RHCSA

Chapter 1 - 2

Virtualisation

• lsmod

  grep kvm # check if kvm is enabled

• svm or vmx should be listed in /proc/cpuinfo if virtualization is enabled in the BIOS.
• modprobe kvm_intel or modprobe kvm_amd to load kvm if the machine is virtualization-capable and svm/vmx is enabled
• virsh can be used to start, shutdown, list, edit config, list capabilities of and autostart (on host power on) VMs.

Kickstart

• Kickstart Config RHEL labs was not giving a complete config. Copying anacodna-ks.cfg from /root in an installed RHEL system was better.

Accessing Servers Remotely

• scp can be used to push a file to a remote computer. To do this, name the local file first and the host second.

  scp example.md james@192.168.1.132:/home/james
  # push example.md to /home/james of the specified host
  

• sftp

• ssh -X user@host can be used to forward graphical applications on the target machine to the local machine.

Tools to Verify Connectivity

• Telnet and nmap verify remote access to ports.
• mutt to as an email client to verify the functionality of an email server
• Cert book suggests elinks as a text-based web browser, however, that is not available on RHEL 8 (nor any other text-based web browser it seems.)
• lftp to access ftp servers.

Telnet

telnet [HOST] 21

• [HOST] can be localhost. i.e. telnet can be used to check if ports are open on the local machine.
• Responds “Connected to localhost” if the port is open.
• Otherwise responds “Connection refused”.
• Some services, such as Postfix, are configured to only accept connections from localhost. Therefore, using telnet locally might be preferable.

Nmap

nmap [HOST]

• [HOST] can be localhost.
• Outputs open ports.

Mail

To mail someone:

MAIL [USER]

lftp - a better FTP client

Chapter 3

Virtual Terminals

• There are three virtual terminals available by default.
• They correspond to the virtual devices /dev/tty1, /dev/tty2, etc.
• The quantity can be increased by changing the NAutoVTs variable in /etc/systemd/logind.conf.

Text Streams and Command Redirection

• The are three basic data streams. Data goes on, data comes out, and errors are sent in a different direction.
• There is standard input (stdin), standard output (stdout) and standard error (stderr).
• Normally, input comes from the keyboard, whereas standard out and errors go to the screen.

Redirecting Input

database < datafile # stream datafile into database

Redirecting Errors

program 2> err-list # redirects errors to err-list

To discard errors:

program 2> /dev/null

Redirecting Both Standard Output and Errors

program &> output-and-error

Environment PATHs

echo $PATH

• The order of output is the order the directories are searched.
• Programs can exist in multiple PATH directories.
• The PATH for individual users can be customised via ~/.bash_profiles or ~/.profile.
• The global PATH script is /etc/profile.
• Custom settings can be added to /etc/profile.d.

ls

• -h: human readable file sizes
• -t: sort by modification time
• -i: inode numbers
• -r: reverse order
• -R: recursive
• -d: list directories, not their contents

cp

• -a: recursive and preserves timestamps

ln

• By default, ln created a hard link.
• Hard links are directory entries that point to the same inode.
• They must be created within the same filesystem.
• You could delete a hard-linked file in on directory and it would still exist in the other direct (files are only deleted when the number of dentry records pointing to them hit -0, which is tracked by a counter per file).
• -s: create a soft (symbolic) link.
• A soft link is just a redirect.
• If the file the link is pointing to is deleted, it has nowhere to go.

rm

• -i: prompt before every removal

mkdir/rmdir

• -p: can be used to create a series of directories. Let’s say there is no directory “all”:

  mkdir -p all/these/directories/are/created
  

• rmdir also has the same -p switch, however, rmdir only works on empty directories.

du

• Estimate file sizes.

