sudo
The sudo command provides a simple and secure way to configure privilege escalation — i.e., letting normal users execute certain (or even all) commands as root or as another user, either with or without giving a password.
To allow some users to perform certain administrative steps on a system without granting them total root access, using sudo is the best option. Using sudo allows control over who can do what.
This article is meant as a quick introduction - the app-admin/sudo package is a lot more powerful than what is described here. It has special features for editing files as a different user (sudoedit), running from within a script (so it can background, read the password from standard input instead of the keyboard, etc.), etc.
Please read the sudo and sudoers manual pages for more information.
Installation
sudo is not part of the system set, so it must be installed manually, if it is needed.
The su command should always be available, in case sudo is not installed.
USE flags
USE flags for app-admin/sudo Allows users or groups to run commands as other users
+secure-path
|
Replace PATH variable with compile time secure paths |
+sendmail
|
Allow sudo to send emails with sendmail |
gcrypt
|
Use message digest functions from dev-libs/libgcrypt instead of sudo's |
ldap
|
Add LDAP support (Lightweight Directory Access Protocol) |
nls
|
Add Native Language Support (using gettext - GNU locale utilities) |
offensive
|
Let sudo print insults when the user types the wrong password |
pam
|
Add support for PAM (Pluggable Authentication Modules) - DANGEROUS to arbitrarily flip |
sasl
|
Add support for the Simple Authentication and Security Layer |
selinux
|
!!internal use only!! Security Enhanced Linux support, this must be set by the selinux profile or breakage will occur |
skey
|
Enable S/Key (Single use password) authentication support |
ssl
|
Add support for SSL/TLS connections (Secure Socket Layer / Transport Layer Security) |
sssd
|
Add System Security Services Daemon support |
verify-sig
|
Verify upstream signatures on distfiles |
Emerge
root #emerge --ask app-admin/sudoConfiguration
Logging activity
One additional advantage of sudo is that it can log any attempt (successful or not) to run an application. This is very useful when tracking who made that one fatal mistake that took 10 hours to fix :)
Granting permissions
The app-admin/sudo package allows the system administrator to grant permission to other users to execute one or more applications they would normally have no right to. Unlike using the setuid bit on these applications sudo gives a more fine-grained control on who can execute a certain command and when.
With sudo a clear list can be made of who can execute a certain application. If the setuid bit is set on an executable, any user would be able to run the application (or any user of a certain group, depending on the permissions used). With sudo the user can (and probably should) be required to provide a password in order to execute the application.
The sudo configuration is managed by the /etc/sudoers file.
Only ever use the visudo command to edit /etc/sudoers. Never edit this file manually with a text editor, shell redirection, or similar means. In other words, never use nano /etc/sudoers, vim /etc/sudoers, etc. Always use visudo - this tool makes sure that no two system administrators are editing this file at the same time, preserves the permissions on the file, and performs some syntax checking to make sure no fatal mistakes can occur in the file.
Basic syntax
The most difficult part of sudo is the /etc/sudoers syntax. The basic syntax is as follows:
user host = command
This line tells sudo that the user, identified by user and logged in on the system host, can execute the command command (which can also be a comma-separated list of allowed commands).
A more real-life example might make this more clear: To allow the user larry to execute emerge when he is logged in on localhost:
larry localhost = /usr/bin/emerge
The hostname must match the name that the hostname command returns.
In the simplest case, commands must be fully qualified paths to executables: hence
/usr/bin/emerge not just emerge. Paths can also contain wildcards and may refer to entire directories. See the manpage for details.Do not allow a normal user to run an application that can enable them to elevate privileges. For instance, allowing users to execute emerge as root can grant them full root access to the system because emerge can be manipulated to change the live file system to the user's advantage. If the sudo users are not trusted, don't grant them any additional rights.
