Authenticating an Stunnel server to an Stunnel client

Stunnel can take care of our network encryption needs for any TCP connection. I blogged about setting this up here. Stunnel makes use off SSL certificates which enables us to achieve these 3 main things, 1. Authentication, 2, Confidentiality and 3, Integrity. If you referenced my first post about stunnel, you may or may not have realized that i did not make use of the "authentication" feat of SSL. In this post, i aim to achieve that.

Essentially, the client will be able to connect to the server and verify that it's indeed connected to the correct server. By default the client will communicate with any stunnel server and accept the server's certificate without taking extra steps to verify it. What this means is that a malicious person can potentially setup a rogue stunnel server with their own certificate and pose as a ligitimate server. Because your client accepts everything blindly, it will make the assumption that it is connected to the right server, when indeed it might  not be. Lets fix that.

Step 1. Generate private key, certificate and PEM file.

# openssl genrsa -out server.key 2048
# openssl req -new -x509 -nodes -days 365 -key server.key -out server.crt
# cat server.key server.crt > server.pem
# chmod 640 server.pem
# chmod 640 server.key

Step 2. Run stunnel server.

# stunnel -f -d 443 -r 127.0.0.1:80 -p server.pem -s nobody -g nogroup -P /tmp/stunnel.pid

Step 3. Copy ONLY server.crt file to clients that would be connecting to the stunnel server. The server.pem and server.key file MUST be kept secret at all times.

Step 4. Run client with extra option -v 3 to enable certificate verification via locally installed cert.

# stunnel -c -f  -d 443 -r remote.server.com:443 -v 3 -A server.crt

Thats it.

If a malicious person tries to impersonate your server,it will fail to pass these checks:

1. When the initial connection between client and server is made, the server sends its certificate to the client. The client would only accept this certificate if it is within it's white list of certs "-A option".
2. If for some reason the malicious person was able to have the server send the correct cert, the client would challenge the server to prove that it has the matching private key for the cert. Because the malicious user does not have the private key and is unable to prove that it does, the client will terminiate the session.

Here is a screenshot of what things look like on the client under normal working conditions.

Here is a screenshot of what will happen on the client if a malicious server is used to impersonate a legitimate one.

The way we have setup stunnel we achieve the three main goals of SSL certificates.

1. Authentication. Server authenticates itself to the client before communication is established.
2. Confidentiality. Session is encrypted using symmetric cryptography.
3. Integrity. Ensures that the encrypted data has not been tampered with via a negotiated hashing algorithm.

More tunneling with SSL and stunnel

We've discussed tunneling in the past with regards to httptunnel and ptunnel. As effective as these methods are for bypassing firewall rules and other purposes, neither ptunnel or httptunnel provide any means of encryption (although shh tunneling does, something i've discussed in a previous post).
Using programs like wiresark, you can easily see the payloads of the tunneled traffic. SSL tunnel provides similar tunneling funtionalities as the other tunneling programs but provides the much needed encryption to mitigate eaves droppers. I'll disscus all the needed steps to set-up a Windows client and a Linux Server.

Server [linux] - 10.0.0.1
Client [Windows XP] - 10.0.0.2

SERVER Setup:

First thing you need to do is generate a x509 certificate file to be used for encrytion if one doesnt already exist [It should be loacated at /etc/stunnel/stunnel.pem]. I usally like to generate my own and customize the certificate with my name, email, location, etc.

To generate an x509 cert, type:
openssl req -new -x509 -days 365 -nodes -out stunnel.pem -keyout stunnel.pem

After the cert is generated the server can be set up using the following commands:
stunnel -d 2222 -r 127.0.0.1:80 -p /root/stunnel.pem

The server would now be listening on port 2222 for incoming client traffic. Traffic connecting to the servers listening port would be forwarded to 127.0.0.1 at port 80.

CLIENT setup:

Download the Stunnel setup for windows and install. Heres a link to the latest compiled binaries:
http://www.stunnel.org/download/stunnel/win32/stunnel-4.29-installer.exe

Rename the original stunnel.conf file to stunnel.conf.bak for backup purposes.

Now make a file named stunnel.conf in that same directory. Input the following using notepad:

Client = yes
[my_https]
accept = 80
connect = 10.0.0.1:2222

Save this file. Now run stunnel.exe (You can also run from the command line: c:\stunnel.exe stunnel.conf). Stunnel looks for stunnel.conf in the same directory by default. If you choose to use a config file with a different name, you would have to open up the command prompt and type as follows to run: c:\stunnel.exe myconfigfile.conf.Your client would now be listening on port 80. To use the tunnel, type in your browser, http://127.0.0.1:80. You should now see the webpage. This webpage was successfully transfered over your securely created ssl tunnel.

Resources/Good reading:
www.stunnel.org
http://freshmeat.net/articles/ssl-encrypting-syslog-with-stunnel
http://librenix.com/?inode=7126