Sudoers Manual

NAME

sudoersdefault sudo security policy module

DESCRIPTION

The sudoers policy module determines a user's sudo privileges. It is the default sudo policy plugin. The policy is driven by the /etc/sudoers file or, optionally in LDAP. The policy format is described in detail in the SUDOERS FILE FORMAT section. For information on storing sudoers policy information in LDAP, please see sudoers.ldap(5).

Authentication and logging

The sudoers security policy requires that most users authenticate themselves before they can use sudo. 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 policy has disabled authentication for the user or command. Unlike su(1), when sudoers 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 flags, described later.

If a user who is not listed in the policy 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 Defaults entry (described later) 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, the sudoers policy will use this value 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.

sudoers 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, sudoers 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.

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

sudoers 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, sudoers 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 deal 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, sudoers 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.

SUDOERS FILE FORMAT

The sudoers file is composed of two types of entries: aliases (basically variables) and user specifications (which specify who may run what).

When multiple entries match for a user, they are applied in order. Where there are multiple matches, the last match is used (which is not necessarily the most specific match).

The sudoers grammar will be described below in Extended Backus-Naur Form (EBNF). Don't despair if you are unfamiliar with EBNF; it is fairly simple, and the definitions below are annotated.

Quick guide to EBNF

EBNF is a concise and exact way of describing the grammar of a language. Each EBNF definition is made up of production rules. E.g.,

symbol ::= definition | alternate1 | alternate2 ...

Each production rule references others and thus makes up a grammar for the language. EBNF also contains the following operators, which many readers will recognize from regular expressions. Do not, however, confuse them with “wildcard” characters, which have different meanings.

?
Means that the preceding symbol (or group of symbols) is optional. That is, it may appear once or not at all.
*
Means that the preceding symbol (or group of symbols) may appear zero or more times.
+
Means that the preceding symbol (or group of symbols) may appear one or more times.

Parentheses may be used to group symbols together. For clarity, we will use single quotes ('') to designate what is a verbatim character string (as opposed to a symbol name).

Aliases

There are four kinds of aliases: User_Alias, Runas_Alias, Host_Alias and Cmnd_Alias.

Alias ::= 'User_Alias'  User_Alias (':' User_Alias)* |
          'Runas_Alias' Runas_Alias (':' Runas_Alias)* |
          'Host_Alias'  Host_Alias (':' Host_Alias)* |
          'Cmnd_Alias'  Cmnd_Alias (':' Cmnd_Alias)*
 
User_Alias ::= NAME '=' User_List
 
Runas_Alias ::= NAME '=' Runas_List
 
Host_Alias ::= NAME '=' Host_List
 
Cmnd_Alias ::= NAME '=' Cmnd_List
 
NAME ::= [A-Z]([A-Z][0-9]_)*

Each alias definition is of the form

Alias_Type NAME = item1, item2, ...

where Alias_Type is one of User_Alias, Runas_Alias, Host_Alias, or Cmnd_Alias. A NAME is a string of uppercase letters, numbers, and underscore characters (‘_’). A NAME must start with an uppercase letter. It is possible to put several alias definitions of the same type on a single line, joined by a colon (‘:’). E.g.,

Alias_Type NAME = item1, item2, item3 : NAME = item4, item5

The definitions of what constitutes a valid alias member follow.

User_List ::= User |
              User ',' User_List
 
User ::= '!'* user name |
         '!'* #uid |
         '!'* %group |
         '!'* %#gid |
         '!'* +netgroup |
         '!'* %:nonunix_group |
         '!'* %:#nonunix_gid |
         '!'* User_Alias

A User_List is made up of one or more user names, user IDs (prefixed with ‘#’), system group names and IDs (prefixed with ‘%’ and ‘%#’ respectively), netgroups (prefixed with ‘+’), non-Unix group names and IDs (prefixed with ‘%:’ and ‘%:#’ respectively) and User_Aliases. Each list item may be prefixed with zero or more ‘!’ operators. An odd number of ‘!’ operators negate the value of the item; an even number just cancel each other out.

A user name, uid, group, gid, netgroup, nonunix_group or nonunix_gid may be enclosed in double quotes to avoid the need for escaping special characters. Alternately, special characters may be specified in escaped hex mode, e.g. \x20 for space. When using double quotes, any prefix characters must be included inside the quotes.

The actual nonunix_group and nonunix_gid syntax depends on the underlying group provider plugin (see the group_plugin description below). For instance, the QAS AD plugin supports the following formats:

  • Group in the same domain: "%:Group Name"
  • Group in any domain: "%:Group Name@FULLY.QUALIFIED.DOMAIN"
  • Group SID: "%:S-1-2-34-5678901234-5678901234-5678901234-567"

Note that quotes around group names are optional. Unquoted strings must use a backslash (‘\’) to escape spaces and special characters. See Other special characters and reserved words for a list of characters that need to be escaped.

Runas_List ::= Runas_Member |
               Runas_Member ',' Runas_List
 
Runas_Member ::= '!'* user name |
                 '!'* #uid |
                 '!'* %group |
                 '!'* %#gid |
                 '!'* %:nonunix_group |
                 '!'* %:#nonunix_gid |
                 '!'* +netgroup |
                 '!'* Runas_Alias

A Runas_List is similar to a User_List except that instead of User_Aliases it can contain Runas_Aliases. Note that user names and groups are matched as strings. In other words, two users (groups) with the same uid (gid) are considered to be distinct. If you wish to match all user names with the same uid (e.g. root and toor), you can use a uid instead (#0 in the example given).

Host_List ::= Host |
              Host ',' Host_List
 
Host ::= '!'* host name |
         '!'* ip_addr |
         '!'* network(/netmask)? |
         '!'* +netgroup |
         '!'* Host_Alias

A Host_List is made up of one or more host names, IP addresses, network numbers, netgroups (prefixed with ‘+’) and other aliases. Again, the value of an item may be negated with the ‘!’ operator. If you do not specify a netmask along with the network number, sudo will query each of the local host's network interfaces and, if the network number corresponds to one of the hosts's network interfaces, the corresponding netmask will be used. The netmask may be specified either in standard IP address notation (e.g. 255.255.255.0 or ffff:ffff:ffff:ffff::), or CIDR notation (number of bits, e.g. 24 or 64). A host name may include shell-style wildcards (see the Wildcards section below), but unless the host name command on your machine returns the fully qualified host name, you'll need to use the fqdn option for wildcards to be useful. Note that sudo only inspects actual network interfaces; this means that IP address 127.0.0.1 (localhost) will never match. Also, the host name “localhost” will only match if that is the actual host name, which is usually only the case for non-networked systems.

Cmnd_List ::= Cmnd |
              Cmnd ',' Cmnd_List
 
command name ::= file name |
                 file name args |
                 file name '""'
 
Cmnd ::= '!'* command name |
         '!'* directory |
         '!'* "sudoedit" |
         '!'* Cmnd_Alias

A Cmnd_List is a list of one or more command names, directories, and other aliases. A command name is a fully qualified file name which may include shell-style wildcards (see the Wildcards section below). A simple file name allows the user to run the command with any arguments he/she wishes. However, you may also specify command line arguments (including wildcards). Alternately, you can specify "" to indicate that the command may only be run without command line arguments. A directory is a fully qualified path name ending in a ‘/’. When you specify a directory in a Cmnd_List, the user will be able to run any file within that directory (but not in any sub-directories therein).