The user name can also be substituted with a group name, in which case the name is prefaced by a % sign. For instance, to allow any one in the wheel group to execute emerge:
%wheel localhost = /usr/bin/emerge
To enable more than one command for a given user on a given machine, multiple commands can be listed on the same line. For instance, to allow larry to not only run emerge but also ebuild and emerge-webrsync as root:
larry localhost = /usr/bin/emerge, /usr/bin/ebuild, /usr/sbin/emerge-webrsync
The precise command line can also be specified (including parameters and arguments) not just the name of the executable. This is useful to restrict the use of a certain tool to a specified set of command options. The sudo tool allows shell-style wildcards (AKA meta or glob characters) to be used in path names as well as command-line arguments in the sudoers file. Note that these are not regular expressions.
Here is an example of sudo from the perspective of a first-time user of the tool who has been granted access to the full power of emerge:
user $sudo emerge -uDN worldWe trust you have received the usual lecture from the local System
Administrator. It usually boils down to these three things:
#1) Respect the privacy of others.
#2) Think before you type.
#3) With great power comes great responsibility.
Password: ## (Enter the user password, not root!)
The password that sudo requires is the user's own password. This is to make sure that no terminal that is accidentally left open to others is abused for malicious purposes.
sudo does not alter the
${PATH} variable: any command placed after sudo is executed within the user's own environment. Thus if a user wants to run a tool in /sbin, for instance, the user must provide sudo with the full path of the command, like so:
user $sudo /usr/sbin/emerge-webrsyncBasic syntax with LDAP
The ldap and pam USE flags are needed for the LDAP support.
When using sudo with LDAP, sudo will read configuration from LDAP Server as well. So two files will need to edited.
/etc/ldap.conf.sudoPlease chmod 400 when done# See ldap.conf(5) and README.LDAP for details
# This file should only be readable by root
# supported directives: host, port, ssl, ldap_version
# uri, binddn, bindpw, sudoers_base, sudoers_debug
# tls_{checkpeer,cacertfile,cacertdir,randfile,ciphers,cert,key}
host ldap.example.com
port 389
base dc=example,dc=com
uri ldap://ldap.example.com/
#uri ldapi://%2fvar%2frun%2fopenldap%2fslapd.sock
ldap_version 3
#ssl start_tls
sudoers_base ou=SUDOers,dc=example,dc=com
#sudoers_debug 2
bind_policy soft
/etc/nsswitch.confPlease add the sudoers linesudoers: ldap files
The following LDAP entry will need to added for sudo.
It was design so that the sudoers branch are on top of the tree for security reason. This allows a different access right from LDAP to read/write to this branch
version: 1
DN: ou=SUDOers,dc=example,dc=com
objectClass: organizationalUnit
objectClass: top
objectClass: domainRelatedObject
associatedDomain: example.com
ou: SUDOers
DN: cn=defaults,ou=SUDOers,dc=example,dc=com
objectClass: top
objectClass: sudoRole
cn: defaults
description: Default sudoOption's go here
sudoOption: env_reset
DN: cn=root,ou=SUDOers,dc=example,dc=com
objectClass: top
objectClass: sudoRole
cn: root
sudoCommand: ALL
sudoHost: ALL
sudoUser: root
DN: cn=%wheel,ou=SUDOers,dc=example,dc=com
objectClass: top
objectClass: sudoRole
cn: %wheel
sudoCommand: ALL
sudoHost: ALL
sudoOption: !authenticate
sudoUser: %wheel
version: 1
DN: cn=wheel,ou=Group,dc=example,dc=com
objectClass: top
objectClass: posixGroup
cn: wheel
description: Wheel Group
gidNumber: 10
memberUid: useradmin1
memberUid: root
The configuration on the sudoer on LDAP are similar to files with some different. Please read more about sudo with LDAP on the link below.[1]
Using aliases
In larger environments having to enter all users over and over again (or hosts, or commands) can be a daunting task. To ease the administration of /etc/sudoers aliases can defined. The format to declare aliases is quite simple:
Host_Alias hostalias = hostname1, hostname2, ...
User_Alias useralias = user1, user2, ...
Cmnd_Alias cmndalias = command1, command2, ...
One alias that always works, for any position, is the ALL alias (to make a good distinction between aliases and non-aliases it is recommended to use capital letters for aliases). The ALL alias is an alias to all possible settings.
