Firewall administrators are challenged to balance flexibility and security when designing a comprehensive rule set. A firewall should provide protection against malfeasants, while allowing trusted users to connect. Unfortunately, it is not always possible to filter out the bad guys, because filtering on the basis of IP addresses and ports does not distinguish connecting users. Bad guys can and do come from trusted IP addresses. Open ports remain a necessary vulnerability: they allow connections to applications but also may turn into open doors for attack. This article presents a new security system, termed port knocking, in which trusted users manipulate firewall rules by transmitting information across closed ports.

Briefly, users make connection attempts to sequences of closed ports. The failed connections are logged by the server-side packet filtering firewall and detected by a dæmon that monitors the firewall log file. When a properly formatted knock sequence, playing the role of the secret used in the authentication, is received, firewall rules are manipulated based on the information content of the sequence. This user-based authentication system is both robust, being mediated by the kernel firewall, and stealthy--it's not possible to detect whether a networked machine is listening for port knocks. Port knocking does not require any open ports, and it can be extended to transmit any type of information encoded in a port sequence.

In commonly deployed firewalls, filtering is done either by the IP address of the connecting host or by the port to which this host is connecting. Firewalls examine and interact with packets before any user authentication takes place; therefore, they do not discriminate amongst the users making the connection. It is expected that once the firewall has approved the packet and allowed it to enter the network, downstream applications will handle user authentication. Normally, this provides a sufficient balance between protection and flexibility. Some IP ranges, say cracker-friendly Internet cafés, may be closed completely to incoming traffic, while hosts in other IP ranges may be allowed to connect to ports otherwise unavailable to the general public (proprietary/sensitive applications). Unfortunately, this type of IP-based filtering has the potential to lock out trusted users from your system. Flexibility is limited by the fact that nobody from the blocked IP ranges can connect, regardless of their trust statuses. At the same time, protection is undermined by the fact that anyone from the blocked IP range physically can travel and connect from an unfiltered host.

In the end, as long as ports remain open, network applications are susceptible to attack. Using intrusion detection systems and keeping applications up to date can go a long way towards providing protection, but they do so against only known, derivative or anticipated attacks.

To eliminate the risk associated with publically open ports, port knocking provides an authentication system that works across closed ports. The use of these ports, however, has to be subverted because all packets are denied. Fortunately, in most firewalls that perform even the most rudimentary logging, information already is flowing across closed ports in the form of entries in a log file indicating connection attempts.

Consider the following example. A handful of ports (100-109) are configured to deny all traffic--no ICMP error packets are sent back to the connecting client--and all attempted connections are logged. In this example, the firewall IP is IPF and the connecting client IP is IPC. The appropriate ipchains command to close the ports and log connections is:

ipchains -A input -p tcp -s 0/0 -d IPF/32 100:109 -j DENY -l

A user attempts to connect from IPC to the following firewall ports in sequence: 102,100,100,103. From the point of view of the user, the connections fail silently. On the firewall, though, the 102,100,100,103 number sequence has been recorded.

Feb 12 00:13:26 ... input DENY eth1 PROTO=6 IPC:64137 IPF:102 ...
Feb 12 00:13:27 ... input DENY eth1 PROTO=6 IPC:64138 IPF:100 ...
Feb 12 00:13:27 ... input DENY eth1 PROTO=6 IPC:64139 IPF:100 ...
Feb 12 00:13:28 ... input DENY eth1 PROTO=6 IPC:64140 IPF:103 ...

The knock sequence appears in the firewall log, and the user has transmitted data across the closed ports.

Any implementation of the port knocking system needs to provide some basic functionality. First, some way to monitor the firewall log file needs to be devised. A simple Perl application that tails the file is presented in Listing 2, discussed more fully later in the article. Second, a method is required to extract the sequences of ports from the log file and translate their payload into usable information. In this step it is important to be able to (a) detect when a port sequence begins and ends, (b) correctly detect a port sequence in the presence of spurious connection attempts that are not part of the sequence and (c) keep track of multiple port sequences arriving at the same time from different remote IPs. The encoding used to generate the port sequence can be designed to minimize the length of the sequence. For example, the sequence 100,102 could correspond to one or a series of predefined operations (for example, open port ssh/22 for 15 minutes for a specific IP and then close the port). Finally, once the information is derived from the sequence, the implementation must provide some way to manipulate the firewall rules.

One of the key features of port knocking is it provides a stealthy method of authentication and information transfer to a networked machine that has no open ports. It is not possible to determine successfully whether the machine is listening for knock sequences by using port probes. Thus, although a brute-force attack could be mounted to try to guess the ports and the form of the sequence, such breach attempts could be detected easily.

Second, because information is flowing in the form of connection attempts rather than in typical packet data payload, without knowing that this system is in place it would be unlikely that the use of this authentication method would be detected by monitoring traffic. To minimize the risk of a functional sequence being constructed by the intercepting party, the information content containing the remote IP of the sequence can be encrypted.

Third, because the authentication is built into the port knock sequence, existing applications need not be changed. Implementing one-time passwords is done easily by adjusting the way particular sequences are interpreted. A sequence could correspond to a request that a port be opened for a specific length of time and then closed and never opened again to the same IP. Furthermore, a one-time pad could be used to encrypt the sequence, making it indecipherable by those without the pad.

To use port knocking, a client script that performs the knock is required. The client and any associated data should be considered a secret and kept on removable media, such as a USB key. The use of the client imposes an overhead for each connection. Certain locations, such as libraries or Internet cafés, may not allow execution of arbitrary programs.

