Discover LinuxSecurity Features
Encrypted Tunnels using SSH and MindTerm
Businesses, schools, and
home users need more secure network services now more than ever. As online
business increases, more people continue to access critical company information
over insecure networks. Companies are using the Internet as a primary means
to communicate with travelling employees in their country and abroad, sending
documents to various field offices around the world, and sending unencrypted
email; this communication can contain a wealth of information that any
malicious person can potentially intercept and sell or give to a rival
company. Good security policies for both users and network administrators
can help to minimize the problems associated with a malicious person intercepting
or stealing critical information within their organization. This paper
will discuss using Secure Shell (SSH) and MindTerm to secure organizational
communication across the Internet.
Home users and business travelers are accessing company resources and sending sensitive data over insecure networks. This opens up a whole new area of security issues for System Administrators (Securing the home office sensible and securely), especially since the number of corporate users from home with high-speed access is expected to "more than double from 24 million in 2000 to 55 million by 2005" (Broadband Access to Increase in Workplace). The increase in the number of airports and hotels offering internet access, especially high-speed access, is increasing and is expected to grow in the future (Broadband Moving On Up). This can also leave a door wide open for a malicious person to hijack or view a person's Internet traffic and access their companies. The malicious person may not be interested in the work the employee is doing but just want access to a high-speed server to launch attacks, store files, or other uses. Business people are really at high risk because they don't know who's monitoring their Internet connection in the hotel, airport, or anywhere in their travels. Users of the new high-speed connections are usually not taught proper security protocols and some companies don't have the staff to help the home user and business traveler set up secure communication. Individual users and, surprisingly, some companies have a mentality that "I don't have anything people want". This is very disturbing considering the amount of sensitive information that travels across the Internet from an employee's home or from travelers. What's more disturbing is the availability of free software to perform these kinds of attacks and the software's ease of use. Dsniff (https://www.monkey.org/~dugsong/dsniff/) is a freely available program that has utilities that can allow anyone with a networked computer to highjack a local network and monitor what others are doing and grab passwords and other sensitive data. In his book Secrets and Lies: Digital Security in a Networked World, Bruce Schneier states that Technique Propagation is one of the main threats to network security: "The Internet is...a perfect medium for propagating successful attack tools. Only the first attacker has to be skilled; everyone else can use his software" (Schneier). The purpose of this paper is not how to secure computers but how to set up virtual tunnels to perform secure communication, whether sending documents or sending email. Business travelers should read Jim Purcell, Frank Reid, and Aaron Weissenfluh's articles on travel security https://www.sans.org/infosecFAQ/travel/travel_list.htm. Home users with high-speed access should read Ted Tang's article at (https://www.sans.org/infosecFAQ/start/free.htm) for information on how to secure your computers with high-speed access. I'd recommend the many resources available on www.sans.org, www.securityfocus.com, www.linuxsecurity.com, or www.securityportal.com for tutorials on how to secure your computers and servers.
The way to ensure that sensitive
data is transmitted securely and quickly is to use encrypted methods of
data delivery. This can be by way of encrypted email, using secure web-based
email services, or establishing encrypted tunnels between two computers.
Also, easy to setup and reliable software need to be used in order to allow
the inexperienced users the ability to quickly establish secure communication
channels. Taten Ylonen 's Secure Shell (www.ssh.com) and MindBright Technology's
(www.mindbright.se) MindTerm are a quick, easy to use, and reliable solution
for securing communication over the Internet.