A sample use of the ALL alias to allow any user to execute the shutdown command if he is logged on locally is:
ALL localhost = /sbin/shutdown
Another example is to allow the user larry to execute the emerge command as root, regardless of where he is logged in from:
larry ALL = /usr/bin/emerge
More interesting is to define a set of users who can run software administrative applications (such as emerge and ebuild) on the system and a group of administrators who can change the password of any user, except root!
User_Alias SOFTWAREMAINTAINERS = larry, john, danny
User_Alias PASSWORDMAINTAINERS = larry, sysop
Cmnd_Alias SOFTWARECOMMANDS = /usr/bin/emerge, /usr/bin/ebuild
Cmnd_Alias PASSWORDCOMMANDS = /usr/bin/passwd [a-zA-Z0-9_-]*, !/usr/bin/passwd root
SOFTWAREMAINTAINERS localhost = SOFTWARECOMMANDS
PASSWORDMAINTAINERS localhost = PASSWORDCOMMANDS
Non-root execution
It is also possible to have a user run an application as a different, non-root user. This can be very interesting when running applications as a different user (for instance apache for the web server) and want to allow certain users to perform administrative steps as that user (like killing zombie processes).
Inside /etc/sudoers list the user(s) in between ( and ) before the command listing:
users hosts = (run-as) commands
For instance, to allow larry to run the kill tool as the apache or gorg user:
Cmnd_Alias KILL = /bin/kill, /usr/bin/pkill
larry ALL = (apache, gorg) KILL
With this set, the user can run sudo -u to select the user he wants to run the application as:
user $sudo -u apache pkill apacheAn alias can set for the user to run an application as using the Runas_Alias directive. Its use is identical to the other _Alias directives we have seen before.
Passwords and default settings
By default, sudo asks the user to identify himself using his own password. Once a password is entered, sudo remembers it for 5 minutes, allowing the user to focus on his tasks and not repeatedly re-entering his password.
Of course, this behavior can be changed: set the Defaults: directive in /etc/sudoers to change the default behavior for a user.
For instance, to change the default 5 minutes to 0 (never remember):
Defaults:larry timestamp_timeout=0
A setting of -1 would remember the password indefinitely (until the system reboots).
A different setting would be to require the password of the user that the command should be run as and not the users' personal password. This is accomplished using runaspw. In the following example we also set the number of retries (how many times the user can re-enter a password before sudo fails) to 2 instead of the default 3:
Defaults:john runaspw, passwd_tries=2
Another interesting feature is to keep the DISPLAY variable set so that graphical tools can be executed:
Defaults:john env_keep=DISPLAY
Dozens of default settings can changed using the Defaults: directive. Fire up the sudoers manual page and search for Defaults.
To allow a user to run a certain set of commands without providing any password whatsoever, start the commands with NOPASSWD:, like so:
larry localhost = NOPASSWD: /usr/bin/emerge
Bash completion
Users that want bash completion with sudo should ensure that the app-shells/bash-completion package is installed.
user $emerge --ask app-shells/bash-completionZshell completion
Users that want zsh completion with sudo should install the app-shells/zsh-completions package.
user $emerge --ask app-shells/zsh-completionsUsage
Listing privileges
To list the current user's capabilities, run sudo -l :
user $sudo -lUser larry may run the following commands on this host:
(root) /usr/libexec/xfsm-shutdown-helper
(root) /usr/bin/emerge
(root) /usr/bin/passwd [a-zA-Z0-9_-]*
(root) !/usr/bin/passwd root
(apache) /usr/bin/pkill
(apache) /bin/kill
Any command in /etc/sudoers that does not require a password to be entered, a password will not be required to list the entries either. Otherwise sudo will ask for a password if it isn't remembered.
Prolonging password timeout
By default, if a user has entered their password to authenticate their self to sudo, it is remembered for 5 minutes. If the user wants to prolong this period, he can run sudo -v to reset the time stamp so that it will take another 5 minutes before sudo asks for the password again.
user $sudo -vThe inverse is to kill the time stamp using sudo -k.
See also
- doas — provides a way to perform commands as another user.
- su — used to adopt the privileges of other users from the system