SUDO

NAME

sudo, sudoedit − execute a command as another user

SYNOPSIS

sudo −h | −K | −k | −L | −V
sudo −v [−AknS] [−g group name | #gid] [−p prompt] [−u user name | #uid]
sudo −l[l] [−AknS] [−g group name | #gid] [−p prompt] [−U user name] [−u user name | #uid] [command]
sudo [−AbEHnPS] [−C fd] [−g group name | #gid] [−p prompt] [−u user name | #uid] [VAR=value] −i | −s [command]

sudoedit [−AnS] [−C fd] [−g group name | #gid] [−p prompt] [−u user name | #uid] file ...

DESCRIPTION

sudo allows a permitted user to execute a command as the superuser or another user, as specified by the sudoers file. See the COMMAND EXECUTION section below for more details.

sudo determines who is an authorized user by consulting the file /private/etc/sudoers. By running sudo with the −v option, a user can update the time stamp without running a command. If authentication is required, sudo will exit if the user’s password is not entered within a configurable time limit. The default password prompt timeout is unlimited.

When invoked as sudoedit, the −e option (described below), is implied.

The options are as follows:

If the specified file does not exist, it will be created. Note that unlike most commands run by sudo, the editor is run with the invoking user’s environment unmodified. If, for some reason, sudo is unable to update a file with its edited version, the user will receive a warning and the edited copy will remain in a temporary file.

−i [command]

The −i (simulate initial login) option runs the shell specified by the password database entry of the target user as a login shell. This means that login-specific resource files such as .profile or .login will be read by the shell. If a command is specified, it is passed to the shell for execution via the shell’s −c option. If no command is specified, an interactive shell is executed. sudo attempts to change to that user’s home directory before running the shell. It also initializes the environment to a minimal set of variables, similar to what is present when a user logs in. The Command environment section below documents in detail how the −i option affects the environment in which a command is run.

−k [command]

When used alone, the −k (kill) option to sudo invalidates the user’s time stamp file. The next time sudo is run a password will be required. This option does not require a password and was added to allow a user to revoke sudo permissions from a .logout file.

When used in conjunction with a command or an option that may require a password, the −k option will cause sudo to ignore the user’s time stamp file. As a result, sudo will prompt for a password (if one is required by sudoers) and will not update the user’s time stamp file.

−l[l] [command]

If no command is specified, the −l (list) option will list the allowed (and forbidden) commands for the invoking user (or the user specified by the −U option) on the current host. If a command is specified and is permitted by sudoers, the fully-qualified path to the command is displayed along with any command line arguments. If command is specified but not allowed, sudo will exit with a status value of 1. If the −l option is specified with an l argument (i.e. −ll), or if −l is specified multiple times, a longer list format is used.

The prompt specified by the −p option will override the system password prompt on systems that support PAM unless the passprompt_override flag is disabled in sudoers.

−s [command]

The −s (shell) option runs the shell specified by the SHELL environment variable if it is set or the shell as specified in the password database. If a command is specified, it is passed to the shell for execution via the shell’s −c option. If no command is specified, an interactive shell is executed.

Environment variables to be set for the command may also be passed on the command line in the form of VAR=value, e.g. LD_LIBRARY_PATH=/usr/local/pkg/lib. Variables passed on the command line are subject to the same restrictions as normal environment variables with one important exception. If the setenv option is set in sudoers, the command to be run has the SETENV tag set or the command matched is ALL, the user may set variables that would otherwise be forbidden. See sudoers(5) for more information.

Authentication and logging
sudo requires that most users authenticate themselves by default. A password is not required if the invoking user is root, if the target user is the same as the invoking user, or if the authentication has been disabled for the user or command in the sudoers file. Unlike su(1), when sudo requires authentication, it validates the invoking user’s credentials, not the target user’s (or root’s) credentials. This can be changed via the rootpw, targetpw and runaspw Defaults entries in sudoers.

