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Linux - Network: Configuration

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Linux - Network: Configuration

Networking Fundamental

Terminologies

IP (Internet Protocol)
- A set of rules governing the format of data sent over the internet or a local network.
- It assigns unique addresses to devices and ensures data packets are routed to their correct destination.
Private IP
- An IP address used within a private network (e.g., home, school, or office LAN) that is not accessible directly from the internet.
Public IP
- An IP address that is accessible over the internet and is assigned by an Internet Service Provider (ISP) to a device or network for communication outside the private network.
Static IP
- An IP address that is manually configured and remains constant over time.
- It is commonly used for servers and devices requiring a permanent address.
Dynamic IP
- An IP address assigned automatically by a Dynamic Host Configuration Protocol (DHCP) server.
- These addresses can change over time as devices connect to and disconnect from the network.

Network Components

Components
- Network Interface
  - a hardware or software component that connects a device to a network.
  - It’s the point where two pieces of network equipment or protocol layers connect.
- MAC address
  - a unique identifier assigned to a network interface controller (NIC) for use as a network address in communications within a network.
- Subnet mask
  - a 32-bit binary number that’s used to determine the destination of a network packet.
  - It’s part of a network address and is always used with an Internet Protocol (IP) address.
- Gateway
  - a piece of networking hardware or software used in telecommunications networks that allows data to flow from one discrete network to another.
- DNS (Domain Name System)
  - The DNS translates human-readable domain names (e.g., www.example.com) into IP addresses (e.g., 192.0.2.1) that computers use to identify each other on a network.
  - It serves as the “phonebook” of the internet, enabling users to access websites using easy-to-remember names instead of numerical IP addresses.

Network Configuration Tools

`nmtui`: Text Interface

Use command nmtui
- can access via ssh
Must as root

nmtui01

`nm-connection-editor`

user command nm-connection-editor
- cannot access via ssh
- only via console

nm-connection-editor01

GNOME Settings

Hostname

`/etc/hostname`

/etc/hostname
- a simple text file that contains the system’s hostname, which is a unique identifier for the machine on a network.
  - the hostname written as a single line.
- Stores the static hostname for the system.
- Used during system boot to set the hostname.

hostname

Command

CMD	Desc
`hostname`	Display the Current Hostname
`hostname -a`	Display the alias name
`hostname -i`	Display IPv4 and IPv6 of the host.
`hostname -I`	Display IPv4 of the host.
`sudo hostname newhostname`	Set a Temporary Hostname
`hostnamectl`	Display Hostname Information
`sudo hostnamectl set-hostname newhostname`	Sets a new hostname and updates the `/etc/hostname`
`sudo hostnamectl set-hostname "pretty_name" --pretty`	Set the Pretty Hostname
`sudo hostnamectl set-hostname newstatichostname --static`	set a static hostname
`sudo hostnamectl set-hostname newtransienthostname --transient`	set a transient hostname (temporary hostname for the current session)

hostname command
- only changes the hostname in the current session.
- Changes made with the hostname command are not persistent after a reboot unless explicitly configured in /etc/hostname and /etc/hosts.
hostnamectl command
- a systemd-based utility for managing the hostname.
- hostname information:
  - Static hostname: The permanent hostname set in /etc/hostname.
  - Pretty hostname: An optional, human-readable version of the hostname.
  - Transient hostname: A temporary hostname assigned dynamically (e.g., by DHCP).
  - Icon name: An optional system identifier, usually for graphical systems.

Lab: Change Hostname

Display current hostname

# display hostname
hostname
# rhelhost.localdomain

# display hostname info
hostnamectl
#  Static hostname: rhelhost.localdomain
#        Icon name: computer-vm
#          Chassis: vm
#       Machine ID: ca299475723c4510bf96994f9b0391d6
#          Boot ID: 8236fb4874fc4e958e092424c1617a84
#   Virtualization: vmware
# Operating System: Red Hat Enterprise Linux 8.10 (Ootpa)
#      CPE OS Name: cpe:/o:redhat:enterprise_linux:8::baseos
#           Kernel: Linux 4.18.0-553.30.1.el8_10.x86_64
#     Architecture: x86-64

