netstat and ss (connection listing) in Linux CLI - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the netstat command do in Linux?

The netstat command shows network connections, routing tables, interface statistics, masquerade connections, and multicast memberships.

intermediate

What is the main difference between netstat and ss commands?

ss is a newer, faster tool that shows socket statistics and network connections, while netstat is older and slower but still widely used.

beginner

How do you list all listening TCP ports using ss?

Use ss -lt to list all listening TCP ports.

beginner

What option with netstat shows all connections including listening and non-listening?

The option -a with netstat shows all connections, both listening and non-listening.

intermediate

Why might you prefer ss over netstat on modern Linux systems?

ss is faster, uses less CPU, and provides more detailed socket information, making it better for quick and detailed network diagnostics.

Which command shows all listening UDP connections using ss?

A netstat -lt

B ss -lt

C netstat -au

D ss -lu

ss -lu lists all listening UDP connections.

What does the -n option do in netstat or ss?

A Shows only established connections

B Shows network statistics summary

C Shows numeric addresses and ports instead of resolving names

D Shows only listening sockets

The -n option prevents name resolution and shows numeric IP addresses and port numbers.

Which command lists all TCP connections including listening and established using netstat?

A ss -au

B netstat -at

C ss -at

D netstat -au

netstat -at lists all TCP connections.

What is the purpose of the -p option in ss or netstat?

A Show the process using the socket

B Show protocol statistics

C Show only passive sockets

D Show port numbers only

The -p option shows the process ID and name using the socket.

Which command is generally faster and recommended for modern Linux systems?

A ss

B netstat

C ifconfig

D ping

ss is faster and more efficient than netstat for socket and connection listing.

Explain how to use ss to find all listening TCP ports and the processes using them.

Think about options to show listening sockets and process details.

Describe the differences and advantages of using ss over netstat for connection listing.

Consider performance and information richness.

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