If a Cmnd has associated command line arguments, then the arguments in the Cmnd must match exactly those given by the user on the command line (or match the wildcards if there are any). Note that the following characters must be escaped with a ‘\’ if they are used in command arguments: ‘,’, ‘:’, ‘=’, ‘\’. The special command “sudoedit” is used to permit a user to run sudo with the ‑e option (or as sudoedit). It may take command line arguments just as a normal command does.

Defaults

Certain configuration options may be changed from their default values at run-time via one or more Default_Entry lines. These may affect all users on any host, all users on a specific host, a specific user, a specific command, or commands being run as a specific user. Note that per-command entries may not include command line arguments. If you need to specify arguments, define a Cmnd_Alias and reference that instead.

Default_Type ::= 'Defaults' |
                 'Defaults' '@' Host_List |
                 'Defaults' ':' User_List |
                 'Defaults' '!' Cmnd_List |
                 'Defaults' '>' Runas_List
 
Default_Entry ::= Default_Type Parameter_List
 
Parameter_List ::= Parameter |
                   Parameter ',' Parameter_List
 
Parameter ::= Parameter '=' Value |
              Parameter '+=' Value |
              Parameter '-=' Value |
              '!'* Parameter

Parameters may be flags, integer values, strings, or lists. Flags are implicitly boolean and can be turned off via the ‘!’ operator. Some integer, string and list parameters may also be used in a boolean context to disable them. Values may be enclosed in double quotes ("") when they contain multiple words. Special characters may be escaped with a backslash (‘\’).

Lists have two additional assignment operators, += and -=. These operators are used to add to and delete from a list respectively. It is not an error to use the -= operator to remove an element that does not exist in a list.

Defaults entries are parsed in the following order: generic, host and user Defaults first, then runas Defaults and finally command defaults.

See SUDOERS OPTIONS for a list of supported Defaults parameters.

User specification

User_Spec ::= User_List Host_List '=' Cmnd_Spec_List \
              (':' Host_List '=' Cmnd_Spec_List)*
 
Cmnd_Spec_List ::= Cmnd_Spec |
                   Cmnd_Spec ',' Cmnd_Spec_List
 
Cmnd_Spec ::= Runas_Spec? SELinux_Spec? Solaris_Priv_Spec? Tag_Spec* Cmnd
 
Runas_Spec ::= '(' Runas_List? (':' Runas_List)? ')'
 
SELinux_Spec ::= ('ROLE=role' | 'TYPE=type')
 
Solaris_Priv_Spec ::= ('PRIVS=privset' | 'LIMITPRIVS=privset')
 
Tag_Spec ::= ('NOPASSWD:' | 'PASSWD:' | 'NOEXEC:' | 'EXEC:' |
              'SETENV:' | 'NOSETENV:' | 'LOG_INPUT:' | 'NOLOG_INPUT:' |
              'LOG_OUTPUT:' | 'NOLOG_OUTPUT:')

A user specification determines which commands a user may run (and as what user) on specified hosts. By default, commands are run as root, but this can be changed on a per-command basis.

The basic structure of a user specification is “who where = (as_whom) what”. Let's break that down into its constituent parts:

Runas_Spec

A Runas_Spec determines the user and/or the group that a command may be run as. A fully-specified Runas_Spec consists of two Runas_Lists (as defined above) separated by a colon (‘:’) and enclosed in a set of parentheses. The first Runas_List indicates which users the command may be run as via sudo's ‑u option. The second defines a list of groups that can be specified via sudo's ‑g option. If both Runas_Lists are specified, the command may be run with any combination of users and groups listed in their respective Runas_Lists. If only the first is specified, the command may be run as any user in the list but no ‑g option may be specified. If the first Runas_List is empty but the second is specified, the command may be run as the invoking user with the group set to any listed in the Runas_List. If both Runas_Lists are empty, the command may only be run as the invoking user. If no Runas_Spec is specified the command may be run as root and no group may be specified.

A Runas_Spec sets the default for the commands that follow it. What this means is that for the entry:

dgb	boulder = (operator) /bin/ls, /bin/kill, /usr/bin/lprm

The user dgb may run /bin/ls, /bin/kill, and /usr/bin/lprm—but only as operator. E.g.,

$ sudo -u operator /bin/ls

It is also possible to override a Runas_Spec later on in an entry. If we modify the entry like so:

dgb	boulder = (operator) /bin/ls, (root) /bin/kill, /usr/bin/lprm

Then user dgb is now allowed to run /bin/ls as operator, but /bin/kill and /usr/bin/lprm as root.

We can extend this to allow dgb to run /bin/ls with either the user or group set to operator:

dgb	boulder = (operator : operator) /bin/ls, (root) /bin/kill,\
	/usr/bin/lprm

Note that while the group portion of the Runas_Spec permits the user to run as command with that group, it does not force the user to do so. If no group is specified on the command line, the command will run with the group listed in the target user's password database entry. The following would all be permitted by the sudoers entry above:

$ sudo -u operator /bin/ls
$ sudo -u operator -g operator /bin/ls
$ sudo -g operator /bin/ls

In the following example, user tcm may run commands that access a modem device file with the dialer group.

tcm	boulder = (:dialer) /usr/bin/tip, /usr/bin/cu,\
	/usr/local/bin/minicom

Note that in this example only the group will be set, the command still runs as user tcm. E.g.

$ sudo -g dialer /usr/bin/cu

Multiple users and groups may be present in a Runas_Spec, in which case the user may select any combination of users and groups via the ‑u and ‑g options. In this example:

alan	ALL = (root, bin : operator, system) ALL

user alan may run any command as either user root or bin, optionally setting the group to operator or system.

SELinux_Spec

On systems with SELinux support, sudoers entries may optionally have an SELinux role and/or type associated with a command. If a role or type is specified with the command it will override any default values specified in sudoers. A role or type specified on the command line, however, will supersede the values in sudoers.

Solaris_Priv_Spec

On Solaris systems, sudoers entries may optionally specify Solaris privilege set and/or limit privilege set associated with a command. If privileges or limit privileges are specified with the command it will override any default values specified in sudoers.

A privilege set is a comma-separated list of privilege names. The ppriv(1) command can be used to list all privileges known to the system. For example:

$ ppriv -l

In addition, there are several “special” privilege strings:

none
the empty set
all
the set of all privileges
zone
the set of all privileges available in the current zone
basic
the default set of privileges normal users are granted at login time

Privileges can be excluded from a set by prefixing the privilege name with either an ‘!’ or ‘-’ character.

