), then you
do not need this module. The multisite functionality in Drupal core
has the ability to share logins for sites like these without
additional modules.

also take a look at CAS module.