Linux -- sudo
SUDO(8) BSD System Manager‘s Manual SUDO(8)
NAME
sudo, sudoedit — execute a command as another user
SYNOPSIS
sudo -h | -K | -k | -V
sudo -v [-AknS] [-g group] [-h host] [-p prompt] [-u user]
sudo -l [-AknS] [-g group] [-h host] [-p prompt] [-U user] [-u user] [command]
sudo [-AbEHnPS] [-C num] [-g group] [-h host] [-p prompt] [-r role] [-t type] [-u user] [VAR=value] [-i | -s] [command]
sudoedit [-AknS] [-C num] [-g group] [-h host] [-p prompt] [-u user] file ...
DESCRIPTION
sudo allows a permitted user to execute a command as the superuser or another user, as specified by the security policy.
sudo supports a plugin architecture for security policies and input/output logging. Third parties can develop and distribute their own policy
and I/O logging plugins to work seamlessly with the sudo front end. The default security policy is sudoers, which is configured via the file
/etc/sudoers, or via LDAP. See the Plugins section for more information.
The security policy determines what privileges, if any, a user has to run sudo. The policy may require that users authenticate themselves
with a password or another authentication mechanism. If authentication is required, sudo will exit if the user‘s password is not entered
within a configurable time limit. This limit is policy-specific; the default password prompt timeout for the sudoers security policy is
unlimited.
Security policies may support credential caching to allow the user to run sudo again for a period of time without requiring authentication.
The sudoers policy caches credentials for 15 minutes, unless overridden in sudoers(5). By running sudo with the -v option, a user can update
the cached credentials without running a command.
When invoked as sudoedit, the -e option (described below), is implied.
Security policies may log successful and failed attempts to use sudo. If an I/O plugin is configured, the running command‘s input and output
may be logged as well.
The options are as follows:
-A, --askpass
Normally, if sudo requires a password, it will read it from the user‘s terminal. If the -A (askpass) option is specified, a (pos‐
sibly graphical) helper program is executed to read the user‘s password and output the password to the standard output. If the
SUDO_ASKPASS environment variable is set, it specifies the path to the helper program. Otherwise, if sudo.conf(5) contains a line
specifying the askpass program, that value will be used. For example:
# Path to askpass helper program
Path askpass /usr/X11R6/bin/ssh-askpass
If no askpass program is available, sudo will exit with an error.
-b, --background
Run the given command in the background. Note that it is not possible to use shell job control to manipulate background processes
started by sudo. Most interactive commands will fail to work properly in background mode.
-C num, --close-from=num
Close all file descriptors greater than or equal to num before executing a command. Values less than three are not permitted. By
default, sudo will close all open file descriptors other than standard input, standard output and standard error when executing a
command. The security policy may restrict the user‘s ability to use this option. The sudoers policy only permits use of the -C
option when the administrator has enabled the closefrom_override option.
-E, --preserve-env
Indicates to the security policy that the user wishes to preserve their existing environment variables. The security policy may
return an error if the user does not have permission to preserve the environment.
-e, --edit Edit one or more files instead of running a command. In lieu of a path name, the string "sudoedit" is used when consulting the
security policy. If the user is authorized by the policy, the following steps are taken:
1. Temporary copies are made of the files to be edited with the owner set to the invoking user.
2. The editor specified by the policy is run to edit the temporary files. The sudoers policy uses the SUDO_EDITOR, VISUAL and
EDITOR environment variables (in that order). If none of SUDO_EDITOR, VISUAL or EDITOR are set, the first program listed in
the editor sudoers(5) option is used.
3. If they have been modified, the temporary files are copied back to their original location and the temporary versions are
removed.
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.
-g group, --group=group
Run the command with the primary group set to group instead of the primary group specified by the target user‘s password database
entry. The group may be either a group name or a numeric group ID (GID) prefixed with the ‘#’ character (e.g. #0 for GID 0).
When running a command as a GID, many shells require that the ‘#’ be escaped with a backslash (‘\’). If no -u option is speci‐
fied, the command will be run as the invoking user. In either case, the primary group will be set to group.