Tag_Spec

A command may have zero or more tags associated with it. There are ten possible tag values: NOPASSWD, PASSWD, NOEXEC, EXEC, SETENV, NOSETENV, LOG_INPUT, NOLOG_INPUT, LOG_OUTPUT and NOLOG_OUTPUT. Once a tag is set on a Cmnd, subsequent Cmnds in the Cmnd_Spec_List, inherit the tag unless it is overridden by the opposite tag (in other words, PASSWD overrides NOPASSWD and NOEXEC overrides EXEC).
NOPASSWD and PASSWD
 
By default, sudo requires that a user authenticate him or herself before running a command. This behavior can be modified via the NOPASSWD tag. Like a Runas_Spec, the NOPASSWD tag sets a default for the commands that follow it in the Cmnd_Spec_List. Conversely, the PASSWD tag can be used to reverse things. For example:

ray	rushmore = NOPASSWD: /bin/kill, /bin/ls, /usr/bin/lprm

would allow the user ray to run /bin/kill, /bin/ls, and /usr/bin/lprm as root on the machine rushmore without authenticating himself. If we only want ray to be able to run /bin/kill without a password the entry would be:

ray	rushmore = NOPASSWD: /bin/kill, PASSWD: /bin/ls, /usr/bin/lprm

Note, however, that the PASSWD tag has no effect on users who are in the group specified by the exempt_group option.

By default, if the NOPASSWD tag is applied to any of the entries for a user on the current host, he or she will be able to run “sudo -l” without a password. Additionally, a user may only run “sudo -v” without a password if the NOPASSWD tag is present for all a user's entries that pertain to the current host. This behavior may be overridden via the verifypw and listpw options.

NOEXEC and EXEC
 
If sudo has been compiled with noexec support and the underlying operating system supports it, the NOEXEC tag can be used to prevent a dynamically-linked executable from running further commands itself.

In the following example, user aaron may run /usr/bin/more and /usr/bin/vi but shell escapes will be disabled.

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

See the Preventing shell escapes section below for more details on how NOEXEC works and whether or not it will work on your system.

SETENV and NOSETENV
 
These tags override the value of the setenv option on a per-command basis. Note that if SETENV has been set for a command, the user may disable the env_reset option from the command line via the ‑E option. Additionally, environment variables set on the command line are not subject to the restrictions imposed by env_check, env_delete, or env_keep. As such, only trusted users should be allowed to set variables in this manner. If the command matched is ALL, the SETENV tag is implied for that command; this default may be overridden by use of the NOSETENV tag.
LOG_INPUT and NOLOG_INPUT
 
These tags override the value of the log_input option on a per-command basis. For more information, see the description of log_input in the SUDOERS OPTIONS section below.
LOG_OUTPUT and NOLOG_OUTPUT
 
These tags override the value of the log_output option on a per-command basis. For more information, see the description of log_output in the SUDOERS OPTIONS section below.

Wildcards

sudo allows shell-style wildcards (aka meta or glob characters) to be used in host names, path names and command line arguments in the sudoers file. Wildcard matching is done via the glob(3) and fnmatch(3) functions as specified by IEEE Std 1003.1 (“POSIX.1”). Note that these are not regular expressions.
*
Matches any set of zero or more characters.
?
Matches any single character.
[...]
Matches any character in the specified range.
[!...]
Matches any character not in the specified range.
\x
For any character ‘x’, evaluates to ‘x’. This is used to escape special characters such as: ‘*’, ‘?’, ‘[’, and ‘]’.

Character classes may also be used if your system's glob(3) and fnmatch(3) functions support them. However, because the ‘:’ character has special meaning in sudoers, it must be escaped. For example:

/bin/ls [[alpha]]*

Would match any file name beginning with a letter.

Note that a forward slash (‘/’) will not be matched by wildcards used in the path name. This is to make a path like:

/usr/bin/*

match /usr/bin/who but not /usr/bin/X11/xterm.

When matching the command line arguments, however, a slash does get matched by wildcards since command line arguments may contain arbitrary strings and not just path names.

Wildcards in command line arguments should be used with care. Because command line arguments are matched as a single, concatenated string, a wildcard such as ‘?’ or ‘*’ can match multiple words. For example, while a sudoers entry like:

%operator ALL = /bin/cat /var/log/messages*

will allow command like:

$ sudo cat /var/log/messages.1

It will also allow:

$ sudo cat /var/log/messages /etc/shadow

which is probably not what was intended.

Exceptions to wildcard rules

The following exceptions apply to the above rules:
""
If the empty string "" is the only command line argument in the sudoers entry it means that command is not allowed to be run with any arguments.
sudoedit
Command line arguments to the sudoedit built-in command should always be path names, so a forward slash (‘/’) will not be matched by a wildcard.

Including other files from within sudoers

It is possible to include other sudoers files from within the sudoers file currently being parsed using the #include and #includedir directives.

This can be used, for example, to keep a site-wide sudoers file in addition to a local, per-machine file. For the sake of this example the site-wide sudoers will be /etc/sudoers and the per-machine one will be /etc/sudoers.local. To include /etc/sudoers.local from within /etc/sudoers we would use the following line in /etc/sudoers:

#include /etc/sudoers.local

When sudo reaches this line it will suspend processing of the current file (/etc/sudoers) and switch to /etc/sudoers.local. Upon reaching the end of /etc/sudoers.local, the rest of /etc/sudoers will be processed. Files that are included may themselves include other files. A hard limit of 128 nested include files is enforced to prevent include file loops.

If the path to the include file is not fully-qualified (does not begin with a ‘/’, it must be located in the same directory as the sudoers file it was included from. For example, if /etc/sudoers contains the line:

#include sudoers.local

the file that will be included is /etc/sudoers.local.

The file name may also include the %h escape, signifying the short form of the host name. In other words, if the machine's host name is “xerxes”, then

#include /etc/sudoers.%h

will cause sudo to include the file /etc/sudoers.xerxes.

The #includedir directive can be used to create a sudo.d directory that the system package manager can drop sudoers rules into as part of package installation. For example, given:

#includedir /etc/sudoers.d

sudo will read each file in /etc/sudoers.d, skipping file names that end in ‘~’ or contain a ‘.’ character to avoid causing problems with package manager or editor temporary/backup files. Files are parsed in sorted lexical order. That is, /etc/sudoers.d/01_first will be parsed before /etc/sudoers.d/10_second. Be aware that because the sorting is lexical, not numeric, /etc/sudoers.d/1_whoops would be loaded after /etc/sudoers.d/10_second. Using a consistent number of leading zeroes in the file names can be used to avoid such problems.

Note that unlike files included via #include, visudo will not edit the files in a #includedir directory unless one of them contains a syntax error. It is still possible to run visudo with the ‑f flag to edit the files directly.

Other special characters and reserved words

The pound sign (‘#’) is used to indicate a comment (unless it is part of a #include directive or unless it occurs in the context of a user name and is followed by one or more digits, in which case it is treated as a uid). Both the comment character and any text after it, up to the end of the line, are ignored.

