How do I emulate su's behavior with sudo?

Question

I'm trying to make sudo ask for target password, except when run by root. Here's what I do:

Defaults:!root    targetpw

This way it asks for my password, and if I make it like this:

Defaults:yuri    targetpw

it asks for target user's password, but that's not what I wanted.

Answer 1

All you can do is set targetpw as default, but this won't require root to enter any password. You can't configure sudo to do this and it wouldn't make any sense, either, since root may always do whatever he wants (su doesn't ask for a password either, does it‽).

So put

Defaults targetpw

into your /etc/sudoers file and you should have su behavior.

Answer 2

From the sudoers man page:

An exclamation point (‘!’) can be used as a logical not operator in a list or
alias as well as in front of a Cmnd.  This allows one to exclude certain values.
For the ‘!’ operator to be effective, there must be something for it to exclude.
For example, to match all users except for root one would use:

     ALL,!root

 If the ALL, is omitted, as in:

     !root

 it would explicitly deny root but not match any other users.  This is different
 from a true “negation” operator.

So,

Defaults:ALL,!root    targetpw

should be what you are looking for.