Configuring Hostname and DNS

1. Viewing Hostname

Command	Description	Example Output
hostname	Displays the system's hostname	`myserver`
hostname -f	Displays the fully qualified domain name (FQDN)	`myserver.example.com`
hostname -I	Displays all IP addresses associated with the host	`192.168.1.100 10.0.0.5`
cat /etc/hostname	Displays the hostname stored in the system configuration file	`myserver`
uname -n	Displays the network node hostname using the uname system call	`myserver`

Example: Displaying Hostname

$ hostname
myserver

$ hostname -f
myserver.example.com

2. Setting Hostname

Method	Description	Command/Config	Notes
/etc/hostname	Static hostname file (systemd-based systems)	`localhost.localdomain`	Reboot required for changes to take effect
hostnamectl	Systemd utility to manage hostname	`hostnamectl set-hostname "new-hostname"`	Applies immediately; persists across reboots
hostname	Legacy command-line utility	`hostname new-hostname`	Does not persist across reboots unless using `/etc/hostname`

Example: Setting Hostname with hostnamectl

hostnamectl set-hostname "webserver-01"

Note: The hostnamectl command is preferred on modern systemd-based systems as it handles both runtime and persistent changes.

Warning: Changing hostname via /etc/hostname or hostnamectl does not update the /etc/hosts file automatically. Manual updates may be necessary for local resolution.

3. Configuring Static Hostname

Method	Description	Command/Configuration	Notes
/etc/hostname	Static hostname file (traditional method)	`localhost.localdomain`	Requires reboot or hostname command to apply changes
hostnamectl	Systemd-based hostname management	`hostnamectl set-hostname "myhost.local"`	Applies immediately, persists across reboots
nmcli (NetworkManager)	Hostname via NetworkManager	`nmcli general hostname myhost.local`	Requires NetworkManager service running

Example: Setting Static Hostname Using hostnamectl

hostnamectl set-hostname "webserver.prod"
hostnamectl status

Note: The static hostname must be a valid DNS label and cannot contain spaces or special characters.

Warning: Changes made via /etc/hostname may not take effect until the system is rebooted or the hostname command is executed manually.

Summary of Hostname Configuration Methods

/etc/hostname: Simple file-based method, requires manual reload
hostnamectl: Modern systemd approach, persistent and immediate
nmcli: NetworkManager integration for dynamic environments

4. Understanding hosts File

Entry Format	Description	Example
IP Address	IPv4 or IPv6 address of the host	`127.0.0.1`
Hostname	Primary name for the IP address	`localhost`
Aliases	Alternative names for the same IP address	`localhost.localdomain`

Example: hosts file entry

127.0.0.1       localhost
127.0.0.1       localhost.localdomain
::1             localhost ip6-localhost ip6-loopback
fe00::0         ip6-localnet
ff00::0         ip6-mcastprefix
ff02::1         ip6-allnodes
ff02::2         ip6-allrouters

Note: The /etc/hosts file is read before DNS lookups and can override DNS resolution for specific hostnames.

Key Points

The /etc/hosts file is a static lookup table for hostnames and IP addresses.
It is processed before DNS queries, allowing local overrides of domain names.
Entries can include IPv4 and IPv6 addresses with aliases.
Changes to /etc/hosts take effect immediately without requiring a service restart.

5. Adding Local DNS Entries

Entry Type	File Location	Example	Description
Local Hosts	`/etc/hosts`	`127.0.0.1 localhost 127.0.1.1 myhostname.localdomain myhostname ::1 localhost ip6-localhost ip6-loopback`	Maps hostnames to IP addresses for local resolution without DNS lookup
Custom Hosts	`/etc/hosts`	`192.168.1.100 app.localdomain 10.0.0.50 database.internal`	Allows defining custom local domain names and their corresponding IPs

Example: Adding a local DNS entry for a development server

# Edit the /etc/hosts file to add a local entry
sudo nano /etc/hosts

# Add the following line to map an IP to a local domain name
127.0.0.1       dev-server.localdomain

Note: Changes to /etc/hosts take effect immediately without requiring a reboot or service restart.

Warning: Modifying /etc/hosts can interfere with network services or cause unexpected behavior if incorrect entries are added.

6. Configuring DNS Resolvers

Configuration File	Content	Description
/etc/resolv.conf	`nameserver 8.8.8.8 nameserver 8.8.4.4 search example.com`	Primary DNS resolver configuration file

Example: Adding DNS Resolver

echo "nameserver 1.1.1.1" >> /etc/resolv.conf

Note: Changes to /etc/resolv.conf may be overwritten by network managers like systemd-resolved.

Warning: Ensure DNS resolvers are accessible and do not block internal traffic.

Resolver Type	Address	Notes
Google Public DNS	`8.8.8.8`	Reliable and fast DNS service
Cloudflare DNS	`1.1.1.1`	Fast and privacy-focused DNS
OpenDNS	`208.67.222.222`	Includes security features

7. Understanding NSSwitch

Service	Source Order	Description
passwd	`files dns`	Looks up user information in /etc/passwd first, then DNS (if configured)
group	`files dns`	Looks up group information in /etc/group first, then DNS
hosts	`files dns`	Resolves hostnames using /etc/hosts first, then DNS
services	`files dns`	Maps service names to port numbers using /etc/services first, then DNS

Example: NSSwitch Configuration

/etc/nsswitch.conf
passwd: files dns
group: files dns
hosts: files dns
services: files dns

Note: The order of sources in /etc/nsswitch.conf determines the lookup priority. If a service is not found in the first source, it will attempt to find it in the next one.

8. Using systemd-resolved

Configuration File	Path	Description
/etc/systemd/resolved.conf	`/etc/systemd/resolved.conf`	Main configuration file for systemd-resolved

Example: Basic systemd-resolved configuration

[Resolve]
DNS=8.8.8.8 8.8.4.4
FallbackDNS=1.1.1.1
Domains=~.

Note: The Domains=~. directive ensures that all domains are resolved via the configured DNS servers.

Option	Values	Description
DNS	`8.8.8.8 8.8.4.4`	Primary DNS servers to use for resolution
FallbackDNS	`1.1.1.1`	Fallback DNS servers if primary ones fail
Domains	`~.`	Domain suffixes to resolve using this resolver

Workflow: Setting up systemd-resolved

Ensure systemd-resolved service is running: sudo systemctl status systemd-resolved
Edit /etc/systemd/resolved.conf to configure DNS servers and domains
Restart the service: sudo systemctl restart systemd-resolved
Verify configuration: systemd-resolve --status

9. Configuring Search Domains

Configuration File	Content	Description
/etc/resolv.conf	`search example.com fallback.org nameserver 8.8.8.8`	Defines search domains and DNS servers for hostname resolution

Example: Setting Search Domains

echo "search company.internal" >> /etc/resolv.conf

Note: Changes to /etc/resolv.conf may be overwritten by network managers like NetworkManager or systemd-resolved.

Warning: Incorrectly configured search domains can lead to unexpected DNS resolution behavior.

10. Testing DNS Resolution

Command	Description	Example
nslookup	Query DNS name servers for IP address	`nslookup example.com`
dig	Query DNS name servers with detailed output	`dig example.com`
host	Resolve domain names to IP addresses	`host example.com`
ping	Test network connectivity and DNS resolution	`ping example.com`

Example: Using dig to query DNS records

dig example.com A

Example: Using nslookup to resolve a hostname

nslookup google.com