1. CheapChat: a Java-based chat application Nicolas Ducheneaut Mike Gebbie Barbara Rosario SIMS, Spring 2000

2. Chat application with Port Scanner 2 modules: –Port Scanner –Chat application acting both as a server and a client

3. Port Scanner Possible Ports: (0–1023) (1024–49151) (49152-65535)

4. Port Scanner Possible Ports: (0–1023) (1024–49151) (49152-45535) findTheApplicationPort( Host Address, Start Port, End Port ) Keep Trying…

5. Port Scanner CheapChat “Client” CheapChat “Server” (Port 50005)

6. Port Scanner CheapChat “Client” CheapChat “Server” (Port 50005) Try 50000… Timeout …

7. Port Scanner CheapChat “Client” CheapChat “Server” (Port 50005) PortScanner Invoke! PS

8. Port Scanner CheapChat “Client” CheapChat “Server” (Port 50005) PortScanner Try 50001… Try 50002… Try 50003… Try 50004… PortScanner Threads

9. Port Scanner CheapChat “Client” CheapChat “Server” (Port 50005) PortScanner PortScanner Threads Connect at 50005 Authenticate: “Are you a CheapChat server?”

10. Port Scanner CheapChat “Client” CheapChat “Server” (Port 50005) PortScanner PortScanner Threads Reply: “I am a CheapChat server!”

11. Port Scanner CheapChat “Client” CheapChat “Server” (Port 50005) Connect at 50005

12. Chat 1: first to start Chat

18. Problems and potential remedies Main weakness: clear-text administrative messages sent over the communication channel –> Solution: use two separate data streams instead No way to know in advance if someone is on-line –> Solution: adapt the port scanner and use it before the application is launched

19. Thank you for your attention Questions? Comments?