Change hostname only for current session

# Display hostname
hostname
# rhelhost.localdomain

# Change hostname temporarily
sudo hostname newhost

# view the hostname in cf
cat /etc/hostname
# rhelhost.localdomain

# reboot
systemctl reboot

# Display hostname
hostname
# rhelhost.localdomain

# view the hostname in cf
cat /etc/hostname
# rhelhost.localdomain

Change hostname permanently

# Display hostname info
hostnamectl
#  Static hostname: rhelhost.localdomain
#        Icon name: computer-vm
#          Chassis: vm
#       Machine ID: ca299475723c4510bf96994f9b0391d6
#          Boot ID: db70d05de8e7435798baa3b443c97ea7
#   Virtualization: vmware
# Operating System: Red Hat Enterprise Linux 8.10 (Ootpa)
#      CPE OS Name: cpe:/o:redhat:enterprise_linux:8::baseos
#           Kernel: Linux 4.18.0-553.30.1.el8_10.x86_64
#     Architecture: x86-64


# Change the static hostname
sudo hostnamectl set-hostname newhostname

# Change pretty name
sudo hostnamectl set-hostname "pretty_name" --pretty

# Change transient name
sudo hostnamectl set-hostname newtransienthostname --transient

# Confirm
# hostname for current session
hostname
# newhostname

# confirm
cat /etc/hostname
# newhostname

hostnamectl
#    Static hostname: newhostname
#    Pretty hostname: pretty_name
# Transient hostname: newtransienthostname
#          Icon name: computer-vm
#            Chassis: vm
#         Machine ID: ca299475723c4510bf96994f9b0391d6
#            Boot ID: db70d05de8e7435798baa3b443c97ea7
#     Virtualization: vmware
#   Operating System: Red Hat Enterprise Linux 8.10 (Ootpa)
#        CPE OS Name: cpe:/o:redhat:enterprise_linux:8::baseos
#             Kernel: Linux 4.18.0-553.30.1.el8_10.x86_64
#       Architecture: x86-64


# reboot
systemctl reboot

# show hostname
hostname

cat /etc/hostname
# newhostname

hostnamectl
#  Static hostname: newhostname
#  Pretty hostname: pretty_name
#        Icon name: computer-vm
#          Chassis: vm
#       Machine ID: ca299475723c4510bf96994f9b0391d6
#          Boot ID: 5a82a02c3a634367991075db25be1cf1
#   Virtualization: vmware
# Operating System: Red Hat Enterprise Linux 8.10 (Ootpa)
#      CPE OS Name: cpe:/o:redhat:enterprise_linux:8::baseos
#           Kernel: Linux 4.18.0-553.30.1.el8_10.x86_64
#     Architecture: x86-64

DNS Service

Configuration File

`/etc/hosts`

/etc/hosts
- a simple text file used for hostname-to-IP address mapping.
- used to resolve hostnames to IP addresses before querying a DNS server.
  - resolve hostnames without relying on DNS.
  - Override DNS for specific hostname-IP mappings.
  - setting custom hostname resolutions

IP_address  hostname  [aliases...]

`/etc/resolv.conf`

/etc/resolv.conf
- a file used to configure DNS (Domain Name System) settings.
- It defines how the system resolves domain names to IP addresses by specifying the DNS servers and search domains.
- It specifies:
  - The DNS servers to use for resolving domain names.
  - The search domains to append to unqualified hostnames.
File format

option value
nameserver 8.8.8.8

Common options:
- search: Defines a list of domain names to append to unqualified hostnames for resolution.
- nameserver: Specifies a DNS server by its IP address.
- domain: Specifies the default search domain.
- options: Allows additional resolver options, such as timeouts.
  - options timeout:2 attempts:3
Example:

domain example.com
search example.net example.org example.edu example.gov
nameserver 192.168.0.1
nameserver 8.8.8.8
nameserver 8.8.4.4

On a system with this file absent, the resolver utilities
- only query the nameserver configured on the localhost,
- determine the domain name from the hostname of the system,
- and construct the search path based on the domain name.

`/etc/nsswitch.conf`

/etc/nsswitch.conf file
- a system configuration file that defines the order and methods used by the system to resolve various types of information, such as hostnames, user accounts, and network services.
- It specifies the name service switch configuration, which dictates how Linux queries different databases like DNS, NIS, and local files to obtain information.
File Structure:
- Each line consists of a database name followed by a list of sources and services to use in order.

database: source1 source2 source3

Common databases:
- passwd: Resolves user account information.
- shadow: Resolves user password information.
- hosts: Resolves hostnames to IP addresses.
- networks: Network names and numbers
- group: Resolves group account information.
- services: Resolves network service names to port numbers.
- ethers: Resolves Ethernet address to hostnames.
- netgroup: Resolves NIS netgroups.
- protocols: Resolves network protocols.
- aliases: Resolves mail aliases.
Common sources:
- files: Refers to local files (e.g., /etc/hosts, /etc/passwd).
- dns: Refers to DNS servers.
- nis: Refers to NIS (Network Information Service).
- mdns: Refers to Multicast DNS.
- ldap: Refers to LDAP (Lightweight Directory Access Protocol).
Default Host
- system will first check the /etc/hosts file for hostname resolution.
- If it doesn’t find the entry there, it will then query DNS.

