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Notes about RHCSA

Mananging disks

In this chapter , which I accidentally deleted by accident….fucking fat fingered my vim keybinds. Anywho… This is how you can manage disks on RHEL 9 !

Step one : Finding out which type of disk parition you would like to use

There’s two types of partitions you can make , the first being an MBR partition table.

MBR

MBR stands for Master Boot Record. It’s an old partitioning scheme from the 80’s which was meant to be read by your BIOS and loaded. This would allow you to actually boot your computer ! Withtin the Master Boot Record is a bootloader and a partition table. The bootloader for your computer by the way is the first 512 bytes of code that loads your OS( kernel ) into memory.

You can create an MBR partitioning table by using fdisk. Run the command and select your disk

Example

$ fdisk /dev/sdx

Side note : replace /dev/sdx with the name of your device

in the options , select “p” for primary partition , use the default first sector , enter +1G for a one gibibyte partition

You can change the type of partition by using the “t” option , by default the parititon type is set to Linux

Once you are done , write your changes to disk with “w”.

You can add extensible partitions which can contain logical partitions which can then be used to create filesystems on them. You cannot create a filesystem on an extensible partition on it’s own !

To do this do the following :

Use the “n” option to create a new partition.

Then use the “e” option to create an extensible partition.

Advanced troubleshooting

Here below I will put some notes while studying the following chapters to see if I retain any info I jot down here.

I’m going to edit the grub2 config to increase the timeout so I can access it for longer periods of time.

I’ve opened up another vm for learning about troubleshooting grub 2. You can modify some of the options during boot. Press ‘e’ to view the command line args that will be passed to your system at boot.

Here are some options you can add to the command args and what they do:

rd.break = stops boot procedure in the initramfs stage. This also means that the root filesystem is not mounted yet.

init=/bin/sh or init=/bin/bash -> specifies that the shell should be started after loading the kernel and initd. Provides earliest possible access to a running system. Only the root fs is mounted and it’s mounted as read-only

Rescue disk

If you are unable to boot from disk , you will more than likely see a blinking cursor. If not , regardless you should be able to use a rescue disk. The rescue disk is apart of the installation disk that comes with RHEL.

If you can’t boot from GRUB , try the “Boot from a Local Drive” option which allows you to boot your machine using a bootloader that is installed to disk

When using the Rescue a Red Hat Entrpriese Linux system , the distro should detect your other installation of RHEL. From there you will be able to access the filesystem your distro is installed on /mnt/sysimage.

from there you can chroot to the filesystem in /mnt/sysimage

chroot /mnt/sysimage

Once you are in the root directory of the rescue disk , you can re-install grub2 if you need to. To do this run the following commands below

grub2-install /dev/sdx

If you’re initramfs is broken , you can use dracut to fix it. This command creates a new initramfs for the currently loaded kernel.

config files for dracuc are stored in /etc/dracut.conf.d

You can tell if the initramfs is messed up since the root filesystem will never be mounted. Nor will you see any systemd units started. To fix it using dracut

just run the following command:

dracut --force

If there’s something wrong with the fstab file and you need to access the filesystem( in the event you can’t for some reason ) you can use the command below to remount the root filesystem

mount -o remount,rw /

This remounts the root fs with the ‘rw’ and the ‘remount’ mount options.

Resetting / forgetting the root password

  1. Start system with init=/bin/bash arg in grub boot menu command line.

  2. mount -o remount,rw /

  3. passwd root

  4. exec /usr/lib/systemd/systemd

And that is how you can change the root password

IF systemd is not started , you can use a similar command to step 4

exec /usr/lib/sytemd/system

Bash scripting

Arguments used in bash scripts can be referenced by a $1-9 character

For example , if you have a script something.bash name1 name2

Name1 would be $1 and name2 would be $2

”$#” represents the number of arguments you have entered “$@” refers to all arguments used when starting the script.

You can use the “read” command to query the user for input

To add a timestamp to something , you can use the following command and add it to a variable

TODAY=$(date +%d-%m-%y)

Most of the syntax for bash scripting is straight forward , with the added exception of the until statement , which acts like a do while loop

Note : don’t use spaces in between the ‘=’ sign and the declaration of the variable Also put spaces in between conditional brackets

Debugging bash scripts

To debug a script and find out what it’s doing , you can run the bash -x script.bash command which will tell you what code is being run. And any errors that might occur during runtime.

Configuring SSH

dictionary attacks are commonly used to break into ssh servers , from what i’ve seen on my machine they typically use usernames and passwords that might be setup by default on the server.

