Homework #0 Solution

Homework #0 Solution

Contact TAs: vegetable@csie.ntu.edu.tw

Network Administration Preliminary

True/False

1. False. The Internet is provided on a “best-effort” basis and 100Mbps is the best possible value.

“Oversubscription” is often the case. If everyone “has” 100Mbps network connection, and then we “have” more network connection than the actual amount. Therefore, congestion can happen.

However, people seldom utilize network 24/7, so it’s often not a problem.

2. False. Ideally, one IP address corresponds to one host, but NAT breaks this rule. NAT enables many people to share one public IP address and thus sharing the same IP address doesn’t imply they are the same person.

3. True. Without DHCP, one can still configure network manually. However, a gateway is required to connect to hosts outside local network.

4. True. MAC addresses are designed to be unique in the same broadcast domain (although an end device is able to be explicitly assigned a custom MAC address).

5. False. Upon receiving a packet, a hub broadcasts the packet to all ports except the incoming one, while a switch maintains a MAC-port lookup table and forwards packets only to corresponding ports, which eliminates redundant traffic.

6. False. When you connect to a domain name, DNS is required to translate that to an IP address so that your computer can recognize that host.

7. False. Many devices only support 802.11b/g/n wifi, so turning it off is not a wise move.

8. True. That’s how firewall works.

9. False. Each DNS server only caches or stores part of them. If a DNS server is asked an unknown domain name, it will ask the DNS server which controls that domain name. (For detailed explanation, see “authoritative DNS server” and “DNS resolver”)

10. True. As an example, windows have the option of “Obtain DNS server address automatically.”

11. True. Though, one may be able to decrypt other people’s data which is also under the same wifi network. If you are typing some confidential data, use VPN or https.

12. True. For the latter part, VPN helps achieve better download speed through bypassing congested network or utilizing a better network route.

13. True. With proper routing.

Select All That Apply

1. (b)(c)(d)(e)

2. (a)(c) are public ones.

3. (a)(d) (a) IPv6 provides much more usable IP addresses. (d) CGNAT is a special case of NAT.

NAT let many hosts share a public IP address, so we don’t have to assign a public ip address to each host.

System Administration Preliminary

1.1

In the welcom screen text

NASA{HW0 1S Y0UR T1CKET T0 NASA C0UR5E!!}

1.2

Type pwd and then the flag is in part of the returned path NASA{Y0U ARE @ Y0UR H0ME!!}

1.3

$ ls -al

Then the flag is there

NASA{1s -al 2 5EE WH4T Y0U H4VE:D}

1.4

First make engine executable

$ chmod +x engine Then execute it

$ ./engine

It will print out the flag

NASA{chmod 4ND ./ 0N THE FLY}

1.5

Use cat to see password of the zip file

$ cat toolbox-key Then unzip it

$ unzip -P WOW IT SEEMS THAT YOU CAN SEE ME toolbox.zip Or simply

$ unzip -P $(cat toolbox-key) toolbox.zip Then use tar to untar the tar file

$ tar zxvf toolbox.tar.gz

1.6

To get into the toolbox

$ cd toolbox

To remove the obstacle

$ rm obstacle

To rename the crisis to chance

$ mv crisis chance Then execute magic

$ ./magic The flag is there

NASA{Cant you believe that you built up my wing}

1.7

To see home directory of user NASA, command

$ getent passwd can be used.

Or check /etc/passwd, user NASA’s data happens to be there, though not necessarily all users’s data will be there.

Or simply

$ echo ∼NASA Then

$ ls -la ∼NASA

etago can be found there.

Execute it with the absolute path

$ ∼NASA/etago

Find that it dumps lots of garbage So pipe is needed

$ ∼NASA/etago | less

Then use / in less to find the flag (or use your eye):

NASA{| AM A|||||||||||||||||||||||||}

Or simply use grep

$ ∼NASA/etago | grep ‘NASA’

1.8

Execute throttle

$ /plane/throttle Then press

$ Ctrl + C

$ Ctrl + Z

$ Ctrl + \ Then get the flag

NASA{ctrl z ctrl c ctrl \}

1.9

$ man man

Then the flag is in the description section:

NASA{M4N 15 THE 4BBREV1AT10N 0F M4NUA1}

1.10

To find out something suspicious, first observe top.

Then you can find that a process periodic appears every 1 minute with differet PIDs running as user nobody, so check /var/spool/cron/nobody and then you can find /opt/wrapper.sh.

Check /opt/wrapper.sh, and then you can find that the program periodic is also there.

Run /opt/periodic, you can see an error message “sh: te1net: command not found”.

Use ltrace and strings to inspect periodic, and then you can find it issues a command te1net nasa-hw0.csie.ntu.edu.tw 9487.

Then grep -r 250F /proc/1/net/tcp to see who is listening on the port, and then you can find it is user 65694. (250F is the hex of 9487)

Check getent, find out that it is user b03902072.

Use ps -u b03902072 to see that he is (I am) running the program throttle.

Apparently, the real throttle does not listen on the port 9487. Therefore, this throttle is not the throttle you know before. Check the permission of ∼b03902072, you will find that x is on. Try

and you can use command file to know that it is a gzip file.

Use gunzip to unzip it, and use command file again. Then you can find that it is a LVM im- age.

Then you can use command losetup -f and mount to mount partition secret-flag on it.

Check the partion, and find a file F14GIO. Again use file to know that it is a bzip file.

Use bzip -d to unzip it and get the file containing the flag.