How to know who is connecting using db_link?

Hi,

There is schema (example USER4LINK) that has SELECT on other schema (same database, example DB1)

There are several databases on other hosts (example HOST1) have created database link using provided user USER4LINK.

Right now , I want to know which databases are connecting through the database link, they comes from the same USER and MACHINE.

From the view v$session, I can only know USERNAME, MACHINE. They are all the same if these connections comming from different databases but the same host.

Thanks,

Xiaogang