Policy plugin API¶

A policy plugin must declare and populate a policy_plugin struct in the global scope. This structure contains pointers to the functions that implement the sudo policy checks. The name of the symbol should be specified in sudo.conf(5) along with a path to the plugin so that sudo can load it.

struct policy_plugin {
#define SUDO_POLICY_PLUGIN     1
    unsigned int type; /* always SUDO_POLICY_PLUGIN */
    unsigned int version; /* always SUDO_API_VERSION */
    int (*open)(unsigned int version, sudo_conv_t conversation,
                sudo_printf_t plugin_printf, char * const settings[],
                char * const user_info[], char * const user_env[],
                char * const plugin_options[]);
    void (*close)(int exit_status, int error);
    int (*show_version)(int verbose);
    int (*check_policy)(int argc, char * const argv[],
                        char *env_add[], char **command_info[],
                        char **argv_out[], char **user_env_out[]);
    int (*list)(int argc, char * const argv[], int verbose,
                const char *list_user);
    int (*validate)(void);
    void (*invalidate)(int remove);
    int (*init_session)(struct passwd *pwd, char **user_env[]);
    void (*register_hooks)(int version,
       int (*register_hook)(struct sudo_hook *hook));
    void (*deregister_hooks)(int version,
       int (*deregister_hook)(struct sudo_hook *hook));
};

The policy_plugin struct has the following fields:

type

The type field should always be set to SUDO_POLICY_PLUGIN.

version

The version field should be set to SUDO_API_VERSION.

This allows sudo to determine the API version the plugin was built against.

open

int (*open)(unsigned int version, sudo_conv_t conversation,
            sudo_printf_t plugin_printf, char * const settings[],
            char * const user_info[], char * const user_env[],
            char * const plugin_options[]);

Returns 1 on success, 0 on failure, -1 if a general error occurred, or -2 if there was a usage error. In the latter case, sudo will print a usage message before it exits. If an error occurs, the plugin may optionally call the conversation() or plugin_printf() function with SUDO_CONF_ERROR_MSG to present additional error information to the user.

The function arguments are as follows:

version

The version passed in by sudo allows the plugin to determine the major and minor version number of the plugin API supported by sudo.

conversation

A pointer to the conversation() function that can be used by the plugin to interact with the user (see below). Returns 0 on success and -1 on failure.

plugin_printf

A pointer to a printf()-style function that may be used to display informational or error messages (see below). Returns the number of characters printed on success and -1 on failure.

settings

A vector of user-supplied sudo settings in the form of “name=value” strings. The vector is terminated by a NULL pointer. These settings correspond to flags the user specified when running sudo. As such, they will only be present when the corresponding flag has been specified on the command line.

When parsing settings, the plugin should split on the first equal sign (‘=’) since the name field will never include one itself but the value might.

bsdauth_type=string

Authentication type, if specified by the -a flag, to use on systems where BSD authentication is supported.

closefrom=number

If specified, the user has requested via the -C flag that sudo close all files descriptors with a value of number or higher. The plugin may optionally pass this, or another value, back in the command_info list.

debug_flags=string

A debug file path name followed by a space and a comma-separated list of debug flags that correspond to the plugin's Debug entry in sudo.conf(5), if there is one. The flags are passed to the plugin exactly as they appear in sudo.conf(5). The syntax used by sudo and the sudoers plugin is subsystem@priority but a plugin is free to use a different format so long as it does not include a comma (‘,’). Prior to sudo 1.8.12, there was no way to specify plugin-specific debug_flags so the value was always the same as that used by the sudo front end and did not include a path name, only the flags themselves. As of version 1.7 of the plugin interface, sudo will only pass debug_flags if sudo.conf(5) contains a plugin-specific Debug entry.

debug_level=number

This setting has been deprecated in favor of debug_flags.

ignore_ticket=bool

Set to true if the user specified the -k flag along with a command, indicating that the user wishes to ignore any cached authentication credentials. implied_shell to true. This allows sudo with no arguments to be used similarly to su(1). If the plugin does not to support this usage, it may return a value of -2 from the check_policy() function, which will cause sudo to print a usage message and exit.

implied_shell=bool

If the user does not specify a program on the command line, sudo will pass the plugin the path to the user's shell and set

login_class=string

BSD login class to use when setting resource limits and nice value, if specified by the -c flag.

login_shell=bool

Set to true if the user specified the -i flag, indicating that the user wishes to run a login shell.

max_groups=int

The maximum number of groups a user may belong to. This will only be present if there is a corresponding setting in sudo.conf(5).

network_addrs=list

A space-separated list of IP network addresses and netmasks in the form “addr/netmask”, e.g. “192.168.1.2/255.255.255.0”. The address and netmask pairs may be either IPv4 or IPv6, depending on what the operating system supports. If the address contains a colon (‘:’), it is an IPv6 address, else it is IPv4.

noninteractive=bool

Set to true if the user specified the -n flag, indicating that sudo should operate in non-interactive mode. The plugin may reject a command run in non-interactive mode if user interaction is required.

plugin_dir=string

The default plugin directory used by the sudo front end. This is the default directory set at compile time and may not correspond to the directory the running plugin was loaded from. It may be used by a plugin to locate support files.

plugin_path=string

The path name of plugin loaded by the sudo front end. The path name will be a fully-qualified unless the plugin was statically compiled into sudo.

preserve_environment=bool

Set to true if the user specified the -E flag, indicating that the user wishes to preserve the environment.

