Security isn't a noun, it's a verb; not a product, but a process. Today, learn the hacks involved in reducing the risks involved in offering services on a Unix-based system. This the first part of chapter one in Network Security Hacks, by Andrew Lockhart (ISBN 0-596-00643-8, O'Reilly & Associates, 2004).
Let others do your work for you without giving away root privileges.
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 userís behalf, after she has entered her 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 administrators 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, this method is truly useful only when all of the sudo users can be completely trusted. When properly configured, the sudo utility provides 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 sudoís Alias syntax. An Alias can be used in place of its respective entry on any line of the sudo configuration:
For that to work at boot time, the default line root ALL=(ALL) ALL must 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 isnít a problem, and itís certainly preferable to giving away undue access to root privileges.
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