netstat and ss (connection listing) in Linux CLI - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the command to list all active TCP connections using netstat.

Linux CLI

netstat [1] tcp

Drag options to blanks, or click blank then click option'

A-r

B-a

C-s

D-n

Attempts:

3 left

💡 Hint

Common Mistakes

Using '-r' which shows routing tables instead of connections.

Using '-s' which shows statistics, not connections.

The '-a' option shows all active connections and listening ports.

2fill in blank

medium

Complete the command to display listening sockets only using ss.

Linux CLI

ss [1]

Drag options to blanks, or click blank then click option'

A-l

B-u

C-t

D-a

Attempts:

3 left

💡 Hint

Common Mistakes

Using '-a' which shows all sockets, not just listening ones.

Using '-t' which filters TCP sockets only.

The '-l' option shows only listening sockets.

3fill in blank

hard

Fix the error in the command to show numeric addresses with netstat.

Linux CLI

netstat [1]

Drag options to blanks, or click blank then click option'

A-a

B-p

C-n

D-r

Attempts:

3 left

💡 Hint

Common Mistakes

Using '-a' which shows all connections but not numeric addresses.

Using '-p' which shows process info but not numeric addresses.

The '-n' option forces numeric output of addresses and ports.

4fill in blank

hard

Fill both blanks to list all UDP connections with ss and show process info.

Linux CLI

ss [1] [2] udp

Drag options to blanks, or click blank then click option'

A-a

B-p

C-u

D-l

Attempts:

3 left

💡 Hint

Common Mistakes

Using '-l' which shows only listening sockets, missing active connections.

Using '-u' which is redundant since 'udp' is already specified.

Use '-a' to show all sockets and '-p' to show process info.

5fill in blank

hard

Fill all three blanks to create a dictionary comprehension that maps ports to states for TCP connections using ss.

Linux CLI

connections = [1]: [2] for [3] in ss_output if 'tcp' in [3]

Drag options to blanks, or click blank then click option'

Aconn['port']

Bconn['state']

Cconn

Dconn['proto']

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'conn['proto']' as key or value instead of port or state.

Using wrong loop variable or missing it.

We map port to state for each connection in ss_output where protocol is TCP.

Practice

1. What is the primary purpose of the netstat and ss commands in Linux?

easy

A. To list installed software packages

B. To edit network configuration files

C. To monitor CPU usage

D. To display active and listening network connections

Solution

1. Step 1: Understand command purpose

   Both netstat and ss are used to show network connections and ports.

2. Step 2: Identify correct function

   They do not edit files, monitor CPU, or list software; they show network status.

3. Final Answer:

   To display active and listening network connections -> Option D

4. Quick Check:

   Command purpose = Display connections [OK]

Hint: Remember: netstat and ss show network connections [OK]

Common Mistakes:

• Confusing netstat/ss with file editors
• Thinking they monitor CPU or memory
• Assuming they list installed software

2. Which of the following is the correct syntax to list all listening TCP and UDP ports using ss?

easy

A. ss -tuln

B. ss -a

C. ss --listening

D. ss -x

Solution

1. Step 1: Understand option meanings

   The options -tuln mean TCP (-t), UDP (-u), Listening (-l), Numeric (-n).

2. Step 2: Match options to goal

   To list listening TCP and UDP ports, ss -tuln is the correct syntax.

3. Final Answer:

   ss -tuln -> Option A

4. Quick Check:

   Listening TCP/UDP = ss -tuln [OK]

Hint: Use -tuln with ss to see listening TCP/UDP ports [OK]

Common Mistakes:

• Using -a shows all but not filtered to listening
• Assuming --listening is a valid option
• Using -x which is for Unix sockets, not TCP/UDP

3. What will be the output of the command netstat -tuln | grep ':22' if the SSH service is running and listening on port 22?

medium

A. No output, because port 22 is not shown

B. An error message about invalid syntax

C. A line showing TCP port 22 in LISTEN state

D. A list of all UDP ports except 22

Solution

1. Step 1: Understand command components

   netstat -tuln lists TCP and UDP ports numerically and listening; grep ':22' filters lines with ':22'.

2. Step 2: Interpret output if SSH runs

   If SSH listens on port 22, output will include a line showing TCP port 22 in LISTEN state.

3. Final Answer:

   A line showing TCP port 22 in LISTEN state -> Option C

4. Quick Check:

   SSH on port 22 = netstat output with :22 line [OK]

Hint: grep ':22' filters netstat output for SSH port [OK]

Common Mistakes:

• Expecting no output if port is open
• Thinking command syntax is invalid
• Confusing UDP ports with TCP port 22

4. You run ss -tuln but get no output. What is the most likely reason?

medium

A. The system has no active or listening TCP/UDP ports

B. The command syntax is incorrect

C. No network interfaces are active

D. You forgot to run the command as root or with sudo

Solution

1. Step 1: Check command correctness

   ss -tuln syntax is correct and should show listening ports if any exist.

2. Step 2: Consider system state

   The most likely reason for no output is no TCP/UDP ports are listening, common in minimal containers or systems without bound services. Permissions are not required for ss -tuln.

3. Final Answer:

   The system has no active or listening TCP/UDP ports -> Option A

4. Quick Check:

   No output = no listening TCP/UDP [OK]

Hint: No ss -tuln output means no listening TCP/UDP ports [OK]

Common Mistakes:

• Assuming syntax error when command is correct
• Thinking no active interfaces means no output
• Thinking need root or sudo

5. You want to write a script that checks if port 80 is listening using ss. Which command correctly returns a non-empty output only if port 80 is listening?

hard

A. ss -tuln | grep ':80$'

B. ss -tuln | grep ':80 '

C. ss -tuln | grep ':80'

D. ss -tuln | grep '80'

Solution

1. Step 1: Understand port matching in grep

   Port 80 appears as IP:80[spaces] in ss output before peer address.

2. Step 2: Compare options

   ':80 ' matches exactly :80 (port 80 + space), avoids partial matches like :8080. ':80$' fails (not end-of-line). ':80' and '80' match substrings.

3. Final Answer:

   ss -tuln | grep ':80 ' -> Option B

4. Quick Check:

   Exact port match with ':80 ' [OK]

Hint: Use grep ':80 ' to match port 80 exactly [OK]

Common Mistakes:

• Using grep ':80$' misses because port not at line end
• Using grep ':80' matches ports like 8080
• Using grep '80' matches unrelated numbers