Practice Exercise: # DNS Troubleshooting with `nslookup` in Linux

Introduction

In this lab, you will learn how to effectively use the nslookup command in Linux for DNS (Domain Name System) troubleshooting. DNS is a critical component of network communication, translating human-readable domain names into IP addresses. Understanding how to diagnose and resolve DNS issues is essential for a System Reliability Engineer (SRE).

Prerequisites

A Linux server with nslookup installed (You can use any Linux distribution like Ubuntu, CentOS, or Debian).
Basic knowledge of the Linux command line.
Internet connectivity from the Linux server.

Scenario

You are a System Reliability Engineer (SRE) responsible for troubleshooting DNS issues on a Linux server acting as a client. Your goal is to use the nslookup command to diagnose and resolve DNS problems. You will perform various exercises to practice troubleshooting DNS issues.

Lab Procedures

Task 1: Verify DNS Configuration

Open a terminal on your Linux server.
Run the following command to check the DNS configuration:
```
nslookup google.com
```
Note down the IP address of the DNS server used.

Task 2: Test DNS Resolution

Use nslookup to resolve the IP address of google.com:
```
nslookup google.com
```
Ensure that the IP address matches the one you noted in Step 1.

Task 3: Test Reverse DNS Lookup

Perform a reverse DNS lookup for an IP address:
```
nslookup 8.8.8.8
```
Verify that it resolves to a meaningful hostname.

Task 4: Query Specific DNS Server

Query a specific DNS server (e.g., 8.8.4.4) for a domain name:
```
nslookup google.com 8.8.4.4
```
Compare the results with the default DNS server.

Task 5: Query a Non-Existent Domain

Try to resolve a non-existent domain:
```
nslookup nonexistentsite123.com
```
Analyze the error message.

Task 6: Diagnose DNS Timeout

Test DNS resolution for a site known to be slow or unavailable:
```
nslookup slowwebsite.com
```
Analyze the timeout message.

Task 7: Debug DNS with Verbose Output

Use verbose mode to get detailed information about DNS queries:
```
nslookup -debug google.com
```
Examine the verbose output for additional insights.

Task 8: Check DNS Cache

Check the local DNS cache for cached records:
```
nslookup -querytype=any google.com
```
Note any cached records and their TTL (Time To Live).

Task 9: Perform DNSSEC Validation

Test DNSSEC validation for a secure domain:
```
nslookup +dnssec isc.org
```
Check if DNSSEC validation was successful.

Task 10: Troubleshooting DNS with `dig`

- Use the `dig` command to troubleshoot DNS:
  ```bash
  dig google.com
  ```
- Compare the output with `nslookup` results.

Sample Troubleshooting Command

To troubleshoot DNS resolution issues, use the following command to trace the DNS query path:

```bash nslookup -debug google.com

Practice Exercise: # DNS Troubleshooting with nslookup in Linux