In order to use port knocking, a number of ports need to be allocated for exclusive use by this system. As the number of such ports increases, the knock sequences becomes shorter for a given amount of information payload, because the number of coding symbols is increased. Practically, 256 free privileged ports (in the 1-1024 range), not necessarily contiguous, usually can be allocated and used to listen for port knocks.

Finally, any system that manipulates firewall rules in an automated fashion requires careful implementation. For the scenario in which no ports are initially open, if the listening dæmon fails or is not able to interpret the knocks correctly, it becomes impossible to connect remotely to the host.

In this section, three examples are outlined that illustrate how the port knocking system can be used.

1. Single Port, Fixed Mapping

Connection to only one port (ssh/22) is required. The ssh dæmon is running; all privileged ports are closed, including ssh/22; and packets addressed to ports 30,31,32 are being logged. The following port sequences are recognized:

31,32,30        open ssh/22 to connecting IP
32,30,31        close ssh/22 to connecting IP
31,30,32        close ssh/22 to connecting IP and disregard further knocks from this IP

The justifiably paranoid administrator can open the ssh/22 port on his system by initiating TCP connections to ports 31,32,30. At the end of the ssh session, the port would be closed by using the second sequence shown above. If the host from which the administrator is connecting is not trusted (if, say, keystrokes may be snooped), the use of the third sequence would deny all further traffic from the IP, preventing anyone from duplicating the session. This assumes the port sequence and system login credentials are not captured by a third party and used before the legitimate session ends.

In this example, only three sequences are understood by the system, as the requirements call for only a handful of well-defined firewall manipulations. The sequences were chosen not to be monotonically increasing (30, 31, 32), so they would not be triggered by remote port scans. If multiple ports are to be protected by this system, a mapping needs to be derived between the port sequence and a flexible firewall rule. This is covered in the next example.

2. Multiple Port, Dynamic Mapping

In this example, a network may be running any number of applications. Ports 100-109 are used to listen to knocks. The port sequence is expected to be of the form:

102,100,110  10a,10b,10c,10d  10(a+b+c+d mod 10)        110,100,102
header          payload             checksum                 footer

The first and last three ports let the port knocking dæmon know that a sequence is starting and ending. The next four ports encode the port (abcd) to be opened. For example, if a connection to port 143 is required, the sequence would be 100,101,104,103. The final element in the sequence is a checksum that validates the sequence payload. In this example, the checksum is 8 (1+4+3 mod 10). The sequence element therefore is 108, and the full sequence would be

102,100,103  100,101,104,103 108 103,100,102

When this sequence is detected, port 143 would be made available to the incoming IP address. If the port is open already, the knock would rendered it closed. The knock can be extended to include additional information, such as an anticipated session length, that can be used to close the port after a set amount of time.

3. Mapping with Encryption

The information contained in the knock sequence can be encrypted to provide an additional measure of security. In this example, 256 ports are allocated and logged. A knock map of the form

remote IP  port   time   checksum

is used where the remote IP, port, time and checksum (sum of other fields mod 255) are encrypted. The encrypted string can be mapped onto eight unsigned chars using Perl's pack("C*",STRING) command, see Listing 1.

Listing 1. Mapping the Encrypted String

A minimal prototype Perl implementation of port knocking is presented. The implementation is comprised of a knockclient, responsible for originating the knock sequence, and a knockdæmon, responsible for monitoring the firewall log and manipulating the rules.

The complete client is shown in Listing 1. Lincoln Stein's Crypt::CBC module is used as proxy to Crypt::Blowfish to carry out encryption. The unencrypted knock sequence is comprised of seven values: four IP bytes, a port (limited to the range 0-255 in this implementation), a time flag and a checksum (mod 255). The time flag determines how the dæmon reacts: 0 to open the port, 255 to close the port and any other value in the 1-254 range to open the port and then close it after that many minutes. The knock on the firewall (IP=IPF) to open port ssh/22 on IP=IPC and then have the port close after 15 minutes would be executed by calling the client as follows:

knockclient -i IPC -r IPF -p 22 -t 15

The client packs the list of seven integers, performs the encryption and unpacks the string into unsigned chars (0-255). These values are then mapped onto a sequence of ports in the 745-1000 range.

The knockdæmon is shown in Listing 2. This application uses File::Tail to look for new lines in the firewall log file. Lines corresponding to connection attempts to ports 745-1000 are parsed for the remote IP and port number. An 8-element queue storing the ports is maintained for each incoming IP. When the queue size reaches 8, its contents are decrypted. If the decryption is successful and the checksum is correct, appropriate action is taken and the queue is cleared. If the decryption fails, the oldest queue port element is removed and the dæmon continues monitoring.

Listing 2. knockdæmon

The firewall rules are manipulated by a system call to the ipchains binary, although the IPChains Perl module by Jonathan Schatz also may be used. If the port is to be closed, as indicated by the time flag, Jose Rodrigues' Schedule::At module is used to schedule the deletion of the rule using the at queue system.

Port knocking is a stealthy authentication system that employs closed ports to carry out identification of trusted users. This novel method provides the means of establishing a connection to an application running on a completely isolated system on which no ports initially are open.

Port Knocking Scripts

Randomness Recommendations for Security (RFC 1750)

Registered Ports List (RFC 3232)

ipchains/iptables

"An Introduction to Using Linux as a Multipurpose Firewall", Jeff Regan, Linux Journal, Issue 71, March 2000.