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https://github.com/moxie0/knockknock

A simple, secure, and stealthy port knocking implementation that does not use libpcap or bind to a socket interface.
https://github.com/moxie0/knockknock

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A simple, secure, and stealthy port knocking implementation that does not use libpcap or bind to a socket interface.

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Some History:



    The port knockingconcept has been around for a while, and there

    are many different port knocking implementations.  The core idea

    is that you send a sequence of innocuous looking packets to a

    server, which have the effect of adjusting the firewall rules to

    allow you to connect through on a port that was previously

    firewalled off.

  

    Originally, this was simply conceived as a series of connection

    attempts to closed ports in a specific order.  The "knock" would

    be something like trying to connect to ports 12, 23, 18, 66, or

    whatever, in that order, over a specific period of time.

  

    The problem with the original concept was that if your port

    sequence was observed by passive eavesdropping, it was easily

    replayable.  The obvious solution was to develop a port knocking

    system that did not allow for replay attacks.  Such a solution

    suggests the use of cryptography.

  

    And this is where the insanity begins.  People started

    implementing all kinds of stuff to achieve this goal, and along

    the way lost sight of the original intent behind port knocking.

  

    Let me tell you what I /don't/ want from a port knocking

    implementation:

    

      * I don't want something written in an an unsafe language.  This

        should be a very small application, and the performance

        requirements should be minimal.  

      * I don't want something that runs /in the kernel/.  

      * I don't want an entirely new service that binds to a port.  

      * I don't want something that uses libpcap and inspects /every/ 

        packet.  

      * I don't want something that uses UDP.  

      * I don't want something that generates obvious port-knock 

        requests and leaves an observer knowing exactly what port is 

        about to open up.  

      * I don't want something which requires that more than one

        packet move over the network.  

      * I don't want something that uses half-baked cryptography and

        isn't provably IND-CCA secure.

    

  

    But from what I can tell, this is exactly what people have

    implemented.  Despite the fact that the main thrust of this is to

    have less services running, the direction that this has been going

    is to have whole servers written in C that bind to sockets and

    exchange UDP packets.  The entire /point/ of port-knocking was for

    it to be stealthy, simple, and secure.  The goal was to eliminate

    network services, not create more of them.  We can use

    cryptography to /simplify/ the initial port knocking concept,

    rather than making it more complex.

  

knockknock Overview:



    So here's how knockknock works:

    

      * Servers run the python app 'knockknock-daemon', and clients

        open ports on those servers by running the python app

        'knockknock'



      * 'knockknock-daemon' simply tails kern.log.  It doesn't bind to

        any sockets, load libpcap and inspect every packet, or send

        anything onto the network at all.



      * When you want to open a port from a client, you run

        'knockknock', which sends a /single/ SYN packet to the server.

        The packet's IP and TCP headers are encoded to represent an

        IND-CCA secure encrypted request to open a specified port from

        the source IP address.



      * Fields from this packet are logged to kern.log and processed

        by 'knockknock-daemon', which validates them.



      * You connect from the client to the now-open port on the

        server.



      * The port closes behind you and doesn't allow any new

        connections.

    

  

    That's it.  Written in python, simple, single-packet, secure.

  

    The request is encrypted using AES in CTR mode, with an HMAC-SHA1

    using the authenticate-then-encrypt paradigm.  It protects against

    evesdropping, replay attacks, and all known forms of cryptanalysis

    against IND-CCA secure schemes.

  

Why Is knockknock Secure?

  

    * The knockknock-daemon code is very simple, and is written in

      python (a 'safe' language).  The code is concise enough to be

      easily audited, and doesn't make use of any crazy libraries like

      libpcap.



    * While knockknock-daemon needs root priviledges to adjust

      iptables rules, it employs privilege separation to isolate the

      code that actually runs as root to ~15 lines.  So even though

      the entire code base is very small and very simple, the only

      part of the code actually running with root privilges is even

      smaller and even simpler.  When you run knockknock-daemon, it

      will fork out the privileged code and drop privilges everywhere

      else before processing knockknock requests.



    * The communication protocol is a simple IND-CCA secure encryption

      scheme that uses standard, contemporary, cryptographic

      constructions.  An observer watching packets is not given any

      indication that the SYN packet transmitted by 'knockknock' is a

      port knocking request, but even if they knew, there would be no

      way for them to determine which port was requested to open.

      Replaying the knock request later does them no good, and in fact

      does not provide any information that might be useful in

      determining the contents of future requets.

  

  

Why Is This Even Necessary?  



    /You are running network services with security vulnerabilities in

    them./ Again, /you are running network services with security

    vulnerabilities in them/.  If you're running a server, this is

    almost universally true.  Most software is complex.  It changes

    rapidly, and innovation tends to make it more complex.  It is

    going to be, forever, hopelessly, insecure.  Even projects like

    OpenSSH that were designed from the ground-up with security in

    mind, where every single line of code is written with security as

    a top priority, where countless people audit the changes that are

    made every day -- even projects like this have suffered from

    remotely exploitable vulnerabilities.  If this is true, what hope

    do the network services that are written with different priorities

    have?

  

    The name of the game is to isolate, compartmentalize, and /expose

    running services as little as possible/.  That's where knockknock

    comes in.  Given that your network services are insecure, you want

    to expose as few of them to the world as possible.  I offer

    knockknock as a possible solution to minimizing exposure.