netstat and ss (connection listing) in Linux CLI - Commands & Configuration

Introduction

Sometimes you want to see which programs are talking over the network on your computer. netstat and ss are tools that show you all the current network connections and listening ports. They help you understand what is communicating and where.

When you want to check if a web server is running and listening on port 80.

When you suspect a program is using the network and want to find its connection details.

When you want to see all active network connections on your machine.

When you need to find the process ID (PID) of a program using a specific port.

When you want to troubleshoot network issues by viewing socket states.

Commands

This command lists all TCP and UDP ports that are currently listening, showing numeric addresses and port numbers for clarity.

Terminal

netstat -tuln

Expected Output

Active Internet connections (only servers)

Proto Recv-Q Send-Q Local Address           Foreign Address         State      

tcp        0      0 0.0.0.0:22              0.0.0.0:*               LISTEN     

tcp6       0      0 :::80                   :::*                    LISTEN     

udp        0      0 0.0.0.0:123             0.0.0.0:*                           

udp6       0      0 :::123                  :::*                                

→

-t - Show TCP connections

→

-u - Show UDP connections

→

-l - Show only listening sockets

→

-n - Show numeric addresses and ports

This command also lists all listening TCP and UDP sockets with numeric addresses, but ss is faster and more modern than netstat.

Terminal

ss -tuln

Expected Output

Netid  State      Recv-Q Send-Q Local Address:Port               Peer Address:Port              

tcp    LISTEN     0      128    0.0.0.0:22                      0.0.0.0:*                    

tcp    LISTEN     0      128    [::]:80                        [::]:*                      

udp    UNCONN     0      0      0.0.0.0:123                     0.0.0.0:*                    

udp    UNCONN     0      0      [::]:123                       [::]:*                      

→

-t - Show TCP sockets

→

-u - Show UDP sockets

→

-l - Show listening sockets

→

-n - Show numeric addresses and ports

This command shows all active TCP and UDP connections with numeric addresses and the process ID and name that owns each connection.

Terminal

netstat -tunp

Expected Output

Active Internet connections (w/o servers)

Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name

tcp        0      0 192.168.1.10:22         192.168.1.5:53422       ESTABLISHED 1234/sshd

udp        0      0 0.0.0.0:123             0.0.0.0:*                           567/ntpd

→

-t - Show TCP connections

→

-u - Show UDP connections

→

-n - Show numeric addresses and ports

→

-p - Show process ID and program name

This command lists all active TCP and UDP connections with numeric addresses and shows the process ID and name for each connection, using the modern ss tool.

Terminal

ss -tunp

Expected Output

Netid  State      Recv-Q Send-Q Local Address:Port               Peer Address:Port              Process

tcp    ESTAB      0      0      192.168.1.10:22                 192.168.1.5:53422              users:("sshd",pid=1234,fd=3)

udp    UNCONN     0      0      0.0.0.0:123                     0.0.0.0:*                      users:("ntpd",pid=567,fd=4)

→

-t - Show TCP sockets

→

-u - Show UDP sockets

→

-n - Show numeric addresses and ports

→

-p - Show process using the socket

Key Concept

If you remember nothing else from this pattern, remember: ss is a faster, modern replacement for netstat that shows detailed network connection info including processes.

Common Mistakes

Running netstat without sudo when trying to see process IDs

Without sudo, netstat cannot show the process ID and program name, so that info is missing.

Run netstat with sudo (e.g., sudo netstat -tunp) to see process details.

Using netstat without the -n flag

Without -n, netstat tries to resolve hostnames and service names, which slows down output and can confuse beginners.

Always use -n to show numeric IP addresses and ports for clarity and speed.

Expecting ss output to be identical to netstat

ss output format is different and more detailed; expecting exact same columns can cause confusion.

Learn ss output format separately; focus on key info like state, local address, and process.

Summary

Use netstat or ss to list current network connections and listening ports.

Add flags like -t, -u, -l, -n, and -p to filter and show details like protocols, listening state, numeric addresses, and processes.

ss is a modern, faster tool that provides similar info to netstat with better performance.

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