Michael Rash, Security Researcher

Port Knocking: Why You Should Give It Another Look

Port Knocking: Why you Should Give It Another Look (Update 10/20/2013: There is a Reddit comment thread going on this post here.)

It has been a decade since Port Knocking was first introduced to the security community in 2003, so it seemed fitting to recap how far the concept has evolved. Much effort has gone into solving architectural problems with PK systems, and today Single Packet Authorization (SPA) embodies the primary benefits of PK while fixing its limitations.

There are noted security researchers on both sides of the debate as to whether PK/SPA has any security value, but it is interesting that researchers who don't find value seem to concentrate on aspects of PK/SPA that have little to do with the chief benefit: cryptographically strong concealment. At least, this is the property offered by Single Packet Authorization but admittedly not necessarily with Port Knocking. Let's first go through some of the more common criticisms of PK/SPA, and show what the SPA answer is to each one. For those that haven't considered SPA in the past, perhaps it is time to give it a second look if for no other reason than to propose a method for breaking it.

1) Isn't PK/SPA just another password?

Suppose I hand you an arbitrary IP, say that is running a default-drop firewall policy. As an attacker, you scan and can't get any information back whatsoever. It doesn't respond to pings, Nmap cannot detect any TCP or UDP service under all scanning techniques, and any Metasploit module that relies on a TCP connect() call is ineffective. In the absence of a routing issue, it is safe to assume there is a firewall or ACL blocking all incoming scans. It is not feasible to tell whether there are any services listening (SSH or otherwise), and it also not feasible to tell whether there is a PK/SPA daemon either - the firewall is being used to do what it does best: block network traffic. The point is that there is no information coming back from the target. A PK/SPA daemon may be deployed, but the passive nature of PK/SPA makes it undiscoverable [1].

As a thought experiment, what would it take to make PK/SPA "just another password"? Well, if the PK/SPA daemon listened on a TCP socket and advertised itself via a server banner (like "fwknop-2.5.1, enter password for SSH access:") this would go a long way. Then Nmap would once again become an effective tool for finding the PK/SPA daemon, and an attacker could start to try different passwords. In other words, the daemon would no longer be passive, which is the whole point of PK/SPA to begin with.

But wait, you might say "but attackers can just try to brute force passive PK/SPA daemons anyway (even though they can't scan for them directly) and see if a port opens up", which brings us to:

2) Can PK/SPA be brute-forced?

Port Knocking implementations that use simple shared sequences are certainly vulnerable to brute forcing, and they are also vulnerable to replay attacks. These vulnerabilities (among other problems) were primary motivators for the development of SPA, and any modern SPA implementation is not vulnerable to either of these attacks. For example, fwknop uses AES in CBC mode authenticated with an HMAC SHA-256 in the encrypt-then-authenticate model, and both the encryption and HMAC keys (256 and 512 bits respectively for a total of 768 bits) are generated from random data in --key-gen mode. Further, fwknop can leverage GnuPG instead of AES, and 2048-bit GnuPG keys are fully supported. If it were practical to brute force fwknop encryption and authentication, then it would also be practical to brute force a lot of other cryptographic software too. Hence, fwknop is not vulnerable to such attacks in any practical sense [2].

Beyond this, from 1) above remember that the very existence of the SPA daemon is not discoverable by an attacker. For the average adversary, interacting with the SPA daemon must be done blindly "by chance". So, the target - even if it is running an SPA daemon which the attacker can't see - which implementation should the attacker try to brute force? If the target is running Moxie Marlinspike's knockknock (which uses AES in CTR mode authenticated with a truncated HMAC SHA-1), then the attacker needs to try and brute force the daemon with crafted TCP SYN packets via the following fields: TCP window size, TCP sequence, TCP acknowledgement, and the network layer IP ID. On the other hand, if the target is running fwknop, then the attacker would have to try and brute force the fwknopd daemon with UDP payloads to a port that the fwknopd pcap filter statement allows (although fwknopd can also be configured to only accept SPA payloads over ICMP instead). Should the attacker try to brute force the fwknop AES-CBC + HMAC SHA-256 mode? Or the GnuPG + HMAC SHA-256 mode? Further, the fwknopd daemon can place restrictions on the services that an authenticated client is authorized to request via the access.conf file. There are a lot of bits adding up, and the entire time the attacker doesn't even know whether an SPA daemon is actually running let alone which one.

Unfortunately for the attacker it gets worse. Even if the attacker could somehow brute force both the encryption and authentication steps in fwknop or other SPA software, to which service should the attacker try to make a connection? No service has to listen on the default port, so if a connection to SSH isn't answered should the attacker scan the target looking for a service? Maybe SPA is being used to conceal an IMAP daemon, a webserver, or OpenVPN instead of SSHD? Further, because the SPA daemon never acknowledges anything to begin with, the attacker can only infer that a brute force attempt was successful by seeing if a service is finally available after each attempt. So, in order for the attacker to be effective, the work flow for every brute force attempt should be: (1) brute force attempt, (2) scan for SSH (just because SPA is usually used to conceal SSH), (3) full scan if the SSH scan doesn't work. This starts to become extremely noisy to say the least. Even if full scans aren't also used, the volume of traffic just to attempt brute force operations by themselves is prohibitively huge.

