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Ctunnel 0.8

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 virtual 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 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:

Source is managed 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/


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

Forwarding VNC

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

On the client machine ( we'll run ctunnel.

    ./ctunnel -c -l -f -C aes-256-cfb

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

    ./ctunnel -s -l -f -C aes-256-cfb

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


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 was only listening on it's local loopback interface of, this way the only way to access it would be via ctunnel.

Forwarding MySQL


    ./ctunnel -c -l -f -C aes-256-cfb


    ./ctunnel -s -l -f -C aes-256-cfb


    mysql -u root -p -h 

Intermediate Proxy / bouncing connections


    ./ctunnel -c -l -f -C aes-256-cfb


    ./ctunnel -s -l -f -C aes-256-cfb &
    ./ctunnel -c -l -f -C aes-256-cfb


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

Forwarding DNS / UDP:


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


    ./ctunnel -U -n -c -l -f -C aes-256-cfb 
    dig @localhost 

Plaintext proxy

Proxy TCP connections with no encryption from port 2202 to port 22

    ./ctunnel -n -c -l -f -C plain

VPN Mode

Please note that if you want machines on either side of the tunnel to be able to contact each other, you'll need to enable IP Forwarding in the kernel, as well as add static routes which point to each network.

TUN/TAP (default)


    ./ctunnel -V -U -n -s -l -C aes-128-cfb -r


    ./ctunnel -V -U -n -c -f -C aes-128-cfb -r

PPP mode


    ./ctunnel -V -U -n -s -t 1 -l -C rc4 \
    -P '/usr/sbin/pppd nodetach noauth unit 1'


    ./ctunnel -V -U -n -c -f -C rc4 \ 
    -P '/usr/sbin/pppd nodetach noauth passive'


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.


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, after which CTunnel won't prompt you again until you remove your passkey file located in ~/.passkey


OpenSSL or libgcrypt

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

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


Set CRYPTO_TYPE in Makefile.cfg . The default is OPENSSL.

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 much 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 an IP instead


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

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

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

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 by default.

If you are still having trouble, please create an issue on our github page:

VPN Checklist:

  • Did you check if IP Forwarding is enabled in the kernel?
  • Do you have routes pointing correctly to your destination network?
  • Did you specify -U? Remember, TCP is the default for ctunnel, but VPN's don't work well in TCP mode.

Copyright (c) 2009-2020 Jess Mahan [email protected]

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