preserve_groups=bool

Set to true if the user specified the -P flag, indicating that the user wishes to preserve the group vector instead of setting it based on the runas user.

progname=string

The command name that sudo was run as, typically “sudo” or “sudoedit”.

prompt=string

The prompt to use when requesting a password, if specified via the -p flag.

remote_host=string

The name of the remote host to run the command on, if specified via the -h option. Support for running the command on a remote host is meant to be implemented via a helper program that is executed in place of the user-specified command. The sudo front end is only capable of executing commands on the local host. Only available starting with API version 1.4.

run_shell=bool

Set to true if the user specified the -s flag, indicating that the user wishes to run a shell.

runas_group=string

The group name or gid to run the command as, if specified via the -g flag.

runas_user=string

The user name or uid to run the command as, if specified via the -u flag.

selinux_role=string

SELinux role to use when executing the command, if specified by the -r flag.

selinux_type=string

SELinux type to use when executing the command, if specified by the -t flag.

set_home=bool

Set to true if the user specified the -H flag. If true, set the HOME environment variable to the target user's home directory.

sudoedit=bool

Set to true when the -e flag is is specified or if invoked as sudoedit. The plugin shall substitute an editor into argv in the check_policy() function or return -2 with a usage error if the plugin does not support sudoedit. For more information, see the check_policy section.

timeout=string

User-specified command timeout. Not all plugins support command timeouts and the ability for the user to set a timeout may be restricted by policy. The format of the timeout string is plugin-specific.

Additional settings may be added in the future so the plugin should silently ignore settings that it does not recognize.

user_info

A vector of information about the user running the command in the form of “name=value” strings. The vector is terminated by a NULL pointer.

When parsing user_info, the plugin should split on the first equal sign (‘=’) since the name field will never include one itself but the value might.

cols=int

The number of columns the user's terminal supports. If there is no terminal device available, a default value of 80 is used.

cwd=string

The user's current working directory.

egid=gid_t

The effective group ID of the user invoking sudo.

euid=uid_t

The effective user ID of the user invoking sudo.

gid=gid_t

The real group ID of the user invoking sudo.

groups=list

The user's supplementary group list formatted as a string of comma-separated group IDs.

host=string

The local machine's hostname as returned by the gethostname(2) system call.

lines=int

The number of lines the user's terminal supports. If there is no terminal device available, a default value of 24 is used.

pgid=int

The ID of the process group that the running sudo process is a member of. Only available starting with API version 1.2.

pid=int

The process ID of the running sudo process. Only available starting with API version 1.2.

plugin_options

Any (non-comment) strings immediately after the plugin path are passed as arguments to the plugin. These arguments are split on a white space boundary and are passed to the plugin in the form of a NULL-terminated array of strings. If no arguments were specified, plugin_options will be the NULL pointer.

NOTE: the plugin_options parameter is only available starting with API version 1.2. A plugin must check the API version specified by the sudo front end before using plugin_options. Failure to do so may result in a crash.

ppid=int

The parent process ID of the running sudo process. Only available starting with API version 1.2.

sid=int

The session ID of the running sudo process or 0 if sudo is not part of a POSIX job control session. Only available starting with API version 1.2.

tcpgid=int

The ID of the foreground process group associated with the terminal device associated with the sudo process or -1 if there is no terminal present. Only available starting with API version 1.2.

tty=string

The path to the user's terminal device. If the user has no terminal device associated with the session, the value will be empty, as in “tty=”.

uid=uid_t

The real user ID of the user invoking sudo.

umask=octal

The invoking user's file creation mask. Only available starting with API version 1.10.

user=string

The name of the user invoking sudo.

user_env

The user's environment in the form of a NULL-terminated vector of “name=value” strings.

When parsing user_env, the plugin should split on the first equal sign (‘=’) since the name field will never include one itself but the value might.

close

void (*close)(int exit_status, int error);

The close() function is called when the command being run by sudo finishes.

The function arguments are as follows:

exit_status

The command's exit status, as returned by the wait(2) system call. The value of exit_status is undefined if error is non-zero.

error

If the command could not be executed, this is set to the value of errno set by the execve(2) system call. The plugin is responsible for displaying error information via the conversation() or plugin_printf() function. If the command was successfully executed, the value of error is 0.

If no close() function is defined, no I/O logging plugins are loaded, and neither the timeout not use_pty options are set in the command_info list, the sudo front end may execute the command directly instead of running it as a child process.

show_version

int (*show_version)(int verbose);

The show_version() function is called by sudo when the user specifies the -V option. The plugin may display its version information to the user via the conversation() or plugin_printf() function using SUDO_CONV_INFO_MSG. If the user requests detailed version information, the verbose flag will be set.

check_policy

int (*check_policy)(int argc, char * const argv[]
                    char *env_add[], char **command_info[],
                    char **argv_out[], char **user_env_out[]);

The check_policy() function is called by sudo to determine whether the user is allowed to run the specified commands.

If the sudoedit option was enabled in the settings array passed to the open() function, the user has requested sudoedit mode. sudoedit is a mechanism for editing one or more files where an editor is run with the user's credentials instead of with elevated privileges. sudo achieves this by creating user-writable temporary copies of the files to be edited and then overwriting the originals with the temporary copies after editing is complete. If the plugin supports sudoedit, it should choose the editor to be used, potentially from a variable in the user's environment, such as EDITOR, and include it in argv_out (note that environment variables may include command line flags). The files to be edited should be copied from argv into argv_out, separated from the editor and its arguments by a “--” element. The “--” will be removed by sudo before the editor is executed. The plugin should also set sudoedit=true in the command_info list.