The reserved word ALL is a built-in alias that always causes a match to succeed. It can be used wherever one might otherwise use a Cmnd_Alias, User_Alias, Runas_Alias, or Host_Alias. You should not try to define your own alias called ALL as the built-in alias will be used in preference to your own. Please note that using ALL can be dangerous since in a command context, it allows the user to run any command on the system.

An exclamation point (‘!’) can be used as a logical not operator in a list or alias as well as in front of a Cmnd. This allows one to exclude certain values. For the ‘!’ operator to be effective, there must be something for it to exclude. For example, to match all users except for root one would use:

ALL,!root

If the ALL, is omitted, as in:

!root

it would explicitly deny root but not match any other users. This is different from a true “negation” operator.

Note, however, that using a ‘!’ in conjunction with the built-in ALL alias to allow a user to run “all but a few” commands rarely works as intended (see SECURITY NOTES below).

Long lines can be continued with a backslash (‘\’) as the last character on the line.

White space between elements in a list as well as special syntactic characters in a User Specification (‘=’, ‘:’, ‘(’, ‘)’) is optional.

The following characters must be escaped with a backslash (‘\’) when used as part of a word (e.g. a user name or host name): ‘!’, ‘=’, ‘:’, ‘,’, ‘(’, ‘)’, ‘\’.

SUDOERS OPTIONS

sudo's behavior can be modified by Default_Entry lines, as explained earlier. A list of all supported Defaults parameters, grouped by type, are listed below.

Boolean Flags:

always_set_home
If enabled, sudo will set the HOME environment variable to the home directory of the target user (which is root unless the ‑u option is used). This effectively means that the ‑H option is always implied. Note that HOME is already set when the the env_reset option is enabled, so always_set_home is only effective for configurations where either env_reset is disabled or HOME is present in the env_keep list. This flag is off by default.
authenticate
If set, users must authenticate themselves via a password (or other means of authentication) before they may run commands. This default may be overridden via the PASSWD and NOPASSWD tags. This flag is on by default.
closefrom_override
If set, the user may use sudo's ‑C option which overrides the default starting point at which sudo begins closing open file descriptors. This flag is off by default.
compress_io
If set, and sudo is configured to log a command's input or output, the I/O logs will be compressed using zlib. This flag is on by default when sudo is compiled with zlib support.
env_editor
If set, visudo will use the value of the EDITOR or VISUAL environment variables before falling back on the default editor list. Note that this may create a security hole as it allows the user to run any arbitrary command as root without logging. A safer alternative is to place a colon-separated list of editors in the editor variable. visudo will then only use the EDITOR or VISUAL if they match a value specified in editor. This flag is off by default.
env_reset
If set, sudo will run the command in a minimal environment containing the TERM, PATH, HOME, MAIL, SHELL, LOGNAME, USER, USERNAME and SUDO_* variables. Any variables in the caller's environment that match the env_keep and env_check lists are then added, followed by any variables present in the file specified by the env_file option (if any). The default contents of the env_keep and env_check lists are displayed when sudo is run by root with the ‑V option. If the secure_path option is set, its value will be used for the PATH environment variable. This flag is on by default.
fast_glob
Normally, sudo uses the glob(3) function to do shell-style globbing when matching path names. However, since it accesses the file system, glob(3) can take a long time to complete for some patterns, especially when the pattern references a network file system that is mounted on demand (auto mounted). The fast_glob option causes sudo to use the fnmatch(3) function, which does not access the file system to do its matching. The disadvantage of fast_glob is that it is unable to match relative path names such as ./ls or ../bin/ls. This has security implications when path names that include globbing characters are used with the negation operator, ‘!’, as such rules can be trivially bypassed. As such, this option should not be used when sudoers contains rules that contain negated path names which include globbing characters. This flag is off by default.
fqdn
Set this flag if you want to put fully qualified host names in the sudoers file when the local host name (as returned by the hostname command) does not contain the domain name. In other words, instead of myhost you would use myhost.mydomain.edu. You may still use the short form if you wish (and even mix the two). This option is only effective when the “canonical” host name, as returned by the getaddrinfo() or gethostbyname() function, is a fully-qualified domain name. This is usually the case when the system is configured to use DNS for host name resolution.

If the system is configured to use the /etc/hosts file in preference to DNS, the “canonical” host name may not be fully-qualified. The order that sources are queried for hosts name resolution is usually specified in the /etc/nsswitch.conf, /etc/netsvc.conf, /etc/host.conf, or, in some cases, /etc/resolv.conf file. In the /etc/hosts file, the first host name of the entry is considered to be the “canonical” name; subsequent names are aliases that are not used by sudoers. For example, the following hosts file line for the machine “xyzzy” has the fully-qualified domain name as the “canonical” host name, and the short version as an alias.

 
192.168.1.1 xyzzy.sudo.ws xyzzy
 
If the machine's hosts file entry is not formatted properly, the fqdn option will not be effective if it is queried before DNS.

Beware that when using DNS for host name resolution, turning on fqdn requires sudoers to make DNS lookups which renders sudo unusable if DNS stops working (for example if the machine is disconnected from the network). Also note that just like with the hosts file, you must use the “canonical” name as DNS knows it. That is, you may not use a host alias (CNAME entry) due to performance issues and the fact that there is no way to get all aliases from DNS.

This flag is off by default.

ignore_dot
If set, sudo will ignore "." or "" (both denoting current directory) in the PATH environment variable; the PATH itself is not modified. This flag is off by default.
ignore_local_sudoers
If set via LDAP, parsing of /etc/sudoers will be skipped. This is intended for Enterprises that wish to prevent the usage of local sudoers files so that only LDAP is used. This thwarts the efforts of rogue operators who would attempt to add roles to /etc/sudoers. When this option is present, /etc/sudoers does not even need to exist. Since this option tells sudo how to behave when no specific LDAP entries have been matched, this sudoOption is only meaningful for the cn=defaults section. This flag is off by default.
insults
If set, sudo will insult users when they enter an incorrect password. This flag is off by default.
log_host
If set, the host name will be logged in the (non-syslog) sudo log file. This flag is off by default.
log_input
If set, sudo will run the command in a pseudo tty and log all user input. If the standard input is not connected to the user's tty, due to I/O redirection or because the command is part of a pipeline, that input is also captured and stored in a separate log file.

Input is logged to the directory specified by the iolog_dir option (/var/log/sudo-io by default) using a unique session ID that is included in the normal sudo log line, prefixed with “TSID=”. The iolog_file option may be used to control the format of the session ID.

Note that user input may contain sensitive information such as passwords (even if they are not echoed to the screen), which will be stored in the log file unencrypted. In most cases, logging the command output via log_output is all that is required.

log_output
If set, sudo will run the command in a pseudo tty and log all output that is sent to the screen, similar to the script(1) command. If the standard output or standard error is not connected to the user's tty, due to I/O redirection or because the command is part of a pipeline, that output is also captured and stored in separate log files.

Output is logged to the directory specified by the iolog_dir option (/var/log/sudo-io by default) using a unique session ID that is included in the normal sudo log line, prefixed with “TSID=”. The iolog_file option may be used to control the format of the session ID.

