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Linux Lab 11: Network Basics & Troubleshooting

This lab focuses on core Linux networking concepts: identifying IP addresses, understanding routing tables, testing connectivity, and troubleshooting network interfaces. We walk through a scenario where a web server (devapp01) is unreachable due to a disabled network interface and a missing default route.

[Image of Linux networking layers]

Table of Contents

Key Concepts

  • IP Address: A unique numerical label (e.g., 172.16.238.187) assigned to each device connected to a computer network.
  • Network Interface: The point of interconnection between a computer and a network (e.g., eth0, eth1).
  • Default Gateway: The node in a computer network (usually a router) that serves as the access point to another network. If a computer doesn't know where to send a packet, it sends it to the default gateway.
  • Routing Table: A data table stored in a router or a network host that lists the routes to particular network destinations.
  • ping: A utility used to test the reachability of a host on an IP network.
  • telnet: A protocol used to provide bidirectional interactive text-oriented communication facility using a virtual terminal connection. Often used to test if a specific port is open.

Step-by-Step Walkthrough

1. Identifying IP Addresses

Question: Which IP addresses are assigned to Bob's Laptop on its primary network interfaces? Answer: 172.16.238.187, 172.16.239.187

Explanation: We used the ip a (or ip addr) command. * We ignored lo (Loopback interface, 127.0.0.1). * We found eth0 with IP 172.16.238.187/24. * We found eth1 with IP 172.16.239.187/24. * We ignored eth2 as it's often an internal Docker bridge or secondary management interface in these labs (though technically it is an interface with 172.17.0.4). The question usually looks for the primary LAN IPs.

2. Identifying Network Interfaces

Question: What is the name of the interface that has the IPs address assigned? Answer: eth0, eth1

Explanation: Directly from the ip a output above: * 2: eth0@if15... -> Interface Name: eth0 * 3: eth1@if19... -> Interface Name: eth1

3. Finding the Default Gateway

Question: What is the default gateway configured in the system? Answer: 172.16.238.1

Explanation: We used the ip r (or ip route) command. * Output: default via 172.16.238.1 dev eth0 * This line tells the system: "To reach any destination not explicitly listed in this table, send the traffic to 172.16.238.1 via interface eth0."

4. Testing Connectivity (Telnet)

Task: Check connection to HTTP port 80 on devapp01-web. Command: telnet devapp01-web 80 Result: No route to host

Explanation: This error is specific. It doesn't mean "Connection Refused" (which means the server is up but the port is closed). "No route to host" usually means the network is unreachable, often due to a firewall blocking ICMP/traffic or a routing issue.

5. Testing Connectivity (Ping)

Task: Ping devapp01-web. Command: ping devapp01-web Result: Destination Host Unreachable

Explanation: This confirms the telnet finding. The local machine (Bob's laptop) cannot find a path to the server's IP (172.16.238.10). This strongly suggests an issue on the target server or the network between them.

6. Alternate Interface Check

Task: Ping the secondary interface devapp01 (172.16.239.10). Command: ping devapp01 Result: 0% packet loss (Success)

Explanation: This is a HUGE clue. The server IS up. We can reach it via 172.16.239.10 (connected to our eth1 network). However, we cannot reach 172.16.238.10 (connected to our eth0 network). This suggests the problem is specific to the eth0 interface on the remote server.

7. Troubleshooting the Interface

Task: SSH into the server and inspect eth0. Command: ip link (run on devapp01) Output: 

16: eth0@if17: <BROADCAST,MULTICAST> mtu 1500 qdisc noqueue state DOWN ...
Diagnosis: The interface state is DOWN. This explains why we couldn't ping 172.16.238.10. The cable is essentially unplugged.

8. Fixing the Interface

Task: Bring up the eth0 interface. Command: sudo ip link set dev eth0 up

Explanation: This command administratively enables the network interface. Running ip link again afterwards confirms the state is now UP.

9. Fixing the Default Route

Task: Add the default route via eth0. Command: sudo ip r add default via 172.16.238.1

Explanation: Even after the interface is up, the server needs to know how to reply to traffic from the outside world. * Why: Without a default route, the server can only talk to local networks (172.16.238.0/24). It wouldn't be able to reach the internet or other subnets. * Result: Adding the route allows the server to route packets back through the gateway (172.16.238.1).

Command Reference

Command Purpose Example
ip a Show IP addresses and interface status ip a
ip r Show the routing table ip r
ping Test connectivity to a host ping google.com
telnet Test connectivity to a specific port telnet server 80
ip link set dev [iface] up Enable a network interface sudo ip link set dev eth0 up
ip link set dev [iface] down Disable a network interface sudo ip link set dev eth0 down
ip r add default via [gateway] Add a default gateway sudo ip r add default via 192.168.1.1