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"Port Knocking" For Added Security
Jeff writes "The process of Port Knocking is a way to allow only people who know the "secret knock" access to a certain port on a system. For example, if I wanted to connect via SSH to a server, I could build a backdoor on the server that does not directly listen on port 22 (or any port for that matter) until it detects connection attempts to closed ports 1026,1027,1029,1034,1026,1044 and 1035 in that sequence within 5 seconds, then listens on port 22 for a connection within 10 seconds.
The web site explains it in some detail, and there is even an experimental perl implementation of it that is available for download. I can't think of any easy ways you could get around a system using this security method - let alone even know that a system is implementing it.
Another article on port knocking is here."
This is secure in the same way 50-character passphrases are secure, sure they are harder to crack but who the hell is goig to remember them. The harder you make something to use, users will start trying to find ways around it.
whats more, connection attempts are easy to sniff, you might as well be using telnet...THIS THING IS BEGGING FOR A "REPLAY ATTACK".
I though about this along time ago as a way of hiding a trojan. Of course I didnt patent it so no money for me : /
Im not here now... Im out KILLING pepperoni
Right now, script kiddies have their computers automatically try to access other peoples' computers, looking for ones without firewalls, etc.. If this happens, wouldn't you expect them to just send out random knocks in the hopes of getting something? If that happens, you will be more secure personally, but the increased traffic may cause more problems that it solves.
G
And? It is still more secure. By using "port knocking" they HAVE to sniff out your network traffic and find the port combo. Without "port knocking" they just need to run nmap and see what ports they can try to attack.
"Have you ever thought about just turning off the TV, sitting down with your kids, and hitting them?"
Interesting. So the next step would be to have one-time port knock sequences a-la one-time passwords (to defeat adversaries who are grabbing a copy of all your packets). But it seems like there is a race condition between the delay after the knock and the actual connection. Anyone have a solution to this?
I was thinking about implementing this a while ago; I guess it's an obvious enough idea that others have been thinking along the same lines. This is equivalent to to putting a password on access to the port.
Ideally, the implementation will only consider connection attempts originating from the same IP address.
I predict a flood of commenters whining about this being "security through obscurity."
Yeah, just like passwords are "security through obscurity."
This is essentially another level of passwords, but sounds useful for hiding those services that could have vulnerabilities *cough* OpenSSH *cough*.
Will this technique itself have possible vulnerabilities?
Think of it this way... it's an extra password combined with bonus security-by-obscurity of not having a visible password prompt.
The "knocking ports" could also be configured that if there are random hits to the standard port without the proper knock, the system could lock down for 30 seconds and even ignore the proper knock so that if somebody's trying to brute force all the possible knocks, they'll never get feedback when they have the right one.
Yeah, this is no substitute for properly securing the original service, but it's an extra layer that means there's even more that needs to be captured for a successful hack...
It should be noted that this is NOT (necessarily) an example of security through obscurity. One could treat the port-knocking sequence as a "key". Long enough keys could make port-scanning impossible for anyone who doesn't know the key. Real mathematical cryptography is based on a similar principle.
Also, this is only a defense against port-scanning. Even if someone did manage to break the knocking sequence, they would still have to use some kind of exploit against the machine on the port they discovered.
-3Suns
~~~~
The Revolution will be Slashdotted
Hmm, lots of people have pointed this out, but it's easy to set up a system of one-time passwords... provided it's done in a cryptographically secure way, there's little point in sniffing for combinations.
Of course, you can still sniff to see what ports are actually in use...
What if multiple attempts from the same IP are made to access the port at the same time? Wouldn't the knocks get all mixed up?
Mad Software: Rantings on Developing So
OpenSSH is a great peice of sodtware - but it's so huge that I can't help but think that their could be flaws in it (like the one of 6 months ago)
I would love to layer another peice of security infront of OpenSSH and this seems like a great idea.
Moneyed corporations, non-working 'poor' and criminal prisoners are turning productive citizens into tax-slaves.
