Scripting and Plugins

Windows login greeter

This recipe is a continuation of the previous recipe. It will demonstrate how to push a message from the OpenVPN server to the client during the connection phase. This message can be used as a legal warning or as a disclaimer message. In order to do this, we use the setenv-safe directive, which is available in OpenVPN 2.1 and higher. This directive can be pushed out to clients, in contrast with the more commonly-used setenv directive.

Getting ready

Install OpenVPN 2.1 or higher on two computers. Make sure the computers are connected over a network. Set up the client and server certificates using the first recipe from Chapter 2, Client-server IP-only Networks. For this recipe, the server computer was running Fedora 12 Linux and OpenVPN 2.1.1. The client was running Windows XP SP3 and OpenVPN 2.1.1. Keep the server configuration file, example6-1-server.conf, from the previous recipe at hand.

How to do it...

1.Append a line to the server configuration file example6-1-server.conf:

push "setenv-safe MSG 'This is a message from the OpenVPN server'"

Note that this is a single line. Save it as example6-2-server.conf.

2.Start the server:

[root@server]# openvpn --config example6-2-server.conf

3.Next, create the client configuration file:

client proto udp

remote openvpnserver.example.com port 1194

dev tun nobind

tls-auth "c:/program files/openvpn/config/ta.key" 1

script-security 2 system

up "c:\\openvpn\\cookbook\\example6-2.vbs"

Save it as example6-2.ovpn.

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4.Next, create the Visual Basic Script, example6-2.vbs:

Set oShell = CreateObject( "WScript.Shell" ) msg=oShell.ExpandEnvironmentStrings("%OPENVPN_MSG%") MsgBox msg, , "Welcome"

Save it in c:\openvpn\cookbook, or more importantly in a location with no spaces in the directory name. Start the OpenVPN client. During the connection phase, a message box will pop up:

How it works...

The following server directive pushes the statement setenv-safe MSG … to the connecting client:

push "setenv-safe MSG 'This is a message from the OpenVPN server'"

The client carries out the directive as if the following was specified:

setenv-safe MSG 'This is a message from the OpenVPN server'

The setenv-safe directive prepends OPENVPN_ to all the environment variables prior to setting them to avoid conflicts with the existing system variables.

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The following directive is required so that we can execute the Visual Basic script directly:

script-security 2 system

If we had specified it without the system, the OpenVPN client would not have been able to execute the VBS file, as it is not a real executable program.

The Visual Basic script picks up the new environment variable and displays a message box with the text.

There's more...

There are a few things to keep in mind when developing scripts for the Windows platform.

Spaces in filenames

An oddity of running scripts is related to the use of spaces in filenames: it is not easy to place a script in a directory with a space in it, nor should the script itself have a space in the filename. OpenVPN gets confused which part of the command is the actual script and which part comprises the command-line parameters to the script. Therefore, it is best to avoid spaces in the full pathnames for scripts altogether. With OpenVPN 2.1, we could also

have used:

cd "c:\\program files\\openvpn\\scripts" up "example6-2.vbs"

Another option is to always use the OpenVPN GUI application. This application switches to the directory C:\Program Files\OpenVPN\config prior to launching the OpenVPN. So,

a script can also be referenced using:

up "..\\scripts\\example6-2.vbs"

Yet another option is to specify the path to the Windows scripting executable directly. You can then pass the name of the actual script as the first argument, which can then be stored in a directory containing spaces:

up "%windir%\\system32\\wscript.exe

\"c:\\program files\\openvpn\\scripts\\example6-2.vbs\""

Note that this statement is a single line.

setenv or setenv-safe

Normally, the following directive is used to set an environment variable that will be available to any script that OpenVPN calls:

setenv env-var value

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However, the setenv directive cannot be pushed to a client from the server side. The setenv-safe directive, which was introduced in OpenVPN 2.1, can be pushed.

Security considerations

This recipe can be used to show a disclaimer or legal text when a user sets up a VPN connection. However, it is trivial for a user to circumvent this by modifying the client configuration file. If more stringent security is required, an application should be developed that can interact with the OpenVPN server itself.

