Linux Lab 10: Configuring DNS Resolution¶

This lab focuses on how Linux systems resolve hostnames to IP addresses. We explore the three critical configuration files that control this process: /etc/resolv.conf, /etc/hosts, and /etc/nsswitch.conf.

[Image of Linux DNS resolution flow]

Table of Contents¶

Linux Lab 10: Configuring DNS Resolution
Table of Contents

Key Concepts¶

DNS (Domain Name System): The system that translates human-readable domain names (like google.com) into IP addresses (like 142.250.190.78).
/etc/resolv.conf: The primary configuration file for the DNS resolver. It tells the system which external DNS server to ask for IP addresses.
/etc/hosts: A local file that maps hostnames to IP addresses manually. It's like a personal, local phonebook that overrides DNS.
/etc/nsswitch.conf (Name Service Switch): This file determines the order in which the system looks for information. It decides whether to check /etc/hosts ("files") first or ask the DNS server ("dns") first.

Step-by-Step Walkthrough¶

1. Identifying the DNS Server¶

Question: What is the IP address of the DNS Server used in this system? Answer: 127.0.0.11

Explanation: We inspected the resolver configuration file:

cat /etc/resolv.conf
# Output:
# search caleston.ca
# nameserver 127.0.0.11
# options ndots:5

The line nameserver 127.0.0.11 indicates the IP address the system queries for DNS lookups.

2. Understanding Local Host Resolution (/etc/hosts)¶

Question: Which file is responsible for host file-based DNS resolution? Answer: /etc/hosts

Explanation: We checked the file content:

cat /etc/hosts

Analyzing the Output: * 127.0.0.1 localhost: Standard loopback entry. * 172.16.238.10 devapp01: Maps the name devapp01 to the IP 172.16.238.10. * 172.16.239.10 devdb01: Maps devdb01 to its IP. This file allows the system to resolve these names locally without needing to ask an external DNS server.

3. DNS Configuration File¶

Question: What is the configuration file used for the DNS Server? Answer: /etc/resolv.conf

Explanation: As seen in step 1, this file configures the resolver library, specifying nameservers and search domains.

4. Changing the DNS Server¶

Task: Change the DNS Server to Google's DNS (8.8.8.8).

Steps: 1. Switch to root user: sudo su (Password: caleston123) 2. Edit the file: vi /etc/resolv.conf (or use nano or sed) 3. Change nameserver 127.0.0.11 to nameserver 8.8.8.8.

Command (One-liner):

echo "nameserver 8.8.8.8" > /etc/resolv.conf

(Note: In a real persistent environment, echo > overwrites the file. If you wanted to keep other settings like search, you would edit it manually.)

5. Checking Resolution Order¶

Question: What is the current order utilized by the system to resolve an IP address? Answer: files dns (Hosts file first, then DNS)

Explanation: We checked /etc/nsswitch.conf:

grep hosts /etc/nsswitch.conf
# Output: hosts:          files dns

* files: Refers to /etc/hosts. * dns: Refers to the nameserver in /etc/resolv.conf. This means the system checks the local file first.

6. Modifying Resolution Order¶

Task: Change the order to DNS first and then hosts.

Steps: 1. Switch to root (sudo su). 2. Edit /etc/nsswitch.conf. 3. Find the line hosts: files dns. 4. Change it to hosts: dns files.

Why do this? Sometimes you want the central DNS server to be the source of truth, overriding any stale entries in a local /etc/hosts file.

7. Understanding Search Domains¶

Question: Which search domain is configured in this system? Answer: caleston.ca

Explanation: We inspected /etc/resolv.conf:

cat /etc/resolv.conf
# Output: search caleston.ca

What does this mean? If you try to ping server1, the system will automatically try to resolve server1.caleston.ca. It saves you from typing the full domain name for servers inside your own network.

Configuration File Reference¶

File	Purpose	Key Directive
`/etc/resolv.conf`	Configures DNS resolver	`nameserver` (DNS IP), `search` (Default domain)
`/etc/hosts`	Local hostname-to-IP mapping	`IP_address hostname`
`/etc/nsswitch.conf`	Determines lookup order	`hosts: files dns`