From Wikipedia, the free encyclopedia
In computer networking, port knocking is a method of externally opening ports on a firewall by generating a connection attempt on a set of prespecified closed ports. Once a correct sequence of connection attempts is received, the firewall rules are dynamically modified to allow the host which sent the connection attempts to connect over specific port(s). A variant called Single Packet Authorization exists, where only a single ‘knock’ is needed, consisting of an encrypted packet.
The primary purpose of port knocking is to prevent an attacker from scanning a system for potentially exploitable services by doing a port scan, because unless the attacker sends the correct knock sequence, the protected ports will appear closed.
This is usually implemented by configuring a daemon to watch the firewall log file for said connection attempts then modify the firewall configuration accordingly. It can also be
performed by a process examining packets at a higher level (using packet capture interfaces such as pcap), allowing the use of already “open” TCP ports to be used within the knock sequence.
The port “knock” itself is similar to a secret handshake and can consist of any number of TCP, UDP or even sometimes ICMP and other protocol packets to numbered ports on the destination machine. The complexity of the knock can be anything from a simple ordered list (e. g. TCP port 1000, TCP port 2000, UDP port 3000) to a complex time-dependent, source-IP-based and other-factor-based encrypted hash.
A portknock daemon on the firewall machine listens for packets on certain ports (either via the firewall log or by packet capture). The client user would carry an extra utility, which could be as simple as netcat or a modified ping program or as complicated as a full hash-generator, and use that before they attempted to connect to the machine in the usual way.
Most portknocks are stateful systems in that if the first part of the “knock” has been received successfully, an incorrect second part would not allow the remote user to continue and, indeed, would give the remote user no clue as to how far through the sequence they failed. Usually the only indication of failure is that, at the end of the knock sequence, the port expected to be open is not opened. No packets are sent to the remote user at any time.
While this technique for securing access to remote network daemons has not been widely adopted by the security community, it has been integrated in newer rootkits.
Benefits of port knocking
Consider that, if an external attacker did not know the port knock sequence, even the simplest of sequences would require a massive brute force effort in order to be discovered. A three-knock simple TCP sequence (e. g. port 1000, 2000, 3000) would require an attacker without prior knowledge of the sequence to test every combination of three ports in the range 1-65535, and then to scan each port in between to see if anything had opened. As a stateful system, the port would not open until after the correct three-digit sequence had been received in order, without other packets in between.
That equates to a maximum of 655363 packets in order to obtain and detect a single successful opening, in the worst case scenario. That’s 281,474,976,710,656 or over 281 trillion packets. On average, an attempt would take approximately 9.2 quintillion packets to successfully open a single, simple three-port TCP-only knock by brute force.