-H, --set-home
Request that the security policy set the HOME environment variable to the home directory specified by the target user‘s password
database entry. Depending on the policy, this may be the default behavior.
-h, --help Display a short help message to the standard output and exit.
-h host, --host=host
Run the command on the specified host if the security policy plugin supports remote commands. Note that the sudoers plugin does
not currently support running remote commands. This may also be used in conjunction with the -l option to list a user‘s privi‐
leges for the remote host.
-i, --login
Run the shell specified by the target user‘s password database entry 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. The command is run with an environment similar to the one a user would receive at log
in. The Command Environment section in the sudoers(5) manual documents how the -i option affects the environment in which a com‐
mand is run when the sudoers policy is in use.
-K, --remove-timestamp
Similar to the -k option, except that it removes the user‘s cached credentials entirely and may not be used in conjunction with a
command or other option. This option does not require a password. Not all security policies support credential caching.
-k, --reset-timestamp
When used without a command, invalidates the user‘s cached credentials. In other words, 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, this option will cause sudo to ignore the user‘s
cached credentials. As a result, sudo will prompt for a password (if one is required by the security policy) and will not update
the user‘s cached credentials.
Not all security policies support credential caching.
-l, --list If no command is specified, list the allowed (and forbidden) commands for the invoking user (or the user specified by the -U
option) on the current host. A longer list format is used if this option is specified multiple times and the security policy sup‐
ports a verbose output format.
If a command is specified and is permitted by the security policy, 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.
-n, --non-interactive
Avoid prompting the user for input of any kind. If a password is required for the command to run, sudo will display an error mes‐
sage and exit.
-P, --preserve-groups
Preserve the invoking user‘s group vector unaltered. By default, the sudoers policy will initialize the group vector to the list
of groups the target user is a member of. The real and effective group IDs, however, are still set to match the target user.
-p prompt, --prompt=prompt
Use a custom password prompt with optional escape sequences. The following percent (‘%’) escape sequences are supported by the
sudoers policy:
%H expanded to the host name including the domain name (on if the machine‘s host name is fully qualified or the fqdn option is
set in sudoers(5))
%h expanded to the local host name without the domain name
%p expanded to the name of the user whose password is being requested (respects the rootpw, targetpw, and runaspw flags in
sudoers(5))
%U expanded to the login name of the user the command will be run as (defaults to root unless the -u option is also specified)
%u expanded to the invoking user‘s login name
%% two consecutive ‘%’ characters are collapsed into a single ‘%’ character
The custom prompt will override the system password prompt on systems that support PAM unless the passprompt_override flag is dis‐
abled in sudoers.
-r role, --role=role
Run the command with an SELinux security context that includes the specified role.
-S, --stdin
Write the prompt to the standard error and read the password from the standard input instead of using the terminal device. The
password must be followed by a newline character.
-s, --shell
Run the shell specified by the SHELL environment variable if it is set or the shell specified by the invoking user‘s password
database entry. 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.
-t type, --type=type
Run the command with an SELinux security context that includes the specified type. If no type is specified, the default type is
derived from the role.
-U user, --other-user=user
Used in conjunction with the -l option to list the privileges for user instead of for the invoking user. The security policy may
restrict listing other users‘ privileges. The sudoers policy only allows root or a user with the ALL privilege on the current
host to use this option.
-u user, --user=user
Run the command as a user other than the default target user (usually root ). The user may be either a user name or a numeric user
ID (UID) prefixed with the ‘#’ character (e.g. #0 for UID 0). When running commands as a UID, many shells require that the ‘#’
be escaped with a backslash (‘\’). Some security policies may restrict UIDs to those listed in the password database. The
sudoers policy allows UIDs that are not in the password database as long as the targetpw option is not set. Other security poli‐
cies may not support this.
-V, --version
Print the sudo version string as well as the version string of the security policy plugin and any I/O plugins. If the invoking
user is already root the -V option will display the arguments passed to configure when sudo was built and plugins may display more
verbose information such as default options.
-v, --validate
Update the user‘s cached credentials, authenticating the user if necessary. For the sudoers plugin, this extends the sudo timeout
for another 15 minutes by default, but does not run a command. Not all security policies support cached credentials.
