Is there a way to get the uid of the other end of a unix socket connection

Question

Is there a way for a UNIX domain socket listener to only accept connection from certain user (chmod/chown does not work for abstract socket afaik), or

Answer 1

Yes -- this operation, along with FD passing, is supported via an ancilliary message with the SCM_CREDENTIALS type. The calls involved are documented in man 7 unix.