How to setup passwordless `sudo` on Linux?

How can passwordless sudo access be setup on either RHEL (Fedora, CentOS, etc) or Ubuntu distributions? (If it's the same across distros, that's even better!)

Setting: personal and/or lab/training equipment with no concern for unauthorized access (ie, the devices are on non-public networks, and any/all users are fully trusted, and the contents of the devices are "plain-vanilla").

EDIT thanks to medina's comment: According to the man page, you should be able to write

ALL            ALL = (ALL) NOPASSWD: ALL

to allow all users to run all commands without a password.

For reference, I'm leaving my previous answer:

If you add a line of the form

%wheel         ALL = (ALL) NOPASSWD: ALL

to /etc/sudoers (using the visudo command, of course), it will let everyone in the group wheel run any commands without providing a password. So I think the best solution is to put all your users in some group and put a line like that in sudoers - obviously you should replace wheel with the actual group you use.

Alternatively, you can define a user alias,

User_Alias     EVERYONE = user1, user2, user3, ...

and use that:

EVERYONE       ALL = (ALL) NOPASSWD: ALL

although you would have to update /etc/sudoers every time you add or remove a user.

I tried the solutions above to no avail. The following solution worked for me Edit the /etc/sudoers file and add the following line

username ALL=(ALL) NOPASSWD: ALL

The key is to add it after the last line which says

#includedir /etc/sudoers.d

I tried all the answers on this page, with no useful results. Eventually I figured it out, use this command to list your sudo rights:

sudo -l

This should give you an output like this:

User gmurphy may run the following commands on this host:
    (root) NOPASSWD: ALL
    (ALL) ALL

It shows that I'm configured with root privileges but that I'm still part of a group (admin) matched to a sudo rule which expects the password ("(ALL) ALL"). This was forcing sudo to prompt me. The rule in question was the admin users:

# Members of the admin group may gain root privileges
%admin ALL=(ALL) ALL

Once I commented this out, I was able to sudo without password. I hope this is of use to someone else.

Within /etc/sudoers there's an example of just that towards the bottom of the file:

## Same thing without a password
# %wheel        ALL=(ALL)       NOPASSWD: ALL