hosts: files dns

Lab: Step-by-Step Domain Name Resolution

Configuration

cat /etc/nsswitch.conf | grep dns
# hosts:      files dns myhostname
# networks:   files dns

cat /etc/resolv.conf
# search localdomain
# nameserver 192.168.204.2
# nameserver 8.8.8.8

cat /etc/hosts
# 192.168.204.10 localdomain

hostname
# localdomain

Ping hostname

ping localdomain

Step-by-Step Domain Name Resolution
- 1. Kernel looks the file /etc/nsswitch.conf
  - hosts: files dns myhostname:
    - indicates that the system will resolve names in the following order
    - files: Local files like /etc/hosts.
    - dns: Queries DNS servers specified in /etc/resolv.conf.
    - myhostname: Resolves using the local machine’s hostname.
- 1. Check /etc/hosts:
    - The system first checks the /etc/hosts file for the localdomain hostname.
    - 192.168.204.10 localdomain
      - Since localdomain is explicitly mapped to 192.168.204.10, the system uses this entry and resolves localdomain without further queries.

Ping a domain

ping abc.com

Step-by-Step Domain Name Resolution
- 1. Kernel looks the file /etc/nsswitch.conf
    - hosts: files dns myhostname:
      - indicates that the system will resolve names in the following order
      - files: Local files like /etc/hosts.
      - dns: Queries DNS servers specified in /etc/resolv.conf.
      - myhostname: Resolves using the local machine’s hostname.
- 1. Check /etc/hosts:
    - this file does not contain the target domain
    - the system moves to the next method.
- 1. Check /etc/resolv.conf
    - search localdomain: directive appends localdomain to single-label hostnames (e.g., resolving abc as abc.localdomain before querying DNS).
    - specify the DNS servers the system queries in order
      - nameserver 192.168.204.2: the gateway, possibly a local DNS server or resolver
      - nameserver 8.8.8.8: Google’s public DNS
Result Handling:
- If a DNS server returns an IP address for abc.com, the system uses that IP.
- If no DNS server can resolve abc.com, the resolution fails, and ping outputs an error like “unknown host.”

Lab: Setup DNS using `nmcli`

Setup DNS

nmcli device status
# DEVICE  TYPE      STATE                   CONNECTION
# ens160  ethernet  connected               ens160
# ens224  ethernet  connected               static_con

nmcli connection down ens160
nmcli connection modify ens160 ipv4.dns 8.8.4.4
nmcli connection show ens160 | grep "ipv4.dns:"
# ipv4.dns:                               8.8.4.4
nmcli connection up ens160

# confirm
grep "nameserver" /etc/resolv.conf
# nameserver 8.8.4.4
# nameserver 192.168.204.2

DNS Lookup

CMD	Desc
`nmcli connection modify "id_name" ipv4.dns "dns_ip"`	Set the DNS ip
`sudo nano /etc/resolv.conf`	Configure DNS server
`grep "nameserver" /etc/resolv.conf`	Get DNS server
`nslookup target_domain`	DNS lookup a domain name
`nslookup target_domain nameserver`	DNS lookup a domain name with a nameserver
`dig target_domain`	DNS lookup a domain name
`host target_domain`	DNS lookup a domain name
`dig -x ip_address`	Reverse lookup
`host -v ip_address`	Reverse lookup
`nslookup ip_address`	Reverse lookup

Lab: Lookup and Reverse Lookup Domain Name

# Get DNS server
grep "nameserver" /etc/resolv.conf
# nameserver 192.168.128.2

# query a domain name
nslookup google.com
# Server:         192.168.128.2
# Address:        192.168.128.2#53

# Non-authoritative answer:
# Name:   google.com
# Address: 142.251.32.78
# Name:   google.com
# Address: 2607:f8b0:400b:807::200e

# DNS lookup using a specific nameserver
nslookup google.com 192.168.128.2
# Server:         192.168.128.2
# Address:        192.168.128.2#53

# Non-authoritative answer:
# Name:   google.com
# Address: 142.251.41.78
# Name:   google.com
# Address: 2607:f8b0:400b:807::200e

dig google.com
# ; <<>> DiG 9.16.23-RH <<>> google.com
# ;; global options: +cmd
# ;; Got answer:
# ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 48899
# ;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

