Sudo
Sudo allows a system administrator to delegate authority to give certain users—or groups of users—the ability to run commands as root or another user while providing an audit trail of the commands and their arguments.
Sudo is an alternative to su for running commands as root. Unlike su, which launches a root shell that allows all further commands root access, sudo instead grants temporary privilege elevation to a single command. By enabling root privileges only when needed, sudo usage reduces the likelihood that a typo or a bug in an invoked command will ruin the system.
Sudo can also be used to run commands as other users; additionally, sudo logs all commands and failed access attempts for security auditing.
Usage
To begin using sudo as a non-privileged user, it must be properly configured. See #Configuration.
To use sudo, simply prefix a command and its arguments with sudo and a space:
$ sudo cmd
For example, to use pacman:
$ sudo pacman -Syu
See sudo(8) for more information.
Configuration
Defaults skeleton
sudoers(5) § SUDOERS OPTIONS lists all the options that can be used with the Defaults command in the file.
See for a list of options (parsed from the version 1.8.7 source code) in a format optimized for .
See for more information, such as configuring the password timeout.
View current settings
Run to print out the current sudo configuration, or sudo -lU user for a specific user.
Using visudo
The configuration file for sudo is . It should always be edited with the command. visudo locks the file, saves edits to a temporary file, and checks it for syntax errors before copying it to .
- It is imperative that
sudoersbe free of syntax errors! Any error makes sudo unusable. Always edit it with visudo to prevent errors. - visudo(8) warns that configuring visudo to honor the user environment variables for their editor of choice may be a security hole, since it allows the user with visudo privileges to run arbitrary commands as root without logging simply by setting that variable to something else.
The default editor for visudo is vi. The sudo package is compiled with and honors the use of the , and variables. is not used when is set.
To establish nano as the visudo editor for the duration of the current shell session, export ; to use a different editor just once simply set the variable before calling visudo:
# EDITOR=nano visudo
Alternatively you may edit a copy of the file and check it using . This might come in handy in case you want to circumvent locking the file with visudo.
To change the editor permanently, see Environment variables#Per user. To change the editor of choice permanently system-wide only for visudo, add the following to (assuming nano is your preferred editor):
# Reset environment by default Defaults env_reset # Set default EDITOR to restricted version of nano, and do not allow visudo to use EDITOR/VISUAL. Defaults editor=/usr/bin/rnano, !env_editor
Example entries
To allow a user to gain full root privileges when they precede a command with sudo, add the following line:
USER_NAME ALL=(ALL:ALL) ALL
To allow a user to run all commands as any user but only on the machine with hostname :
USER_NAME HOST_NAME=(ALL:ALL) ALL
To allow members of group wheel sudo access:
%wheel ALL=(ALL:ALL) ALL
wheel group and add the user to it, since by default Polkit treats the members of the wheel group as administrators. If the user is not a member of wheel, software using Polkit may ask to authenticate using the root password instead of the user password.To disable asking for a password for user :
Defaults:USER_NAME !authenticate
Enable explicitly defined commands only for user on host :
USER_NAME HOST_NAME=/usr/bin/halt,/usr/bin/poweroff,/usr/bin/reboot,/usr/bin/pacman -Syu
Enable explicitly defined commands only for user on host without password:
USER_NAME HOST_NAME= NOPASSWD: /usr/bin/halt,/usr/bin/poweroff,/usr/bin/reboot,/usr/bin/pacman -Syu
A detailed example is available at /usr/share/doc/sudo/examples/sudoers. Otherwise, see the for detailed information.
Sudoers default file permissions
The owner and group for the file must both be 0. The file permissions must be set to 0440. These permissions are set by default, but if you accidentally change them, they should be changed back immediately or sudo will fail.
# chown -c root:root /etc/sudoers # chmod -c 0440 /etc/sudoers
Tips and tricks
Disable password prompt timeout
A common annoyance is a long-running process that runs on a background terminal somewhere that runs with normal permissions and elevates only when needed. This leads to a sudo password prompt which goes unnoticed and times out, at which point the process dies and the work done is lost or, at best, cached. Common advice is to enable passwordless sudo, or extend the timeout of sudo remembering a password. Both of these have negative security implications. The prompt timeout can also be disabled and since that does not serve any reasonable security purpose it should be the solution here:
Defaults passwd_timeout=0
Add terminal bell to the password prompt
To draw attention to a sudo prompt in a background terminal, users can simply make it echo a bell character:
Defaults passprompt="^G[sudo] password for %p: "
Note the is a literal bell character. E.g. in vim, insert using the sequence . If is mapped, e.g. for pasting, one can usually use instead. In nano, .