SSH and MindTerm
SSH (Secure Shell) is a secure replacement for remote login and file transfer programs like telnet, rsh, and ftp, which transmit data in clear, human-readable text. SSH uses a public-key authentication method to establish an encrypted and secure connection from the user's machine to the remote machine. When the secure connection is established then the username, password, and all other information is sent over this secure connection. You can read more details of how ssh works, the algorithms it uses, and the protocols implemented for it to maintain a high level of security and trust at the ssh website: www.ssh.com. The OpenBSD team has created a free alternative called OpenSSH available at: www.openssh.com. It maintains the high security standards of the OpenBSD team and the IETF specifications for Secure Shell (see the Secure Shell IETF drafts: Upload ), except it uses free public domain algorithms. SSH is becoming a standard for remote login administration. It has become so popular that there are many ports of ssh to various platforms and there are free clients available to login to an ssh server from many platforms as well. See linuxmafia for a list of clients and Securityportal.com has an excellent two-part article on ssh and links to ports for different platforms available at . There are programs that also use an ssh utility called Secure Copy (scp) in the background that provide the same functionality of a full ftp client, like WinSCP ( ) and the Java SSH/SCP Client (https://www.isnetworks.com/ssh/), which has a modified scp interface for MindTerm. Please read the licenses carefully to determine if you are legally allowed to download ssh in your country. SSH is free for academic institutions please. Please read the licenses available at the ssh.com website.
MindTerm is an ssh client
written entirely in Java by MindBright Technology. One of the key practices
of developing security software is proper implementation of the underlying
algorithms and protocols it uses. MindBright Technology has implemented
the ssh protocol very well in this small application file. It is a self-contained
archive that only needs to be unzipped into a directory of your choice
and it is ready to be used. It can be used as a standalone program or as
a web page applet or both. It is available at: .
MindTerm is an excellent and inexpensive client to secure communication
to and from a local and remote location. The MindTerm program located at
the download address above is available free for non-commercial and academic
use, commercial use is available on a case to case basis. However, the
modifications made by the ISNetwork (www.isnetworks.net)
team "is based on the MindTerm 1.21 codebase, which MindBright released
under the GPL [General Public License -- see https://www.gnu.org].
Since our version is released under the GPL you can use it commercially
for free" (Eckels). ISNetwork's implementation has all the features
of MindBright's MindTerm except it has a nicer scp interface for more user-friendly
file transfers. MindTerm does have some drawbacks in that it doesn't support
UDP tunneling. In order to secure UDP traffic, a program called Zebedee
will work nicely. Zebedee's server and client program is available for
Windows and Linux platforms. It is freely distributed under the GPL License
too. You can connect to either Windows or Linux machines using Zebedee.
MindTerm will not check to see if your system is secure. It is up to the
administrators and users to take care of securing the computer systems.
It is easy to implement and it is very effective at maintaining the high
level of security implemented in the ssh protocol. This paper will show
how easy it is to set up and establish secure communication channels for
almost any user and by almost any user. Documents, email, and other data
communication can be easily and securely sent to users a few feet away
or around the world.
How SSH and MindTerm work together
SSH and MindTerm will work
together to use a technique called port forwarding. Port forwarding is
forwarding traffic from one host and a given port to another host and port.
In other words, the MindTerm application will open a port on the client's
machine (local machine) and any connection to that local port is forwarded
to the remote host and its listening port over an encrypted ssh session.
Whether or not the connection is accepted depends on the type of request
you are sending to the remote host. For example, you wouldn't forward POP
requests to a remote host listening on port 21 because port 21 is reserved
for ftp requests. Port forwarding is also used to allow connections to
a server that is behind a firewall and/or has a private IP address. Essentially
this is creating a Virtual Private Network (VPN). A VPN is "a private data
network that makes use of the public telecommunication infrastructure,
maintaining privacy through the use of a tunneling protocol and security
procedures" (www.whatis.com). The port-forwarding
can only be done with TCP services.
In order to follow along with this tutorial you will have to install a few packages. This tutorial assumes you have ssh already installed on your server or workstation. If not then you can read the documentation that comes with the ssh or the OpenSSH package for installation instructions for your platform. For the examples that follow, OpenSSH was installed on a Red Hat 7.0 server and workstation. OpenSSH was installed on Red Hat 6.0- 7.0 and worked the same. The client machine used in the following tutorial is a Windows 2000 machine. Windows 95/98, NT 4.0, NT 5.0, Red Hat 6.0-7.0 workstation were all tested as client machines and worked the same. On a side note, the exact same MindTerm jar archive was used on all client systems tested.
