This can be configured with the sudoers file in /etc/sudoers. However, it is not recommended to edit this file directly. Instead, as of Debian version 1.7.2p1-1, the default /etc/sudoers file now includes the directive:

#includedir /etc/sudoers.d

This will cause sudo to read and parse any files in the /etc/sudoers.d directory that do not end in ‘~’ or contain a ‘.’ character.

Note that there must be at least one file in the sudoers.d directory (there should be a README file by default), and all files in this directory should be mode 0440.

Thus, configuring a user with passwordless sudo can be done in 3 steps:

1. Create a file file in the /etc/sudoers.d directory:

   sudo vim /etc/sudoers.d/<user>
   

2. Add: <user> ALL=(ALL) NOPASSWD: ALL , where <user> is your passwordless sudo username;
3. Change the permission of the file to be mode 0440:

   sudo chmod 0440 /etc/sudoers.d/<user>
   

From now on, your <user> will be able to execute sudo any_command without providing a password.