Master Network Connectivity Checks: Essential Linux & Windows Tools Explained

Physical Link Check

Use ethtool to inspect the physical connection status of a network interface. The command ethtool <device_name> displays a Link detected field; yes indicates a healthy link.

Network Reachability

ping

The ping command tests connectivity between the local host and a remote host. Important output fields include:

icmp_seq – sequence number of the sent packet

ttl – Time To Live, decremented by each router

time – round‑trip time in milliseconds

rtt – round‑trip time

min / avg / max – minimum, average, and maximum RTT

mdev – standard deviation of RTT, indicating stability

On Linux you can bind the ping to a specific interface with the -I (capital i) option; on Windows the -S option specifies the source address.

Linux example: ping -I eth0 8.8.8.8 Windows example:

ping -S 192.168.1.10 8.8.8.8

Path Tracing

tracepath

tracepath

works like traceroute but provides a simpler, more concise output. It shows each hop the packets traverse and helps locate where a connection fails.

Example:

tracepath -n 192.168.1.241

Interpretation notes:

"[LOCALHOST] pmtu 1500" – the first detection point (local machine)

"no reply" – no response from a hop, possibly due to firewall filtering

HTTP/HTTPS Connectivity

wget

Use wget to test URL reachability without downloading content by adding the --spider option. The -S flag shows server response headers.

curl

Check a URL with curl by requesting only the headers using -I. Adding -f makes curl fail silently on HTTP errors, outputting only a short error message.

TCP/UDP Port Checks

nc (netcat)

Netcat probes TCP or UDP connectivity.

TCP check (scan mode, no data transfer): nc -zv remote_host port UDP check (requires -u and may need manual termination with Ctrl+C):

echo test | nc -uv remote_host port

telnet

Simple TCP port test:

telnet remote_host port

nmap

Powerful scanner for port states and OS detection. Common states:

open – service listening

closed – no service

filtered – probe blocked by firewall

Basic scan: nmap 192.168.0.104 Scan specific ports: nmap -p 80,443 remote_host ACK scan ( -sA) determines firewall presence without revealing open ports.

Local Socket Inspection

netstat and ss

Both tools display active connections, listening sockets, and associated processes.

Common options (identical for netstat and ss): -a – show all sockets -t – TCP sockets -u – UDP sockets -n – numeric addresses (no DNS lookup) -l – listening sockets only -p – show owning process

netstat -a

netstat -tn

netstat -un

netstat -ln

netstat -tulnp

The ss command uses the same flags and provides additional columns such as State , Recv‑Q , Send‑Q , Local Address:Port , Peer Address:Port , and Process .

DNS Availability

Verify DNS resolution with dig, host, or nslookup. Example: dig www.baidu.com Specify a DNS server using @:

dig www.baidu.com @180.76.76.76