- SSH or OpenSSH
- FTP Client - Any ftp client should work for this tutorial. Ws-FTP and Leech-ftp are the two most popular for Windows.
- Netscape Communicator - or any other mail client should work
- Optional: NTOP
- Optional: vlock
- Install NTOP to see how other TCP services can be encrypted as well. I downloaded the latest rpm from RPM resource ntop
- Vlock is optional because users may do work from the console after they are authenticated. However, if a user will only be using the tunnels then the command: vlock -c can be typed at the console or it can be added to the users startup script so when the user logs in, it will automatically lock their console.
First, make sure that your server is secure. Though traffic is encrypted as it travels over the Internet, it can be sniffed if someone has root access on the local machine and uses a program like ngrep ( ) to sniff traffic on a local machine. For example, in conjunction with the dsniff program mentioned above, the following command could sniff all traffic on the local interface network: ngrep -d lo. Securing the server is, however, beyond the scope of this paper.
We'll use the POP (port 110), IMAP (port 143), SMTP (port 25), VNC (Virtual Network Computing) (5901+), and NTOP (default port 3000) services for this example. All traffic will be forwarded to each service's respective port on the remote host running the ssh server. All services listening on the remote host listen on all interfaces, unless the service binds to a specific port by default or if manually configured. In order to show how effective this technique of tunneling over ssh is, we will only allow particular services to listen on the local interface.
You don't have to change
your current security configurations, however. We will use tcp_wrappers,
that is installed by default with Red Hat 7.0 (and previous versions), to
connect to the network services. In the /etc/hosts.deny file add the following
ALL : ALL
And in your /etc/hosts.allow file add the following lines:
sshd : ALL
in.ftpd : 127.0.0.1
ipop3d : 127.0.0.1
imapd : 127.0.0.1
This sets sshd (the ssh server) to allow connections from anywhere any IP address. The other services only allow connections from the local interface. You can verify this by configuring a mail client to connect to your remote pop or imap server and/or an ftp client to connect to your ftp server, right now. It won't allow you to connect. You'll also need to set up any user accounts to allow access to these services. (Note: The setup above is only useful if the services are only for internal use and remote users need to access the internal services to send and receive email or transfer files. The services can be available for public use and be encrypted with ssh and MindTerm.)
The only client configuration
that is needed is to be sure that a Java Runtime Environment (JRE) is installed
for your platform. Windows and MacOS 8 and later have a JRE already installed.
It is recommended to install Sun's JRE on Windows. IBM has a list of ports
of JRE's to various plaforms: https://www-105.ibm.com/developerworks/tools.nsf/dw/java-devkits-byname
as well as Sun: Oracle Java Technologies | Oracle.
(You don't need the entire Java package with the debuggers and compilers
you just need the Java Virtual Machine to run java applications.) Also,
for the tutorial that follows, unzip the MindTerm archive, MindBright's
or ISNetwork's implementation, archive into "c:\mindterm" for windows.
Creating the Tunnels
MindTerm can be started a
few ways. If you have the JRE installed then you can double-click on the
mindtermfull.jar application file. Another way is to open up a dos-shell
and type the command:
jview -cp c:\mindterm\mindtermfull.jar mindbright.application.MindTerm or javaw -cp c:\mindterm\mindtermfull.jar mindbright.application.MindTerm
java -cp c:\mindterm\mindtermfull.jar mindbright.application.MindTerm
(jview is used if you are using Windows and you don't download the JRE. Javaw comes with the Windows JRE download and is used because a dos-shell box won't be needed in order to run MindTerm so there is one less window open)
UPDATE: MindTerm 2.0 release candidate 1 is out. The argument to start it has changed slightly. Instead of the command above:
java -cp c:\mindterm\mindtermfull.jar mindbright.application.MindTerm
this will start MindTerm from the commandline:
java -cp c:\mindterm\mindtermfull.jar com.mindbright.application.MindTerm
Only the "com." was added to the applet parameter.
