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Limitations of Port Knocking

Limitations of Port Knocking. Software Project Presentation Paper Study – Part III Group Member: Liew Jiun Hau (20086034) Lee Shirly (20095815) Ong Ivy (20095040). Agenda. Out-of-Order Delivery Network Address Translation (NAT) Authentication-Connection Association.

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Limitations of Port Knocking

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  1. Limitations of Port Knocking Software Project Presentation Paper Study – Part III Group Member: LiewJiunHau (20086034) Lee Shirly (20095815) Ong Ivy (20095040)

  2. Agenda • Out-of-Order Delivery • Network Address Translation (NAT) • Authentication-Connection Association

  3. Out-of-Order Delivery • Problem • Attackers can perform DoS on a client: • Send one packet per second to a random port of server • Spoofing the client IP as the source IP • Knock sequence broken  authentication fail • Solution • Divide the bits representing port number into data bits and sequence number bits • Server will be able to reorder packets correctly before decoding the knock sequence • Use SPA mechanism – only a single packet is sent

  4. About Network Address Translation • A method to solve the IPv4 address space problem • Internet was growing fast back in late 1990s • There was not enough (public) IP address to be assigned to all hosts • NAT can map a large group of private IP addresses into a single public IP address • Each host will be translated into same public IP address • This slows down the consumption of public IP addresses • However NAT causes some hidden issue for port knocking

  5. NAT – Issues • So what if a port knock client is behind NAT? • Who is actually authorized at the end of the port knocking process? • One client successfully port-knock the server • But everyone with the same public IP now has access • This defeats the purpose of port knocking!

  6. NAT – Issues (cont) • In SPA, IP address information is hashed together with Timestamp, ID and Password • How do a client know about its public IP address? • Can it know in a secure way?

  7. NAT – Possible Solutions • Set up a simple server machine at the boundary of the private network • Client can securely discover its public IP address by querying the boundary server machine • Using some external service • www.whatismyip.com • Use by fwknop (one of the SPA implementation)

  8. NAT – Possible Solutions (cont) • The other alternative will be a three-pass mutual authentication • Originally proposed by RenniedeGraaf, John Aycock and Michael Jacobson Jr • The notations used are as follow • A = Client • B = Server • NX = Nonce of X • PIDX = Public IP of X • MAC = Message Authentication Code (e.g. HMAC-SHA-1) • K = Pre-shared key between client and server • The steps are as follow: • 1: A  B req, NA,MACK(req, NA) • 2: B  A PIDA, NB, MACK(NA, PIDA, PIDB) • 3: A  B MACK (NB, PIDA, PIDB) • This approach is more secure but make port knocking less “stealthy”

  9. NAT – Possible Solutions • We can also add some configurations to further mitigate risk • Set a short timeout • Close the port after timeout • Close the port immediately after a connection is made • This leave only a small window during which the client can connect • This also avoid a problem where adversaries could eavesdrop • They can wait for the port to be opened and try to make connection after the authentic client did • There might still exist a race condition • Adversaries can send request faster than client • So how can we solve this issue?

  10. Authentication-Connection Association • A successfully opened port can still be hijacked by an attacker • Attacker will be able to impersonating the client • This is the unsolved issue in previous slide • To counter this, we can wrap post-authentication connections within an encrypted session • Use a pre-shared key that only known by the client and server • Attacker has no way of crafting valid packets without the key • Invalid packets will be dropped at server side • This however increase the load on server

  11. Authentication-Connection Association (cont) • The other possible solution would be to generate TCP Sequence Numbers (SN) in a randomized & authentication-dependent manner • Server can verify the TCP sequence number based on previous authentication • But this require client to have heightened privileges to manipulate the TCP / IP packet fields • Powerful adversaries who can perform MITM is not affected

  12. Next Presentation • Continue to study and analyze other known problems and their possible solutions/improvements in details • Present and discuss in next session • Questions?

  13. References • Jeanquier S (2006) An Analysis of Port Knocking and Single Packet Authorization. M.Sc. Thesis. Royal Holloway, University of London. Sept 2006. • deGraaf R, Aycock C, and Jacobson M. ‘Improved Port Knocking with Strong Authentication’. ACSAC 2005, pp. 409-418. • Rash M. ‘Single Packet Authorization with Fwknop’. The USENIX Magazine. Feb 2006, Vol. 31, No. 1, pp 63-69. • William Stallings. Cryptography and Network Security: Principles and Practices (3rd Edition). Pearson Education. 2003. • James. F. Kurose, Keith W. Ross. Computer Networking: A Top-Down Approach Featuring the Internet (3rd Edition). Addison Wesley. May 23, 2004.

  14. Thank you

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