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sys, an alternative tool to sudo.

TL;DR

sys is a sudo-like tool optimized for multi-users environments where users need to gain temporarily another user's privilege based on a set of conditions.

The sys configuration files are command-based, not user-based like sudo but, like sudo, sys can allow certain categories of users to run any command if they meet all the required conditions.

sys also has some additional features such as the possibility to force or restrict the environment and the command line arguments using patterns and regular expressions.

Read on to find out more about its features.

Disclaimer.

Please feel free to comment, especially on security aspects and potential vulnerabilities.

Remark.

Some specifications are still subject to change, so please let me know if you have any comments.

Concepts.

sys runs commands as root or as another user. in order to do that, it locates and interprets rules stored in data files or in its cache. It was designed for an environment with many end users but is easy to use even if there is only one user in addition to root.

Here is a correct example of a possible rule to launch a root bash shell. All users, except pierre, will be asked to enter a password:

bash
  cmd:bash $*
  environment:
  users:pierre
  password:

According to this rule, you can enter at the shell prompt (here $ is the non-root shell prompt):

$ sys bash [bash_options...]

Here is another way to do a similar thing using ksh instead of bash with a more elaborate rule in a .dat file. The first four lines do not belong to the rule but explain how variables can be created and used in data files:

@ADM_GRP:sys,wheel
@ESC:$'\e'
@RED:@{ESC}[32m
@NO_COLOR:@{ESC}[0m

root_shell
  cmd:ksh $*
  uid:root
  gid:root
  $ENV:false
  $PS1:{@{RED}sys@{NO_COLOR}}@${HOSTNAME}" # "
  environment:
  users:pierre
  groups:@{ADM_GRP}
  paths:/usr/bin,/bin,/usr/local/bin

This rule can be searched and interpreted using the following command line:

$ sys root_shell [ksh_options...]

which will run the ksh executable as root if the current user belongs to the sys or wheel group and if the ksh executable is in one of the listed paths. The current environment will be transmitted unchanged at the exception of the PS1 variable which will be added or overridden.

Direct command invocation, as in sudo, is possible with a wildcard like rule.

Data files are interpreted once and cached. They are, of course, interpreted again if their content changes, and the cache is then updated.

See the manual for details on rule syntax, wildcard rule and so on.

A global configuration file names sys.cfg must also be provided.

Compilation and installation.

The script build.sh, based on the traditional configure from GNU, is provided. You can get some help using the --help option.

Building sys requires some attention as configuration directories have to be given to the build mechanism:

  • The directory where sys will search for the file sys.cfg (CFGDIR). This directory also defaults to /etc.

  • On most systems it is also highly recommended to enable the authentication using PAM using the --enable-pam option.

    When not using --enable-pam nor --enable-pam=yes or when PAM is not available or its development components are not installed, the standard authentication method using the password/shadow mechanism is automatically selected as if --enable-pam=no has been used``.

  • Plugins, as dynamically loadable code, can be used to add custom filtering parameters in a rule. They are disabled by default.

Example:

$ ./build.sh --with-cfgdir=/etc         \
             --enable-pam               \
             --enable-plugins           \
             --other_configure_settings...

A list of directories in which sys will search the data files containing the rules must also be added in the sys.cfg file, as in the example below.

...
[Directories]

Data    = /opt/etc/sys /etc/sys
Logs    = /var/log/sys
Plugins = /etc/sys/plugins
...

Configuration.

Refer to the sys(8) and sys.cfg(5) manual pages to create and populate data directories with rules and fill in the configuration file.

Permissions.

The permissions of the directory identified as CFGDIR in the previous section must be 0700. Their owner must be root:root. This is not automatically done during the installation to avoid accidentally changing the permission of existing directories.

The directories indicated in the Directories section of sys.cfg must also belong to root:root and have their permissions set to 0700. All the files in these directories must have their permissions set to 0600.

Example using the setting above:

[root:root drwx------] /opt/etc/sys
[root:root -rw-------]   base.dat
[root:root -rw-------]   default.dat

[root:root drwx------] /etc/sys
[root:root -rw-------]   main.dat