Computer Science

Question 1

FTP active vs. passive

Answer 1

STEP 1
The client connects to the FTP server on port 21

STEP 2
Active: The client also sends a port on which the server may connect to the client to open the data channel (problem: the clients ports are usually firewalled)

Passive: The client also sends the passive command, on which the server replies a port on which the client is allowed to open the data channel (unfortunate result is that the server needs a range of ports to be opened, to support multiple clients)

Question 2

IP sweep: UDP scan

Answer 2

Send 0 byte packets to Target. If ICMP port unreachable is sent back, port is closed or filtered

Question 3

IP sweep: Idle scan

Answer 3

Use a second host, called the zombie.

1) Check his IP fragmentation ID
2) Send a SYN to Target, with source IP of zombie
- If port open: Z will receive SYNACK and reply RST. Fragmentation ID of Z will be +2
- If port closed, Z receives RST. Fragmentation ID will be +1.
- If no response, fragmentation ID of Z will be equal
3) Check Fragmentation ID of Z again to see what happened.

Question 4

Benefit of SSL over IPSec

Answer 4

Authenticates on the level of the application, while IPSec does this at host level. Rejection can be done earlier though, so IPSec is less vulnerable to DoS attacks