The check_policy() function returns 1 if the command is allowed, 0 if not allowed, -1 for a general error, or -2 for a usage error or if sudoedit was specified but is unsupported by the plugin. In the latter case, sudo will print a usage message before it exits. If an error occurs, the plugin may optionally call the conversation() or plugin_printf() function with SUDO_CONF_ERROR_MSG to present additional error information to the user.

The function arguments are as follows:

argc

The number of elements in argv, not counting the final NULL pointer.

argv

The argument vector describing the command the user wishes to run, in the same form as what would be passed to the execve(2) system call. The vector is terminated by a NULL pointer.

env_add

Additional environment variables specified by the user on the command line in the form of a NULL-terminated vector of “name=value” strings. The plugin may reject the command if one or more variables are not allowed to be set, or it may silently ignore such variables.

When parsing env_add, the plugin should split on the first equal sign (‘=’) since the name field will never include one itself but the value might.

command_info

Information about the command being run in the form of “name=value” strings. These values are used by sudo to set the execution environment when running a command. The plugin is responsible for creating and populating the vector, which must be terminated with a NULL pointer. The following values are recognized by sudo:

chroot=string

The root directory to use when running the command.

closefrom=number

If specified, sudo will close all files descriptors with a value of number or higher.

command=string

Fully qualified path to the command to be executed.

cwd=string

The current working directory to change to when executing the command.

exec_background=bool

By default, sudo runs a command as the foreground process as long as sudo itself is running in the foreground. When exec_background is enabled and the command is being run in a pty (due to I/O logging or the use_pty setting), the command will be run as a background process. Attempts to read from the controlling terminal (or to change terminal settings) will result in the command being suspended with the SIGTTIN signal (or SIGTTOU in the case of terminal settings). If this happens when sudo is a foreground process, the command will be granted the controlling terminal and resumed in the foreground with no user intervention required. The advantage of initially running the command in the background is that sudo need not read from the terminal unless the command explicitly requests it. Otherwise, any terminal input must be passed to the command, whether it has required it or not (the kernel buffers terminals so it is not possible to tell whether the command really wants the input). This is different from historic sudo behavior or when the command is not being run in a pty.

For this to work seamlessly, the operating system must support the automatic restarting of system calls. Unfortunately, not all operating systems do this by default, and even those that do may have bugs. For example, macOS fails to restart the tcgetattr() and tcsetattr() system calls (this is a bug in macOS). Furthermore, because this behavior depends on the command stopping with the SIGTTIN or SIGTTOU signals, programs that catch these signals and suspend themselves with a different signal (usually SIGTOP) will not be automatically foregrounded. Some versions of the linux su(1) command behave this way. Because of this, a plugin should not set exec_background unless it is explicitly enabled by the administrator and there should be a way to enabled or disable it on a per-command basis.

This setting has no effect unless I/O logging is enabled or use_pty is enabled.

execfd=number

If specified, sudo will use the fexecve(2) system call to execute the command instead of execve(2). The specified number must refer to an open file descriptor.

iolog_compress=bool

Set to true if the I/O logging plugins, if any, should compress the log data. This is a hint to the I/O logging plugin which may choose to ignore it.

iolog_group=string

The group that will own newly created I/O log files and directories. This is a hint to the I/O logging plugin which may choose to ignore it.

iolog_mode=octal

The file permission mode to use when creating I/O log files and directories. This is a hint to the I/O logging plugin which may choose to ignore it.

iolog_user=string

The user that will own newly created I/O log files and directories. This is a hint to the I/O logging plugin which may choose to ignore it.

iolog_path=string

Fully qualified path to the file or directory in which I/O log is to be stored. This is a hint to the I/O logging plugin which may choose to ignore it. If no I/O logging plugin is loaded, this setting has no effect.

iolog_stdin=bool

Set to true if the I/O logging plugins, if any, should log the standard input if it is not connected to a terminal device. This is a hint to the I/O logging plugin which may choose to ignore it.

iolog_stdout=bool

Set to true if the I/O logging plugins, if any, should log the standard output if it is not connected to a terminal device. This is a hint to the I/O logging plugin which may choose to ignore it.

iolog_stderr=bool

Set to true if the I/O logging plugins, if any, should log the standard error if it is not connected to a terminal device. This is a hint to the I/O logging plugin which may choose to ignore it.

iolog_ttyin=bool

Set to true if the I/O logging plugins, if any, should log all terminal input. This only includes input typed by the user and not from a pipe or redirected from a file. This is a hint to the I/O logging plugin which may choose to ignore it.

iolog_ttyout=bool

Set to true if the I/O logging plugins, if any, should log all terminal output. This only includes output to the screen, not output to a pipe or file. This is a hint to the I/O logging plugin which may choose to ignore it.

login_class=string

BSD login class to use when setting resource limits and nice value (optional). This option is only set on systems that support login classes.

nice=int

Nice value (priority) to use when executing the command. The nice value, if specified, overrides the priority associated with the login_class on BSD systems.

noexec=bool

If set, prevent the command from executing other programs.

preserve_fds=list

A comma-separated list of file descriptors that should be preserved, regardless of the value of the closefrom setting. Only available starting with API version 1.5.

preserve_groups=bool

If set, sudo will preserve the user's group vector instead of initializing the group vector based on runas_user.

runas_egid=gid

Effective group ID to run the command as. If not specified, the value of runas_gid is used.

