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 (SPA) 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.
Overview Port knocking is usually implemented by configuring a daemon to watch the firewall log file for connection attempts to certain points, and then to modify the firewall configuration accordingly. It can also be performed on the kernel level (using a kernel-level packet filter such as iptables) or by a userspace 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 actively used in many rootkits even before year 2000.
Benefits Defeating port knocking protection requires large-scale brute force attacks in order to discover even simple sequences. An anonymous brute force attack against a three-knock TCP sequence (e.g. port 1000, 2000, 3000) would require an attacker to test every three port combination in the 1–65535 range and then scan each port between attacks to uncover any changes in port access on the target system. Since port knocking is by definition stateful, the requested port would not open until the correct three-port number sequence had been received in the correct order and without receiving any other intervening packets from the source. The average case scenario requires approximately 141 trillion (655353 / 2) packets to determine a correct three-port number. This technique, in combination with knock attempt-limiting, longer or more complex sequences and cryptographic hashes, makes successful port access attempts extremely difficult. Once the successful port knock sequence is supplied to open a port, firewall rules generally only open the port to the IP address that supplied the correct knock, adding dynamic functionality to firewall behaviour. Instead of using a preconfigured static IP whitelist on the firewall, an authorised user situated anywhere in the world would be able to open any necessary port without assistance from the server administrator. The system could also be configured to allow the authenticated user to manually close the port once the session is over or to have it close automatically using a timeout mechanism. To establish a new session, the remote user would be required to reauthenticate using the correct sequence. The stateful behaviour of port knocking allows several users from different source IP addresses to be at varying levels of port knock authentication simultaneously, allowing a legitimate user with the correct knock sequence through the firewall while the firewall itself is in the middle of a port attack from multiple IP addresses (assuming the bandwidth of the firewall is not completely consumed). From any other attacking IP address, the ports on the firewall will still appear to be closed. Using cryptographic hashes inside the port knock sequence defends against packet sniffing between the source and target machines, preventing discovery of the port knock sequence or using the information to create traffic replay attacks to repeat prior port knock sequences. Port knocking is used as part of a defense in depth strategy. Even if the attacker were to successfully gain port access, other port security mechanisms are still in place, along with the assigned service authentication mechanisms on the opened ports. Implementation of the technique is straightforward, using at the bare minimum a shell script on the server and a Windows batch file or command line utility on the client. Overhead on both the server and client in terms of traffic, CPU and memory consumption is minimal. Port knock daemons are not complex to code, with a low audit burden. A port knock system implemented on password-authenticated services, like SSH, sidesteps the issue of brute force password attacks on logins. In the case of SSH, the SSH daemon is not activated without the correct port knock, and the attack is filtered by the TCP/IP stack rather than using SSH authentication resources. To the attacker, the daemon is inaccessible until the correct port knock is supplied.
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