Output logs may be viewed with the sudoreplay(8) utility, which can also be used to list or search the available logs.

log_year
If set, the four-digit year will be logged in the (non-syslog) sudo log file. This flag is off by default.
long_otp_prompt
When validating with a One Time Password (OTP) scheme such as S/Key or OPIE, a two-line prompt is used to make it easier to cut and paste the challenge to a local window. It's not as pretty as the default but some people find it more convenient. This flag is off by default.
mail_always
Send mail to the mailto user every time a users runs sudo. This flag is off by default.
mail_badpass
Send mail to the mailto user if the user running sudo does not enter the correct password. If the command the user is attempting to run is not permitted by sudoers and one of the mail_always, mail_no_host, mail_no_perms or mail_no_user flags are set, this flag will have no effect. This flag is off by default.
mail_no_host
If set, mail will be sent to the mailto user if the invoking user exists in the sudoers file, but is not allowed to run commands on the current host. This flag is off by default.
mail_no_perms
If set, mail will be sent to the mailto user if the invoking user is allowed to use sudo but the command they are trying is not listed in their sudoers file entry or is explicitly denied. This flag is off by default.
mail_no_user
If set, mail will be sent to the mailto user if the invoking user is not in the sudoers file. This flag is on by default.
noexec
If set, all commands run via sudo will behave as if the NOEXEC tag has been set, unless overridden by a EXEC tag. See the description of NOEXEC and EXEC below as well as the Preventing shell escapes section at the end of this manual. This flag is off by default.
path_info
Normally, sudo will tell the user when a command could not be found in their PATH environment variable. Some sites may wish to disable this as it could be used to gather information on the location of executables that the normal user does not have access to. The disadvantage is that if the executable is simply not in the user's PATH, sudo will tell the user that they are not allowed to run it, which can be confusing. This flag is on by default.
passprompt_override
The password prompt specified by passprompt will normally only be used if the password prompt provided by systems such as PAM matches the string “Password:”. If passprompt_override is set, passprompt will always be used. This flag is off by default.
preserve_groups
By default, sudo will initialize the group vector to the list of groups the target user is in. When preserve_groups is set, the user's existing group vector is left unaltered. The real and effective group IDs, however, are still set to match the target user. This flag is off by default.
pwfeedback
By default, sudo reads the password like most other Unix programs, by turning off echo until the user hits the return (or enter) key. Some users become confused by this as it appears to them that sudo has hung at this point. When pwfeedback is set, sudo will provide visual feedback when the user presses a key. Note that this does have a security impact as an onlooker may be able to determine the length of the password being entered. This flag is off by default.
requiretty
If set, sudo will only run when the user is logged in to a real tty. When this flag is set, sudo can only be run from a login session and not via other means such as cron(8) or cgi-bin scripts. This flag is off by default.
root_sudo
If set, root is allowed to run sudo too. Disabling this prevents users from “chaining” sudo commands to get a root shell by doing something like “sudo sudo /bin/sh”. Note, however, that turning off root_sudo will also prevent root from running sudoedit. Disabling root_sudo provides no real additional security; it exists purely for historical reasons. This flag is on by default.
rootpw
If set, sudo will prompt for the root password instead of the password of the invoking user. This flag is off by default.
runaspw
If set, sudo will prompt for the password of the user defined by the runas_default option (defaults to root) instead of the password of the invoking user. This flag is off by default.
set_home
If enabled and sudo is invoked with the ‑s option the HOME environment variable will be set to the home directory of the target user (which is root unless the ‑u option is used). This effectively makes the ‑s option imply ‑H. Note that HOME is already set when the the env_reset option is enabled, so set_home is only effective for configurations where either env_reset is disabled or HOME is present in the env_keep list. This flag is off by default.
set_logname
Normally, sudo will set the LOGNAME, USER and USERNAME environment variables to the name of the target user (usually root unless the ‑u option is given). However, since some programs (including the RCS revision control system) use LOGNAME to determine the real identity of the user, it may be desirable to change this behavior. This can be done by negating the set_logname option. Note that if the env_reset option has not been disabled, entries in the env_keep list will override the value of set_logname. This flag is on by default.
set_utmp
When enabled, sudo will create an entry in the utmp (or utmpx) file when a pseudo-tty is allocated. A pseudo-tty is allocated by sudo when the log_input, log_output or use_pty flags are enabled. By default, the new entry will be a copy of the user's existing utmp entry (if any), with the tty, time, type and pid fields updated. This flag is on by default.
setenv
Allow the user to disable the env_reset option from the command line via the ‑E option. Additionally, environment variables set via the command line are not subject to the restrictions imposed by env_check, env_delete, or env_keep. As such, only trusted users should be allowed to set variables in this manner. This flag is off by default.
shell_noargs
If set and sudo is invoked with no arguments it acts as if the ‑s option had been given. That is, it runs a shell as root (the shell is determined by the SHELL environment variable if it is set, falling back on the shell listed in the invoking user's /etc/passwd entry if not). This flag is off by default.
stay_setuid
Normally, when sudo executes a command the real and effective UIDs are set to the target user (root by default). This option changes that behavior such that the real UID is left as the invoking user's UID. In other words, this makes sudo act as a setuid wrapper. This can be useful on systems that disable some potentially dangerous functionality when a program is run setuid. This option is only effective on systems that support either the setreuid(2) or setresuid(2) system call. This flag is off by default.
targetpw
If set, sudo will prompt for the password of the user specified by the ‑u option (defaults to root) instead of the password of the invoking user. In addition, the time stamp file name will include the target user's name. Note that this flag precludes the use of a uid not listed in the passwd database as an argument to the ‑u option. This flag is off by default.
tty_tickets
If set, users must authenticate on a per-tty basis. With this flag enabled, sudo will use a file named for the tty the user is logged in on in the user's time stamp directory. If disabled, the time stamp of the directory is used instead. This flag is on by default.
umask_override
If set, sudo will set the umask as specified by sudoers without modification. This makes it possible to specify a more permissive umask in sudoers than the user's own umask and matches historical behavior. If umask_override is not set, sudo will set the umask to be the union of the user's umask and what is specified in sudoers. This flag is off by default.
use_loginclass
If set, sudo will apply the defaults specified for the target user's login class if one exists. Only available if sudo is configured with the --with-logincap option. This flag is off by default.
use_pty
If set, sudo will run the command in a pseudo-pty even if no I/O logging is being gone. A malicious program run under sudo could conceivably fork a background process that retains to the user's terminal device after the main program has finished executing. Use of this option will make that impossible. This flag is off by default.
utmp_runas
If set, sudo will store the name of the runas user when updating the utmp (or utmpx) file. By default, sudo stores the name of the invoking user. This flag is off by default.
visiblepw
By default, sudo will refuse to run if the user must enter a password but it is not possible to disable echo on the terminal. If the visiblepw flag is set, sudo will prompt for a password even when it would be visible on the screen. This makes it possible to run things like “ssh somehost sudo ls” since by default, ssh(1) does not allocate a tty when running a command. This flag is off by default.