runas_euid=uid

Effective user ID to run the command as. If not specified, the value of runas_uid is used.

runas_gid=gid

Group ID to run the command as.

runas_groups=list

The supplementary group vector to use for the command in the form of a comma-separated list of group IDs. If preserve_groups is set, this option is ignored.

runas_uid=uid

User ID to run the command as.

selinux_role=string

SELinux role to use when executing the command.

selinux_type=string

SELinux type to use when executing the command.

set_utmp=bool

Create a utmp (or utmpx) entry when a pseudo-tty is allocated. 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.

sudoedit=bool

Set to true when in sudoedit mode. The plugin may enable sudoedit mode even if sudo was not invoked as sudoedit. This allows the plugin to perform command substitution and transparently enable sudoedit when the user attempts to run an editor.

sudoedit_checkdir=bool

Set to false to disable directory writability checks in sudoedit. By default, sudoedit 1.8.16 and higher will check all directory components of the path to be edited for writability by the invoking user. Symbolic links will not be followed in writable directories and sudoedit will refuse to edit a file located in a writable directory. These restrictions are not enforced when sudoedit is run by root. The sudoedit_follow option can be set to false to disable this check. Only available starting with API version 1.8.

sudoedit_follow=bool

Set to true to allow sudoedit to edit files that are symbolic links. By default, sudoedit 1.8.15 and higher will refuse to open a symbolic link. The sudoedit_follow option can be used to restore the older behavior and allow sudoedit to open symbolic links. Only available starting with API version 1.8.

timeout=int

Command timeout. If non-zero then when the timeout expires the command will be killed.

umask=octal

The file creation mask to use when executing the command.

use_pty=bool

Allocate a pseudo-tty to run the command in, regardless of whether or not I/O logging is in use. By default, sudo will only run the command in a pty when an I/O log plugin is loaded.

utmp_user=string

User name to use when constructing a new utmp (or utmpx) entry when set_utmp is enabled. This option can be used to set the user field in the utmp entry to the user the command runs as rather than the invoking user. If not set, sudo will base the new entry on the invoking user's existing entry.

Unsupported values will be ignored.

argv_out

The NULL-terminated argument vector to pass to the execve(2) system call when executing the command. The plugin is responsible for allocating and populating the vector.

user_env_out

The NULL-terminated environment vector to use when executing the command. The plugin is responsible for allocating and populating the vector.

list

int (*list)(int verbose, const char *list_user,
            int argc, char * const argv[]);

List available privileges for the invoking user. Returns 1 on success, 0 on failure and -1 on error. On error, the plugin may optionally call the conversation() or plugin_printf() function with SUDO_CONF_ERROR_MSG to present additional error information to the user.

Privileges should be output via the conversation() or plugin_printf() function using SUDO_CONV_INFO_MSG,

verbose

Flag indicating whether to list in verbose mode or not.

list_user

The name of a different user to list privileges for if the policy allows it. If NULL, the plugin should list the privileges of the invoking user.

argc

The number of elements in argv, not counting the final NULL pointer.

argv

If non-NULL, an argument vector describing a command the user wishes to check against the policy in the same form as what would be passed to the execve(2) system call. If the command is permitted by the policy, the fully-qualified path to the command should be displayed along with any command line arguments.

validate

int (*validate)(void);

The validate() function is called when sudo is run with the -v flag. For policy plugins such as sudoers that cache authentication credentials, this function will validate and cache the credentials.

The validate() function should be NULL if the plugin does not support credential caching.

Returns 1 on success, 0 on failure and -1 on error. On error, the plugin may optionally call the conversation() or plugin_printf() function with SUDO_CONF_ERROR_MSG to present additional error information to the user.

invalidate

void (*invalidate)(int remove);

The invalidate() function is called when sudo is called with the -k or -K flag. For policy plugins such as sudoers that cache authentication credentials, this function will invalidate the credentials. If the remove flag is set, the plugin may remove the credentials instead of simply invalidating them.

The invalidate() function should be NULL if the plugin does not support credential caching.

init_session

int (*init_session)(struct passwd *pwd, char **user_envp[);

The init_session() function is called before sudo sets up the execution environment for the command. It is run in the parent sudo process and before any uid or gid changes. This can be used to perform session setup that is not supported by command_info, such as opening the PAM session. The close() function can be used to tear down the session that was opened by init_session.

The pwd argument points to a passwd struct for the user the command will be run as if the uid the command will run as was found in the password database, otherwise it will be NULL.

The user_env argument points to the environment the command will run in, in the form of a NULL-terminated vector of “name=value” strings. This is the same string passed back to the front end via the Policy Plugin's user_env_out parameter. If the init_session() function needs to modify the user environment, it should update the pointer stored in user_env. The expected use case is to merge the contents of the PAM environment (if any) with the contents of user_env. NOTE: the user_env parameter is only available starting with API version 1.2. A plugin must check the API version specified by the sudo front end before using user_env. Failure to do so may result in a crash.

Returns 1 on success, 0 on failure and -1 on error. On error, the plugin may optionally call the conversation() or plugin_printf() function with SUDO_CONF_ERROR_MSG to present additional error information to the user.

register_hooks

void (*register_hooks)(int version,
   int (*register_hook)(struct sudo_hook *hook));

The register_hooks() function is called by the sudo front end to register any hooks the plugin needs. If the plugin does not support hooks, register_hooks should be set to the NULL pointer.

The version argument describes the version of the hooks API supported by the sudo front end.