Another option is to set the environment variable. For example, add the following to you shell configuration file:
export SUDO_PROMPT=
Passing aliases
Aliases in Zsh and Bash are normally only expanded for the first word in a command. This means that your aliases will not normally get expanded when running the sudo command. One way to make the next word expand is to make an alias for sudo ending with a space. Add the following to your shell's configuration file:
alias sudo='sudo '
describes how this works:
- If the replacement text ends with a space, the next word in the shell input is always eligible for purposes of alias expansions.
As well as :
- If the last character of the alias value is a blank, then the next command word following the alias is also checked for alias expansion.
Disable per-terminal sudo
If you are annoyed by sudo's defaults that require you to enter your password every time you open a new terminal, set to :
Defaults timestamp_type=global
Reduce the number of times you have to type a password
If you are annoyed that you have to re-enter your password every 5 minutes (default), you can change this by setting a longer value for (in minutes):
Defaults timestamp_timeout=10
If you are using a lot of sudo commands on a row, it is more logical to refresh the timeout every time you use sudo than
to increase . Refreshing the timeout can be done with (whereas sudo -K revokes immediately).
You might want to automate this by adding the following to your :
alias sudo='sudo -v; sudo '
It is also possible to use a bash function; for more details see stackexchange.
Environment variables
If you have a lot of environment variables, or you export your proxy settings via export http_proxy="...", when using sudo these variables do not get passed to the root account unless you run sudo with the option.
$ sudo -E pacman -Syu
The recommended way of preserving environment variables is to append them to :
Root password
Users can configure sudo to ask for the root password instead of the user password by adding (target user, defaults to root) or to the Defaults line in :
Defaults targetpw
To prevent exposing your root password to users, you can restrict this to a specific group:
Defaults:%wheel targetpw %wheel ALL=(ALL) ALL
Disable root login
Users may wish to disable the root login. Without root, attackers must first guess a user name configured as a sudoer as well as the user password. See for example OpenSSH#Deny.
The account can be locked via :
# passwd -l root
A similar command unlocks root.
$ sudo passwd -u root
Alternatively, edit and replace the root's encrypted password with "!":
root:!:12345::::::
To enable root login again:
$ sudo passwd root
sudo -i.kdesu
kdesu may be used under KDE to launch GUI applications with root privileges. It is possible that by default kdesu will try to use su even if the root account is disabled. Fortunately one can tell kdesu to use sudo instead of su. Create/edit the file ~/.config/kdesurc:
[super-user-command] super-user-command=sudo
or use the following command:
$ kwriteconfig5 --file kdesurc --group super-user-command --key super-user-command sudo
Harden with sudo example
Let us say you create 3 users: admin, devel, and archie. The user "admin" is used for journalctl, systemctl, mount, kill, and iptables; "devel" is used for installing packages, and editing configuration files; and "archie" is the user you log in with. To let "archie" reboot, shutdown, and use netctl we would do the following:
Edit and . Require user be in the wheel group, but do not put anyone in it.
#%PAM-1.0 auth sufficient pam_rootok.so # Uncomment the following line to implicitly trust users in the "wheel" group. #auth sufficient pam_wheel.so trust use_uid # Uncomment the following line to require a user to be in the "wheel" group. auth required pam_wheel.so use_uid auth required pam_unix.so account required pam_unix.so session required pam_unix.so
Limit SSH login to the 'ssh' group. Only "archie" will be part of this group.
# groupadd -r ssh # gpasswd -a archie ssh # echo 'AllowGroups ssh' >> /etc/ssh/sshd_config
Restart .
Add users to other groups.
# for g in power network ;do ;gpasswd -a archie $g ;done # for g in network power storage ;do ;gpasswd -a admin $g ;done
Set permissions on configs so devel can edit them.