If a user who is not listed in sudoers tries to run a command via sudo, mail is sent to the proper authorities. The address used for such mail is configurable via the mailto sudoers Defaults entry and defaults to root.

Note that mail will not be sent if an unauthorized user tries to run sudo with the −l or −v option. This allows users to determine for themselves whether or not they are allowed to use sudo.

If sudo is run by root and the SUDO_USER environment variable is set, its value will be used to determine who the actual user is. This can be used by a user to log commands through sudo even when a root shell has been invoked. It also allows the −e option to remain useful even when invoked via a sudo-run script or program. Note, however, that the sudoers lookup is still done for root, not the user specified by SUDO_USER.

sudo uses time stamp files for credential caching. Once a user has been authenticated, the time stamp is updated and the user may then use sudo without a password for a short period of time (5 minutes unless overridden by the timeout option) . By default, sudo uses a tty-based time stamp which means that there is a separate time stamp for each of a user’s login sessions. The tty_tickets option can be disabled to force the use of a single time stamp for all of a user’s sessions.

sudo can log both successful and unsuccessful attempts (as well as errors) to syslog(3), a log file, or both. By default, sudo will log via syslog(3) but this is changeable via the syslog and logfile Defaults settings.

sudo also supports logging a command’s input and output streams. I/O logging is not on by default but can be enabled using the log_input and log_output Defaults flags as well as the LOG_INPUT and LOG_OUTPUT command tags.

Command environment
Since environment variables can influence program behavior, sudo provides a means to restrict which variables from the user’s environment are inherited by the command to be run. There are two distinct ways sudoers can be configured to handle with environment variables.

By default, the env_reset option is enabled. This causes commands to be executed with a new, minimal environment. On AIX (and Linux systems without PAM), the environment is initialized with the contents of the /etc/environment file. On BSD systems, if the use_loginclass option is enabled, the environment is initialized based on the path and setenv settings in /etc/login.conf. The new environment contains the TERM, PATH, HOME, MAIL, SHELL, LOGNAME, USER, USERNAME and SUDO_* variables in addition to variables from the invoking process permitted by the env_check and env_keep options. This is effectively a whitelist for environment variables.

If, however, the env_reset option is disabled, any variables not explicitly denied by the env_check and env_delete options are inherited from the invoking process. In this case, env_check and env_delete behave like a blacklist. Since it is not possible to blacklist all potentially dangerous environment variables, use of the default env_reset behavior is encouraged.

In all cases, environment variables with a value beginning with () are removed as they could be interpreted as bash functions. The list of environment variables that sudo allows or denies is contained in the output of ‘‘sudo -V’’ when run as root.

Note that the dynamic linker on most operating systems will remove variables that can control dynamic linking from the environment of setuid executables, including sudo. Depending on the operating system this may include _RLD*, DYLD_*, LD_*, LDR_*, LIBPATH, SHLIB_PATH, and others. These type of variables are removed from the environment before sudo even begins execution and, as such, it is not possible for sudo to preserve them.

As a special case, if sudo’s −i option (initial login) is specified, sudo will initialize the environment regardless of the value of env_reset. The DISPLAY, PATH and TERM variables remain unchanged; HOME, MAIL, SHELL, USER, and LOGNAME are set based on the target user. On AIX (and Linux systems without PAM), the contents of /etc/environment are also included. On BSD systems, if the use_loginclass option is enabled, the path and setenv variables in /etc/login.conf are also applied. All other environment variables are removed.

Finally, if the env_file option is defined, any variables present in that file will be set to their specified values as long as they would not conflict with an existing environment variable.

COMMAND EXECUTION

When sudo executes a command, the sudoers file specifies the execution envionment for the command. Typically, the real and effective uid and gid are set to match those of the target user, as specified in the password database, and the group vector is initialized based on the group database (unless the −P option was specified).

The sudoers file settings affect the following execution parameters:

See the Command environment section for details on how the environment list is constructed.