Also, under the section "MindTerm over the Web" can you add this code
directly under the first code example:
UPDATE: MindTerm 2.0 release candidate 1 is out. The argument to start the web applet has changed slightly. Instead of the applet parameter above, and the code example below, change the line:
code=mindbright.application.MindTerm width=700 height=400>
code=com.mindbright.application.MindTerm width=700 height=400>
Only the "com." needs to be added to the applet parameter "code=". So the code below will be changed to:
<applet archive="mindterm_ns.jar" code=com.mindbright.application.MindTerm.class width=1 height=1>
This will start the MindTerm program and you can then type the server name when prompted and it will prompt you to "Save as Alias". You can type a short server name so when you start the applet again you can simply type the "Alias" you created. You will then be prompted for your login name. After you type it, hit enter and a dialog box will appear informing you that the host doesn't exist and prompt you to create it. Click "Yes". Another dialog will appear prompting you if you want to add that host to your "known_host" file. Click "Yes". Then you are prompted for your password. Type your password and hit enter. If you supplied the proper username and password then you should be at a command line on the server you specified.
Creating the Tunnels
We'll create a tunnel to
the POP and SMTP server, first. After you have successfully logged in (and
optionally enabled vlock) click on "Tunnels"
on the menu and then click "Basic".
A dialog box will appear. Add the following
settings to each box, respectively:
- Local port: 2010
- Open up Netscape
- Click on the "Edit" menu ---> "Preferences".
- On the left column click on "Mail & Newsgroups", if the contents aren't already displayed.
- Click on "Identity" and type your information in each box.
- Click on "Mail Servers" in the left column. The default install of Netscape has "mail" in the box underneath "Incoming mail servers".
- Click on "mail"
- Click "Edit" to the right of that box and a dialog box should appear
- If POP is not already selected in that drop down box, select it now.
- In the "Server Name" box type "localhost:2010" (remember we chose that local port in the MindTerm tunnel creation menu to forward to the remote servers POP (110) port) and then your username. Set any other options as you see fit.
- Click "OK".
- In the box "Outgoing mail (SMTP) server" type your smtp server name and underneath that type your "Outgoing mail server user name".
- Click "OK". (Don't do anything to the "Use Secure Socket Layer (SSL) or TLS for outgoing messages" option).
- Now click on "Communicator" on the menu and
- Click "Messenger"
- You should then be prompted for your password. Type your password and hit enter. If you have mail you should now be able to read it.
Remote Hosts: Your remote host (this should be the server running the sshd server).
Remote port: 110
Now click "Add".
A dialog box should appear stating "The tunnel is now open and operational". (Note: If you select a port that is already open an error message will appear stating "Could not open tunnel. Error creating tunnel. Error setting up local forward on port XXXX, Address in use.) Click "OK" and the tunnel configuration should appear in the box now. Click "Close Dialog". Open up your email client's options or preferences menu. We'll use Netscape Messenger for this example.
As long as you have a MindTerm ssh session open, this should work with most email clients. Remember that the remote server name or POP server name will be "localhost:
With the MindTerm client still running click on "Tunnels" again then "Basic" and add these settings.
Local Port: 2025 (just type over the settings set from what we did previously)
Remote Host: Your remote smtp server.
Remote Port: 25
Then click "OK" on the confirmation menu. Now smtp should be added to the list underneath the settings for POP. In the Netscape Messenger mail server settings add: localhost:2025 as your "Outgoing mail (SMTP) server"
All email you send to the remote host will be encrypted. However, if you send mail to someone outside of the remote host's mail server, your email will be encrypted only from your local machine to your remote smtp server. From the remote smtp server to any other host, will not be encrypted, unless you've configured a tunnel to the other hosts.
To enable encrypted ftp sessions add these settings to a new tunnel.