Has anyone implemented a system where a service would be stopped if the ports next to it were scanned? i.e. if 1024, 1025, 1026, 1027 were scanned, a service running on 1028 would stop.
This isn't going to catch on. It's not more secure and it wastes more resources.
Why would this be any more secure than listening on a single port for the "unique knock sequence?" Any good admin knows the most secure system is one that is listening on as few ports as possible.
Could this be implemented with IP Tables under Linux? I remember seeing a set of rules to detect a port scan; could a similar set of rules be used to unlock a port for a given remote IP number?
Of course, this won't take off unless there's also knocking support built into the clients (like ssh).
one could also change the sequence of ports that are used to be based on some key/progression/timestamp? so the knock is constantly changing... so even if someone sniffs the knock, it wont help.
"Nyquil - The stuffy, sneezy, why-the-hell-is-the-room-spinning medicine."
Improperly done, the knock sentry could become a security/QOS issue in itself.
This definitely is security through obscurity and perhaps would work in the same way as a car alarm. There's lots more systems out there that are easier to break into, and if someone does try, just hope that they get fed up and moves on to the next one.
If you've gone this far, why not do something like they do on radio. Open up severable ports at the same time and multiplex your signal over several of them while sending noise over the ununsed ports randomly switching between ports using a syncronized random selector.
Implement it in combination with a onetime type password arrangement. You look up what the series of knocks is supposed to be on your secureID card (or whatever), then knock in the combination it tells you to use. Tie it in with the server you want to get into, and the port you actually connect to for ssh can be different every time.
IE, secureID says sequence is "1234 1441 1114 5123", you knock on the first three, and 5123 is the ssh port activated for you only.
Casca
It doesn't have to be listening on the 'knock' ports, it can be dropping the packets and either logging the drop or setting a flag via some daemon. There are a million ways to tell if someone attempted to access a closed port without having to open the port. All of this, by my calculation, makes port knocking indeed more secure.
Speak for yourself.
Actually, an interesting potential of this is to have you "knock" at the NAT gateway. Proper knocking opens up a given service and knock ports to an internal system.
Different knock patterns at the NAT open you to different internal hosts. Quite interesting possibilities there.
That is the point.
1. Many ports getting a sequence is much more like noise than one port getting it -> much harder to identify an attempt of intrusion.
2. If you have a backdoor, as mentioned in the article, how will you know it has not been accessed? It was not listening, it gets activated, does its duty, deactivates. If it is a good backdoor it is invisible to that system (only visible though an additional layer).
So it is a better way of getting a connection, but not a solve-all for the intruder, and I doubt the intruder cares about any waste of your resources.
--
FreeNET user? Comfortable with the adverse selection?
One interesting way to use this would be to forward incorrect knocks to a honeypot instead of the legitamite service. Then the attacker could never determine if he had indeed knocked successfully and would waste time running around in a fake system giving you valuable data about there intrusion methods and freeing up the actual service for legit users.
Wouldn't he have had to hack a computer closer to his target in order to be successful?
Wouldn't the best option be to have some type of SecurID based password in order to access the port? Unless there is a bug or hole in the software isn't a randomly changing password that requires a pin about as good as it gets?
And as they say, Security through obscurity isn't.
"Thanks to the remote control I have the attention span of a gerbil."
Interesting concept... I thought of this 2 years ago and I'm now kicking myself in the balls for not acting on it! (not literally)
In my version of "port knocking", everything was going to be controled via ICMP echo packets.. aka "ping".
A single Ping packet can contain arbitrary data of an arbitrary length less than 64k. Through a config file, the system admin could define ping sequences using time, data, and/or packet size, along with a specified script to execute on each successful reception of the ping sequence.
Then, remotely, people who know the ping sequences could use almost any available ping utility on any machine to open remote ports, etc.
The concept of executing a script, rather than opening or closing ports, allows for more flexibility. Not only can the admin open and close ports via scripts, but could do other useful things.
Skiers and Riders -- http://www.snowjournal.com
Of course you could also have a new combination generated every minute for the super paranoid.
