Diagnosing connectivity issues in a Linux environment is a crucial skill for any systems engineer or administrator. Understanding how to troubleshoot network problems can save time and resources. This article will guide you through the process of diagnosing connectivity issues using various Linux commands and tools.

Examples:

1. Ping: The ping command is one of the simplest and most commonly used tools for checking the reachability of a host on an IP network.

   ping -c 4 google.com

   This command sends four ICMP echo requests to google.com and waits for a reply, helping you determine if the host is reachable.

2. Traceroute: Use the traceroute command to trace the path packets take to a network host, which can help identify where a connection is failing.

   traceroute google.com

   This command will display each hop along the route to google.com, showing where delays or failures occur.

3. Netstat: The netstat command displays network connections, routing tables, and interface statistics.

   netstat -tuln

   This command lists all listening ports and their associated services, helping you identify if a service is running and accessible.

4. Ifconfig / Ip: Use ifconfig or ip to check the network configuration of your interfaces.

   ifconfig
   # or
   ip addr show

   These commands provide details about your network interfaces, such as IP addresses and MAC addresses, which are crucial for diagnosing connectivity issues.

5. Nslookup / Dig: Use nslookup or dig to query DNS servers and diagnose DNS-related issues.

   nslookup google.com
   # or
   dig google.com

   These commands will show you the IP address associated with a domain name, helping you ensure that DNS resolution is functioning correctly.

6. Curl / Wget: Use curl or wget to test HTTP connectivity and download content from web servers.

   curl -I http://google.com
   # or
   wget --spider http://google.com

   These commands check if a web server is reachable and responding as expected.

7. Nmap: The nmap command is a powerful network scanning tool that can be used to discover hosts and services on a computer network.

   nmap -sP 192.168.1.0/24

   This command scans the specified subnet for active hosts, helping you identify which devices are online.