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Linux Server Hacks by Rob Flickenger

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Hack #12. Make sudo Work Harder

Use sudo to let other users do your evil bidding, without giving away the machine

The sudo utility can help you delegate some system responsibilities to other people, without giving away full root access. It is a setuid root binary that executes commands on an authorized user's behalf, after they have entered their current password.

As root, run /usr/sbin/visudo to edit the list of users who can call sudo. The default sudo list looks something like this:

root ALL=(ALL) ALL

Unfortunately, many system admins tend to use this entry as a template and grant unrestricted root access to all other admins unilaterally:

root ALL=(ALL) ALL
rob ALL=(ALL) ALL
jim ALL=(ALL) ALL
david ALL=(ALL) ALL

While this may allow you to give out root access without giving away the root password, it is really only a useful method when all of the sudo users can be completely trusted. When properly configured, the sudo utility allows for tremendous flexibility for granting access to any number of commands, run as any arbitrary uid.

The syntax of the sudo line is:

            user machine=(effective user) command

The first column specifies the sudo user. The next column defines the hosts in which this sudo entry is valid. This allows you to easily use a single sudo configuration across multiple machines.

For example, suppose you have a developer who needs root access on a development machine, but not on any other server:

peter beta.oreillynet.com=(ALL) ALL

The next column (in parentheses) specifies the effective user that may run the commands. This is very handy for allowing users to execute code as users other than root:

peter lists.oreillynet.com=(mailman) ALL

Finally, the last column specifies all of the commands that this user may run:

david ns.oreillynet.com=(bind) /usr/sbin/rndc,/usr/sbin/named

If you find yourself specifying large lists of commands (or, for that matter, users or machines), then take advantage of sudo's Alias syntax. An Alias can be used in place of its respective entry on any line of the sudo configuration:

User_Alias ADMINS=rob,jim,david
User_Alias WEBMASTERS=peter,nancy

Runas_Alias DAEMONS=bind,www,smmsp,ircd

Host_Alias WEBSERVERS=www.oreillynet.com,www.oreilly.com,www.perl.com

Cmnd_Alias PROCS=/bin/kill,/bin/killall,/usr/bin/skill,/usr/bin/top
Cmnd_Alias APACHE=/usr/local/apache/bin/apachectl

WEBMASTERS WEBSERVERS=(www) APACHE
ADMINS ALL=(DAEMONS) ALL

It is also possible to specify system groups in place of the user specification to allow any user who belongs to that group to execute commands. Just preface the group with a %, like this:

%wwwadmin WEBSERVERS=(www) APACHE

Now any user who is part of the wwwadmin group can execute apachectl as the www user on any of the web server machines.

One very useful feature is the NOPASSWD: flag. When present, the user won't have to enter his password before executing the command:

rob ALL=(ALL) NOPASSWD: PROCS

This will allow the user rob to execute kill, killall, skill, and top on any machine, as any user, without entering a password.

Finally, sudo can be a handy alternative to su for running commands at startup out of the system rc files:

(cd /usr/local/mysql; sudo -u mysql ./bin/safe_mysqld &)

sudo -u www /usr/local/apache/bin/apachectl start

For that to work at boot time, you'll need the default line root ALL=(ALL) ALL to be present.

Use sudo with the usual caveats that apply to setuid binaries. Particularly if you allow sudo to execute interactive commands (like editors) or any sort of compiler or interpreter, you should assume that it is possible that the sudo user will be able to execute arbitrary commands as the effective user. Still, under most circumstances this isn't a problem and is certainly preferable to giving away undue access to root privileges.

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