# chown -R devel:root /etc/{http,openvpn,cups,zsh,vim,screenrc}
Cmnd_Alias POWER = /usr/bin/shutdown -h now, /usr/bin/halt, /usr/bin/poweroff, /usr/bin/reboot Cmnd_Alias STORAGE = /usr/bin/mount -o nosuid\,nodev\,noexec, /usr/bin/umount Cmnd_Alias SYSTEMD = /usr/bin/journalctl, /usr/bin/systemctl Cmnd_Alias KILL = /usr/bin/kill, /usr/bin/killall Cmnd_Alias PKGMAN = /usr/bin/pacman Cmnd_Alias NETWORK = /usr/bin/netctl Cmnd_Alias FIREWALL = /usr/bin/iptables, /usr/bin/ip6tables Cmnd_Alias SHELL = /usr/bin/zsh, /usr/bin/bash %power ALL = (root) NOPASSWD: POWER %network ALL = (root) NETWORK %storage ALL = (root) STORAGE root ALL = (ALL) ALL admin ALL = (root) SYSTEMD, KILL, FIREWALL devel ALL = (root) PKGMAN archie ALL = (devel) SHELL, (admin) SHELL
With this setup, you will almost never need to login as the root user.
"archie" can connect to their home WiFi.
$ sudo netctl start home $ sudo poweroff
"archie" can not use netctl as any other user.
$ sudo -u admin -- netctl start home
When "archie" needs to use journalctl or kill run away process they can switch to that user.
$ sudo -i -u devel $ sudo -i -u admin
But "archie" cannot switch to the root user.
$ sudo -i -u root
If "archie" want to start a gnu-screen session as admin they can do it like this:
$ sudo -i -u admin [admin]$ chown admin:tty `echo $TTY` [admin]$ screen
Configure sudo using drop-in files in /etc/sudoers.d
sudo parses files contained in the directory . This means that instead of editing , you can change settings in standalone files and drop them in that directory. This has two advantages:
- There is no need to edit a file;
- If there is a problem with a new entry, you can remove the offending file instead of editing (but see the warning below).
The format for entries in these drop-in files is the same as for itself. To edit them directly, use . See for details.
The files in directory are parsed in lexicographical order, file names containing or ~ are skipped. To avoid sorting problems, the file names should begin with two digits, e.g. .
/etc/sudoers.d/ are just as fragile as /etc/sudoers itself: any improperly formatted file will prevent sudo from working. Hence, for the same reason it is strongly advised to use visudoEditing files
sudo provides the command (equivalent to ). This is useful for editing files which can be edited by root only while still running the editor as a normal user, and using that user’s configuration.
To edit a file, set to the name of the editor and pass the file name to . For example:
$ SUDO_EDITOR=vim sudoedit /etc/file
See #Using visudo and for ways to set the editor, but beware of possible security issues.
If multiple names are passed to sudo, all files are opened in the editor in a single invocation. A feature useful for merging files:
$ SUDO_EDITOR=vimdiff sudoedit /etc/file /etc/file.pacnew
Enable insults
Users can enable the insults easter egg in sudo by adding the following line in the file with .
Upon entering an incorrect password this will replace message with humorous insults.
Troubleshooting
SSH problem without TTY
SSH does not allocate a tty by default when running a remote command. Without an allocated tty, sudo cannot prevent the password from being displayed. You can use ssh's option to force it to allocate a tty.
The Defaults option only allows the user to run sudo if they have a tty.
# Disable "ssh hostname sudo <cmd>", because it will show the password in clear text. You have to run "ssh -t hostname sudo <cmd>". # #Defaults requiretty
Permissive umask
Sudo will union the user's umask value with its own umask (which defaults to 0022). This prevents sudo from creating files with more open permissions than the user's umask allows. While this is a sane default if no custom umask is in use, this can lead to situations where a utility run by sudo may create files with different permissions than if run by root directly. If errors arise from this, sudo provides a means to fix the umask, even if the desired umask is more permissive than the umask that the user has specified. Adding this (using ) will override sudo's default behavior:
Defaults umask = 0022 Defaults umask_override
This sets sudo's umask to root's default umask (0022) and overrides the default behavior, always using the indicated umask regardless of what umask the user as set.