Process model
If sudo has been configured with PAM support or if I/O logging is enabled, sudo must wait until the command has completed before it will exit. In the case of PAM, sudo must remain running so that it can close the PAM session when the command is finished. If neither PAM nor I/O logging are configured, sudo will execute the command without calling fork(2). In either case, sudo sets up the execution environment as described above, and calls the execve system call (potentially in a child process). If I/O logging is enabled, a new pseudo-terminal (‘‘pty’’) is created and a second sudo process is used to relay job control signals between the user’s existing pty and the new pty the command is being run in. This extra process makes it possible to, for example, suspend and resume the command. Without it, the command would be in what POSIX terms an ‘‘orphaned process group’’ and it would not receive any job control signals.

Signal handling
If the command is run as a child of the sudo process (due to PAM or I/O logging), sudo will relay signals it receives to the command. Unless the command is being run in a new pty, the SIGHUP, SIGINT and SIGQUIT signals are not relayed unless they are sent by a user process, not the kernel. Otherwise, the command would receive SIGINT twice every time the user entered control-C. Some signals, such as SIGSTOP and SIGKILL, cannot be caught and thus will not be relayed to the command. As a general rule, SIGTSTP should be used instead of SIGSTOP when you wish to suspend a command being run by sudo.

As a special case, sudo will not relay signals that were sent by the command it is running. This prevents the command from accidentally killing itself. On some systems, the reboot(8) command sends SIGTERM to all non-system processes other than itself before rebooting the systyem. This prevents sudo from relaying the SIGTERM signal it received back to reboot(8), which might then exit before the system was actually rebooted, leaving it in a half-dead state similar to single user mode. Note, however, that this check only applies to the command run by sudo and not any other processes that the command may create. As a result, running a script that calls reboot(8) or shutdown(8) via sudo may cause the system to end up in this undefined state unless the reboot(8) or shutdown(8) are run using the exec() family of functions instead of system() (which interposes a shell between the command and the calling process).

EXIT VALUE

Upon successful execution of a program, the exit status from sudo will simply be the exit status of the program that was executed.

Otherwise, sudo exits with a value of 1 if there is a configuration/permission problem or if sudo cannot execute the given command. In the latter case the error string is printed to the standard error. If sudo cannot stat(2) one or more entries in the user’s PATH, an error is printed on stderr. (If the directory does not exist or if it is not really a directory, the entry is ignored and no error is printed.) This should not happen under normal circumstances. The most common reason for stat(2) to return ‘‘permission denied’’ is if you are running an automounter and one of the directories in your PATH is on a machine that is currently unreachable.

LOG FORMAT

sudo can log events using either syslog(3) or a simple log file. In each case the log format is almost identical.

Accepted command log entries
Commands that sudo runs are logged using the following format (split into multiple lines for readability):

date hostname progname: username : TTY=ttyname ; PWD=cwd ; \
USER=runasuser ; GROUP=runasgroup ; TSID=logid ; \
ENV=env_vars COMMAND=command

Where the fields are as follows:

Messages are logged using the locale specified by sudoers_locale, which defaults to the ‘‘C’’ locale.

Denied command log entries
If the user is not allowed to run the command, the reason for the denial will follow the user name. Possible reasons include:
user NOT in sudoers

The user is not listed in the sudoers file.

user NOT authorized on host

The user is listed in the sudoers file but is not allowed to run commands on the host.

command not allowed

The user is listed in the sudoers file for the host but they are not allowed to run the specified command.

3 incorrect password attempts

The user failed to enter their password after 3 tries. The actual number of tries will vary based on the number of failed attempts and the value of the passwd_tries sudoers option.

a password is required

The −n option was specified but a password was required.

sorry, you are not allowed to set the following environment variables

The user specified environment variables on the command line that were not allowed by sudoers.

