Using sudo

Using sudo

If a server needs to be administered by a number of people, it is normally not a good idea for them all to use the root account. This is because it becomes difficult to determine exactly who did what, when, and where if everyone logs in with the same credentials. The sudo utility was designed to overcome this difficulty.

The sudo utility allows users defined in the /etc/sudoers configuration file to have temporary access to run commands they would not normally be able to due to file permission restrictions. The commands can be run as user root or as any other user defined in the /etc/sudoers configuration file.

The privileged command you want to run must first begin with the word sudo followed by the command's regular syntax. When running the command with the sudo prefix, you will be prompted for your regular password before it is executed. You may run other privileged commands using sudo within a five-minute period without being re-prompted for a password. All commands run as sudo are logged in the log file /var/log/messages.

Example of a User Using sudo

In this example, user bob attempts to view the contents of the /etc/sudoers file, which is an action that normally requires privileged access. Without sudo, the command fails:

     [bob@bigboy bob]$ more /etc/sudoers
     /etc/sudoers: Permission denied
     [bob@bigboy bob]$

Bob tries again using sudo and his regular user password and is successful:

     [bob@bigboy bob]$ sudo more /etc/sudoers
     Password:
     ...
     ...
     [bob@bigboy bob]$

The details of configuring and installing sudo are covered in later sections.

Downloading and Installing the sudo Package

Fortunately the package is installed by default by Red Hat/Fedora, which eliminates the need to anything more in this regard.

The visudo Command

The visudo command is a text editor that mimics the vi editor that is used to edit the /etc/sudoers configuration file. It is not recommended that you use any other editor to modify your sudo parameters because the sudoers file isn't located in the same directory on all versions of Linux. visudo uses the same commands as the vi text editor. The visudo command must run as user root and should have no arguments:

     [root@bigboy tmp]# visudo

The /etc/sudoers File

The /etc/sudoers file contains all the configuration and permission parameters needed for sudo to work. There are a number of guidelines that need to be followed when editing it with visudo.

General /etc/sudoers Guidelines

The /etc/sudoers file has the general format shown in Figure.

There are some general guidelines when editing this file:

• Groups are the same as user groups and are differentiated from regular users by a % at the beginning. The Linux user group users would be represented by %users.

• You can have multiple usernames per line, separated by commas.

• Multiple commands also can be separated by commas. Spaces are considered part of the command.

• The keyword ALL can mean all usernames, groups, commands, and servers.

• If you run out of space on a line, you can end it with a back slash (\) and continue on the next line.

• sudo assumes that the sudoers file will be used networkwide, and therefore offers the option to specify the names of servers, which will be using it in the servername position in Figure. In most cases, the file is used by only one server and the keyword ALL suffices for the servername.

• The NOPASSWD keyword provides access without prompting you for your password.

Simple /etc/sudoers Examples

This section presents simple examples of how to do many commonly required tasks using the sudo utility.

Granting All Access to Specific Users

You can grant users bob and bunny full access to all privileged commands with this sudoers entry:

     bob, bunny ALL=(ALL) ALL

This is generally not a good idea because this allows bob and bunny to use the su command to grant themselves permanent root privileges, thereby bypassing the command logging features of sudo. The example on using aliases in the sudoers file shows how to eliminate this problem.

Granting Access to Specific Users to Specific Files

This entry allows user peter and all the members of the group operator to gain access to all the program files in the /sbin and /usr/sbin directories, plus the privilege of running the command /usr/local/apps/check.pl. Notice how the trailing slash (/) is required to specify a directory location:

     peter, %operator ALL= /sbin/, /usr/sbin, /usr/local/apps/check.pl

Notice also that the lack of any username entries within parentheses () after the = sign prevents the users from running the commands automatically masquerading as another user. This is explained further in the next example.

Granting Access to Specific Files as Another User

The sudo -u entry enables you to execute a command as if you were another user, but first you have to be granted this privilege in the sudoers file.

This feature can be convenient for programmers who sometimes need to kill processes related to projects they are working on. For example, programmer peter is on the team developing a financial package that runs a program called monthend as user accounts. From time to time the application fails, requiring peter to stop it with the /bin/kill, /usr/bin/kill, or /usr/bin/pkill commands but only as user accounts. The sudoers entry would look like this:

     peter ALL=(accounts) /bin/kill, /usr/bin/kill /usr/bin/pkill

User peter is allowed to stop the monthend process with this command:

     [peter@bigboy peter]# sudo -u accounts pkill monthend

Granting Access Without Needing Passwords

This example allows all users in the group operator to execute all the commands in the /sbin directory without the need for entering a password. This has the added advantage of being more convenient to the user:

     %operator ALL= NOPASSWD: /sbin/

Using Aliases in the sudoers File

Sometimes you need to assign random groupings of users from various departments very similar sets of privileges. The sudoers file allows users to be grouped according to function with the group and then be assigned a nickname or alias, which is used throughout the rest of the file. Groupings of commands can also be assigned aliases.

In the next example, users peter, bob, and bunny and all the users in the operator group are made part of the user alias ADMINS. All the command shell programs are then assigned to the command alias SHELLS. Users ADMINS are then denied the option of running any SHELLS commands and su:

     Cmnd_Alias   SHELLS = /usr/bin/sh, /usr/bin/csh, \
                           /usr/bin/ksh, /usr/local/bin/tcsh, \
                           /usr/bin/rsh, /usr/local/bin/zsh

     User_Alias   ADMINS = peter, bob, bunny, %operator
     ADMINS       ALL    = !/usr/bin/su, !SHELLS

This attempts to ensure that users don't permanently su to become root, or enter command shells that bypass sudo's command logging. It doesn't prevent them from copying the files to other locations to be run. The advantage of this is that it helps to create an audit trail, but the restrictions can be enforced only as part of the company's overall security policy.

Other Examples

You can view a comprehensive list of /etc/sudoers file options by issuing the command man sudoers.

Using syslog to Track All sudo Commands

All sudo commands are logged in the log file /var/log/messages, which can be very helpful in determining how user error might have contributed to a problem. All the sudo log entries have the word sudo in them, so you can easily get a thread of commands used by using the grep command to filter the output accordingly.

Here is sample output from a user bob failing to enter his correct sudo password when issuing a command, immediately followed by the successful execution of the command /bin/more sudoers.

     [root@bigboy tmp]# grep sudo /var/log/messages
     Nov 18 22:50:30 bigboy sudo(pam_unix)[26812]: authentication failure;
     logname=bob uid=0 euid=0 tty=pts/0 ruser= rhost= user=bob
     Nov 18 22:51:25 bigboy sudo: bob : TTY=pts/0 ; PWD=/etc ; USER=root ;
     COMMAND=/bin/more sudoers
     [root@bigboy tmp]#