No, if you were "super paranoid" you'd have two identical one-time pads, one residing on the computer to be accessed, one in the hands of a single person trying to connect.
Every minute, the computer would consult its copy of the pad to determine what that minute's secret knock sequence would be.
The person connecting would look up in his copy of the pad that minute's sequence. You'd need to synchronize both participants' clocks, of course.
Less secure would be generating a new combination -- using some continuous function -- every minute.
Less secure than the pad, but more secure than a continuous function, would be a cellular automaton -- life Conway's Game of Life -- where any particular minute's knock sequence could only determined by first determining the previous minute's sequence.
Of course, to prevent the rules from being extrapolated by anyone analyzing your traffic, you'd have to agree on a new function or automaton to use for the next connection before ending the current connection. To guard against interruption of the connection, agreeing on the next connection's function would be the first thing you'd do after making a connection, and to guard against that being interrupted you'd need some fallback combinations -- which brings us back to the one-time pad.
Opinions on the Twiddler2 hand-held keyboard?
But you're missing the point that this doesn't require any extra ports being open. The listener waits for the correct series of attempted connections to closed ports, and then transiently opens an otherwise closed port. Thus the approach lets you leave a port open temporarily when you would otherwise have to leave it open all the time, so it should add safety by your known good administration practices.
There's no point in questioning authority if you aren't going to listen to the answers.
This looks similar to how frequency-hopping is used on secure radios.
Two radios synchronize, based on a key, and both change frequency every so many milliseconds. If you don't know the key, you can't send or receive to either of them.
I would like to see this extended to a port-hopping system for all ports and services. Sure -- it will burn some clock cycles, but I like the approach.
- Sam
http://www.iamsam.com
ignore the proper knock so that if somebody's trying to brute force all the possible knocks, they'll never get feedback when they have the right one.
, 655,932,542,976 possible knocks, a.k.a. "way the hell more then can be brute-forced".
Re "brute forcing"... the number of possible knocks is (ports used for knocking) ** (ports in knock sequence). Yes, that's exponentiation.
In fact, I'd suggest making the knock sequence much longer then in the article; ten might be good. Then, if you allocate 100 ports to the knocked and randomly select a 10 port sequence for the knocking, you get 100 ** 10 possible knocks, or 100,000,000,000,000,000,000 (100 sextillion) possible knocks.
With just a few more ports in the sequence and just a modest investment in ports, you can make brute forcing impossible.
(And if you mix up the ports so they aren't sequential and the attacker has to guess THOSE ports, it goes to approx. (2**16)**(number of knock), so for a 10-port sequence on potentially all TCP ports it's 1,461,501,637,330,902,918,203,684,832,716,283,019
(I love posting big numbers on Slashdot.)
You need to worry about sniffers way more then brute forcers. (And as this is another layer of security, hopefully on top of an already fairly secure protocol like SSH, it's a good thing; now the 'man in the middle' has to have advanced knowlege to even know there's something to get into the middle of!)
It isn't more secure. It's just more obfuscated, because it's more complex. But that doesn't make it more secure, it makes it potentially _less_ secure.
I think the idea is that an attacker would install a hidden sshd on your system, and you wouldn't know. You wouldn't even see an open port. You wouldn't see concentrations of connections to one "closed" port, but sporatic attempts at a few seemingly random ports. The attacker would use this as a backdoor into your system. I think you got the wrong impression that this would be a keen tool for system administrators for some reason. I didn't RTFA, but jeez.. at least read the summary.
It's like saying that if sshd asked consecutively for two passwords before granting access, that would be more secure.
In a way it does. It firsts asks for a username, and then a password. If one of them is incorrect, you don't get access. But SSH doesn't tell you which one was incorrect.
Port knocking is very similar (but not exactly the same). You need to have both keys (port combination) and login info in order to get access.
