sudoers examples

Since the sudoers file is parsed in a single pass, order is important. In general, you should structure sudoers such that the Host_Alias, User_Alias, and Cmnd_Alias specifications come first, followed by any Default_Entry lines, and finally the Runas_Alias and user specifications. The basic rule of thumb is you cannot reference an Alias that has not already been defined.

Below are example sudoers entries. Admittedly, some of these are a bit contrived. First, we define our aliases:

 User_Alias     FULLTIMERS = millert, mikef, dowdy
 User_Alias     PARTTIMERS = bostley, jwfox, crawl
 User_Alias     WEBMASTERS = will, wendy, wim

 Runas_Alias    OP = root, operator
 Runas_Alias    DB = oracle, sybase

 Host_Alias     SPARC = bigtime, eclipse, moet, anchor :\
                SGI = grolsch, dandelion, black :\
                ALPHA = widget, thalamus, foobar :\
                HPPA = boa, nag, python
 Host_Alias     CUNETS = 128.138.0.0/255.255.0.0
 Host_Alias     CSNETS = 128.138.243.0, 128.138.204.0/24, 128.138.242.0
 Host_Alias     SERVERS = master, mail, www, ns
 Host_Alias     CDROM = orion, perseus, hercules

 Cmnd_Alias     DUMPS = /usr/bin/mt, /usr/sbin/dump, /usr/sbin/rdump,\
                        /usr/sbin/restore, /usr/sbin/rrestore
 Cmnd_Alias     KILL = /usr/bin/kill
 Cmnd_Alias     PRINTING = /usr/sbin/lpc, /usr/bin/lprm
 Cmnd_Alias     SHUTDOWN = /usr/sbin/shutdown
 Cmnd_Alias     HALT = /usr/sbin/halt
 Cmnd_Alias     REBOOT = /usr/sbin/reboot
 Cmnd_Alias     SHELLS = /usr/bin/sh, /usr/bin/csh, /usr/bin/ksh, \
                         /usr/local/bin/tcsh, /usr/bin/rsh, \
                         /usr/local/bin/zsh
 Cmnd_Alias     SU = /usr/bin/su

Here we override some of the compiled in default values. We want sudo to log via using the auth facility in all cases. We don't want to subject the full time staff to the sudo lecture, user millert need not give a password, and we don't want to reset the LOGNAME or USER environment variables when running commands as root. Additionally, on the machines in the SERVERS Host_Alias, we keep an additional local log file and make sure we log the year in each log line since the log entries will be kept around for several years.

 Defaults               syslog=auth
 Defaults>root          !set_logname
 Defaults:FULLTIMERS    !lecture
 Defaults:millert       !authenticate
 Defaults@SERVERS       log_year, logfile=/var/log/sudo.log

The User specification is the part that actually determines who may run what.

 root           ALL = (ALL) ALL
 %wheel         ALL = (ALL) ALL

We let root and any user in group wheel run any command on any host as any user.

 FULLTIMERS     ALL = NOPASSWD: ALL

Full time sysadmins (millert, mikef, and dowdy) may run any command on any host without authenticating themselves.

 PARTTIMERS     ALL = ALL

Part time sysadmins (bostley, jwfox, and crawl) may run any command on any host but they must authenticate themselves first (since the entry lacks the NOPASSWD tag).

 jack           CSNETS = ALL

The user jack may run any command on the machines in the CSNETS alias (the networks 128.138.243.0, 128.138.204.0, and 128.138.242.0). Of those networks, only 128.138.204.0 has an explicit netmask (in CIDR notation) indicating it is a class C network. For the other networks in CSNETS, the local machine's netmask will be used during matching.

 lisa           CUNETS = ALL

The user lisa may run any command on any host in the CUNETS alias (the class B network 128.138.0.0).

 operator       ALL = DUMPS, KILL, SHUTDOWN, HALT, REBOOT, PRINTING,\
                sudoedit /etc/printcap, /usr/oper/bin/

The operator user may run commands limited to simple maintenance. Here, those are commands related to backups, killing processes, the printing system, shutting down the system, and any commands in the directory /usr/oper/bin.

 joe            ALL = /usr/bin/su operator

The user joe may only su(1) to operator.

 pete           HPPA = /usr/bin/passwd [A-z]*, !/usr/bin/passwd root

The user pete is allowed to change anyone's password except for root on the HPPA machines. Note that this assumes passwd(1) does not take multiple usernames on the command line.

 bob            SPARC = (OP) ALL : SGI = (OP) ALL

The user bob may run anything on the SPARC and SGI machines as any user listed in the OP Runas_Alias (root and operator).

 jim            +biglab = ALL

The user jim may run any command on machines in the biglab netgroup. Sudo knows that ``biglab'' is a netgroup due to the '+' prefix.