-- The -- option indicates that sudo should stop processing command line arguments.
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 restrictions imposed by the security policy plugin.
The sudoers policy subjects variables passed on the command line 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.
COMMAND EXECUTION
When sudo executes a command, the security policy specifies the execution environment for the command. Typically, the real and effective user
and group and IDs 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 following parameters may be specified by security policy:
· real and effective user ID
· real and effective group ID
· supplementary group IDs
· the environment list
· current working directory
· file creation mode mask (umask)
· SELinux role and type
· scheduling priority (aka nice value)
Process model
When sudo runs a command, it calls fork(2), sets up the execution environment as described above, and calls the execve system call in the
child process. The main sudo process waits until the command has completed, then passes the command‘s exit status to the security policy‘s
close function and exits. If an I/O logging plugin is configured or if the security policy explicitly requests it, 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. As a special case, if the policy plugin does
not define a close function and no pty is required, sudo will execute the command directly instead of calling fork(2) first. The sudoers pol‐
icy plugin will only define a close function when I/O logging is enabled, a pty is required, or the pam_session or pam_setcred options are
enabled. Note that pam_session and pam_setcred are enabled by default on systems using PAM.
Signal handling
When the command is run as a child of the sudo process, 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 sys‐
tem. 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).
If no I/O logging plugins are loaded and the policy plugin has not defined a close() function, set a command timeout or required that the com‐
mand be run in a new pty, sudo may execute the command directly instead of running it as a child process.
Plugins
Plugins may be specified via Plugin directives in the sudo.conf(5) file. They may be loaded as dynamic shared objects (on systems that sup‐
port them), or compiled directly into the sudo binary. If no sudo.conf(5) file is present, or it contains no Plugin lines, sudo will use the
traditional sudoers security policy and I/O logging. See the sudo.conf(5) manual for details of the /etc/sudo.conf file and the
sudo_plugin(8) manual for more information about the sudo plugin architecture.
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 auto‐
mounter and one of the directories in your PATH is on a machine that is currently unreachable.
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.
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). To aid in debugging sudo crashes, you may wish to re-enable core dumps by setting “disable_coredump” to false
in the sudo.conf(5) file as follows:
Set disable_coredump false
See the sudo.conf(5) manual for more information.
ENVIRONMENT
sudo utilizes the following environment variables. The security policy has control over the actual content of the command‘s environment.
EDITOR Default editor to use in -e (sudoedit) mode if neither SUDO_EDITOR nor VISUAL is set.
MAIL In -i mode or when env_reset is enabled in sudoers, set to the mail spool of the target user.
HOME Set to the home directory of the target user if -i or -H are specified, env_reset or always_set_home are set in sudoers, or
when the -s option is specified and set_home is set in sudoers.
PATH May be overridden by the security policy.
SHELL Used to determine shell to run with -s option.
SUDO_ASKPASS Specifies the path to a helper program used to read the password if no terminal is available or if the -A option is speci‐
fied.
SUDO_COMMAND Set to the command run by sudo.
SUDO_EDITOR Default editor to use in -e (sudoedit) mode.
SUDO_GID Set to the group ID of the user who invoked sudo.
SUDO_PROMPT Used as the default password prompt.
SUDO_PS1 If set, PS1 will be set to its value for the program being run.
SUDO_UID Set to the user ID of the user who invoked sudo.
SUDO_USER Set to the login name of the user who invoked sudo.
USER Set to the target user (root unless the -u option is specified).
VISUAL Default editor to use in -e (sudoedit) mode if SUDO_EDITOR is not set.
FILES
/etc/sudo.conf sudo front end configuration
EXAMPLES
Note: the following examples assume a properly configured security policy.
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
su(1), stat(2), passwd(5), sudo.conf(5), sudoers(5), sudo_plugin(8), 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 con‐
tributed 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 pro‐
grams (such as editors) allow the user to run commands via shell escapes, thus avoiding sudo‘s checks. However, on most systems it is possi‐
ble to prevent shell escapes with the sudoers(5) plugin‘s noexec functionality.
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 com‐
plete details.
Sudo 1.8.9p5 December 8, 2013 Sudo 1.8.9p5
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