These are things you can do to harden your ssh server

To configure these things , you can make all configuration changes in the /etc/ssh/sshd_config file. This config file is the default config used for an ssh server.

Config option for disallowing the root user to login to ssh

PermitRootLogin prohibit-password

This only allows the root user to login if they have a valid pub/private key pair

Changing default ssh port

Edit config file in /etc/ssh/sshd_config

Add port to SELinux label semanage port -a -t ssh_port_t -p tcp <PORTNUMBER>

Allow port through firewall-cmd

firewall-cmd --add-port=<port>/tcp --permanent

Then reload the configuration for the service

systemctl reload sshd

Then reload the firewall-cmd config

firewall-cmd --reload

You shoulden’t really use the MaxAuthTries option since it’s suseptible to ddos attacks. When this option is enabled it locks out the specified user trying to login. So someone could just try to login as that usera bunch to lock up the account. It’s still good for monitoring security events as this option starts logging the failed attempts.

Logs are written to the AUTHPRIV syslog utility. By default it writes the logs to /var/log/secure

Restriciting which users can login to your ssh server

You can enable access to your server by the usernames of the users. To do this , you must add the AllowUsers field in the /etc/ssh/sshd_config file

Other useful sshd options

UseDNS -> Very inefficient if other users connections are slow , turn off for performance. This option verifies remote hostname is same as remote address

MaxSessions -> specifies max number of sessions from the same remote IP. If you have users connecting to the server with multiple sessions it’s a good practice to increase the amount of max sessions.

TCPKeepAlive -> ensures clients that are unavailable are released ClientALiveInterval -> time before the server sends a packet to the client when no activity is detected.

ClientAliveCountMax -> specifies how many of these packets are sent. if set to 30 and ClientAliveCountMax is set to 10 , connections are kept alive for about 5 minutes.

The two options above can only be used for the server side of ssh connections. You can try using two options for clients which try to do the same thing

ServerAliveInterval & ServerAliveCountMax

modify the ~/.ssh/config for local users

I’d also like to note other options that are used with ssh as well

HostBasedAuthentication -> Allows only users who’s keys are already present in /etc/ssh/ssh_known_hosts. To enable this , add the key in the users .ssh/authorized_keys and copy it over to the directory mentioned previously.

Key locations

Client/Sever public/private keys -> /etc/ssh

Clients pub pri key -> /home/user/.ssh

Key pair authentication

Generate public / private key ssh-keygen

Then copy the key over to your server , make sure password auth is enabled ssh-copy-id -p 1234 user@192.1.1.1

If you set a passphrase , cache the passphrase to save time

ssh-agent /bin/bash

ssh-add

You can set a passphrase for the private key which is in most cases more secure, however it is inconveinient for the user as they will have to enter this key everytime they attempt to connect to the server. However you can cache the key for a short amount of time using the ssh-agent and ssh-add commands. To do so , see below

SELinux

SELinux prevents system calls from being executed unless allowed. In selinux you have policies which contain rules that specify what source domain is allowed to access which target domain You have to be root user to use any of the commands

Check all active ports on SELinux

semange port -l

The names listed for these ports are labels , for example the label for the SSH port is ssh_port_t. The label relates to the service and port used.

If you want to change the port for an existing label , you must modify the port listed on the label.

Source domains can be a port or a process.

Target domains could be a file , directory , or network port

labels define access rules also called context labels

SELinux has two modes, enforcing mode , and permissive mode.

IN enforcing mode, SELinux is fully operational and enforces all rules in the polic

in permissive mode, activity is logged however no access is blocked to any services so this mode isn’t really secure

Logging messages are written to /var/log/audit/audit.log

you can change the mode for selinux in /etc/sysconfig/selinux

On RHEL , by default the setting is set to enforcing

To put selinux in disabled mode on RHEL 9 , you need to change the kernel boot argument to selinux=0 , you can also set permissive mode in the kernel boot options as well with enforcing=0

You can use setenforce to check what mode selinux is in , you can switch between the two modes by specifying a number that relates to the mode

Set permissive mode

setenforce 0

Set enforce mode

setenforce 1

This only changes the mode temporarily , to keep the changes you will need to write it to /etc/sysconfig/selinux

You can also check the status of selinux with the sestatus command sestatus -v

If anyone tells you you need to disable selinux for an application , they’re wrong as a policy can be added for any application.

you can use sepolicy generate to generate a new policy for an application

Context labels

Files and directories Ports Processes Users

Context labels define the nature of the object. Rules are created to match context labels of source objects ( or source domains ) to labels of target objects ( target domains )
So setting up these labels correctly is very important as mismatching context labels can cause alot of issues with selinux.

