NRPE SECURITY README
====================

TCP Wrapper Support
-------------------

NRPE 2.x includes native support for TCP wrappers. Once you
compile NRPE you can check to see if it has wrapper support
built in by running the daemon from the command line without
any arguments like this:

    ./nrpe --help


Command Arguments
-----------------

NRPE 2.0 includes the ability for clients to supply arguments to
commands which should be run.  Please note that this feature
should be considered a security risk, and you should only use
it if you know what you're doing!


Bash Command Substitution
-------------------------

Even with the metacharacter restrictions below, if command arguments 
are enabled, it is still possible to send bash command substitutions 
in the form `$(...)` as an argument. This is explicitly disabled by 
default, but can be enabled by a configure-time option and a
configuration file option. Enabling this option is **VERY RISKY**
and its use is **HIGHLY DISCOURAGED**.


Enabling Arguments
------------------

To enable support for command argument in the daemon, you must
do two things:

   1.  Run the configure script with the `--enable-command-args`
       option

   2.  Set the `dont_blame_nrpe` directive in the NRPE config
       file to `1`.


Enabling Bash Command Substitution
----------------------------------

To enable support for arguments containing bash command substitutions, 
you must do two things:

   1.  Enable arguments as described above

   2.  Include the `--enable-bash-command-substitution` configure
       option when running the configure script

   3.  Set the `allow_bash_command_substitutions` directive in the 
       NRPE config file to `1`.


Nasty Metacharacters
--------------------

To help prevent some nasty things from being done by evil 
clients, the following metacharacters are not allowed
in client command arguments:

    | ` & > < ' \ [ ] { } ; ! \r \n

You can override these defaults by adjusting the `nasty_metachars`
flag in the config file.

Any client request which contains the above mentioned metachars
is discarded.


User/Group Restrictions
-----------------------

The NRPE daemon cannot be run with (effective) root user/group
privileges.  You must run the daemon with an account that does
not have superuser rights.  Use the `--with-nrpe-user` and 
`--with-nrpe-group` flags during `./configure`, or the `nrpe_user`
and `nrpe_group` config file options to specify which user/group 
the daemon should run as.


Encryption
----------

If you do enable support for command arguments in the NRPE daemon,
make sure that you encrypt communications either by using:

   1.  Stunnel (see http://www.stunnel.org for more info)
   2.  Native SSL support (See the [SSL Readme](README.SSL.md) file for more info)

Do **NOT** assume that just because the daemon is behind a firewall
that you are safe! ***Always encrypt NRPE traffic!***


Using Arguments
---------------

How do you use command arguments?  Well, lets say you define a
command in the NRPE config file that looks like this:

    command[check_users]=/usr/local/nagios/libexec/check_users -w $ARG1$ -c $ARG2$

You could then call the check_nrpe plugin like this:

    ./check_nrpe -H <host> -c check_users -a 5 10

The arguments '5' and '10' get substituted into the appropriate
`$ARGx$` macros in the command (`$ARG1$` and `$ARG2$`, respectively).
The command that would be executed by the NRPE daemon would look
like this:

    /usr/local/nagios/libexec/check_users -w 5 -c 10

You can supply up to 16 arguments to be passed to the command
for substitution in `$ARG$` macros (`$ARG1$` - `$ARG16$`).