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🗓️ 04102025 0030

PORTS & CONNECTIONS TOOLS

Tools for checking what ports are open and what's connected. Essential for debugging "port already in use" errors.

netstat - Network Statistics (Legacy)​

Commands​

netstat -tuln                # Listening ports (TCP/UDP)
netstat -an                  # All connections, numeric
netstat -anp                 # All connections with process names (Linux)
netstat -r                   # Routing table
netstat -i                   # Network interfaces
netstat -s                   # Protocol statistics

Use for: Checking open ports, viewing active connections, routing info.

Note: Being replaced by ss on modern Linux systems. Still common on macOS and older systems.

Interpreting Output​

$ netstat -tuln
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
tcp        0      0 127.0.0.1:5432          0.0.0.0:*               LISTEN
tcp6       0      0 :::80                   :::*                    LISTEN
udp        0      0 0.0.0.0:68              0.0.0.0:*

Key parts:

  • Proto: Protocol (tcp, tcp6 = IPv6, udp)
  • Recv-Q/Send-Q: Bytes in receive/send queue (should be ~0)
  • Local Address: What's listening locally
    • 0.0.0.0:22 - Listening on all interfaces, port 22 (SSH)
    • 127.0.0.1:5432 - Listening on localhost only, port 5432 (PostgreSQL)
    • :::80 - IPv6 listening on all interfaces, port 80 (HTTP)
  • Foreign Address: Remote connection (for established connections)
  • State:
    • LISTEN - Waiting for connections
    • ESTABLISHED - Active connection
    • TIME_WAIT - Connection closing
    • CLOSE_WAIT - Remote end closed connection

Common scenarios:

Find what's using port 8080:

$ netstat -tuln | grep :8080
tcp        0      0 127.0.0.1:8080          0.0.0.0:*               LISTEN

→ Something is listening on port 8080 (localhost only)

See all established connections:

$ netstat -tn | grep ESTABLISHED
tcp        0      0 192.168.1.100:54321     93.184.216.34:443       ESTABLISHED

→ Your machine (192.168.1.100:54321) connected to remote server (93.184.216.34:443)

ss - Socket Statistics (Modern)​

Commands​

ss -tuln                     # Listening ports (TCP/UDP)
ss -tun                      # All TCP/UDP sockets
ss -t state established      # Established TCP connections
ss -p                        # Show process names
ss -s                        # Summary statistics
ss dst 192.168.1.1          # Filter by destination
ss sport = :80              # Filter by source port

Use for: Same as netstat but much faster and more powerful filtering.

What it does: Display socket statistics and network connections.

Interpreting Output​

$ ss -tuln
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    127.0.0.1:5432       0.0.0.0:*
tcp   ESTAB   0      0      192.168.1.100:54321  93.184.216.34:443

Similar to netstat, but:

  • Netid: Socket type (tcp, udp, etc.)
  • State: Connection state (LISTEN, ESTAB = established)
  • Faster performance on systems with many connections
  • Better filtering capabilities

Show process names (-p requires sudo):

$ sudo ss -tulnp
Netid State  Local Address:Port  Process
tcp   LISTEN 0.0.0.0:22           users:(("sshd",pid=1234,fd=3))
tcp   LISTEN 127.0.0.1:5432       users:(("postgres",pid=5678,fd=5))
tcp   LISTEN 0.0.0.0:8080         users:(("node",pid=9012,fd=21))

→ Now you can see which process is using each port!

Filter by state:

$ ss -t state established
# Shows only active connections

lsof - List Open Files/Ports​

Commands​

lsof -i :8080               # What's using port 8080?
lsof -i tcp                 # All TCP connections
lsof -i -P                  # Show port numbers (no service names)
lsof -i -n                  # No hostname resolution (faster)
lsof -i tcp:1-1024          # Privileged ports
lsof -u username            # Network connections by user

Use for: Finding which specific process is using a port. Most direct way to answer "what's using port X?"

What it does: Lists open files, including network sockets (in Unix, network connections are files!).

Interpreting Output​

$ lsof -i :8080
COMMAND   PID   USER   FD   TYPE  DEVICE SIZE/OFF NODE NAME
node    12345  user   21u  IPv4  0x123   0t0     TCP *:8080 (LISTEN)

Key parts:

  • COMMAND: Process name (node, python, etc.)
  • PID: Process ID (use kill 12345 to stop it)
  • USER: Which user owns the process
  • FD: File descriptor (21u = descriptor 21, read+write)
  • TYPE: IPv4 or IPv6
  • NAME: Connection details
    • *:8080 (LISTEN) - Listening on all interfaces, port 8080
    • localhost:8080->localhost:54321 (ESTABLISHED) - Active connection

Common use cases:

Port already in use error?

$ lsof -i :3000
COMMAND   PID  USER   FD   TYPE   NODE NAME
node    12345  user   21u  IPv4   TCP *:3000 (LISTEN)

# Kill it:
$ kill 12345
# Or force kill:
$ kill -9 12345

See all connections for an app:

$ lsof -c nginx  # All files/connections for nginx

Find what's making external connections:

$ lsof -i tcp -s TCP:ESTABLISHED

Comparison: When to Use Which?​

TaskBest ToolCommand
What's on port 8080?lsoflsof -i :8080
List all listening portsss or netstatss -tuln
Which process? (with PID)lsof or ss -psudo ss -tulnp
Quick check on old systemnetstatnetstat -tuln
Fast on many connectionsssss -tuln

References​