Make sure you know about context types as they are important for the exam

Configuring a firewall

firewall-cmd uses the netfilter subsystem , it allows kernel modules to inspect incoming , outgoing , or forwarded packets and either allow the packet or block it.

iptables used to be the default firewall , however it has since been replaced by nftables.

You can use the nft command to rules directly to nftables

Firewalld is a system service that can configure firewall rules by using different interfaces.

Apps can request ports to be opened by using the DBus messaging system. Without requireing any action from the sys admin.

This allows apps to address the firewall from user space.

firewall-cmd manages the firewalld system service

Understanding Firewalld Zones

a zone is a collection of rules that are applied to incoming packets matching a specific source address or network interface

Firewalld applies to incoming packets by default , outgoing packets must be configured

Zones are useful for servers that have multiple interfaces. Multiple zones allow you to set a specific set of rules for each interface.

With servers with only one interface, you may only need 1 zone.

This zone is already set by default , and if no zone is available , packets are handled through the default zone.

Firewalld default zones

Block

incoming connections are rejected with an “icmp-host-prohibited”. Only connections that were initiated on this system are allowed

Dmz

For use on computers in the demilitarized zone , only selected incoming connections are accepted, limited access to the internal network is allowed.

Drop

Any incoming packets are dropped and there is no reply.

External

For use on external networks with NAT enabled, used on routers. Only selected incoming packets are acccepted.

Home

For use with home networks , computers on the same network are truested and only selected incoming connections are accepted.

Internal

Most computers on the same network are trusted , only selected incoming connections are accepted.

Public

Other computers in the same network are not trusted, limited connections are accepted. Used as the default zone for newly created network interfaces.

Trusted

All network connections are accepted

Work

Most computers on the same network are trusted, only selected incoming connections are accepted.

Understanding Firewalld Services

Not the same as a service in systemd.

A Firewalld service specifies what exactly should be accepted as incoming and outgoing traffic in the firewall.

This typically includes ports to be opened and kernel modules that should be loaded

XML files define the service. And they can be found in /usr/lib/firewalld/services

By looking at the service file for git.xml , you can see the port being defined and the protocol used for the service.

You can also use the firewall-cmd --get-services command to see what services are available

Services are added to zones

To add your own services , you can put the XML files in the /etc/firewalld/services directory. They will be automatically added once the firewalld service is restarted.

Working with Firewalld

Remember to commit changes to on-disk once you are finished

On the exam , only use ports if no services contain the ports that you want to open.

NFS

You should use NFS with an authentication service such as Lightweight Directory Access Protocol ( LDAP for short ) or kerberos

Setting up NFS

  1. Create local directory you would like to share
  2. Edit the /etc/exports file to define NFS share
  3. Start NFS server
  4. Configure firewall to allow incoming NFS traffic

If you encounter issues while mounting , keep in mind that NFSv4 may have issues with servers behind a firewall.

Showmount relies on the portmapper service which uses random UDP ports to make a connection. Therefore if you have issues it may be due to the showmounts UDP port being blocked by the firewall. To circumvent this, you can simply perform a pseudo root mount on the

Mounting NFS Shares Through fstab

Add the following line :

server1:/share /nfs/mount/point nfs sync 0 0

Using Automount to mount Remote File Systems

autofs supports wildcard mounts which are not supported by systemd

unlike mount , no root perms are required

You define mount points and the directory of the secondary file

Example :

/nfsdata /etc/auto.nfsdata

automount works completely in user space

The configurations for the mount are defined in the /etc/auto.nfsdata file you created

In the secondary file you created, you define the subdir that will be created using a relative file name. In the example in the book , they are using the subdir “files”

files -rw server2:/nfsdata

Instead of “server2” you can also put the IP of the nfs host

Configuring Automount for NFS

  1. Install the autofs package
  2. showmount -e <hostname/ip>
  3. edit the /etc/auto.nfsdata
  4. systemctl enable --now autofs
  5. cd /nfsdata/files
  6. type mount

Using Wildcard in Automount

good for automounting home directories

This allows you to store the home dir of a specific user on a central NFS server

You can create a wildcard mount by specifying the mount in the secondary config file Example :

* -rw server2:/users/&

the “*” represents the local mount point while the “\&” matches the item on the remote server

you could usually mount the /home directory in the config , but that would mean other users can access each others home directories.

