Copyright (c) 2009-2018 Jess Mahan ctunnel@alienrobotarmy.com

ctunnel 0.7

What it is: ctunnel is a software for proxying and forwarding TCP or UDP connections via a cryptographic or plain tunnel.

ctunnel can also operate as a VPN creating a private network 
between ctunnel hosts.

ctunnel can be used to simply proxy TCP or UDP traffic, proxy
and compress, or to secure any existing TCP or UDP based 
protocol( such as HTTP, Telnet, FTP, RSH, MySQL, etc).
You can even tunnel SSH! (if you are really paranoid!).

You can also chain/bounce connections to any number of intermediary
hosts (including VPN mode).

Where to get it

Official releases and snapshots may be obtained from the following location: http://alienrobotarmy.com/ctunnel

You may also fork us on github!

How it works

In tunnel mode (default) ctunnel works by listening on the client machine, encrypting the TCP or UDP traffic, and then forwarding the encrypted traffic to the server, where another instance of ctunnel will decrypt that traffic in turn and forward the decrypted traffic to the destination port.

In VPN mode ctunnel has a point-to-point VPN mode (multiple clients may work but your milage may vary). A tun device on both the client and server are required (using ppp instead of tuntap is experimental and routes must be added to /etc/ppp/ip-up). Once the ctunnel is connected and the VPN is established, it is up to you to add any IPTABLES/Forwarding rules on the client or server. (Examples for post-up forwarding are including in libexec/up.sh)

Examples

How about an example?

** Note, the examples below are for OpenSSL ** ** Substitute '-C aes-256-cfb' with '-C aes256 -M cfb' ** ** when compiled with libgcrypt **

For instance, your local machine has an IP of 10.0.0.2. Now let's say you've got a VNC server running on 10.0.0.4, listening on 5901 (the default port for vnc) and you want to secure it.

On the client machine (10.0.0.2) we'll run ctunnel.

./ctunnel -c -l 127.0.0.1:2221 -f 10.0.0.4:2222 -C aes-256-cfb

On the server machine (10.0.0.4 running the vnc server) we'll also run ctunnel.

./ctunnel -s -l 10.0.0.4:2222 -f 127.0.0.1:5901 -C aes-256-cfb

On the client machine (10.0.0.2) we run vncviewr throught the tunnel.

./vncviewer 127.0.0.1::2221

Ta DA! You've got an encrypted tunnel right to your VNC Server.

An even more secure example would be to make sure that VNC Server on 10.0.0.4 was only listening on it's local loopback interface of 127.0.0.1, this way the only way to access it would be via ctunnel.

MySQL:

Client/10.0.0.2

./ctunnel -c -l 127.0.0.1:2221 -f 10.0.0.4:2222 -C aes-256-cfb

Server/10.0.0.4

./ctunnel -s -l 10.0.0.4:2222 -f 127.0.0.1:3306 -C aes-256-cfb

Client

mysql -u root -p -h 127.0.0.1 

You can also bounce connections off an intermediary proxy, like this:

Client/10.0.0.2

./ctunnel -c -l 127.0.0.1:2221 -f 10.0.0.3:2222 -C aes-256-cfb

Proxy/10.0.0.3

./ctunnel -s -l 10.0.0.3:2222 -f 127.0.0.1:2223 -C aes-256-cfb &
./ctunnel -c -l 127.0.0.1:2223 -f 10.0.0.4:2224 -C aes-256-cfb

Server/10.0.0.4

./ctunnel -s -l 10.0.0.4:2224 -f localhost:3306 -C aes-256-cfb

DNS:

Server/10.0.0.3

./ctunnel -U -n -s -l 0.0.0.0:5001 -f localhost:53 -C aes-256-cfb

Client/10.0.0.2 ./ctunnel -U -n -c -l 0.0.0.0:53 -f 10.0.0.3:5001 -C aes-256-cfb dig @localhost alienrobotarmy.com