The register_hook() function should be used to register any supported hooks the plugin needs. It returns 0 on success, 1 if the hook type is not supported and -1 if the major version in struct hook does not match the front end's major hook API version.

See the Hook function API section below for more information about hooks.

NOTE: the register_hooks() function is only available starting with API version 1.2. If the sudo front end doesn't support API version 1.2 or higher, register_hooks will not be called.

deregister_hooks

void (*deregister_hooks)(int version,
   int (*deregister_hook)(struct sudo_hook *hook));

The deregister_hooks() function is called by the sudo front end to deregister any hooks the plugin has registered. If the plugin does not support hooks, deregister_hooks should be set to the NULL pointer.

The version argument describes the version of the hooks API supported by the sudo front end.

The deregister_hook() function should be used to deregister any hooks that were put in place by the register_hook() function. If the plugin tries to deregister a hook that the front end does not support, deregister_hook will return an error.

See the Hook function API section below for more information about hooks.

NOTE: the deregister_hooks() function is only available starting with API version 1.2. If the sudo front end doesn't support API version 1.2 or higher, deregister_hooks will not be called.

Policy Plugin Version Macros

/* Plugin API version major/minor. */
#define SUDO_API_VERSION_MAJOR 1
#define SUDO_API_VERSION_MINOR 2
#define SUDO_API_MKVERSION(x, y) ((x << 16) | y)
#define SUDO_API_VERSION SUDO_API_MKVERSION(SUDO_API_VERSION_MAJOR,\
                                            SUDO_API_VERSION_MINOR)

/* Getters and setters for API version */
#define SUDO_API_VERSION_GET_MAJOR(v) ((v) >> 16)
#define SUDO_API_VERSION_GET_MINOR(v) ((v) & 0xffff)
#define SUDO_API_VERSION_SET_MAJOR(vp, n) do { \
    *(vp) = (*(vp) & 0x0000ffff) | ((n) << 16); \
} while(0)
#define SUDO_API_VERSION_SET_MINOR(vp, n) do { \
    *(vp) = (*(vp) & 0xffff0000) | (n); \
} while(0)

I/O plugin API¶

struct io_plugin {
#define SUDO_IO_PLUGIN 2
    unsigned int type; /* always SUDO_IO_PLUGIN */
    unsigned int version; /* always SUDO_API_VERSION */
    int (*open)(unsigned int version, sudo_conv_t conversation,
                sudo_printf_t plugin_printf, char * const settings[],
                char * const user_info[], char * const command_info[],
                int argc, char * const argv[], char * const user_env[],
                char * const plugin_options[]);
    void (*close)(int exit_status, int error); /* wait status or error */
    int (*show_version)(int verbose);
    int (*log_ttyin)(const char *buf, unsigned int len);
    int (*log_ttyout)(const char *buf, unsigned int len);
    int (*log_stdin)(const char *buf, unsigned int len);
    int (*log_stdout)(const char *buf, unsigned int len);
    int (*log_stderr)(const char *buf, unsigned int len);
    void (*register_hooks)(int version,
       int (*register_hook)(struct sudo_hook *hook));
    void (*deregister_hooks)(int version,
       int (*deregister_hook)(struct sudo_hook *hook));
    int (*change_winsize)(unsigned int lines, unsigned int cols);
};

When an I/O plugin is loaded, sudo runs the command in a pseudo-tty. This makes it possible to log the input and output from the user's session. If any of the standard input, standard output or standard error do not correspond to a tty, sudo will open a pipe to capture the I/O for logging before passing it on.

The log_ttyin function receives the raw user input from the terminal device (note that this will include input even when echo is disabled, such as when a password is read). The log_ttyout function receives output from the pseudo-tty that is suitable for replaying the user's session at a later time. The log_stdin(), log_stdout() and log_stderr() functions are only called if the standard input, standard output or standard error respectively correspond to something other than a tty.

Any of the logging functions may be set to the NULL pointer if no logging is to be performed. If the open function returns 0, no I/O will be sent to the plugin.

If a logging function returns an error (-1), the running command will be terminated and all of the plugin's logging functions will be disabled. Other I/O logging plugins will still receive any remaining input or output that has not yet been processed.

If an input logging function rejects the data by returning 0, the command will be terminated and the data will not be passed to the command, though it will still be sent to any other I/O logging plugins. If an output logging function rejects the data by returning 0, the command will be terminated and the data will not be written to the terminal, though it will still be sent to any other I/O logging plugins.

The io_plugin struct has the following fields:

type

The type field should always be set to SUDO_IO_PLUGIN.

version

The version field should be set to SUDO_API_VERSION.

This allows sudo to determine the API version the plugin was built against.

open

int (*open)(unsigned int version, sudo_conv_t conversation,
            sudo_printf_t plugin_printf, char * const settings[],
            char * const user_info[], int argc, char * const argv[],
            char * const user_env[], char * const plugin_options[]);

The open() function is run before the log_ttyin(), log_ttyout(), log_stdin(), log_stdout(), log_stderr(), or show_version() functions are called. It is only called if the version is being requested or if the policy plugin's check_policy() function has returned successfully. It returns 1 on success, 0 on failure, -1 if a general error occurred, or -2 if there was a usage error. In the latter case, sudo will print a usage message before it exits. If an error occurs, the plugin may optionally call the conversation() or plugin_printf() function with SUDO_CONF_ERROR_MSG to present additional error information to the user.