Using client-connect/client-disconnect scripts

This recipe will demonstrate how to set up a client-connect script that gets executed on the server side when a new client connects. Similarly, a client-disconnect script can be specified that is executed when a client disconnects from the server. Client-connect and client-disconnect scripts can be used for several purposes:

Extra authentication

Opening and closing firewall ports

Assigning specific IP address to special clients

Writing out connection-specific configuration lines for a client

In this recipe, we will use a client-connect script to push a custom message to an OpenVPN client, based on the time of the day when the client connects.

Getting ready

Install OpenVPN 2.1 or higher on two computers. Make sure the computers are connected over a network. Set up the client and server certificates using the first recipe from Chapter 2. For this recipe, the server computer was running CentOS 5 Linux and OpenVPN 2.1.1. The client was running Windows XP SP3 and OpenVPN 2.1.1. Keep the server configuration file example6-1-server.conf from the first recipe of this chapter at hand.

How to do it...

1.Append the following lines to the example6-1-server.conf server configuration file:

script-security 2

client-connect /etc/openvpn/cookbook/example6-3-connect.sh

Save it as example6-3-server.conf.

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2.Next , create the connect script:

#!/bin/bash

hour=`/bin/date +"%H"` if [ $hour -lt 6 ] then

msg1="You're up at a weird hour" elif [ $hour -le 12 ]

then

msg1="Good morning" elif [ $hour -lt 18 ] then

msg1="Good afternoon"

else

msg1="Good evening"

fi

OPENVPN_MSG1="$msg1 $common_name" OPENVPN_MSG2=`/bin/date +"Local time at the VPN server is %H:%M:%S"`

# now write out the extra configuration lines to $1 echo "push \"setenv-safe MSG1 '$OPENVPN_MSG1'\"" > $1 echo "push \"setenv-safe MSG2 '$OPENVPN_MSG2'\"" >> $1

Save this file as example6-3-connect.sh.

3.Make sure the script is executable:

[root@server]# chmod 755 example6-3-connect.sh

4.Start the server:

[root@server]# openvpn --config example6-3-server.conf

5.The client configuration file is very similar to the one from the previous recipe:

client proto udp

remote openvpnserver.example.com port 1194

dev tun nobind

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script-security 2 system

up "c:\\openvpn\\cookbook\\example6-3.vbs"

Save it as example6-3.ovpn.

6.Create the VB Script file:

Set oShell = CreateObject( "WScript.Shell" ) msg1=oShell.ExpandEnvironmentStrings("%OPENVPN_MSG1%") msg2=oShell.ExpandEnvironmentStrings("%OPENVPN_MSG2%") MsgBox msg1 + vbcrlf + msg2, , "Welcome"

Save it as c:\openvpn\cookbook\example6-3.vbs.

7.Start the OpenVPN client:

During the connection phase a message box will pop up:

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How it works...

When a client connects, the OpenVPN server executes the client-connect script with several environment variables set that are related to the client connecting. The script writes out two lines to the connect-specific configuration file, which is passed as the first and only parameter to the client-connect script. This configuration file is then processed by the OpenVPN server as if it's a normal configuration file. The two lines that we use are:

push "setenv-safe MSG1 '$OPENVPN_MSG1'" push "setenv-safe MSG2 '$OPENVPN_MSG2'"

This means that two environment variables are pushed out to the client. These environment variables are picked up by the OpenVPN client and are displayed in a dialog box using a Windows VBS script.

There's more...

In this section, we focus on client-disconnect and the many environment variables that are available to all OpenVPN scripts.

'client-disconnect' scripts

A client-disconnect script can be specified using:

client-disconnect /etc/openvpn/cookbook/disconnect.sh

This script is executed when the client disconnects from the server. Be aware that when a client first disconnects and a explicit-exit-notify is not specified on the client side, then the OpenVPN server will first try to reconnect several times to the client. If a client does not respond after several attempts then the client-disconnect script will be executed.

Depending on the server configuration, this might be several minutes after the client has actually disconnected.

Environment variables

There is a multitude of environment variables available inside a client-connect and clientdisconnect script. It is very instructive to write a client-connect script that does a little more than:

#!/bin.bash env >> /tmp/log

Also, similar to the up and down script, is the environment variable script_type that contains the type of script as configured in the server configuration file. This gives the server administrator the option to write a single script for both client-connect and

client-disconnect.

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