Error log entries
If an error occurs, sudo will log a message and, in most cases, send a message to the administrator via email. Possible errors include:
parse error in /private/etc/sudoers near line N

sudo encountered an error when parsing the specified file. In some cases, the actual error may be one line above or below the line number listed, depending on the type of error.

problem with defaults entries

The sudoers file contains one or more unknown Defaults settings. This does not prevent sudo from running, but the sudoers file should be checked using visudo.

timestamp owner (username): No such user

The time stamp directory owner, as specified by the timestampowner setting, could not be found in the password database.

unable to open/read /private/etc/sudoers

The sudoers file could not be opened for reading. This can happen when the sudoers file is located on a remote file system that maps user ID 0 to a different value. Normally, sudo tries to open sudoers using group permissions to avoid this problem.

unable to stat /private/etc/sudoers

The /private/etc/sudoers file is missing.

/private/etc/sudoers is not a regular file

The /private/etc/sudoers file exists but is not a regular file or symbolic link.

/private/etc/sudoers is owned by uid N, should be 0

The sudoers file has the wrong owner.

/private/etc/sudoers is world writable

The permissions on the sudoers file allow all users to write to it. The sudoers file must not be world-writable, the default file mode is 0440 (readable by owner and group, writable by none).

/private/etc/sudoers is owned by gid N, should be 1

The sudoers file has the wrong group ownership.

unable to open /var/db/sudo/username/ttyname

sudoers was unable to read or create the user’s time stamp file.

unable to write to /var/db/sudo/username/ttyname

sudoers was unable to write to the user’s time stamp file.

unable to mkdir to /var/db/sudo/username

sudoers was unable to create the user’s time stamp directory.

Notes on logging via syslog
By default, sudoers logs messages via syslog(3). The date, hostname, and progname fields are added by the syslog daemon, not sudoers itself. As such, they may vary in format on different systems.

On most systems, syslog(3) has a relatively small log buffer. To prevent the command line arguments from being truncated, sudo will split up log messages that are larger than 960 characters (not including the date, hostname, and the string ‘‘sudo’’). When a message is split, additional parts will include the string ‘‘(command continued)’’ after the user name and before the continued command line arguments.

Notes on logging to a file
If the logfile option is set, sudoers will log to a local file, such as /var/log/sudo. When logging to a file, sudoers uses a format similar to syslog(3), with a few important differences:

SECURITY NOTES

sudo tries to be safe when executing external commands.

To prevent command spoofing, sudo checks "." and "" (both denoting current directory) last when searching for a command in the user’s PATH (if one or both are in the PATH). Note, however, that the actual PATH environment variable is not modified and is passed unchanged to the program that sudo executes.

sudo will check the ownership of its time stamp directory (/var/db/sudo by default) and ignore the directory’s contents if it is not owned by root or if it is writable by a user other than root. On systems that allow non-root users to give away files via chown(2), if the time stamp directory is located in a world-writable directory (e.g., /tmp), it is possible for a user to create the time stamp directory before sudo is run. However, because sudo checks the ownership and mode of the directory and its contents, the only damage that can be done is to ‘‘hide’’ files by putting them in the time stamp dir. This is unlikely to happen since once the time stamp dir is owned by root and inaccessible by any other user, the user placing files there would be unable to get them back out.

sudo will not honor time stamps set far in the future. Time stamps with a date greater than current_time + 2 * TIMEOUT will be ignored and sudo will log and complain. This is done to keep a user from creating his/her own time stamp with a bogus date on systems that allow users to give away files if the time stamp directory is located in a world-writable directory.

On systems where the boot time is available, sudo will ignore time stamps that date from before the machine booted.