Example auto.master file

/users /etc/auto.users

Example /etc/auto.users file

* -rw server2:/users/&

Example auto.home file which mounts each users home dir

#/etc/auto.home
user1   server:/home/user1
user2   server:/home/user2
user3   server:/home/user3

Exmaple auto.home mounting home dirs with wildcard mounts

* server:/home/&

Configuring Time Services

During boot, the hardware clock or the real-time clock (RTC) is read , which resides in the computer hardware. The time it defines is the “hardware time”

The time on the hardware clock on linux servers is set to Coordinated Universal Time (UTC)

System time is maintained by the OS , independent of the hardware clock

When system time is changed , it’s not synced with the harware clock.

System time is kept in UTC . Apps convert system time into local time

Local time is the actual timezone , Daylight Savings Time (DST ) is also considered so the accurate time will be displayed.

Real Time Clock = same thing as a hardware clock

Using Network Time Protocol

Stratum defines the reliability of an NTP time source , the lower the stratum, the more reliable it is. Internet servers use stratum 1 or 2. You can use a higher stratum as a backup

it’s good practice to set stratum 5 on a local time server with a reliable hardware clock and stratum 8 on a server that is not reliable

the /etc/chrony.conf file is a config file with a standard list of NTP servers on the internet.

All you need to do is switch on NTP

timedatectl set-ntp 1

On linux , time is calculated as an offset of epoch time , Epoch time is the numebr of seconds since January 1, 1970

Timestamps in /var/log/audit/audit.log are in epoch time , not human time.

To convert epoch time to human time , use the following command

The epoch timestamp is in the “msg=audit” field in /var/log/audit/audit.log

date --date '@1720893005

There is a problem with epoch time however, on a 32-bit system , the number of seconds that can be counted in the field reserved for time notation will be exceeded in 2037. You can test this by attempting to set the time to 2050

64-bit systems however can notate time far into the twenty second century

You can also use date to show time in different formats

Show the current system day of month, month , and year

date +%d-%m-%y

Set the current time 3 minutes past 4 pm

date -s 16:03

Using hwclock

hwclock --systohc - synchronizes curent system time to the hardware clock

hwclock --hctosys - synchronizes current hardware time to the system clock

Using timedatectl

timedatectl , when used without any arguments the command shows you detailed info about the current time and date , it also shows the time zone you are in.

timedatectl commands

status - shows current time settings

set-time TIME - sets the current time

set-timezone ZONE - sets the current timezone

list-timezone - shows a list of all time zones

set-local-rtc [0|1] - controls whether the RTC ( hardware clock )

set-ntp [0|1] - Controls whether NTP is enabled

timedatectl is used to switch on NTP time , it talks to the chronyd process

Between linux servers , time is normally communicated in UTC to allow servers to use the same time settings

Setting the correct time zone

  1. Create a symbolic link to a file in the /usr/share/zoneinfo and name it /etc/localtime

  2. Use the tzselect utility

  3. use timedatectl

Configuring Time Service Clients

by default , chrony is configured to get the right time from the internet. Servers from pool.ntp.org are used.

you can add your own local time servers to be used in the /etc/chrony.conf file

Configuring local NTP time client

  1. Comment out pool 2.rhel.pool.ntp.org
  2. include the line allow 192.168.0.0/16 to allow access from all clients that use a local IP address starting with 192.168
  3. include the line local stratum 8 to ensure your local time server will advertise itself with a stratum of 8
  4. restart the chronyd service
  5. add the ntp service to the firewall firewall-cmd --add-service ntp --permanent followed by firewall-cmd --reload
  6. on the secondary server , add the hostname or IP of the NTP server to the clients config file.
  7. restart the chronyd service
  8. run chronyc sources

Containers

Container images contain all dependencies to run an application.

Container runs on top of a container engine , the container is run from a container image. These container images are found in a container registry.