Local Port: 2021 (just type over the settings set from what we did previously)
Remote Host: Your remote ftp server.
Remote Port: 21
Then click "OK" on the confirmation menu. Now ftp (see the leech ftp example and wsftp--picture 1 and picture 2) should be added to the list underneath the settings for SMTP.
Local Port: 2043 (just type over the settings set from what we did previously)
Remote Host: Your remote imap server.
Remote Port: 143
Then click "OK" on the confirmation menu. Now ftp should be added to the list underneath the settings for POP.
All these settings can be automated in a batch file. Simply add the following to a startup script to automatically create a tunnel to your pop server after authentication:
jview (or java or javaw) -cp c:\mindterm\mindtermfull.jar mindbright.application.MindTerm -server
Here is an example based on what we've done above. Add the following to a file in an editor:
jview (or java or javaw) -cp c:\mindterm\mindtermfull.jar mindbright.application.MindTerm -server
-local0 2010:localhost:110 -local1 2025:localhost:25 -local2 /ftp/2021:localhost:21 -local3 2043:localhost:143
now save it with a .bat extension. Double-click on it. You should be prompted for your login name when MindTerm starts up then type your password. After you are authenticated click on the "Tunnels" menu and click "Basic". You should see the tunnels in the box that opens up. This is an easy way to allow remote users to start up the tunnels without many configurations on their part. They only need to click the .bat file and type their username and password and optionally run vlock. Their client software can be pre-configured for remote profiles that connect to the tunnels automatically.
When you are finished using the MindTerm, be sure to close all applications that are using a tunnel. If you forget to close the programs using the tunnels, MindTerm will display a message when you attempt to exit from the console or quit the program.
What about VNC and NTOP? These services work the same way. Here the VNC server was running on a Red Hat 7.0 workstation. When you start the VNC server, it first listens on port 5901 and each server after that increments up 1 port so the second instance of VNC will listen on port 5902, and the third 5903, etc.. On Linux, you can run multiple VNC servers and people can connect to each VNC server as well. In MindTerm you can simply add a VNC tunnel with the following settings:
Local Port: 2001
Remote Host: Your remote VNC server host name.
Remote Port: 5901 (If this is the first server instance running)
Then click "OK" on the confirmation menu.
Run the vncviewer application on your local machine and type: localhost:2001, and then the password, when prompted, for the VNC desktop and you have an encrypted VNC session.
Ntop works the same way. If you want to run ntop in web mode as a network monitor, you can tunnel connections to your local machine and view the stats in your local browser, without having to install a webserver or opening port 3000 on your remote server. By default, ntop in web mode listens on port 3000 and waits for an http connection to display network stats. Simply create a tunnel to the server running the ssh server and ntop. First run ntop in web mode: ntop -d -w 3000 Then add the settings to the MindTerm tunnel:
Local Port: 2080
Host: Server running ntop.
Remote Port: 3000
Then click "OK" on the confirmation menu.
Open up your web browser and in the location bar type: You should now see the network stats page for ntop (see the ntop man pages to add password protected access to the ntop display). Similarly, if you want to install a web server so you can use web-based applications to control your server or firewall, then just create a tunnel to port 80. You don't have to open up a port on the public interface. Simply bind the webserver to the local interface and create a tunnel to the remote hosts' port 80. For Apache, edit the httpd.conf file and change the "BindAddress *" option to BindAddress 127.0.0.1. Then add localhost to the "ServerName" directive: ServerName localhost.
As you can see by now MindTerm
can secure almost any TCP service. It can be used on a remote server to
run Webmin (),
which is an excellent web-application to administer your servers. It comes
with its own perl-based webserver and listens on port 10000 by default.
Simply create a tunnel to it using MindTerm and it should work without
any changes to the Webmin application or your local web browser. The MindTerm
download zip file contains many useful examples, such as using it from
the command line and an explanation of all the menu options. MindTerm has
more features than outlined in this tutorial but the tunnel option is well
worth spending time focusing on.
