Table of Contents

About

sudo is an command utility that:

  • su: switch user su
  • and do: execute a command

In other words, it executes a command as another user. (ie it's a proxy authentication utility)

Authorization

It determines who is an authorized user by consulting the sudoers configuration files

Example

sudo -E -i -H -u UserOtherThanRoot

where:

  • - H: sets the HOME environment variable to the homedir of the target user
  • - u: run the specified command as a user other than root
  • -E: takes the environment of the actual user
  • -i: run as a login shell. Reading the user environment files.

Configuration Files

sudoers syntax

The sudoers file is composed of two types of entries:

When multiple entries match for a user, they are applied in order

sudoers

Alias

aliases are basically variables that can be user in the user_specification

User_Alias
User_Alias ADMINS = jsmith, mikem
User_Alias WEBMASTERS = will, wendy, wim
RunAsAlias

Runas_Alias - determines the user and/or the group that a command may be run as. 

Runas_Alias DB = oracle, sybase
Runas_Alias ADMINGRP = adm, oper

Example with a user_specification 

dgb boulder = (ADMINGRP) /bin/ls, (root) /bin/kill, /usr/bin/lprm

The user dgb may run on the host boulder:

  • /bin/ls as ADMINGRP,
  • /bin/kill, and /usr/bin/lprm as root

ie 

sudo -u oper /bin/ls
sudo -u adm /bin/ls
sudo /bin/kill
Host Alias
  • Host_Alias 
Host_Alias SERVERS = master, mail, www, ns
Command alias

Cmnd_Alias: A command alias defines one or more glob expressions that need to match the command entered to allow it to run.

The alis name must be in uppercase

Example:

  • The kill command with any arguments
Cmnd_Alias KILL = /usr/bin/kill*
  • All shells
Cmnd_Alias SHELLS = /usr/bin/sh, /usr/bin/csh, /usr/bin/ksh,\

User specification

User specifications specify who may run what.

Syntax: 

user	MACHINE=COMMANDS

user MACHINE=(AS) TAGS COMMAND

More specifically in regular expression EBNF (see the sudoers and search for the section User specification)

Example: 

%wheel	ALL=(ALL) NOPASSWD: ALL

every user in the group wheel may run:

  • on any machine ALL (machine)
  • as any user (ALL) (run as)
  • without any password NOPASSWD: (tag)
  • any command ANY (command)

sudoers files

/etc/sudoers

The default security policy is sudoers, which is configured via the file /etc/sudoers, or via LDAP.

Open the sudoers file 

sudo visudo

/etc/sudoers.d/

The last line of the /etc/sudoers files include others configuration that can be added in the directory /etc/sudoers.d/

The last line is not a comment. A comment in the sudoers file as a space after the hash tag 

## Read drop-in files from /etc/sudoers.d (the # here does not mean a comment)
#includedir /etc/sudoers.d

Example in Azure they will add the file waagent 

sshuser ALL=(ALL) NOPASSWD: ALL

/etc/sudo.conf

The sudo configuration is in the file /etc/sudo.conf

sudo.conf

Management

Language of the configuration file

wheel / sudo admin group

If the wheel line is uncommented, you got an admin group. 

## Allows people in group wheel to run all commands
%wheel	ALL=(ALL) ALL

Any user in the group wheel can run any command on any host as any user.

Example: 

cat /etc/group | grep wheel

wheel:x:27:testuser,sshuser

Allow a user to run a command

In a sudoers file add the following rules: 

userName ALL=(ALL) NOPASSWD: /full/path/to/command
# or with the alias command named ALIAS_CMD
userName ALL=(ALL) NOPASSWD: ALIAS_CMD
# or
userName ALL=(ALL) NOPASSWD: /full/path/to/command ARG1 ARG2

Example allow the powercenter user to start and stop its services 

powercenter  ALL=(ALL) NOPASSWD: /sbin/service infa start
powercenter  ALL=(ALL) NOPASSWD: /sbin/service infa stop

Disable password prompt

Disable password prompt for all command.

  • Open the sudoers file.
sudo visudo
  • Append the following line at the bottom of the sudoers file:
<username> ALL=NOPASSWD: ALL
  • Save the file and exit the editor.
  • Log out and log in to apply the changes.

Test if allowed

run sudo with the -l or -v flags

Example with the su command 

sudo -l su

[sudo] password for gerard:
/bin/su

If a user who is not listed in the sudoers file tries to run a command via sudo without the -l or -v flags, mail is sent to the proper authorities, as defined at configure time or the sudoers file (defaults to root).

Documentation / Reference