Integers:

closefrom
Before it executes a command, sudo will close all open file descriptors other than standard input, standard output and standard error (ie: file descriptors 0-2). The closefrom option can be used to specify a different file descriptor at which to start closing. The default is 3.
passwd_tries
The number of tries a user gets to enter his/her password before sudo logs the failure and exits. The default is 3.

Integers that can be used in a boolean context:

loglinelen
Number of characters per line for the file log. This value is used to decide when to wrap lines for nicer log files. This has no effect on the syslog log file, only the file log. The default is 80 (use 0 or negate the option to disable word wrap).
passwd_timeout
Number of minutes before the sudo password prompt times out, or 0 for no timeout. The timeout may include a fractional component if minute granularity is insufficient, for example 2.5. The default is 5.
timestamp_timeout
Number of minutes that can elapse before sudo will ask for a passwd again. The timeout may include a fractional component if minute granularity is insufficient, for example 2.5. The default is 5. Set this to 0 to always prompt for a password. If set to a value less than 0 the user's time stamp will never expire. This can be used to allow users to create or delete their own time stamps via “sudo -v” and “sudo -k” respectively.
umask
Umask to use when running the command. Negate this option or set it to 0777 to preserve the user's umask. The actual umask that is used will be the union of the user's umask and the value of the umask option, which defaults to 0022. This guarantees that sudo never lowers the umask when running a command. Note: on systems that use PAM, the default PAM configuration may specify its own umask which will override the value set in sudoers.

Strings:

badpass_message
Message that is displayed if a user enters an incorrect password. The default is Sorry, try again. unless insults are enabled.
editor
A colon (‘:’) separated list of editors allowed to be used with visudo. visudo will choose the editor that matches the user's EDITOR environment variable if possible, or the first editor in the list that exists and is executable. The default is vi.
iolog_dir
The top-level directory to use when constructing the path name for the input/output log directory. Only used if the log_input or log_output options are enabled or when the LOG_INPUT or LOG_OUTPUT tags are present for a command. The session sequence number, if any, is stored in the directory. The default is /var/log/sudo-io.

The following percent (‘%’) escape sequences are supported:

%{seq}
expanded to a monotonically increasing base-36 sequence number, such as 0100A5, where every two digits are used to form a new directory, e.g. 01/00/A5
%{user}
expanded to the invoking user's login name
%{group}
expanded to the name of the invoking user's real group ID
%{runas_user}
expanded to the login name of the user the command will be run as (e.g. root)
%{runas_group}
expanded to the group name of the user the command will be run as (e.g. wheel)
%{hostname}
expanded to the local host name without the domain name
%{command}
expanded to the base name of the command being run

In addition, any escape sequences supported by the system's strftime(3) function will be expanded.

To include a literal ‘%’ character, the string ‘%%’ should be used.

iolog_file
The path name, relative to iolog_dir, in which to store input/output logs when the log_input or log_output options are enabled or when the LOG_INPUT or LOG_OUTPUT tags are present for a command. Note that iolog_file may contain directory components. The default is “%{seq}”.

See the iolog_dir option above for a list of supported percent (‘%’) escape sequences.

In addition to the escape sequences, path names that end in six or more Xs will have the Xs replaced with a unique combination of digits and letters, similar to the mktemp(3) function.

limitprivs
The default Solaris limit privileges to use when constructing a new privilege set for a command. This bounds all privileges of the executing process. The default limit privileges may be overridden on a per-command basis in sudoers. This option is only available if sudoers is built on Solaris 10 or higher.
mailsub
Subject of the mail sent to the mailto user. The escape %h will expand to the host name of the machine. Default is “*** SECURITY information for %h ***”.
noexec_file
This option is no longer supported. The path to the noexec file should now be set in the /etc/sudo.conf file.
passprompt
The default prompt to use when asking for a password; can be overridden via the ‑p option or the SUDO_PROMPT environment variable. The following percent (‘%’) escape sequences are supported:
%H
expanded to the local host name including the domain name (only if the machine's host name is fully qualified or the fqdn option is set)
%h
expanded to the local host name without the domain name
%p
expanded to the user whose password is being asked for (respects the rootpw, targetpw and runaspw flags in sudoers)
%U
expanded to the login name of the user the command will be run as (defaults to root)
%u
expanded to the invoking user's login name
%%
two consecutive % characters are collapsed into a single % character

The default value is “Password:”.

privs
The default Solaris privileges to use when constructing a new privilege set for a command. This is passed to the executing process via the inherited privilege set, but is bounded by the limit privileges. If the privs option is specified but the limitprivs option is not, the limit privileges of the executing process is set to privs. The default privileges may be overridden on a per-command basis in sudoers. This option is only available if sudoers is built on Solaris 10 or higher.
role
The default SELinux role to use when constructing a new security context to run the command. The default role may be overridden on a per-command basis in sudoers or via command line options. This option is only available when sudo is built with SELinux support.
runas_default
The default user to run commands as if the ‑u option is not specified on the command line. This defaults to root.
syslog_badpri
Syslog priority to use when user authenticates unsuccessfully. Defaults to alert.

The following syslog priorities are supported: alert, crit, debug, emerg, err, info, notice, and warning.

syslog_goodpri
Syslog priority to use when user authenticates successfully. Defaults to notice.

See syslog_badpri for the list of supported syslog priorities.

sudoers_locale
Locale to use when parsing the sudoers file, logging commands, and sending email. Note that changing the locale may affect how sudoers is interpreted. Defaults to “C”.
timestampdir
The directory in which sudo stores its time stamp files. The default is /var/adm/sudo.
timestampowner
The owner of the time stamp directory and the time stamps stored therein. The default is root.
type
The default SELinux type to use when constructing a new security context to run the command. The default type may be overridden on a per-command basis in sudoers or via command line options. This option is only available when sudo is built with SELinux support.

Strings that can be used in a boolean context:

env_file
The env_file option specifies the fully qualified path to a file containing variables to be set in the environment of the program being run. Entries in this file should either be of the form “VARIABLE=value” or “export VARIABLE=value”. The value may optionally be surrounded by single or double quotes. Variables in this file are subject to other sudo environment settings such as env_keep and env_check.
exempt_group
Users in this group are exempt from password and PATH requirements. The group name specified should not include a % prefix. This is not set by default.
group_plugin
A string containing a sudoers group plugin with optional arguments. This can be used to implement support for the nonunix_group syntax described earlier. The string should consist of the plugin path, either fully-qualified or relative to the /usr/local/libexec directory, followed by any configuration arguments the plugin requires. These arguments (if any) will be passed to the plugin's initialization function. If arguments are present, the string must be enclosed in double quotes ("").

For example, given /etc/sudo-group, a group file in Unix group format, the sample group plugin can be used:

Defaults group_plugin="sample_group.so /etc/sudo-group"

For more information see sudo_plugin(5).

lecture
This option controls when a short lecture will be printed along with the password prompt. It has the following possible values:
always
Always lecture the user.
never
Never lecture the user.
once
Only lecture the user the first time they run sudo.