• du [DIRECTORY/FILE]
  

• -s: summarise; totals for a directory rather than each item listed

• -h: human readable

• -a: list files and directories. By default, only directories are listed.

less and more

• ? can be used to search backwards through a file, like / can be used to search forwards.

head and tail

• By default, head looks at the first 10 lines of a file and tail looks a the last 10 lines.
• -n X: specify the number of lines in head/tail.
• -f: follow output as the file grows. Especially useful for monitoring error output in real time.

sort

• By default, sorts in alphabetical order by the first character in each line.
• Not case sensitive by default.

grep

• -E or egrep: extended regex support. Without this option, it will be necessary to escape sequences like {3}.
• -v: invert the matching logic.
• -i: case insensitive.

regex

wc

• -w: word count

sed

• sed ‘s/Find/Replace’ [FILE] > [NEW_FILE] # replaces the first occurrence
• sed ‘s/Find/Replace/g’ [FILE] > [NEW_FILE] # replaces all instances. g = global

Changing the Default Text Editor

For the current user:

export EDITOR=/bin/nano

For all users:

echo 'export EDITOR=/bin/nano' >> /etc/environment

Reading Documentation

• /usr/share/doc
• –help or -h
• man -k or apropos
• man [COMMAND]
• info [COMMAND] and ls /usr/share/info to list what is available
• Use whatis [COMMAND] to find man pages
• /etc/cron.daily/man-db.cron # a script to update the man cache for newly installed applications
• pinfo is a prettier version of info with a UI similar to Lynx

IPv4

IP Address Classes

Note: this is somewhat legacy but some parts, such as class E, are still relevant.

• IPv4 addresses are 32 bits.
• Each decimal octet represents 8 bits.
• Common private network ranges are defined in RFC 1918.
• 10.0.0.0 - 10.255.255.255, 172.168.16.0 - 172.168.31.255 and 192.168.0.0 - 192.168.1255.255 are the private IP address ranges.
• Network addresses 127.0.0.0 through 127.255.255.255 are used for loopback communication on a localhost.

Networking and Routing

• An IP address has two parts, a network prefix and a host identifier.
• To determine the network and host part, IP addresses are associated with a subnet masks (also known as a netmask or prefix).
• A subnet mask is a 32-bit number made up of a sequence of binary ones followed by zeros.
• A subnet mask can be represented in the same dot-decimal notation used by IPv4 addresses. For example, 255.255.255.0 is equivalent to 24 binary ones and 8 zeros.
• An alternative notation is Classless Inter-Domain Routing (CIDR). The CIDR equivalent to 255.255.255.0 is /24.
• To determine the network portion of an IP address, you use a logical AND between the IP address and netmask. For example, given a netmask of /24 and an IP address 102.168.122.50, the first three bytes represent the network portion and the last byte represents the host portion.
• Three key IP addresses define a network: the network address, the broadcast address and the subnet mask.
• The network address is always the first IP address in the range.
• The broadcast address is always the last IP address in the same range.
• A host with multiple network interface that forwards traffic across different networks is called a router.
• A gateway is a junction between the local network and other networks.

Networking Tools and Commands

• The default name of network interfaces is now enoX and emX, rather than ethX. ethX is a fallback.
• enpXsY and pXpY is the modern name for PCI slots.
• ifconfig, arp, netstat and route are now legacy commands. Replacements:

• traceroute has an -n flag that displays IP addresses as opposed to hostnames.
• It’s important to test the integrity of TCP/IP locally by pinging 127.0.0.1
• traceroute can require root privileges, tracepath is a similar utility that doesn’t require root privileges.
• traceroute used UDP, which can be blocked by firewalls along the path. In which case, the -I (for ICMP) and -T (for TCP) switches can be used.

ip

• link (l) - Display and modify network interfaces.
• address (a) - Display and modify IP Addresses.
• route (r) - Display and alter the routing table.
• neigh (n) - Display and manipulate neighbor objects (ARP table).
• ip address show eth0 # show the current configuration of the first Ethernet network adaptor.

Configure a Network Adaptor with ip

ip addr add 192.168.122.150/24 dev eth0

Some ip command options:

• Changes listed thus far will not persist through a reboot – that depends on the configuration files in /etc/sysconfig/network-scripts. Any changes made by the ip command are temporary by definition.