The function arguments are as follows:

version

The version passed in by sudo allows the plugin to determine the major and minor version number of the plugin API supported by sudo.

conversation

A pointer to the conversation() function that may be used by the show_version() function to display version information (see show_version() below). The conversation() function may also be used to display additional error message to the user. The conversation() function returns 0 on success and -1 on failure.

plugin_printf

A pointer to a printf()-style function that may be used by the show_version() function to display version information (see show_version below). The plugin_printf() function may also be used to display additional error message to the user. The plugin_printf() function returns number of characters printed on success and -1 on failure.

settings

A vector of user-supplied sudo settings in the form of “name=value” strings. The vector is terminated by a NULL pointer. These settings correspond to flags the user specified when running sudo. As such, they will only be present when the corresponding flag has been specified on the command line.

When parsing settings, the plugin should split on the first equal sign (‘=’) since the name field will never include one itself but the value might.

See the Policy plugin API section for a list of all possible settings.

user_info

A vector of information about the user running the command in the form of “name=value” strings. The vector is terminated by a NULL pointer.

When parsing user_info, the plugin should split on the first equal sign (‘=’) since the name field will never include one itself but the value might.

See the Policy plugin API section for a list of all possible strings.

argc

The number of elements in argv, not counting the final NULL pointer.

argv

If non-NULL, an argument vector describing a command the user wishes to run in the same form as what would be passed to the execve(2) system call.

user_env

The user's environment in the form of a NULL-terminated vector of “name=value” strings.

When parsing user_env, the plugin should split on the first equal sign (‘=’) since the name field will never include one itself but the value might.

plugin_options

Any (non-comment) strings immediately after the plugin path are treated as arguments to the plugin. These arguments are split on a white space boundary and are passed to the plugin in the form of a NULL-terminated array of strings. If no arguments were specified, plugin_options will be the NULL pointer.

NOTE: the plugin_options parameter is only available starting with API version 1.2. A plugin must check the API version specified by the sudo front end before using plugin_options. Failure to do so may result in a crash.

close

void (*close)(int exit_status, int error);

The close() function is called when the command being run by sudo finishes.

The function arguments are as follows:

exit_status

The command's exit status, as returned by the wait(2) system call. The value of exit_status is undefined if error is non-zero.

error

If the command could not be executed, this is set to the value of errno set by the execve(2) system call. If the command was successfully executed, the value of error is 0.

show_version

int (*show_version)(int verbose);

The show_version() function is called by sudo when the user specifies the -V option. The plugin may display its version information to the user via the conversation() or plugin_printf() function using SUDO_CONV_INFO_MSG. If the user requests detailed version information, the verbose flag will be set.

log_ttyin

int (*log_ttyin)(const char *buf, unsigned int len);

The log_ttyin() function is called whenever data can be read from the user but before it is passed to the running command. This allows the plugin to reject data if it chooses to (for instance if the input contains banned content). Returns 1 if the data should be passed to the command, 0 if the data is rejected (which will terminate the running command) or -1 if an error occurred.

The function arguments are as follows:

buf

The buffer containing user input.

len

The length of buf in bytes.

log_ttyout

int (*log_ttyout)(const char *buf, unsigned int len);

The log_ttyout() function is called whenever data can be read from the command but before it is written to the user's terminal. This allows the plugin to reject data if it chooses to (for instance if the output contains banned content). Returns 1 if the data should be passed to the user, 0 if the data is rejected (which will terminate the running command) or -1 if an error occurred.

The function arguments are as follows:

buf

The buffer containing command output.

len

The length of buf in bytes.

log_stdin

int (*log_stdin)(const char *buf, unsigned int len);

The log_stdin() function is only used if the standard input does not correspond to a tty device. It is called whenever data can be read from the standard input but before it is passed to the running command. This allows the plugin to reject data if it chooses to (for instance if the input contains banned content). Returns 1 if the data should be passed to the command, 0 if the data is rejected (which will terminate the running command) or -1 if an error occurred.

The function arguments are as follows:

buf

The buffer containing user input.

len

The length of buf in bytes.

log_stdout

int (*log_stdout)(const char *buf, unsigned int len);

The log_stdout() function is only used if the standard output does not correspond to a tty device. It is called whenever data can be read from the command but before it is written to the standard output. This allows the plugin to reject data if it chooses to (for instance if the output contains banned content). Returns 1 if the data should be passed to the user, 0 if the data is rejected (which will terminate the running command) or -1 if an error occurred.

The function arguments are as follows:

buf

The buffer containing command output.

len

The length of buf in bytes.

log_stderr

int (*log_stderr)(const char *buf, unsigned int len);

The log_stderr() function is only used if the standard error does not correspond to a tty device. It is called whenever data can be read from the command but before it is written to the standard error. This allows the plugin to reject data if it chooses to (for instance if the output contains banned content). Returns 1 if the data should be passed to the user, 0 if the data is rejected (which will terminate the running command) or -1 if an error occurred.

The function arguments are as follows:

buf

The buffer containing command output.

len

The length of buf in bytes.

register_hooks

See the Policy plugin API section for a description of register_hooks.

deregister_hooks

See the Policy plugin API section for a description of deregister_hooks.

change_winsize

int (*change_winsize)(unsigned int lines, unsigned int cols);

The change_winsize() function is called whenever the window size of the terminal changes from the initial values specified in the user_info list. It returns 1 on success, 0 on failure, -1 if an error occurred (which will terminate the running command).

I/O Plugin Version Macros

Same as for the Policy plugin API.