The difference is, without the port combinations, you can't tell if the service is up, or your port combination is wrong.
i've been running SSH on a nonstandard port with this in the way:
iptables -N ${SSH_TABLE}
iptables -Z ${SSH_TABLE}
iptables -A ${SSH_TABLE} -m state --state NEW -m limit --limit 2/minute --limit-
burst 2 -j DROP
iptables -A ${SSH_TABLE} -m state --state NEW -m limit --limit 7/hour --limit-bu
rst 7 -j DROP
iptables -A ${SSH_TABLE} -m state --state NEW -m limit --limit 10/day --limit-bu
rst 10 -j ACCEPT
iptables -A ${SSH_TABLE} -j DROP
well, I thought it was cool...
Keep your packets off my GNU/Girlfriend!
But if your knock sequence was generated similar to a one time password, the knock sequence would be unique each time and not replayable.
does it only open the port for that one IP somehow, using also advanced IP filtering, cause otherwise this is dumb, it would be like unlocking your door for the first customer to knock right, but having to leave it open the whole time the customer is shopping.
Anonymous Cowards - Oh God, How I hate you
I wrote proof of concept code for this a few years ago. Mine is implemented as modified network sniffer code (with promisc disabled), so it doesn't require any weird firewall configs. I was considering some other things which could be useful, such as transmitting data via a fake network scan.
In the context of the theoretical shifting door in the wall, part of what makes it secure is that outsiders don't know how the door works; they only know that it moves away when an unauthorized person tries to open it.
However, people who oppose "security through obscurity", especially when in conjunction with blind open source advocacy, would argue that giving would-be intruders the blueprints of the door system would somehow deter them and make the system more secure.
There's a Mercedes gap too. I want one and can't afford one, but it's not government's job to do anything about it.
Sadly, most people who probably think port knocking is great security probably have yet to learn how to use DSA keys.
Think "in addition to" and not "instead of". No reason why you couldn't do both. In fact, you could rotate your port knocks to be different everytime you connect. That way, if someone does try to fake their way in, you could detect it and react.
That doesn't seem right. If the order of the knocks is important, how do you get around that there's never a guarantee in what order network packets arrive? If no packets are sent back at all, how do you know when to send the next knock or even if the knock made it to the server?
Most security is based on secrets of one kind or another---that doesn't make it "obscurity."
I think that's the point the grandparent is making. The key to this is that folks around here aren't real clear on the difference between "obscurity" and "secrets". One is touted as being worthless by itself, the other is accepted as the cornerstone of electronic security in general (I posit that the cornerstone of physical security is violence).
the phrase "security through obscurity" means obscurity of system design.
This is an excellent point, and deserves to be modded up because *way* too many people both here and elsewhere miss this fundamental concept. In my opinion any method that makes accessible hosts look the same as unaccessible hosts to a port scan is a great idea, no matter what folks here choose to call it.
Dan
I see a lot of comments saying "Well, why not just have two passwords?". It seems that people didn't read the article (the first link is /.ed, the second is not). The whole point is that with this, until you knock, the machine appears as a closed machine. No ports open. All ports will simply drop packets on the floor, meaning that a hacker scanning your subnet will not bother with that machine. The machine essentially appears invisible until knocked. Even with the most secure system, the hacker can still see that you're running, say, sshd, Apache, CUPS, and a few other services. And if a buffer overflow was announced 5 minutes ago for, say, sshd, they know that they can attempt to exploit the machine, since they see port 22 open. If you are using Port Knocking, you can have a vulnerable sshd, and it's a hell of a lot less likely to get exploited since the cracker has no way of knowing that you're running sshd...
There is no sig, there is only Zuul.
By that definition port knocking isn't security by obscurity either. The secret is the sequence of knocks and the server port number, the protocol is that connections to the server port will only be accepted after the knock sequence has arrived from the same ip address as the connection to the server port. The security of this protocol doesn't drop if the protocol is revealed. If you know about the port knocking protocol (you do now), can you tell if a server uses it without knowing the secret?
