From WPKG | Open Source Software Deployment and Distribution

[edit] Theory

If you want to execute a local command (be it a script or a program) remotely, you can upload it via SCP first, and then execute it via SSH. But why make two connections, when one is enough?

To execute a local script or a program on a remote machine, do something like below:

cat /some/script | ssh user@server "cat > /tmp/script ; chmod 755 /tmp/script ; /tmp/script --arguments"

or

cat /usr/bin/program | ssh user@server "cat > /tmp/program ; chmod 755 /tmp/program ; /tmp/program --arguments" 

It will copy the command, and execute it using just one SSH connection.

You can also remove the command after its execution (just add rm -f as the last command), "randomize" its name, etc.

It's useful for remote monitoring, with nagios, etc.

[edit] Example usage with nagios

Nagios already has a plugin which enables checking services using SSH. Unfortunately, one of the prerequisites is that the plugin you are about to execute needs to be installed on the remote server.

[edit] Example plugin

This nagios plugin will upload and execute any nagios check on a remote machine, using SSH.

• user nagios needs to be able to login to the remote machine using keys,
• if the plugin already exists on the remote side, it won't be uploaded,
• the plugin will be uploaded only once; all subsequent times it will use already uploaded plugin,
• the plugins from your nagios server need to be exacutable on the remote server (they won't run if the remote architecture is different, i.e. your nagios server is a x86 box, and the remote machine runs on a MIPS CPU, or, when your glibc versions differ a lot) - if it's the case, you have to install correct plugins on the remote servers,
• use only on the remote servers you trust; if you want to use it on the remote servers you don't trust, change the REMOTEDIR to the path only accessible by the nagios user.

Save the below script as check_remote to your nagios plugin directory.

#!/bin/bash
 
# Executes a local nagios plugin on a remote server.
 
# Some variables you are likely to change
SSH_USER=checkuser                      # user for SSH logins, default user running nagios
SSH_KEY=~nagios/.ssh/checkuser.rsa      # SSH key, can be empty
 
PLUGINDIR=/usr/lib/nagios/plugins       # nagios plugin dir
REMOTEDIR=/tmp                          # remote dir for file uploads
 
# Strips off hostname (first two arguments, $1 and $2), and leaves options only
ARGS=$@
ARGS=${ARGS##*$2}                               # plugin arguments
 
# Display help if no arguments given, or paths wrong
if [ ! "$1" -o ! "$2" -o "$1" == "--help" -o "$1" == "-h" ] ; then
cat <<EOF
 
Executes a local nagios plugin on a remote server.
 
Usage:
 
    $0 <hostname> <nagios_check_plugin> [plugin arguments]
 
EOF
exit 0
fi
 
# Fetch the SSH key; if file exists, set SSH options to use this key
[ -f "$SSH_KEY" ] && SSH_OPTIONS="-i "$SSH_KEY""
 
 
# Fetch the user, if exists, it is used, otherwise, use the current user
[ ! "$SSH_USER" ] && SSH_USER=$USERNAME
 
# Calculate md5sum of the plugin - we want to be sure that we run it remotely, and not something else
MD5_LOCAL=$(md5sum $PLUGINDIR/$2 | cut -f1 -d " ")
 
# Our plugin executed remotely
 
# We login to the remote server, check if the plugin is there - if yes, we run it.
# If not, we "cat" the plugin, and pipe it via ssh to the remote server, and execute it.
 
cat $PLUGINDIR/$2 | ssh $SSH_OPTIONS $SSH_USER@$1 \
    "# if the plugin exists on the remote side in $PLUGINDIR, execute it
    if [ -x $PLUGINDIR/$2 ] ; then
        $PLUGINDIR/$2 $ARGS
        EXITCODE=\$?
        exit \$EXITCODE
 
    # if it doesn't exist in $PLUGINDIR, is it in $REMOTEDIR?
    elif [ -x $REMOTEDIR/nagios-$2 ] ; then
 
    # is it the same as the plugin on the server? if yes, execute it
        MD5_REMOTE=\$(md5sum < $REMOTEDIR/nagios-$2)
            if [ \"$MD5_LOCAL\" == \"\$MD5_REMOTE\" ] ; then
                $REMOTEDIR/nagios-$2 $ARGS
                EXITCODE=\$?
                exit \$EXITCODE
            else
                UPLOAD=1
        fi
    fi
 
    # it's not in $PLUGINDIR nor $REMOTEDIR, upload it
    if [ ! -x \"$REMOTEDIR/nagios-$2\" -a ! -x \"$PLUGINDIR/$2\" -o \"\$UPLOAD\" ] ; then
        rm -f $REMOTEDIR/nagios-$2 # remove if it exists; someone might made a symlink to some other file
        cat > $REMOTEDIR/nagios-$2 
        chmod 700 $REMOTEDIR/nagios-$2
        $REMOTEDIR/nagios-$2 $ARGS
        EXITCODE=\$?
        exit \$EXITCODE
    fi"
 
EXITCODE=$?
 
# Note that the remote command should not return exit code 255,
# it is used by ssh if it fails to connect
if [ $EXITCODE -eq 255 ] ; then
    echo "CRITICAL - Unable to connect ($1)"
    exit 2
else
    exit $EXITCODE
fi

[edit] Example nagios command definition

These two commands check system load and free space on remote servers.

define command{
        command_name    check_remote_load
        command_line    $USER1$/check_remote $HOSTADDRESS$ check_load -w $ARG1$ -c $ARG2$
        }

define command{
        command_name    check_remote_disk
        command_line    $USER1$/check_remote $HOSTADDRESS$ check_disk -w $ARG1$ -c $ARG2$
        }

[edit] Example nagios service definition

Two example service definitions - for checking system load and free space on remote servers.

define service {
        use                             generic-service
#       host_name                       host1, host2
        service_description             System load
        is_volatile                     0
        check_period                    24x7
        max_check_attempts              3
        normal_check_interval           60
        retry_check_interval            30
        contact_groups                  localhost-admins
        notification_interval           240
        notification_period             24x7
        notification_options            w,u,c,r
        check_command                   check_remote_load!5.0,4.0,3.0!10.0,6.0,4.0
}

define service{
        use                             generic-service
#       host_name                       host1, host2
        service_description             Free Space
        is_volatile                     0
        check_period                    24x7
        max_check_attempts              3
        normal_check_interval           60
        retry_check_interval            30
        contact_groups                  localhost-admins
        notification_interval           240
        notification_period             24x7
        notification_options            w,u,c,r
        check_command                   check_remote_disk!20%!10%
        }

Keywords: SSH, SCP, Linux, UNIX, nagios, remote, local