Signal handlers¶

The sudo front end installs default signal handlers to trap common signals while the plugin functions are run. The following signals are trapped by default before the command is executed:

• SIGALRM
• SIGHUP
• SIGINT
• SIGPIPE
• SIGQUIT
• SIGTERM
• SIGTSTP
• SIGUSR1
• SIGUSR2

If a fatal signal is received before the command is executed, sudo will call the plugin's close() function with an exit status of 128 plus the value of the signal that was received. This allows for consistent logging of commands killed by a signal for plugins that log such information in their close() function. An exception to this is SIGPIPE, which is ignored until the command is executed.

A plugin may temporarily install its own signal handlers but must restore the original handler before the plugin function returns.

Hook function API¶

Beginning with plugin API version 1.2, it is possible to install hooks for certain functions called by the sudo front end.

Currently, the only supported hooks relate to the handling of environment variables. Hooks can be used to intercept attempts to get, set, or remove environment variables so that these changes can be reflected in the version of the environment that is used to execute a command. A future version of the API will support hooking internal sudo front end functions as well.

Hook structure

Hooks in sudo are described by the following structure:

typedef int (*sudo_hook_fn_t)();

struct sudo_hook {
    unsigned int hook_version;
    unsigned int hook_type;
    sudo_hook_fn_t hook_fn;
    void *closure;
};

The sudo_hook structure has the following fields:

hook_version

The hook_version field should be set to SUDO_HOOK_VERSION.

hook_type

The hook_type field may be one of the following supported hook types:

SUDO_HOOK_SETENV

The C library setenv(3) function. Any registered hooks will run before the C library implementation. The hook_fn field should be a function that matches the following typedef:

typedef int (*sudo_hook_fn_setenv_t)(const char *name,
   const char *value, int overwrite, void *closure);

If the registered hook does not match the typedef the results are unspecified.

SUDO_HOOK_UNSETENV

The C library unsetenv(3) function. Any registered hooks will run before the C library implementation. The hook_fn field should be a function that matches the following typedef:

typedef int (*sudo_hook_fn_unsetenv_t)(const char *name,
   void *closure);

SUDO_HOOK_GETENV

The C library getenv(3) function. Any registered hooks will run before the C library implementation. The hook_fn field should be a function that matches the following typedef:

typedef int (*sudo_hook_fn_getenv_t)(const char *name,
   char **value, void *closure);

If the registered hook does not match the typedef the results are unspecified.

SUDO_HOOK_PUTENV

The C library putenv(3) function. Any registered hooks will run before the C library implementation. The hook_fn field should be a function that matches the following typedef:

typedef int (*sudo_hook_fn_putenv_t)(char *string,
   void *closure);

If the registered hook does not match the typedef the results are unspecified.

hook_fn

sudo_hook_fn_t hook_fn;

The hook_fn field should be set to the plugin's hook implementation. The actual function arguments will vary depending on the hook_type (see hook_type above). In all cases, the closure field of struct sudo_hook is passed as the last function parameter. This can be used to pass arbitrary data to the plugin's hook implementation.

The function return value may be one of the following:

SUDO_HOOK_RET_ERROR

The hook function encountered an error.

SUDO_HOOK_RET_NEXT

The hook completed without error, go on to the next hook (including the native implementation if applicable). For example, a getenv(3) hook might return SUDO_HOOK_RET_NEXT if the specified variable was not found in the private copy of the environment.

SUDO_HOOK_RET_STOP

The hook completed without error, stop processing hooks for this invocation. This can be used to replace the native implementation. For example, a setenv hook that operates on a private copy of the environment but leaves environ unchanged.

Note that it is very easy to create an infinite loop when hooking C library functions. For example, a getenv(3) hook that calls the snprintf(3) function may create a loop if the snprintf(3) implementation calls getenv(3) to check the locale. To prevent this, you may wish to use a static variable in the hook function to guard against nested calls. For example:

static int in_progress = 0; /* avoid recursion */
if (in_progress)
    return SUDO_HOOK_RET_NEXT;
in_progress = 1;
...
in_progress = 0;
return SUDO_HOOK_RET_STOP;

Hook API Version Macros

/* Hook API version major/minor */
#define SUDO_HOOK_VERSION_MAJOR 1
#define SUDO_HOOK_VERSION_MINOR 0
#define SUDO_HOOK_VERSION SUDO_API_MKVERSION(SUDO_HOOK_VERSION_MAJOR,\
                                              SUDO_HOOK_VERSION_MINOR)

For getters and setters see the Policy plugin API.

Remote command execution¶

The sudo front end does not have native support for running remote commands. However, starting with sudo 1.8.8, the -h option may be used to specify a remote host that is passed to the policy plugin. A plugin may also accept a runas_user in the form of “user@hostname” which will work with older versions of sudo. It is anticipated that remote commands will be supported by executing a “helper” program. The policy plugin should setup the execution environment such that the sudo front end will run the helper which, in turn, will connect to the remote host and run the command.

For example, the policy plugin could utilize ssh to perform remote command execution. The helper program would be responsible for running ssh with the proper options to use a private key or certificate that the remote host will accept and run a program on the remote host that would setup the execution environment accordingly.

Note that remote sudoedit functionality must be handled by the policy plugin, not sudo itself as the front end has no knowledge that a remote command is being executed. This may be addressed in a future revision of the plugin API.

Conversation API¶

If the plugin needs to interact with the user, it may do so via the conversation() function. A plugin should not attempt to read directly from the standard input or the user's tty (neither of which are guaranteed to exist). The caller must include a trailing newline in msg if one is to be printed.