# ;; OPT PSEUDOSECTION:
# ; EDNS: version: 0, flags:; MBZ: 0x0005, udp: 4000
# ;; QUESTION SECTION:
# ;google.com.                    IN      A

# ;; ANSWER SECTION:
# google.com.             5       IN      A       172.217.165.14

# ;; Query time: 17 msec
# ;; SERVER: 192.168.128.2#53(192.168.128.2)
# ;; WHEN: Sat Feb 08 17:22:12 EST 2025
# ;; MSG SIZE  rcvd: 55

host google.com
# google.com has address 142.251.32.78
# google.com has IPv6 address 2607:f8b0:400b:807::200e
# google.com mail is handled by 10 smtp.google.com.

perform a reverse lookup

# perform a reverse lookup
dig -x 172.217.165.14
# ; <<>> DiG 9.16.23-RH <<>> -x 172.217.165.14
# ;; global options: +cmd
# ;; Got answer:
# ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 7501
# ;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

# ;; OPT PSEUDOSECTION:
# ; EDNS: version: 0, flags:; MBZ: 0x0005, udp: 4000
# ;; QUESTION SECTION:
# ;14.165.217.172.in-addr.arpa.   IN      PTR

# ;; ANSWER SECTION:
# 14.165.217.172.in-addr.arpa. 5  IN      PTR     yyz12s06-in-f14.1e100.net.

# ;; Query time: 8 msec
# ;; SERVER: 192.168.128.2#53(192.168.128.2)
# ;; WHEN: Sat Feb 08 17:26:16 EST 2025
# ;; MSG SIZE  rcvd: 95

host -v 8.8.8.8
# Trying "8.8.8.8.in-addr.arpa"
# ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 63281
# ;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0

# ;; QUESTION SECTION:
# ;8.8.8.8.in-addr.arpa.          IN      PTR

# ;; ANSWER SECTION:
# 8.8.8.8.in-addr.arpa.   5       IN      PTR     dns.google.

# Received 62 bytes from 192.168.128.2#53 in 9 ms

nslookup 8.8.8.8
# 8.8.8.8.in-addr.arpa    name = dns.google.
# Authoritative answers can be found from:

Connection

`/etc/sysconfig/network-scripts/`

/etc/sysconfig/network-scripts/ directory
- directory in primarily Red Hat-based distributions such as CentOS, Fedora, and RHEL
- a directory contains configuration files and scripts used to manage network interfaces and related settings.
Configuration Files
- Network Interface Configuration Files
  - File name: ifcfg-<interface_name>
  - define the settings for each network interface, such as IP address, subnet mask, gateway, DNS servers, and whether the interface is enabled on boot.
  - Common fields:
    - TYPE: The type of connection (e.g., Ethernet, Bridge).
    - BOOTPROTO: The method of assigning an IP address (none, dhcp, static).
    - NAME: Interface name.
    - DEVICE: Name of the physical device (matches the interface name).
    - ONBOOT: Whether to activate the interface during boot (yes or no).
    TYPE=Ethernet PROXY_METHOD=none BROWSER_ONLY=no BOOTPROTO=dhcp DEFROUTE=yes IPV4_FAILURE_FATAL=no IPV6INIT=yes IPV6_AUTOCONF=yes IPV6_DEFROUTE=yes IPV6_FAILURE_FATAL=no IPV6_ADDR_GEN_MODE=eui64 NAME=ens160 UUID=bea1745d-c7f0-430a-a575-9c06f5d420a7 DEVICE=ens160 ONBOOT=yes DNS1=8.8.4.4
- Network Management Scripts
  - used to start, stop, and manage network interfaces
- Route Configuration Files
  - File name: route-<interface> or route6-<interface>
  - define static IPv4 or IPv6 routes for the associated interface.

Commond

Use nmcli connection command

TOP

Note_Tech

Linux - Network: Configuration

Networking Fundamental

Terminologies

Network Components

Network Configuration Tools

nmtui: Text Interface

nm-connection-editor

GNOME Settings

Hostname

/etc/hostname

Command

Lab: Change Hostname

DNS Service

Configuration File

/etc/hosts

/etc/resolv.conf

/etc/nsswitch.conf

Lab: Step-by-Step Domain Name Resolution

Lab: Setup DNS using nmcli

DNS Lookup

Lab: Lookup and Reverse Lookup Domain Name

Connection

/etc/sysconfig/network-scripts/

Commond

`nmtui`: Text Interface

`nm-connection-editor`

`/etc/hostname`

`/etc/hosts`

`/etc/resolv.conf`

`/etc/nsswitch.conf`

Lab: Setup DNS using `nmcli`

`/etc/sysconfig/network-scripts/`