Congratulations. You've just extended your "secure" corporate network beyond the physical walls of the office, and into the house of one of your employees. Are you sure that the machine they're using as a VPN client hasn't been rooted? VPNs have their uses, but they're far from solving this problem, and in many ways weaken your overall security. The correct solution is to change the authorisation criteria from things you know (password, "secret handshake") to things you know plus something else, for example, things you have. We do this with one time passwords sent to a user's mobile phone. Once that's been entered, they're prompted for their normal password. Thus even if the box has been rooted, and has keystroke and network sniffers galore, it doesn't matter. So long as the black hats don't have my trusted employee's mobile phone, they're not going to get in (and furthermore, the unexpected passwords being sent to the phone act as an early warning system to let us know someone's trying to break in). Of course, no security measures are perfect, and theoretically, with root access, they could hijack an existing ssh connection once it's been opened, but it's non-trivial, and we've raised the bar considerably.
"The invisible and the non-existent look very much alike." -- Delos B. McKown
"open port Y for ip Z using key K"
if the port opening policy accepts this command then it is opened otherwise it is not. Better yet 'REAL' crypto could be used to protect the ports. Fore example...
"open port Y for ip Z @ TIMESTAMP" Encrypted by K.
This simpler implementation (more likely to be correct) will provide equivalent security.
This is a great way to hinder people from randomly picking to hack your box by scanning for open ports. However, if someone is dedicated to hack YOU specifically, they could still use a man-in-the-middle attack, sniff the packets to see which port sequence you were using, etc, etc.
Of course, there really is no way to block a skilled hacker who is intent on breaking into a specific network/box by any means necessary.
An additional step here would be to have both machines (server, client) seeded with the same randomly-generated number and then, utilizing the same algorithm, generate a random port sequence for knocking. This sequence would be valid for 10 to 30 seconds, at which point the sequence for proper knocking is re-generated.
I'm guessing this would need to be tied to Time of Day, necessitating accurate time; so either an external GPS device or an NTP connection would be required.
This is just some off-the-cuff speculation, so don't flame me for the holes.
I have something in common with Stephen Hawking...
This is security by obscurity, but it is useful. Don't repeat this mantra just because "the experts" say so.
Since some still don't understand its use, i'll be speaking metaphorical:
Assume you need to have a special key to open a certain otherwise secure door. OpenSSH might be that door and your passphrase and your certificate are the key.
An attacker can still forge the key or attack the lock with a different approach, picking etc. - comparable to "social engineering" to get the password, brute forcing or exploits.
And that port knocking sequence now effectively hides the lock, leaving an attacker without a first approach to pick or break the lock. It just adds another layer of security. You just don't know where to start your attack. You can't use exploits, you can't try brute force - nothing, heck you don't even know what type of daemon your target is.
A clean stainless steel door with a covert RFID-detector one square inch in size, hidden somewhere, sure as hell beats the same door with a clearly visible lock. You still need to pick the lock, but you can't poke your lockpicking tools into solid steel and you can't crack something you cannot discern.
--- Still one addition to say: having a machine connected to the internet with no ports open makes you a prime suspect for the port knocking scheme.
A good stealth scheme may be implemented, so a potential attacker (excuse for this metaphor again) does not even see the door (or the building, for that matter).
I did this about 5 years ago. But my method was a bit different. Instead of using port numbers to contain the information (and that's all it really is, is just information), I sent a single UDP packet, with a source port of 53 (so it looks like a DNS answer), formatted like a DNS answer, that contained the information in the DNS answer data. Then it opened the SSH filter for that IP address to come in (I did it for 5 minutes, not 10 seconds). It still had to fully authenticate via SSH, so even if someone sniffed my DNS packet and tried to fake it, they could at most have a locked door to jiggle the handle on. Next time I do this, it will be to generate an MD5 checksum from the client IP and a secret salt, and send that as an IPv6 address in the packet. Then it can't even be faked from some other IP address.
now we need to go OSS in diesel cars
What if I turn this whole thing around and install fake services on a number of ports?
For example, whenever you make a connection to a port between 1025 and 2048 on my system, you're greeted with "OpenSSH ...", and prompted to authenticate. But only behind one among those 1024 ports is the real SSH. On any other port, the fake service takes the username and password you've entered, wait a few seconds (just idling around), and tell you "Authentication failed". If you try too often to connect to faked services, you're put on the black list to avoid DOS, of course.