If no value is specified, a value of once is implied. Negating the option results in a value of never being used. The default value is once.

lecture_file
Path to a file containing an alternate sudo lecture that will be used in place of the standard lecture if the named file exists. By default, sudo uses a built-in lecture.
listpw
This option controls when a password will be required when a user runs sudo with the ‑l option. It has the following possible values:
all
All the user's sudoers entries for the current host must have the NOPASSWD flag set to avoid entering a password.
always
The user must always enter a password to use the ‑l option.
any
At least one of the user's sudoers entries for the current host must have the NOPASSWD flag set to avoid entering a password.
never
The user need never enter a password to use the ‑l option.

If no value is specified, a value of any is implied. Negating the option results in a value of never being used. The default value is any.

logfile
Path to the sudo log file (not the syslog log file). Setting a path turns on logging to a file; negating this option turns it off. By default, sudo logs via syslog.
mailerflags
Flags to use when invoking mailer. Defaults to ‑t.
mailerpath
Path to mail program used to send warning mail. Defaults to the path to sendmail found at configure time.
mailfrom
Address to use for the “from” address when sending warning and error mail. The address should be enclosed in double quotes ("") to protect against sudo interpreting the @ sign. Defaults to the name of the user running sudo.
mailto
Address to send warning and error mail to. The address should be enclosed in double quotes ("") to protect against sudo interpreting the @ sign. Defaults to root.
secure_path
Path used for every command run from sudo. If you don't trust the people running sudo to have a sane PATH environment variable you may want to use this. Another use is if you want to have the “root path” be separate from the “user path”. Users in the group specified by the exempt_group option are not affected by secure_path. This option is not set by default.
syslog
Syslog facility if syslog is being used for logging (negate to disable syslog logging). Defaults to auth.

The following syslog facilities are supported: authpriv (if your OS supports it), auth, daemon, user, local0, local1, local2, local3, local4, local5, local6, and local7.

verifypw
This option controls when a password will be required when a user runs sudo with the ‑v option. It has the following possible values:
all
All the user's sudoers entries for the current host must have the NOPASSWD flag set to avoid entering a password.
always
The user must always enter a password to use the ‑v option.
any
At least one of the user's sudoers entries for the current host must have the NOPASSWD flag set to avoid entering a password.
never
The user need never enter a password to use the ‑v option.

If no value is specified, a value of all is implied. Negating the option results in a value of never being used. The default value is all.

Lists that can be used in a boolean context:

env_check
Environment variables to be removed from the user's environment if the variable's value contains ‘%’ or ‘/’ characters. This can be used to guard against printf-style format vulnerabilities in poorly-written programs. The argument may be a double-quoted, space-separated list or a single value without double-quotes. The list can be replaced, added to, deleted from, or disabled by using the =, +=, -=, and ! operators respectively. Regardless of whether the env_reset option is enabled or disabled, variables specified by env_check will be preserved in the environment if they pass the aforementioned check. The default list of environment variables to check is displayed when sudo is run by root with the ‑V option.
env_delete
Environment variables to be removed from the user's environment when the env_reset option is not in effect. The argument may be a double-quoted, space-separated list or a single value without double-quotes. The list can be replaced, added to, deleted from, or disabled by using the =, +=, -=, and ! operators respectively. The default list of environment variables to remove is displayed when sudo is run by root with the ‑V option. Note that many operating systems will remove potentially dangerous variables from the environment of any setuid process (such as sudo).
env_keep
Environment variables to be preserved in the user's environment when the env_reset option is in effect. This allows fine-grained control over the environment sudo-spawned processes will receive. The argument may be a double-quoted, space-separated list or a single value without double-quotes. The list can be replaced, added to, deleted from, or disabled by using the =, +=, -=, and ! operators respectively. The default list of variables to keep is displayed when sudo is run by root with the ‑V option.

LOG FORMAT

sudoers 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:

date
The date the command was run. Typically, this is in the format “MMM, DD, HH:MM:SS”. If logging via syslog(3), the actual date format is controlled by the syslog daemon. If logging to a file and the log_year option is enabled, the date will also include the year.
hostname
The name of the host sudo was run on. This field is only present when logging via syslog(3).
progname
The name of the program, usually sudo or sudoedit. This field is only present when logging via syslog(3).
username
The login name of the user who ran sudo.
ttyname
The short name of the terminal (e.g. “console”, “tty01”, or “pts/0”) sudo was run on, or “unknown” if there was no terminal present.
cwd
The current working directory that sudo was run in.
runasuser
The user the command was run as.
runasgroup
The group the command was run as if one was specified on the command line.
logid
An I/O log identifier that can be used to replay the command's output. This is only present when the log_input or log_output option is enabled.
env_vars
A list of environment variables specified on the command line, if specified.
command
The actual command that was executed.

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 option.
a password is required
sudo's ‑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, sudoers will log a message and, in most cases, send a message to the administrator via email. Possible errors include:
parse error in /etc/sudoers near line N
sudoers 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 /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, sudoers tries to open sudoers using group permissions to avoid this problem. Consider changing the ownership of /etc/sudoers by adding an option like “sudoers_uid=N” (where ‘N’ is the user ID that owns the sudoers file) to the sudoers plugin line in the /etc/sudo.conf file.
unable to stat /etc/sudoers
The /etc/sudoers file is missing.
/etc/sudoers is not a regular file
The /etc/sudoers file exists but is not a regular file or symbolic link.
/etc/sudoers is owned by uid N, should be 0
The sudoers file has the wrong owner. If you wish to change the sudoers file owner, please add “sudoers_uid=N” (where ‘N’ is the user ID that owns the sudoers file) to the sudoers plugin line in the /etc/sudo.conf file.
/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). The default mode may be changed via the “sudoers_mode” option to the sudoers plugin line in the /etc/sudo.conf file.
/etc/sudoers is owned by gid N, should be 1
The sudoers file has the wrong group ownership. If you wish to change the sudoers file group ownership, please add “sudoers_gid=N” (where ‘N’ is the group ID that owns the sudoers file) to the sudoers plugin line in the /etc/sudo.conf file.
unable to open /var/adm/sudo/username/ttyname
sudoers was unable to read or create the user's time stamp file.
unable to write to /var/adm/sudo/username/ttyname
sudoers was unable to write to the user's time stamp file.
unable to mkdir to /var/adm/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, sudoers 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:
  1. The progname and hostname fields are not present.
  2. If the log_year option is enabled, the date will also include the year.
  3. Lines that are longer than loglinelen characters (80 by default) are word-wrapped and continued on the next line with a four character indent. This makes entries easier to read for a human being, but makes it more difficult to use grep(1) on the log files. If the loglinelen option is set to 0 (or negated with a ‘!’), word wrap will be disabled.

SUDO.CONF

The /etc/sudo.conf file determines which plugins the sudo front end will load. If no /etc/sudo.conf file is present, or it contains no Plugin lines, sudo will use the sudoers security policy and I/O logging, which corresponds to the following /etc/sudo.conf file.

