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Simplify the Use of Sudo

A powerful tool for enforcing role-based access control

Being in the security business, I am constantly surprised by how many companies continue to rely on security practices that they know to be poor. Organizations, large and small, make excessive use of the root user account to perform routine maintenance on their UNIX and Linux computers. Even though companies often chose UNIX or Linux for better security, they employ practices that seriously undermine this advantage.

There are many reasons to not rely on the use of the root account:

  • Because you can do anything, it's easy to make mistakes with dire consequences
  • When something goes wrong, it's impossible to figure out who was responsible
  • If someone leaves the company or the IT group, you have to change the root password and let everyone know the new one
  • The opportunity for mischief is high
  • You'll never pass a security audit

Companies use the root account because the alternative requires a lot of work. All too often, easy beats smart.

The main alternative to using root is to make use of the sudo utility. With sudo, you can allow non-privileged accounts to execute privileged commands. The sudoers configuration file lets you specify which users or user groups are allowed to run which privileged programs. Instead of logging in as root, an administrator logs in with a personal account and then prefixes any privileged command with sudo:

sudo /etc/init.d/network restart

The sudo utility checks the sudoers file to determine whether the current user has been granted the privilege of running /etc/init.d/network and, if so, runs the command. When it runs the command, it does so as root to ensure significant privileges. The sudo utility may first prompt the administrator to reenter his or her password to make sure the command wasn't typed by someone who hijacked the administrator's workstation. This behavior is also configurable in sudoers.

In addition to executing privileged commands, sudo performs another valuable function: it logs all privileged operations. For example, when I perform the command above, the following appears in the system log:

Feb  5 13:29:25 rh5test sudo:  mvellon : TTY=pts/2 ; PWD=/etc ; USER=root ;\ COMMAND=/etc/init.d/network restart

Now, when something goes wrong, you can analyze the system logs to determine whodunit.

Of course, to make effective use of sudo, some work is required:

  • You have to figure out which users need to run what commands
  • You have to write and test the sudoers file
  • You have to install sudoers on all your systems
  • You have to maintain sudoers when users join or leave the organization or when users need a change in privilege

Many of the companies that I deal with know that sudo exists and that they should be using it, but they have yet to take the first step. Others have tried to use it but have either given up on it entirely or found that they frequently subvert it by directly logging in as root. Few have eliminated all use of the root account.

Let's first understand why using sudo can require a lot of work and then let's consider how two software products, Likewise Open and Likewise Enterprise, can simplify its use.

Configuring Sudo
To illustrate why maintaining sudo can be time-consuming, let's first consider a trivial sudoers file.

## Command AliasesCmnd_Alias NETCMD = /sbin/route,
/sbin/ifconfig, /bin/ping,\ 
/sbin/dhclient, /usr/bin/net, /sbin/iptables, /usr/bin/rfcomm,\ 
/usr/bin/wvdial, /sbin/iwconfig, /sbin/mii-tool /etc/init.d/network

## Main section: who can do what

## Allow root to run any commands anywhere
root       ALL=(ALL)            ALL

## Allow mvellon to restart the network on any machine
mvellon ALL=/etc/init.d/network restart

## Allow members of the netadmins group to perform networking commands
%netadmins  ALL=NETCMDS

In Listing 1, the sudoers file defines a set of commands called NETCMDS. It explicitly allows the mvellon user to restart the network but allows all members of the netadmins group to execute any of the commands in NETCMDS. (Preceding a group name with a percent sign [%] indicates that the name refers to an operating system group name instead of a user name or alias.)

The main reason using sudo requires a lot of maintenance is that every time a new user joins or leaves the organization or a user needs a change in privileges, the sudoers file or the /etc/group file, or both, needs to be changed. If we want to allow ajones to restart the network, we need to add her to the sudoers file or we need to change the /etc/group file to add her to the netadmins group. This change needs to be made to all the machines on which we want to give ajones elevated privileges.

At one large company that I've worked with, the sudoers file is more than 30,000 lines long. The company makes an average of 10 changes to it every day. Maintaining an up-to-date sudoers on their 15,000 Linux workstations is a massive chore.

Enter Likewise Open
Many of the hindrances to using sudo stem from the lack of a centralized authentication mechanism. Few of the companies that I deal with have any centralized directory or authentication system. The most prevalent mechanism in use is NIS and, because of security concerns, companies that use it are inevitably looking for alternatives.

Likewise Open is an open source product that allows non-Windows computers to authenticate users against Microsoft Active Directory (AD). Authenticating users against a central directory can greatly simplify the use of sudo.

Although it may seem like an unusual choice to authenticate UNIX and Linux users against a Microsoft directory, a large percentage of companies are already using AD as their primary source of user authentication.

Authenticating UNIX and Linux users against AD provides several benefits:

  • Simplified account management
  • Single username and password for all machines
  • Kerberos authentication and single sign-on
  • Centralized enforcement of security policies (password age, length, complexity, etc.)

All this and easier use of sudo! How does Likewise Open help with sudo? First, the sudoers file can now refer to AD users and groups. Consider a one-line change to the previous sudoers file.

...
## Allow members of the netadmins group to perform networking commands
%CORP\\netadmins  ALL=NETCMDS

In the code snippet above, we've replaced the reference to the local netadmins group with a reference to an AD group called CORP\netadmins. The CORP\ prefix informs the Likewise software that the group is part of the CORP domain in Active Directory. When sudo calls the operating system to determine whether the current user is a member of this group, the Likewise software communicates with AD and responds to the call.