This is a neat trick, but it does not really increase protection against targeted attacks.
It really is nothing more than a password to get access to the front gate... When somebody eavesdrops (sniffing), they will know the passwords, thus get access to the gate. They can sucessfully detect the knocking sequence because it is followed by a successfull ssh connection (duh!).
The password is this 'secret sequence' in this 'port knocking'. Why not just use a daemon that listens on a UDP socket for a packet with an encrypted password in its payload? The payload could even be an RSA-signed and/or encrypted request (that includes a timestamp). That would be unscannable too, because UDP is connectionless, and be a lot more secure because of the real encryption/protection of the request data (the server can verify the identity of the sender of the request from the RSA signature, and can deny the request if one was made earlier with the same time stamp, twarting even sniffing of the UDP packet).
Except for not having the ssh daemon 'connected' to the internet at all times and thus evading many port-scanning worms/scripts, this port knocking is nothing more than just some security through obscurity: At best it will delay the attacker somewhat, but probably not at all while giving the user a false sense of being secured.
--- Hindsight is 20/20, but walking backwards is not the answer.
You can also open up inbound ports from specific external IP addresses only, and do many at once. So ten inbound connections can reach ten different internal webservers, and at the next request, reach the same one again.
This can be done dynamically as a form of load balancing which is a neat hack. Expire the specific forward rule after 30s or something. Means similar requests cluster - less DB traffic.
But, combine this with knocking and you've got the next step. Secret services on a 'stealthed' IP, where you can request which quake server (for instance) by knocking in a different way.
Port scanning isn't what it once was. Especially once you factor in time-sensitive keys (easily doable - both machines need a net connect to reach each other, ntp is then trivial) and ID-sensitive keys (so my key isn't like yours, even at the same time). Even if you managed to snoop on a 'knock' you couldn't repeat it.
I suppose a way around this would be to have an old 486 running BSD or Linux act as a first layer firewall and knocking device between your nat/router and ISP. You could just keep open port 22 on your nat/router and have the BSD box decide based on knocks which client to forward the packets the packets too.
Another method I would like is some sort of handshake authentication where the firewall would reconigze who you were after logging in and would open ports up appropriately depending on your private key. I think MS proxy server does this with NTLM authentication but I am not too sure. There is IPSEC but this does not work over the internet or many campus wide networks if your at a unviversity.
A unix equilivant would be nice with Kerbos, LPAD or Samba support.
Having a dual firewall this way is rudant but it would work since I use my nat/router more as a hub then just a firewall.
http://saveie6.com/
Back when I depended on a dialup connection, I used xringd. By having the phone line the modem was on ring in a certain pattern, I could command the computer to dial up (using diald). Then a line in /etc/ppp/ip-up.local would mail me the IP address the PPP server assigned the computer (this was before DynIP or DynDNS). I'd then be able to log into the machine. Pretty darn cool for 1996.
It doesn't actually use significant resources unless it's getting pounded with lots of packets, and you can limit this by only listening on a few ports, blocklisting IP addresses that knock on the wrong ports, and limiting the rate that you actually respond to requests from a given IP address. On the other hand, you have to be careful not to let the attacker spoof a bunch of _bad_ requests, causing you to blocklist a real site. Depending on how much eavesdropping capability the attacker has, this may be easy or hard.
The security advantage of this method over a single-port method with a password is that there are applications that you run which may have bugs in them, such as your SSH server or SNMP monitors, which you're not going to rewrite, and it lets you block access to them except from authorized users. It's a defense-in-depth strategy, possibly good (though it looks clumsy to me.) It can cut down on lots of the script kiddies.
Also, this doesn't have to be in your main server. This is the kind of application you could build into your firewall box, so it reduces the number of ports that can pass through the firewall, except when somebody knocked successfully, and the firewall doesn't allow passthough on the knocking ports. Of course, you could accomplish almost the same thing wiht better security by accepting SSL requests to an application on the firewall...
