Kali Linux Hash Sum Mismatch while doing apt update

https://askubuntu.com/questions/1235914/hash-sum-mismatch-error-due-to-identical-sha1-and-md5-but-different-sha256

Looks like the problem is caused by the Windows Hypervisor Platform. Work around is to disable it.

In the Start menu search for “turn Windows features on or off”

Once open, disable “Virtual Machine Platform” and “Windows Hypervisor Platform”

Disable Virtual Machine Platform

You will need to reboot after it finishes.

Notes on OLED on Linux

OLED screens have had issues on Linux. Main problem is unable to control the screen brightness. It looks like there are some ways to do it now

Looks like icc-brightness should help. Install instructions are from here

sudo apt install -y liblcms2-dev
git clone https://github.com/udifuchs/icc-brightness.git
cd icc-brightness
make

You can run the utility with

./icc-brightness

Installing it will set it up to automatically run on log in.

sudo make install

Log out and back in and it should work.

https://appaper.com/oled-laptop-screen-brightness-control-linux/
https://www.reddit.com/r/linux/comments/cmf0vi/the_state_of_oled_brightness_on_linux/

UFW Allow ICMP (ping) Traffic

https://askubuntu.com/questions/6995/how-to-enable-ufw-firewall-to-allow-icmp-response

Open up the UFW before.rules config file

vi /etc/ufw/before.rules 

And make sure you have these rules in it

# ok icmp codes for INPUT
-A ufw-before-input -p icmp --icmp-type destination-unreachable -j ACCEPT
-A ufw-before-input -p icmp --icmp-type time-exceeded -j ACCEPT
-A ufw-before-input -p icmp --icmp-type parameter-problem -j ACCEPT
-A ufw-before-input -p icmp --icmp-type echo-request -j ACCEPT
# ok icmp code for FORWARD
-A ufw-before-forward -p icmp --icmp-type destination-unreachable -j ACCEPT
-A ufw-before-forward -p icmp --icmp-type time-exceeded -j ACCEPT
-A ufw-before-forward -p icmp --icmp-type parameter-problem -j ACCEPT
-A ufw-before-forward -p icmp --icmp-type echo-request -j ACCEPT

Cannot load Zend OPcache – it was already loaded

Started getting the following error while running ./validate.php for LibreNMS

Cannot load Zend OPcache - it was already loaded

Looks like the problem arises out of PHP trying to load two ini files for OPcache. Was trying to enable OPcache for LibreNMS so I created an opcache.ini file and put the settings in it. I missed the default 10-opcache.ini file.

Moving all the settings into the 10-opcache.ini file and deleting the created opcache.ini file resolved the issue for me.

https://unix.stackexchange.com/questions/253448/php-7-install-throws-cannot-load-zend-opcache-it-was-already-loaded-error/253484

Configure UFW Firewall on Ubuntu

Show status

sudo ufw status

Disable UFW Service

sudo systemctl stop ufw && sudo systemctl disable ufw

Stop UFW Service

sudo systemctl stop ufw

Start UFW service

sudo systemctl stop ufw

Enable UFW

sudo ufw enable

Allow SSH

sudo ufw allow 22/tcp

Show status

sudo ufw status numbered

Example output

sudo ufw status numbered
Status: active
To            Action   From 
--            ------   ----
[1] 3478/udp  ALLOW IN  Anywhere
[2] 5514/udp  ALLOW IN  Anywhere
[3] 8080/tcp  ALLOW IN  Anywhere
[4] 8443/tcp  ALLOW IN  Anywhere
[5] 8880/tcp  ALLOW IN  Anywhere
[6] 8843/tcp  ALLOW IN  Anywhere
[7] 6789/tcp  ALLOW IN  Anywhere
[8] 27117/tcp ALLOW IN  Anywhere
[9] 22/tcp    ALLOW IN  Anywhere

Delete rule

You need to know the number of the rule you want to delete. Replace number with the number of the rule from the status command

sudo ufw delete number

Reset rules

sudo ufw reset

Mount disk by UUID in Linux

Find UUID

blkid

Example output

/dev/xvdb1: UUID="42fbe9a1-eea1-34bc-439d-19a0b48e7df1" TYPE="xfs"

Mount drive using the UUID

[root@host ~]# mount -U 42fbe9a1-eea1-34bc-439d-19a0b48e7df1 /mnt

Add to fstab to automatically mount on system boot up

vi /etc/fstab

Add the following, swap out the UUID for your devices UUID

UUID="42fbe9a1-eea1-34bc-439d-19a0b48e7df1" /backup xfs defaults 0 0

Save and exit. Now when the system reboots it should automatically mount the drive. You should also be able to call “mount -a” to automatically mount everything in fstab.

Linux Screen – Create, Connect, Disconnect, Terminate Sessions

Screen is a handy tool that can help you run scripts on servers remotely without having to worry about the session getting terminated. It seems to operate kinda like a virtual console.

Create Screen Session

Create a new session with a specified name

screen -S SessionName

Example output below. Create session named testsession and print screen sessions.

[bob@localhost imapsync]$ screen -S testsession
[bob@localhost imapsync]$ screen -ls
There are screens on:
3313.testsession (Attached)
1 Sockets in /var/run/screen/S-bob.
[bob@localhost imapsync]$

Disconnect from Screen Session

You can disconnect from a screen session by hitting ctrl + a and then ctrl +d

“ctrl + a” then “ctrl + d”

List Screen Sessions

You can list the screen sessions with

screen -ls

Example

[bob@localhost imapsync]$ screen -ls
There are screens on:
3212.testsession (Detached)
2556.xap (Detached)

2 Sockets in /var/run/screen/S-bob.
[bob@localhost imapsync]$

Connect to screen Session

You can reconnect to a screen session with

screen -r testsession 

Terminate Screen Session

To terminate a screen session, connect to that session and then on a clear line hit ctrl + d

Same way as if you were closing a remote ssh connection.