Since time stamp files live in the file system, they can outlive a user’s login session. As a result, a user may be able to login, run a command with sudo after authenticating, logout, login again, and run sudo without authenticating so long as the time stamp file’s modification time is within 5 minutes (or whatever the timeout is set to in sudoers). When the tty_tickets sudoers option is enabled, the time stamp has per-tty granularity but still may outlive the user’s session. On Linux systems where the devpts filesystem is used, Solaris systems with the devices filesystem, as well as other systems that utilize a devfs filesystem that monotonically increase the inode number of devices as they are created (such as Mac OS X), sudo is able to determine when a tty-based time stamp file is stale and will ignore it. Administrators should not rely on this feature as it is not universally available.

Please note that sudo will normally only log the command it explicitly runs. If a user runs a command such as sudo su or sudo sh, subsequent commands run from that shell are not subject to sudo’s security policy. The same is true for commands that offer shell escapes (including most editors). If I/O logging is enabled, subsequent commands will have their input and/or output logged, but there will not be traditional logs for those commands. Because of this, care must be taken when giving users access to commands via sudo to verify that the command does not inadvertently give the user an effective root shell. For more information, please see the PREVENTING SHELL ESCAPES section in sudoers(5).

To prevent the disclosure of potentially sensitive information, sudo disables core dumps by default while it is executing (they are re-enabled for the command that is run).

For information on the security implications of sudoers entries, please see the SECURITY NOTES section in sudoers(5).

ENVIRONMENT

sudo utilizes the following environment variables:

FILES

EXAMPLES

Note: the following examples assume suitable sudoers(5) entries.

To get a file listing of an unreadable directory:

$ sudo ls /usr/local/protected

To list the home directory of user yaz on a machine where the file system holding ~yaz is not exported as root:

$ sudo -u yaz ls ~yaz

To edit the index.html file as user www:

$ sudo -u www vi ~www/htdocs/index.html

To view system logs only accessible to root and users in the adm group:

$ sudo -g adm view /var/log/syslog

To run an editor as jim with a different primary group:

$ sudo -u jim -g audio vi ~jim/sound.txt

To shut down a machine:

$ sudo shutdown -r +15 "quick reboot"

To make a usage listing of the directories in the /home partition. Note that this runs the commands in a sub-shell to make the cd and file redirection work.

$ sudo sh -c "cd /home ; du -s * | sort -rn > USAGE"

SEE ALSO

grep(1), su(1), stat(2), passwd(5), sudoers(5), sudoreplay(8), visudo(8)

HISTORY

See the HISTORY file in the sudo distribution (http://www.sudo.ws/sudo/history.html) for a brief history of sudo.

AUTHORS

Many people have worked on sudo over the years; this version consists of code written primarily by:

Todd C. Miller

See the CONTRIBUTORS file in the sudo distribution (http://www.sudo.ws/sudo/contributors.html) for an exhaustive list of people who have contributed to sudo.

CAVEATS

There is no easy way to prevent a user from gaining a root shell if that user is allowed to run arbitrary commands via sudo. Also, many programs (such as editors) allow the user to run commands via shell escapes, thus avoiding sudo’s checks. However, on most systems it is possible to prevent shell escapes with sudo ’ s noexec functionality. See the sudoers(5) manual for details.

It is not meaningful to run the cd command directly via sudo, e.g.,

$ sudo cd /usr/local/protected

since when the command exits the parent process (your shell) will still be the same. Please see the EXAMPLES section for more information.

Running shell scripts via sudo can expose the same kernel bugs that make setuid shell scripts unsafe on some operating systems (if your OS has a /dev/fd/ directory, setuid shell scripts are generally safe).

BUGS

If you feel you have found a bug in sudo, please submit a bug report at http://www.sudo.ws/sudo/bugs/

SUPPORT

Limited free support is available via the sudo-users mailing list, see http://www.sudo.ws/mailman/listinfo/sudo-users to subscribe or search the archives.

DISCLAIMER

sudo is provided ‘‘AS IS’’ and any express or implied warranties, including, but not limited to, the implied warranties of merchantability and fitness for a particular purpose are disclaimed. See the LICENSE file distributed with sudo or http://www.sudo.ws/sudo/license.html for complete details.