Regardless of the attacker work flow, which service is concealed, or how heavily the attacker scans the target after every attempt, the brute force resistance offered by fwknop is fundamentally provided through the strength of cryptography. Getting past the authentication step alone would require breaking the 512-bit HMAC SHA-256 key (or forcing a hash collision against SHA-256), and fwknop even supports HMAC SHA-512 if one prefers that instead. Beyond this, the encryption key would also need to be brute-forced. For all intents and purposes it is not practical to brute force fwknop, and similar arguments apply to other SPA software.

3) Does PK/SPA add intolerable complexity?

Every security measure has some associated complexity. Firewalls add complexity. Encryption adds complexity. SSH itself adds complexity. If complexity were always the trump card against higher levels of security, then people would connect admin shells to sockets directly via netcat (or just run telnet) and not worry about encryption. People would not run firewalls because vanilla IP stacks without firewalling hooks would be simpler. Filesystem permissions overlays would be considered insecure because they add complexity. Obviously such viewpoints don't pass muster in the real world. The point is that using more complex code sometimes enables higher levels of security against widely understood threat models. For example, people use SSH and SSL because they want authentication and confidentiality over untrusted networks. Firewalls are used to reduce the attack surface that an adversary can easily communicate with. Application and/or filesystem layer policy controls are engineered to place restrictions on classes of users. The list continues.

This is not to say that complexity is not an important consideration - far from it. Rather, a real security benefit must be realized in order to justify increasing the complexity of a system. In the SPA community, we assert there is a security benefit afforded by passive, cryptographically strong service concealment. How would an attacker try to brute force user passwords via SSH when it is concealed by SPA? How would an attacker exploit even a zero-day vulnerability in a service protected by SPA? How would an attacker exploit a vulnerability in the SPA daemon itself when it is indistinguishable from a system that is running a default-drop firewall policy? (An attacker may interact with the SPA daemon, but it is more or less "by chance" [3].)

In the context of PK/SPA, the real issue is whether the complexity of the code that an attacker can interact with is more or less when PK/SPA is deployed. Anyone protecting networked services is probably already running a firewall, so the firewall usage by the PK/SPA software isn't adding to complexity that wasn't already there. Next, if PK/SPA is used to conceal multiple services (say, SSH, an IMAP daemon, and a webserver all at the same time), a would-be attacker cannot interact with any of those code bases without first getting past the PK/SPA daemon. It is a good bet that the complexity of the PK/SPA daemon is a lot less than the aggregate complexity of all three of those services if they were open to the world. Further, this may still be true even for a single daemon such as SSH as well. In essence, the effective complexity of code that an attacker can interact with may actually go down with PK/SPA deployed - that is, until the PK/SPA daemon can be circumvented (and hence a method for doing this becomes an important question for an attacker).

4) What happens if the PK/SPA daemon dies?

This is a solved problem. Process monitoring software has been around for decades. Many options exist for any OS on which a PK/SPA daemon is deployed. For example, on Ubuntu systems fwknopd is monitored by upstart. Having said this, fwknopd is extremely stable anyway so this feature is hardly ever needed in practice. Still, it is certainly important to ensure that PK/SPA usage does not cause a single point of failure, so using process monitoring software is a good idea.


There are other criticisms of SPA that are not included in this blog post, and certainly some of them are legitimate such as the fact that SPA requires a specialized client to access concealed services and the fact that "NAT piggybacking" is possible for users on the same network from which an SPA client is used when behind a NAT. However, these points don't generally rise to the level that they invalidate the SPA strategy. This blog post attempts to address those criticisms that could rise to this level were it not for the effort that has gone into solid SPA design by fwknop and other projects. More information on the design goals that guide fwknop can be found in the project tutorial.

In conclusion, SSL uses cryptography to provide authentication and confidentiality, Tor uses cryptography to provide anonymity, and SPA uses cryptography to conceal service existence. For those that assert there is no security value in the later strategy, it should consequently not be difficult to circumvent. To those in this camp, given the material in this post, please propose a method for breaking SPA.

[1] At least, a PK/SPA daemon is not discoverable by attackers who aren't already in a privileged position to sniff all traffic to and from the target. Clearly, most attackers - including password-guessing botnets - do not fall into this category.

[2] It is possible to weaken the security of fwknop SPA communications by not using --key-gen mode to generate random encryption and HMAC keys and thereby make them more susceptible to brute force attacks. However, this type of problems similarly affects other cryptographic software so it isn't unique to fwknop. And, even if a user doesn't use --key-gen mode, it is still not as easy to brute force fwknopd (which never confirms its existence to an attacker) as other software which an attacker can readily see is available to exploit.

[3] The security of fwknopd code itself is nonetheless quite important, and this is why the fwknop project uses static analysis provided by Coverity (and has a Coverity scan score of zero), the CLANG static analyzer, and also implements dynamic analysis with valgrind via a comprehensive test suite.