sudo -K, then retry your test. If it starts asking again, all that was happening is that you had
sudo configured to remember your password for some time.
On top of this, Ubuntu's default
sudo configuration makes it remember your credentials across
ttys. This affects
ssh sessions, as you've discovered, since each new
ssh connection looks like a new terminal to the low-level OS code.
This also affects things like the graphical Terminal app. If you authenticate with
sudo, then create a new tab with Ctrl-Shift-T, you'll find that you don't need to give a password to
sudo again in that tab, despite the fact that it also creates a new
tty. You can even close the Terminal app entirely, and as long as you restart it within the normal password timeout,
sudo will run without requiring you to re-enter your password. This behavior may be enough to make you decide you want to keep this feature enabled.
Mac OS X works this way these days, too.
Not all *ixes do. Red Hat Enterprise Linux (and derivatives like CentOS) insist on getting the password on each new
You can disable both behaviors by changing the
Defaults line in
/etc/sudoers to something like this:
env_reset bit should be there already, and isn't relevant here
timestamp_timeout directive tells it to immediately time out each
sudo session. It's like saying
sudo -K after every normal
tty_tickets directive ensures that it associates credentials with the
tty they were used on, not just the user name. This is supposed to be the default already, and is documented as such on Ubuntu, but they must have built their distribution of
sudo to disable this option, for the convenience reasons given above.