 +secretaries   ALL = PRINTING, /usr/bin/adduser, /usr/bin/rmuser

Users in the secretaries netgroup need to help manage the printers as well as add and remove users, so they are allowed to run those commands on all machines.

 fred           ALL = (DB) NOPASSWD: ALL

The user fred can run commands as any user in the DB Runas_Alias (oracle or sybase) without giving a password.

 john           ALPHA = /usr/bin/su [!-]*, !/usr/bin/su *root*

On the ALPHA machines, user john may su to anyone except root but he is not allowed to give su(1) any flags.

 jen            ALL, !SERVERS = ALL

The user jen may run any command on any machine except for those in the SERVERS Host_Alias (master, mail, www and ns).

 jill           SERVERS = /usr/bin/, !SU, !SHELLS

For any machine in the SERVERS Host_Alias, jill may run any commands in the directory /usr/bin/ except for those commands belonging to the SU and SHELLS Cmnd_Aliases.

 steve          CSNETS = (operator) /usr/local/op_commands/

The user steve may run any command in the directory /usr/local/op_commands/ but only as user operator.

 matt           valkyrie = KILL

On his personal workstation, valkyrie, matt needs to be able to kill hung processes.

 WEBMASTERS     www = (www) ALL, (root) /usr/bin/su www

On the host www, any user in the WEBMASTERS User_Alias (will, wendy, and wim), may run any command as user www (which owns the web pages) or simply su(1) to www.

 ALL            CDROM = NOPASSWD: /sbin/umount /CDROM,\
                /sbin/mount -o nosuid\,nodev /dev/cd0a /CDROM

Any user may mount or unmount a CD-ROM on the machines in the CDROM Host_Alias (orion, perseus, hercules) without entering a password. This is a bit tedious for users to type, so it is a prime candidate for encapsulating in a shell script.

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SECURITY NOTES

It is generally not effective to ``subtract'' commands from ALL using the '!' operator. A user can trivially circumvent this by copying the desired command to a different name and then executing that. For example:

    bill        ALL = ALL, !SU, !SHELLS

Doesn't really prevent bill from running the commands listed in SU or SHELLS since he can simply copy those commands to a different name, or use a shell escape from an editor or other program. Therefore, these kind of restrictions should be considered advisory at best (and reinforced by policy).

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Once sudo executes a program, that program is free to do whatever it pleases, including run other programs. This can be a security issue since it is not uncommon for a program to allow shell escapes, which lets a user bypass sudo's restrictions. Common programs that permit shell escapes include shells (obviously), editors, paginators, mail and terminal programs.

Many systems that support shared libraries have the ability to override default library functions by pointing an environment variable (usually LD_PRELOAD) to an alternate shared library. On such systems, sudo's noexec functionality can be used to prevent a program run by sudo from executing any other programs. Note, however, that this applies only to native dynamically-linked executables. Statically-linked executables and foreign executables running under binary emulation are not affected.

To tell whether or not sudo supports noexec, you can run the following as root:

    sudo -V | grep "dummy exec"

If the resulting output contains a line that begins with:

    File containing dummy exec functions:

then sudo may be able to replace the exec family of functions in the standard library with its own that simply return an error. Unfortunately, there is no foolproof way to know whether or not noexec will work at compile-time. Noexec should work on SunOS, Solaris, *BSD, Linux, IRIX, Tru64 UNIX, MacOS X, and HP-UX 11.x. It is known not to work on AIX and UnixWare. Noexec is expected to work on most operating systems that support the LD_PRELOAD environment variable. Check your operating system's manual pages for the dynamic linker (usually ld.so, ld.so.1, dyld, dld.sl, rld, or loader) to see if LD_PRELOAD is supported.

To enable noexec for a command, use the NOEXEC tag as documented in the User Specification section above. Here is that example again:

 aaron  shanty = NOEXEC: /usr/bin/more, /usr/bin/vi

This allows user aaron to run /usr/bin/more and /usr/bin/vi with noexec enabled. This will prevent those two commands from executing other commands (such as a shell). If you are unsure whether or not your system is capable of supporting noexec you can always just try it out and see if it works.

Note that disabling shell escapes is not a panacea. Programs running as root are still capable of many potentially hazardous operations (such as changing or overwriting files) that could lead to unintended privilege escalation. In the specific case of an editor, a safer approach is to give the user permission to run sudoedit.