VPN:

(default uses TUN/TAP) Server/192.168.1.2 ./ctunnel -V -U -n -s -l 192.168.1.2:1024 -C aes-128-cfb -r 192.168.1.0/24

Client/10.0.0.50 ./ctunnel -V -U -n -c -f 192.168.1.2:1024 -C aes-128-cfb -r 10.0.0.0/24

(PPP mode) Server/192.168.1.2 ./ctunnel -V -U -n -s -t 1 -l 192.168.1.2:1024 -C rc4
-P '/usr/sbin/pppd nodetach noauth unit 1'

Client/10.0.0.50 ./ctunnel -V -U -n -c -f 192.168.1.2:1024 -C rc4 \ -P '/usr/sbin/pppd nodetach noauth passive 10.0.5.2:10.0.5.1'

Ciphers

ctunnel currently allows you to specifcy any OpenSSL/libgcrypt cipher via the -C switch (-C and -M for libgcrypt). ctunnel does not check wether you are using a stream or block cipher, but you MUST use a stream cipher for it to work.

******** YOU MUST USE A STREAM CIPHER ******** (or a block cipher in cfb,ofb,ctr mode - stream)

In the example above we use aes-256-cfb, which is the Cipher Feeback mode for aes-256.

Keys

So, how do we securely make a tunnel with a stream cipher? I'll bet you're thinking Keys, and you're correct, partly! Thinking passwords? You're correct there also.

Let's explain: CTunnel does not rely on traditional PEM format keys, or a CA authority. It uses pre shared keys (passwords). CTunnel will store your "Passkey" in ~/.passkey. It stores a 16 character Key and IV in this file. SO PROTECT IT!

On your first run of CTunnel you will be prompted to enter your Key and IV in, after which CTunnel won't prompt you again until you remove your passkey file located in ~/.passkey

Requirments

OpenSSL http://www.openssl.org or libgcrypt http://www.gnupg.org

Typically you can just apt-get install libssl-dev or grab the openssl or libgcrypt development libraries and headers for your distro.

VPN Mode requires a TUNTAP Driver or pppd. TUNTAP is standard on Linux. For win32 and OSX, you will need a 3rd party tuntap driver such as the one budled with OpenVPN http://openvpn.net

VPN Mode may be used with PPP in place of TUNTAP. In this case you need a working pppd binary compiled for your system

Building

Yup, you guessed it... If you have met all the requirements then just do: make; make install

Known Issues

aes-256 cfb in mixed openssl / gcrypt implementations does not work, use aes-128 instead.

Using PPP mode, routes are not exchanged between endpoints. Routes should not be added to the post up exec scrip. Routes should be added to ppp's internal hook-script /etc/ppp/ip-up (or script passed to ipparam option to pppd).

using PPP mode is slower than tun/tap - this is to be expected.

Win32: using an asterisk when trying to bind to an interface with -l may result in segfault or "bind(): Result to large". Specify IP instead

Roadmap

Next release the -C encryption option will be replaced with per endpoint encryption options. For instnace:

-l localhost:22:aes-128-cfb -f 10.0.0.1:22:rc4

This will allow greater flexibility especially when ctunnel is the intermediary proxy and each remote endpoint have different encryption.

Perhaps adding the ability for individual keys per endpoint.

Add options for per endpoint protocol: -l localhost:22:udp:aes-128-cfb -f 10.0.0.1:22:tcp:rc4

Getting Help

If you need help, please make sure before asking a question that you do indeed have the ssl development libraries installed, and that you have read and understand the section "Examples" and the section "Ciphers".

More often than not you are either getting your -c/-s switches mixed up, or you are not using a stream cipher as specified in the "Ciphers" section.

NOTE: If you do not specify the -U switch (to operate in UDP mode), CTunnel will operate in TCP mode.

If you are still having trouble, feel free to contact me. I can be reached via ctunnel-@nardcore.org