MindTerm over the Web
MindTerm can be used over
the web as well. Users don't have to download the application. Simply copy
the mindtermfull.jar file to a directory into a web directory and the users
can simply use it as a built-in application or as a stand-alone java applet.
For example, create a folder named "mindterm" under your web directory.
Copy the mindtermfull.jar file, that was used above, into the web directory
folder "mindterm". Then add the file index.html to the directory with the
following content (from the README):
<applet archive="mindtermfull.jar" code=mindbright.application.MindTerm.class width=1 height=1>
<param name=server value="
<param name=port value="22">
<param name=cipher value="blowfish">
<param name=gm value="80x32+0-0">
<param name=forcpty value="true">
<param name=local0 value="2080:<yourserver name>:80">
<param name=sepframe value="true"><!-- wheter to run in a separate frame or not -->
<param name=autoprops value="both"><!-- enable/disable automatic save/load of settings -->
Now browse to the location of the directory in your web browser ( name>/mindterm/index.html). This will start MindTerm as a standalone java applet, the same as if it was started from the command line. Notice the one tunnel that is already created from the applet tag: " <param name=local0 value="2080:<yourserver name>:80"><br>" Tunnels can be created using the applet tags so that users don't have to do anything but browse to the page and then login. Then they would access their services just as explained in the above examples. They can, however, create their own tunnels or new tunnels from the "Tunnels" menu as explained above. The README that comes with the MindTerm zip archive has many more applet parameters that can be added.
A couple of security notes
here are you can't connect to another server using the initial login applet.
You can only login to the server where the applet is located. However,
after you have logged in successfully you can then log in to other servers
from the command line. Also, this MindTerm applet is not signed so you
need to contact the
When an ssh session starts, the public-keys are being sent over an insecure connection until the authentication process is established.. This allows a person to intercept an ssh session and place their own public key in the connection process. SSH is designed to warn the user if a public-key has changed from what exists in their known_host file. The warning that is given is quite noticeable and ssh will drop the connection if the public keys are different, but user's may still trust the certificate because they may think that their company has changed the server's public key. This kind of attack isn't difficult because the dsniff package mentioned earlier contains the tools to perform it. This attack is more commonly called a "man-in-the-middle attack" (The End of SSL and SSH).
A temporary and easy fix
for this is to first teach the user's how to recognize the signs that the
host key has changed and what to do to get the proper host(s) public key.
Second, post the public key for the ssh server(s) on a website, ftp server,
or distribute it some other way so that users have access to it at all
SSH and MindTerm together can provide local and remote users with a high-level of security with a simple and small drop-in application. It can also be used from nearly any platform available. Java was chosen because of its cross-platform compatibility. If there is a JRE available for a platform that someone uses then they can use the MindTerm application to communicate securely over long distances. Since ssh is becoming the standard for remote administration and logins, soon nearly all platforms will be able to run an ssh server. MindBright is currently working on a Java SSH server.
This tutorial also shows how someone can tunnel through a firewall. This is by no means the intention of this paper. It is hoped people will use it for a secure, quick, and free drop-in VPN-like replacement for remote administration, traveling business people, and a hope that other sectors can see the usefulness in this excellent program. As long as you are allowed to make ssh connections then you can tunnel services through to a remote machine. System and Security Administrators should establish policies against tunneling through firewalls because that can cause internal security breaches if used improperly. Remember that the communication is secured but the commands and files that you access and/or download are still being executed on your local and remote machines. Also, any commands you type on most servers are being logged as well. SSH will protect the data over the network or the Internet but what is done on the remote machines can be logged. SSH and MindTerm will not protect against someone gaining access to a remote user's computer and installing key logging programs or other snooping devices.
It is very simple and quick to set up secure communications but the only way to increase the use of secure communication is for users to encourage their company, financial institutions, health care providers, and other businesses to offer secure services.
Special thanks to Patty Pitz for her editing and helping to organize the paper and to Doug Eyman for his technical editing.
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