#
# Default /etc/sudo.conf file
#
# Format:
#   Plugin plugin_name plugin_path plugin_options ...
#   Path askpass /path/to/askpass
#   Path noexec /path/to/sudo_noexec.so
#   Debug sudo /var/log/sudo_debug all@warn
#   Set disable_coredump true
#
# The plugin_path is relative to /usr/local/libexec unless
#   fully qualified.
# The plugin_name corresponds to a global symbol in the plugin
#   that contains the plugin interface structure.
# The plugin_options are optional.
#
Plugin sudoers_policy sudoers.so
Plugin sudoers_io sudoers.so

Plugin options

Starting with sudo 1.8.5, it is possible to pass options to the sudoers plugin. Options may be listed after the path to the plugin (i.e. after sudoers.so); multiple options should be space-separated. For example:

Plugin sudoers_policy sudoers.so sudoers_file=/etc/sudoers sudoers_uid=0 sudoers_gid=0 sudoers_mode=0440

The following plugin options are supported:

sudoers_file=pathname
The sudoers_file option can be used to override the default path to the sudoers file.
sudoers_uid=uid
The sudoers_uid option can be used to override the default owner of the sudoers file. It should be specified as a numeric user ID.
sudoers_gid=gid
The sudoers_gid option can be used to override the default group of the sudoers file. It should be specified as a numeric group ID.
sudoers_mode=mode
The sudoers_mode option can be used to override the default file mode for the sudoers file. It should be specified as an octal value.

Debug flags

Versions 1.8.4 and higher of the sudoers plugin supports a debugging framework that can help track down what the plugin is doing internally if there is a problem. This can be configured in the /etc/sudo.conf file as described in sudo(8).

The sudoers plugin uses the same debug flag format as the sudo front-end: subsystem@priority.

The priorities used by sudoers, in order of decreasing severity, are: crit, err, warn, notice, diag, info, trace and debug. Each priority, when specified, also includes all priorities higher than it. For example, a priority of notice would include debug messages logged at notice and higher.

The following subsystems are used by sudoers:

alias
User_Alias, Runas_Alias, Host_Alias and Cmnd_Alias processing
all
matches every subsystem
audit
BSM and Linux audit code
auth
user authentication
defaults
sudoers Defaults settings
env
environment handling
ldap
LDAP-based sudoers
logging
logging support
match
matching of users, groups, hosts and netgroups in sudoers
netif
network interface handling
nss
network service switch handling in sudoers
parser
sudoers file parsing
perms
permission setting
plugin
The equivalent of main for the plugin.
pty
pseudo-tty related code
rbtree
redblack tree internals
util
utility functions

FILES

/etc/sudo.conf
Sudo front end configuration
/etc/sudoers
List of who can run what
/etc/group
Local groups file
/etc/netgroup
List of network groups
/var/log/sudo-io
I/O log files
/var/adm/sudo
Directory containing time stamps for the sudoers security policy
/etc/environment
Initial environment for ‑i mode on AIX and Linux systems

EXAMPLES

Below are example sudoers entries. Admittedly, some of these are a bit contrived. First, we allow a few environment variables to pass and then define our aliases:

# Run X applications through sudo; HOME is used to find the
# .Xauthority file.  Note that other programs use HOME to find
# configuration files and this may lead to privilege escalation!
Defaults env_keep += "DISPLAY HOME"
 
# User alias specification
User_Alias	FULLTIMERS = millert, mikef, dowdy
User_Alias	PARTTIMERS = bostley, jwfox, crawl
User_Alias	WEBMASTERS = will, wendy, wim
 
# Runas alias specification
Runas_Alias	OP = root, operator
Runas_Alias	DB = oracle, sybase
Runas_Alias	ADMINGRP = adm, oper
 
# Host alias specification
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 specification
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
Cmnd_Alias	PAGERS = /usr/bin/more, /usr/bin/pg, /usr/bin/less

Here we override some of the compiled in default values. We want sudo to log via syslog(3) 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, USER or USERNAME 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. Lastly, we disable shell escapes for the commands in the PAGERS Cmnd_Alias (/usr/bin/more, /usr/bin/pg and /usr/bin/less).

# Override built-in defaults
Defaults		syslog=auth
Defaults>root		!set_logname
Defaults:FULLTIMERS	!lecture
Defaults:millert	!authenticate
Defaults@SERVERS	log_year, logfile=/var/log/sudo.log
Defaults!PAGERS		noexec

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-Za-z]*, !/usr/bin/passwd root
 
%opers		ALL = (: ADMINGRP) /usr/sbin/

Users in the opers group may run commands in /usr/sbin/ as themselves with any group in the ADMINGRP Runas_Alias (the adm and oper groups).

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 user names 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 specify any options to the su(1) command.

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.

SECURITY NOTES

Limitations of the ‘!’ operator

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).

In general, if a user has sudo ALL there is nothing to prevent them from creating their own program that gives them a root shell (or making their own copy of a shell) regardless of any ‘!’ elements in the user specification.

Security implications of fast_glob

If the fast_glob option is in use, it is not possible to reliably negate commands where the path name includes globbing (aka wildcard) characters. This is because the C library's fnmatch(3) function cannot resolve relative paths. While this is typically only an inconvenience for rules that grant privileges, it can result in a security issue for rules that subtract or revoke privileges.

For example, given the following sudoers entry:

john	ALL = /usr/bin/passwd [a-zA-Z0-9]*, /usr/bin/chsh [a-zA-Z0-9]*,\
              /usr/bin/chfn [a-zA-Z0-9]*, !/usr/bin/* root

User john can still run /usr/bin/passwd root if fast_glob is enabled by changing to /usr/bin and running ./passwd root instead.

Preventing shell escapes

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 access control and logging. Common programs that permit shell escapes include shells (obviously), editors, paginators, mail and terminal programs.

There are two basic approaches to this problem:

restrict
Avoid giving users access to commands that allow the user to run arbitrary commands. Many editors have a restricted mode where shell escapes are disabled, though sudoedit is a better solution to running editors via sudo. Due to the large number of programs that offer shell escapes, restricting users to the set of programs that do not is often unworkable.
noexec
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.

The noexec feature is known to work on SunOS, Solaris, *BSD, Linux, IRIX, Tru64 UNIX, MacOS X, HP-UX 11.x and AIX 5.3 and above. It should be supported 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.

On Solaris 10 and higher, noexec uses Solaris privileges instead of the LD_PRELOAD environment variable.

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 check whether shell escapes work when noexec is enabled.

Note that restricting 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.

Time stamp file checks

sudoers will check the ownership of its time stamp directory (/var/adm/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 sudoers 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.

sudoers 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, sudoers 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 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), sudoers 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.

CAVEATS

The sudoers file should always be edited by the visudo command which locks the file and does grammatical checking. It is imperative that sudoers be free of syntax errors since sudo will not run with a syntactically incorrect sudoers file.

When using netgroups of machines (as opposed to users), if you store fully qualified host name in the netgroup (as is usually the case), you either need to have the machine's host name be fully qualified as returned by the hostname command or use the fqdn option in sudoers.

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.