A printf()-style function is also available that can be used to display informational or error messages to the user, which is usually more convenient for simple messages where no use input is required.

Conversation function structures

The conversation function takes as arguments pointers to the following structures:

struct sudo_conv_message {
#define SUDO_CONV_PROMPT_ECHO_OFF  0x0001 /* do not echo user input */
#define SUDO_CONV_PROMPT_ECHO_ON   0x0002 /* echo user input */
#define SUDO_CONV_ERROR_MSG        0x0003 /* error message */
#define SUDO_CONV_INFO_MSG         0x0004 /* informational message */
#define SUDO_CONV_PROMPT_MASK      0x0005 /* mask user input */
#define SUDO_CONV_PROMPT_ECHO_OK   0x1000 /* flag: allow echo if no tty */
    int msg_type;
    int timeout;
    const char *msg;
};

#define SUDO_CONV_REPL_MAX      255

struct sudo_conv_reply {
    char *reply;
};

typedef int (*sudo_conv_callback_fn_t)(int signo, void *closure);
struct sudo_conv_callback {
    unsigned int version;
    void *closure;
    sudo_conv_callback_fn_t on_suspend;
    sudo_conv_callback_fn_t on_resume;
};

Pointers to the conversation() and printf()-style functions are passed in to the plugin's open() function when the plugin is initialized. The following type definitions can be used in the declaration of the open() function:

typedef int (*sudo_conv_t)(int num_msgs,
             const struct sudo_conv_message msgs[],
             struct sudo_conv_reply replies[],
	     struct sudo_conv_callback *callback);

typedef int (*sudo_printf_t)(int msg_type, const char *fmt, ...);

To use the conversation() function, the plugin must pass an array of sudo_conv_message and sudo_conv_reply structures. There must be a struct sudo_conv_message and struct sudo_conv_reply for each message in the conversation. The struct sudo_conv_callback pointer, if not NULL, should contain function pointers to be called when the sudo process is suspended and/or resumed during conversation input. The on_suspend and on_resume functions are called with the signal that caused sudo to be suspended and the closure pointer from the struct sudo_conv_callback. These functions should return 0 on success and -1 on error. On error, the conversation will end and the conversation function will return a value of -1. The intended use is to allow the plugin to release resources, such as locks, that should not be held indefinitely while suspended and then reacquire them when the process is resumed. Note that the functions are not actually invoked from within a signal handler.

The plugin is responsible for freeing the reply buffer located in each struct sudo_conv_reply, if it is not NULL. SUDO_CONV_REPL_MAX represents the maximum length of the reply buffer (not including the trailing NUL character). In practical terms, this is the longest password sudo will support. It is also useful as a maximum value for the memset_s() function when clearing passwords filled in by the conversation function.

The printf()-style function uses the same underlying mechanism as the conversation() function but only supports SUDO_CONV_INFO_MSG and SUDO_CONV_ERROR_MSG for the msg_type parameter. It can be more convenient than using the conversation() function if no user reply is needed and supports standard printf() escape sequences.

See the sample plugin for an example of the conversation() function usage.

Sudoers group plugin API¶

The sudoers plugin supports its own plugin interface to allow non-Unix group lookups. This can be used to query a group source other than the standard Unix group database. Two sample group plugins are bundled with sudo, group_file and system_group, are detailed in sudoers(5). Third party group plugins include a QAS AD plugin available from Quest Software.

A group plugin must declare and populate a sudoers_group_plugin struct in the global scope. This structure contains pointers to the functions that implement plugin initialization, cleanup and group lookup.

struct sudoers_group_plugin {
   unsigned int version;
   int (*init)(int version, sudo_printf_t sudo_printf,
               char *const argv[]);
   void (*cleanup)(void);
   int (*query)(const char *user, const char *group,
                const struct passwd *pwd);
};

The sudoers_group_plugin struct has the following fields:

version

The version field should be set to GROUP_API_VERSION.

This allows sudoers to determine the API version the group plugin was built against.

init

int (*init)(int version, sudo_printf_t plugin_printf,
            char *const argv[]);

The init() function is called after sudoers has been parsed but before any policy checks. It returns 1 on success, 0 on failure (or if the plugin is not configured), and -1 if a error occurred. If an error occurs, the plugin may call the plugin_printf() function with SUDO_CONF_ERROR_MSG to present additional error information to the user.

The function arguments are as follows:

version

The version passed in by sudoers allows the plugin to determine the major and minor version number of the group plugin API supported by sudoers.

plugin_printf

A pointer to a printf()-style function that may be used to display informational or error message to the user. Returns the number of characters printed on success and -1 on failure.

argv

A NULL-terminated array of arguments generated from the group_plugin option in sudoers. If no arguments were given, argv will be NULL.

cleanup

void (*cleanup)();

The cleanup() function is called when sudoers has finished its group checks. The plugin should free any memory it has allocated and close open file handles.

query

int (*query)(const char *user, const char *group,
             const struct passwd *pwd);

The query() function is used to ask the group plugin whether user is a member of group.

The function arguments are as follows:

user

The name of the user being looked up in the external group database.

group

The name of the group being queried.

pwd

The password database entry for user, if any. If user is not present in the password database, pwd will be NULL.

Group API Version Macros

/* Sudoers group plugin version major/minor */
#define GROUP_API_VERSION_MAJOR 1
#define GROUP_API_VERSION_MINOR 0
#define GROUP_API_VERSION ((GROUP_API_VERSION_MAJOR << 16) | \
                           GROUP_API_VERSION_MINOR)