Doas - Alternative to Sudo

We, the siduction team, have decided to use a real root account and have not configured Sudo. For users who are used to Sudo and don’t want to do without its functionality, the slim alternative Doas is a good choice. Doas is tailored to desktop systems, having only about 1/100 of code lines in comparison to Sudo. With siduction 2021.3 wintersky, Doas is automatically installed in version 6.8.1-3, but is not yet fully configured.

Configure Doas

The only thing missing to be able to use Doas is the configuration file /etc/doas.conf. It contains line-by-line rules that assign actions to a user. A # introduces comments. Doas reads the lines one after the other, executing the action of the last applicable rule. To understand the rules in the configuration file, there are a few things to keep in mind.
- Only actions for which at least one rule applies are executed. - By the fact that Doas evaluates the rules line by line one after the other, hierarchies can be built up. - For rules that contain commands with arguments, the arguments must be specified exactly and completely. - Rules with commands that require variable arguments are not possible. - Doas checks the syntax of the configuration file before executing the requested action. In case of incorrect rules, Doas outputs doas: syntax error at line 4 and then exits. The write access to the configuration file is then only possible with the root account.

The configuration is particularly simple if only one user account exists on the siduction system. A single line is sufficient to execute commands with root privileges using the prefix “doas”.
Log in to a terminal as root and execute the following command, replacing “tux” with the name of your user account.

tux@sidu:~$ su
root@sidu:/home/tux# echo "permit keepenv nopass tux" > /etc/doas.conf
root@sidu:/home/tux# exit

The configuration line consists of:
The action permit|deny with
the option keepenv (this allows to start graphical programs like gparted),
the option nopass|persist (no password request|the one-time password entry remains valid for a limited time), and
the user tux to which the action is to be applied.

If the username stands alone, tux may execute commands as any user present on the system. The default is root. If the execution of the action is to be allowed only with the rights of a user other than root, the name must be specified within the rule (e.g. tux as anne). Instead of the user, a group (e.g. :vboxusers) can gain permissions by prepending a :.

Doas and multiple users

On the workstation PC, in addition to tux, three other users named anne, bob, and lisa are allowed to log in.
Anne only wants to allow Bob to run two of her scripts from her /home directory. Anne has restrictively set the permissions on her scripts to 700.
Lisa is especially trustworthy, so she should be in charge of system upgrades.

Now, as user tux, we use Doas in a terminal to edit the configuration file.

tux@sidu:~$ doas mcedit /etc/doas.conf

We convert the previously mentioned permissions into rules and add some comments to the file.

# doas config file /etc/doas.conf

# tux gets root privileges
permit keepenv nopass tux

# bob may execute anne's script
permit bob as anne cmd /home/anne/bin/script1 args -n
permit bob as anne cmd /home/anne/bin/script2 args

# lisa may execute system upgrade
permit persist lisa cmd init
deny lisa cmd init args 1
deny lisa cmd init args 5
permit persist lisa cmd apt args update
permit persist lisa cmd apt args full-upgrade

bob may execute the scripts script1 and script2 inside Anne’s /home/anne/bin directory (the former exclusively with the argument -n, the latter must not be given any argument). Specifying args in the rule line for the script2 without a following argument forces the file to be called without an argument and thus without potentially malicious code. bob must supply the username when calling scripts, using the -u option.

bob@sidu:~$ doas -u anne /home/anne/bin/script1 -n
doas (bob@sidu) password:


The script was executed without comment after Bob entered his user password.

To allow Lisa to perform the system upgrade, she should switch to (init 3) and perform a systemctl reboot (init 6) after completion. The rule line permit persist lisa as root cmd init without specifying args causes all other calls of init are allowd, except those that are prohibited by the following rules below. Therefore, she cannot go directly from the to the (init 5). Here we see the structure of a hierarchy.

If you keep typing sudo, the line alias sudo="doas" in your .bashrc will help.
Doas plays its decisive advantage where only one user is granted root rights by doas. The above example with Lisa shows how extensive the configuration for a restricted rights assignment can become. Furthermore, a rule for a program call with variable arguments (e.g. apt install <package name>) is not possible.

man doas
man doas.conf
github, doas
DE: LinuxNews, Linux Rechtemanagement, sudo durch doas ersetzen
DE: LinuxUser 08.2021, Kleiner Bruder

Page last updated 2022/03/06