Activate and Deactivate Network Adaptors

• It’s possible to use the ip command activate and deactivate network adaptors, however, there are more intuitive scripts, that call the appropriate configuration file in /etc/sysconfig/network-scripts, that might be better suited to this task.
• ip link set dev eth0 down
• ip link set dev etho up
• idown [INTERFACE]
• ipup [INTERFACE]

ip as a Diagnostic Tool

• The Address Resolution Protocol (ARP) associates the hardware address of a network interface (MAC) with an IP address.
• The ip neigh command displays a table of hardware and IP addresses on the local computer.
• A STALE entry may indicate that its ARP cache timeout has expired since a packet was least seen from the host.
• If the ARP table is empty, no recent connections exist to other systems on the local network.

Routing Tables with ip route

• The -r switch of ip, when used with route, looks to /etc/hosts and DNS servers to display hostnames rather than numeric IP addresses.

DHCP with dhclient

• dhclient calls a DHCP server for an IP address (among other things):

# dhclient [INTERFACE]

• Generally, the network options configured through a DHCP server include the IP address, the network mask, the gateway address and any IP addresses for DHCP servers for that network.
• dhclient also adds the address of the DNS server to /etc/resolv.conf.

Display Network Connections with ss

• ss replaces netstat.
• ss can display listening and nonlistening TDP and UDP sockets.
• -a: shows all netwrok sockets
• -4: IPv4
• -t: TCP
• -u: UDP
• -n: numeric format
• -p: show PID of the process using the socket

Network Configuration and Troubleshooting

• systemctl status NetworkManager

• Network Manager monitors and manages network settings.

• nmcli dev status

• Key configuration files are located in /etc/sysconfig/network-scripts

• nmtui # terminal UI

• nm-connection-editor # GTK+ 3 GUI

• Network manager can store different profiles, known as connections.

• Show all configured connections in Network Manager:

  nmcli con show
  

/etc/sysconfig/network

• If the ip addr show command shows no output, that means all network devices are currently inactive.
• The first thing to check is in /etc/sysconfig/network file.
• On systems that are configured to retrieve addressing via DHCP, the file is usually empty.
• /etc/sysconfig/network-scripts/ contains network config files that are now parsed via Network Manager. In RHEL 7 and earlier, this directory contained network configuration scripts.

Chapter 4

• umask sets default file permissions.
• The dot at the end of an ls -l permissions line specifies that the file has an SELinux security context.
• The dot is replaced by a + if there are ACLs configured, however, that doesn’t override SELinux.

Special Permisssions

• The SUID bit appears as an s in the space for the executable bit for a user owner of a file. It means the file can be executed by other users with the authority of the file owner.
• The SGID bit appears as an s in the space for the group executable bit.
• A sticky bit appears as a t in the space for other executable.
• —s–s–t.
• vi can be used to (for all intents and purposes) overwrite files with no write permissions sessions using the bang character (!). For example, w!

Commands to Change Permissions Ownership

• chgrp is much like chown but for groups. Group ownership can be changed by the user that owns a file.

• There is an equals operator for chmod that sets permissions, rather than adjusting. For example:

  # chmod g=rw [FILE] # sets group permissions to rw, clearing x if it was set
  

• To set special bits there is a 4^th^ bit at the start of the octal permissions. SUID = 4, SGID = 2 and sticky bit = 1.

Special File Attributes

• lsattr lists current attributes.
• chattr allows attributes to be changed.

• There are other attribute such as s (secure deletion), c (compressed) and u (undeletable) but they don’t work in the ext4 and XFS filesystems.

umask

• Every time a new file is created, the default permissions are based on the value of umask.
• Typing umask returns a four-digit octal number.
• If a bit in umask is set, the corresponding bit will be DISABLED in any newly created files.
• For example, a umask of 0245 results in newly created directories having 0532 octal permissions.
• Files can no longer get executable permission through umask, so a umask of 000 will result in files with octal permissions of 666. This was done for security reasons.
• The default umask is set in /etc/profile and /etc/bashrc.
• An if statement is used to set different default umasks depending on UID.
• The default umask for UIDs between 2 and 200 is 002.
• UIDs above 200 have a default umask of 022.
• root creates 644 files and 755 directories by default.
• Users can override UID in ~/.bashrc and ~/.bash_profile.