The significance of this one line change is that we can greatly reduce the need to modify sudoers. If we want to give ajones the privilege of running network commands, we can simply add her to the CORP\netadmins group in Active Directory and not make any changes on our Linux computers.

Similarly, when a user leaves the organization, the account can be disabled, or if the user has simply lost elevated privileges, he or she can be removed from the AD group memberships that grant these privileges via sudoers.

In the example above, I've used "NT style" user and group names: CORP\mvellon and CORP\netadmins, for example. I've done this for clarity - to better distinguish local and AD-based names. You can alternatively use fully qualified domain names, for example, [email protected] and [email protected], or you can configure Likewise Open to assume that short names are in the default domain. With the latter option, mvellon and netadmins would automatically be mapped to their AD forms.

Likewise Enterprise
In addition to our free software, Likewise Open, we also have a commercial product: Likewise Enterprise. Likewise Enterprise provides two other features that help simplify the use of sudo:

  • Group Policy features that can automatically distribute sudoers to groups of UNIX and Linux computers
  • The ability to group computers into related cells that share an authentication configuration

Group Policy is a mechanism frequently used to manage large numbers of Windows computers. Rather than configuring each one separately, Group Policy lets an administrator store common settings in AD and lets computers retrieve this information from the directory on a regular basis. Likewise Enterprise extends the Group Policy mechanism to include numerous UNIX- and Linux-specific settings and provides a daemon to retrieve these settings from AD and apply them to UNIX and Linux computers. One such setting allows an administrator to specify a sudoer file that gets stored in AD and is automatically distributed to all computers running the Likewise Enterprise agent.

The Likewise Enterprise cell feature can also simplify the use of sudo by allowing the design of a sudoer file that can apply across all the computers in your organization. To understand the significance of this, let's first take a step back and understand what it is that administrators are trying to accomplish with sudo.

Role-Based Access Control
Role-based access control, or RBAC, is the holy grail of security (well, coupled with separation of duties, but that's another article). The proponents of RBAC (for example, security auditors) seek to separate the concepts of users and privileges by introducing the Role abstraction. Rather than thinking, "Jane needs to be able to restart the network," RBAC proponents want us to think: "network administrators need to be able to restart the network. Jane is a network administrator." An organization practicing RBAC should be able to define a set of roles and a set of privileges needed by those roles. Finally, individuals should be assigned to the role or roles that they need to perform their job functions. Ideally, assignment is automatic based on job description, department and other business rules. As individuals join or leave the organization or simply change jobs, the RBAC system should dynamically adjust their associated roles.

Likewise Enterprise does not provide a mechanism to design roles, but it is easy to see how AD-based groups coupled with sudo can be used to enforce them. A role can be mapped to a group name and the group name can be used in sudoers to allow designated privileges. Group Policy can be used to automatically distribute sudoers to computers participating in RBAC. Role membership can be easily controlled through AD-based group membership.

In an RBAC system, however, there is one remaining challenge: not all users have the same roles on all systems. A software developer, for example, might be considered a network administrator on all the computers in Research and in Staging but not in Production. Let us consider two different ways of dealing with this challenge.

First, rather than having a role called network administrator, we could have three roles: Research network administrator, Staging network administrator and Production network administrator. These would have three different AD groups associated with them and we could employ a different sudoers file for each of Research, Staging and Production. Likewise Enterprise's Group Policy features would be able to help with the deployment of the appropriate sudoers file.

The second approach is a variation of the first. The Likewise Enterprise cell feature allows us to define aliases for groups. Different cells, containing different Linux computers, can have different aliases. To solve our problem, we could create three cells (Research, Staging and Production) and different group aliases in each. In the Research cell, for example, we could set an alias that said:

netadmins = CORP\ResearchNetAdmins

In the Production alias, however, we would say:

netadmins = CORP\ProductionNetAdmins

The benefit of this approach is that the same sudoers file can be used on all computers. A single Likewise Enterprise Group Policy setting can distribute the same sudoers file. Roles could be defined in the abstract, without concern for which users have those roles on which computers.

Summary
It's clear to IT managers and security staff that excessive use of root is a dangerous vulnerability. sudo provides a great mechanism for giving non-root users elevated privileges. Without centralized authentication, however, effective use of sudo can be cumbersome and time-consuming.

Likewise products provide centralized authentication through Microsoft's Active Directory. Using AD-based authentication can simplify the use of sudo. In addition, the Enterprise version of the Likewise software adds additional functionality that can make sudo even easier to use and a powerful tool for enforcing role-based access control.

More Stories By Manny Vellon

Manny Vellon is co-founder and CTO of Likewise Software and has 24 years of software development and investment experience. He began his professional career at Hewlett-Packard before joining Microsoft in 1987. At HP he was a developer for the Personal Software Division, where he worked on embedded terminal software, word processors and database products. At Microsoft, Manny first worked as a program manager on the software development kit for Microsoft Windows 1.0 with the mission of facilitating the development of Windows applications. He joined the Developer Tools Group as group program manager and then later moved back to R&D as a development manager. Manny co-founded the Virtual Worlds Group in Microsoft Research to study social interaction, collaborative work and real-time communication on the Internet. After leaving Microsoft in 1998, Manny started his own consulting business to help startups and venture capital companies with the due-diligence process. Having worked with Northwest Venture Associates on various projects, he joined the company as an Entrepreneur-in-Residence in 2003. Manny earned a bachelor's degree in electrical engineering and computer science from Princeton University.

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