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RHCSA Notes

A collection of notes on RHCSA tasks and tools.

NTP (Network Time Protocol) and Chrony

On RHEL10, Chrony is the standard NTP implementation.
Things to remember for the exam:

  • chrony: The RPM package name (dnf install -y chrony)
  • chronyd: The systemd service and daemon name (systemctl enable --now chronyd)
  • chronyc: The CLI tool used to inspect and control chronyd
  • /etc/chrony.conf: The main config file for chrony

A typical RHCSA task might include setting up NTP (Chrony) and configuring it to use a specific source (e.g., time.google.com).

dnf install -y chrony
vi /etc/chrony.conf
Add or modify an NTP source in this file by using the following syntax.
server time.google.com iburst
The server keyword is used to specify a server to use, followed by the address. Options can be added afterwards, e.g., iburst, which will start with 4-8 requests in order to make the first update of the clock sooner.

After adding the server, start (or restart) the chronyd service and check the status.

systemctl enable --now chronyd
# or
systemctl restart chronyd

Verify that the new time server is being used.

chronyc sources -v
The ^* at the beginning of the line indicates the active synchronization source.

The chronyc tracking command can also be used to see some additional info. 

chronyc tracking

After setting up Chrony, ensure that NTP is enabled at the system level.

timedatectl set-ntp true
timedatectl # verify

Network Configuration

Another common RHCSA task is to configure a system's current local network. Set up a static IPv4 address for it and configure the system's DNS settings.

RHEL uses NetworkManager for all of this. Two tools are primarily used for interacting with NM, nmcli and nmtui.

Network Modification Objectives

As an example, we'll say the following conditions are on a task:

  • Configure a static IPv4 address using:

    • An IP address within the same network as the current configuration, with a host ID of 50
    • Netmask: 255.255.255.0
    • Default gateway within the same network, with a host ID of 1

  • Configure the system to use the following DNS settings:

    • DNS server: 8.8.8.8
    • DNS search domain: example.local

  • Set the system hostname to:

    • rhel-node1.example.com

Identifying Interface and NetworkManager Profile

First, two things must be identified.

  1. The active network interface
  2. The active NetworkManager profile for that interface

There are a couple commands that can show this information.

ip -br a            # Shows the interfaces 
nmcli device status # Can be shortened to `nmcli d s`

The nmcli output will be something like: 

DEVICE  TYPE      STATE                   CONNECTION
ens18   ethernet  connected               ens18
lo      loopback  connected (externally)  lo

The DEVICE column shows the network interface name.

The CONNECTION column shows the NetworkManage profile name. Here, it's the same name as the interface.

So now we have:

  • Interface: ens18
  • Active NM profile: ens18

Now, confirm which interface carries the default route.

ip r show default
Output will look like: 
default via 192.168.4.1 dev ens18 proto dhcp src 192.168.4.55 metric 100
This shows that the default route is handled by the ens18 interface.

Modifying the NetworkManager Profile

After identifying the interface and NetworkManager profile, modify the NetworkManager profile with nmcli.

Current network info can be found by inspecting the interface: 

ip -r addr show dev ens18

The objectives for the profile's config are as follows.

  • An IP address within the same network as the current configuration, with a host ID of 50

    • The current config shows the IP as 192.168.4.55, so we'd need to change it to 192.168.4.50 (change the last number to 50, that's the host ID).

  • Netmask: 255.255.255.0

    • Gives us the entire 192.168.4.0/24 range.

  • Default gateway within the same network, with a host ID of 1

    • Gateway will be 192.168.4.1.

  • Configure the system to use the following DNS settings:

    • DNS server: 8.8.8.8
    • DNS search domain: example.local

Modify the existing NetworkManager profile.

sudo nmcli connection modify "ens18" \
  ipv4.method manual \
  ipv4.addresses 192.168.4.50/24 \
  ipv4.gateway 192.168.4.1 \
  ipv4.dns 8.8.8.8 \
  ipv4.dns-search example.local \
  connection.autoconnect yes

  • connection modify: Changes a persistent NetworkManager profile.
  • ipv4.method manual: Disables DHCP for IPv4 and uses static addressing.
  • ipv4.address: Specifies the address and CIDR.
  • ipv4.gateway: Configures the default IPv4 gateway.
  • ipv4.dns 8.8.8.8: Specifies the DNS resolver.
  • ipv4.dns-search example.local: Specifies the DNS search suffix.
  • connection.autoconnect yes: Activate the profile automatically during boot.

nmcli con modify updates the persistent connection profile. The /etc/resolv.conf file should never be edited manually when making changes like this on RedHat systems.

Setting The New Hostname

After modifying the profile, we can go to the other objective of changing the system's hostname.

sudo hostnamectl set-hostname rhel-node1.example.com
This changes the hostname immediately and persists it across reboots.

Verify: 

hostnamectl
# or
hostname

Activate the Modified NetworkManager Profile

Changes to a connection profile don't immediately alter the current active connection until it's reactivated.

Reactivate the profile: 

sudo nmcli connection up ens18
Here, ens18 is the profile name. This applies any changes made to the profile. It may interrupt networking briefly.

If an SSH session is connected, it'll probably disconnect because the host's IP is changing.
On the exam, perform this operation from the system console instead of via SSH.

An alternative to the previous command: 

sudo nmcli device reapply ens18
Reapplying cannot always apply every type of change cleanly. For an address and gateway change, bringing the connection profile up again is the dependable exam approach.

Verify the Changes

Check the IP address: 

ip -4 addr show dev ens18
Check for 192.168.4.50/24.

Check the default gateway:

ip route show default
Check for 192.168.4.1.

Check NetworkManager's applied settings: 

nmcli device show ens18
Check those values against the ones required.

Check the resolver config.

cat /etc/resolv.conf
The NetworkManager-generated entries should be present: 
search example.local
nameserver 8.8.8.8

Check the hostname.

hostname
Check for rhel-node1.example.com.

Test the local gateway.

ping -c 3 192.168.4.1

Exam Workflow

The exam workflow will look something like this: 

nmcli device status
ip -4 address show dev ens18
ip route show default
nmcli connection show --active

sudo nmcli connection modify "ens18" \
  ipv4.method manual \
  ipv4.addresses 192.168.4.50/24 \
  ipv4.gateway 192.168.4.1 \
  ipv4.dns 8.8.8.8 \
  ipv4.dns-search example.local \
  connection.autoconnect yes

sudo hostnamectl set-hostname rhel-node1.example.com
sudo nmcli connection up "ens18"

ip -4 address show dev ens18
ip route
nmcli device show ens18
hostnamectl hostname

Misc

Tools:

  • audit2why