Bill Stewart
New Fast-Compression-only CPR http://preview.tinyurl.com/dy575ks
This is a great idea.
It adds security to any existing methods (passwords, etc.).
It can be implemented behind a firewall that doesn't even respond on any port probes, so an attacker can't even tell if the firewall was just unplugged.
If the firewall stays closed, the protocol can't be used by an exploited machine, unless a method for exploiting the firewall is also known.
Or the method can be implemented in user space of a machine behind a completely closed firewall, just by pre-arranging for the logging of firewall port probes, and the forwarding of appropriately filtered contents of the firewall logs into user space.
They key sequence can also be made long enough to make it just as hard to crack as a long pgp private key, e.g. nobody except (3 letter agency) and distributed.net will even bother to try.
The sequence key can be from a one-time pad, meaning that even if the protocol is completely revealed to a local sniffer, they'll just end up with a useless password.
And lastly, it's possible to additionally encode the key sequence with a modulation wrapper and enough redundancy to withstand a given signal to noise ratio and mis-sequencing rate, which means one could even make the sequence key usable in the face of probing or an outright DoS attack against the protocol up to a certain attack bandwidth and knowledge of which ports might be in the sequence.
Where's my coding textbook and patent attorney...
Sorry, wrong. There are several messages in this thread that mention REJECT (response to packet) instead of DROP (total silence). With this scheme in place, you need not listen on *any* ports, and you need not respond in any way. You can have a totally silent box, even with 10 or 20 services "listening". Nothing gets through until your iptables/ipchains software allows the traffic through.
Admittedly, if you're running a public site, you're mixing two kinds of solution --- publicly available vs secured, but analogous statements can be made here - you can't tell a public site using port knocking for some special services from a public site that doesn't support same.
This is like a void fn() in C (no return status). You knock on the 5, 10, or 25 ports in the right sequence to "send your message". You get nothing back. You then try to open the port that is your ultimate destination - if it's open, you're fine, if it's not, you have issues. This isn't a full-duplex kind of protocol, folks. I love it :-)
Thus, it is impossible to distinguish a totally silent box (listening on no ports, dropping all packets) that has implemented port knocking from a box that is merely totally silent.
As a two-laptop user who attaches to corporate LANs and public high-speed networks in hotels, I just love the idea of having all packets dropped until someone sends "shave & a haircut!" - then letting them in for a bit.
It would certainly be better than my current approach - using ethernet addresses (maclist in Shorewall! :-) to allow ftp and http etc to my linux box.
Redundancy is good; triple redundancy is twice as good! - Me.
i think by watching the traffic, it would be possible to figure out what the knock is. logging the sequence of port accesses by a particular ip address would make the pattern apparent.
Seems vulnerable to traffic analysis, as someone mentioned you don't use a combination lock that sends packets all over the world. People will want more complicated sequences, but it will take more time to send them and they may have to resend due to TCP packets coming in different order. But even so anybody on your network or the server's network should be able to see what's going on, how secure is that? And if the server ever responds to your ip from any port then you likewise hosed.
On the other hand this does seem to be an interesting way to one-way send information to the server. I was thinking of playing solitaire using Bruce Schneier's algorithm using a port for each card of a deck.
IANA cypherpunk, but there seem to be a number of ways to treat the set of all closed ports as a numerical space that would be interesting for encrypted communications.
For example you could convert a one-time pad, a private key, or a set of communication channels into a list of port numbers. For a short message at least, you could send with pretty good security (although the list of ports, if not their hash values, would be known to the outside world).
To me this knocking stuff sounds like it only *reduces* security and provides lots of interesting clues to men in the middle. The intriguing part seems to be that you can send a good deal of information through a large number of half-connections in parallel, but this may have already been tried by other people. Of course if the message is simple enough that a single ping to a single prearranged port number is enough to convey it, then you would seem to have a pretty strong system though its existence would certainly be uncovered sooner or later. But if this became popular I suppose the advantage would be in being able to assign certain ports to prearranged values, both for encryption purposes and also to reduce the amount of data you actually need to send.