To run containers , you need an OS that includes a conatiner engine , docker was used initially but it has since been replaced by CRI-o

There are three tools you can use to manage containers

podman - used to start, stop , and manage containers

buildah - tool that helps you create custom images

skopeo - tool used for managing and testing container images

Containers are not always pure linux , although they do depend on a few features offered by the linux kernel

Namely :

A namespace provides isolation for system resources , for example chroot jails, which presents the contents of a directory as it it is the root directory of your system. So process running in the chroot jail can’t see anything but the contents of that directory. This stops the process from accessing other parts of the OS.

chroot jails still exist , except now they are leveraged and part of what’s called the mount namespace

Overview of namespaces

Mount - equivalent to chroot namespace , contents of a directory are presented in such a way that no other directories can be accessed

Process - makes sure that processes running the this namespace cannot reach

Network - Network namespaces can be compared to VLAN’s. Nodes connected to a specific network namespace cannot see what is happening in other network namespaces. Contact to other Network namespaces is only possible through routers.

User - used to separate user ID’s and group ID’s between namespaces. user accounts are specific to each namespace.

Interprocess Communication (ipc) - what processes use to connect to one another, namespace ensures only connections can be made to processes in the same namespace.

Network namespace is not enabled by default

Control groups or Cgroups are a kernel feature that enables resource access limitation. By default , there is no restriction to the amount of memory of number of CPU cycles a process can access. These make it possible to limit the resources a container can use.

With SELinux , a context label is added to ensure that containers can access only the resources they need access to and nothing else.

Containers on RHEL 9

Advantages over Docker with CRI-o:

Container Orchestration

When a host goes down , the containers will go down , however there are additional features to fix this

To implement these features , Kuberenetes was developed , redhat has it’s own kubernetes distro called OpenShift. Which can build a scalable, flexible , reliable infra based on containers.

Running Containers

Install tools required for using containers

sudo dnf install container-tools

You can run your first container with

podman run

You supply the name of the container image you would like to run.

You can specify the registry with podman run as well if you would like to select a specific one

/bin/sh will usually be available in most container images

Use Ctrl-P or Ctrl-Q to detach

non-root container files are copied to ~/.local/share/containers/sotrage

Average size for each container is about 60 MB

container is a running instance of an image , while running a writable layer is added to store changes made to the container.

Container images are created in the Docker format.

Using Registries

Continer registries are defined in the /etc/containers/registries.conf file

Users hwo run a rootless container can create a file ~/.config/containers/registries.conf

settings in the user-specific file will override settings in the generic file

in registries.conf , all container registries are listed as unqualified-seach-registries , this is due to redhat requiring the complete image name and registry to avoid ambiguity.

So instead of using :

podman run -d nginx

You use this :

podman run -d docker.io/library/nginx

Finding Images

To find available images , you can use the podman search command j

If you are trying to search for containers in Redhat registries , you will need to login using the podman login command with the registry name following the prior commands.

You can filter the results with podman search by using the --filter args

Example:

podman search --filter official=true alpine

podman search --filter stars=5 alpine

You can also search for the UBI in the Redhat registries for an image

UBI = Universal Base Image

Inspecting Images

You can inspect images using the spokeo tool

skopeo inspect

Inspection happens from the registry without any need to pull the image

You can inspect images on your local system by using the podman command

podman inspect

You can remove images by using the podman rmi command.

You must stop all instances of a container image in order to remove it.

Building Images from a Containerfile

Conatinerfile is the same as a Dockerfile

To build container images , you can use the podman build command

Containerfile example :

FROM registry.access.redhat.com/ubi8/ubi:latest
RUN dnf install nmap
CMD  ["/usr/sbin/nmap", "-sn", "192.168.29.0/24"]

THe FROM keyword identifies the base image to use

RUN keyword specifies commands to run in the base image while building the custom image

The CMD command specifies the default command that should be started by the custom image

Alpine is a good base image to use as it is lightweight.

Manging Container Status

podman stop - sends SIGTERM signal to the container , if no results after 10 seconds , the SIGKILL signal is sent.

podman kill - immediately sends the SIGKILL command

podman restart - restarts container

podman rm - removes container files written to the writable layer

podman run --rm - runs the container and deletes container files automatically

Running Commands in a Container

You can run commands in the container using the podman exec command

Command output is written to STDOUT

Open port for container

podman run --name <container> -d -p <hostport>:<forwardport> <processname>

Then add the port to your firewall and reload the config

Start container with env variables

Mariadb example:

podman run -d -e MYSQL_ROOT_PASSWORD=password -e MYSQL_USER=anna

Set persistant storage for container

semanage fcontext -a -t container_file_t "hostdir(/.*)?"; restorecon

To do so automatically :

-v host_dir:container_dir

If root container or if user is owner of the container

-v host_dir:container_dir:Z

Run commands inside container namespace

podman unshare

Map root user UID with current UID

podman unshare cat /proc/self/uid_map