Category: Linux

Out Of Memory (OOM) Error
OOM (Out Of Memory) error happens when a server run out of Memory. When a server runs out of memory, processes get randomly killed to free up memory. This will result in the service that gets killed stop functioning.

How to avoid OOM Error

Here are a few things you can do to avoid server running out of memory.
- Reduce memory usage by your application if possible. Poorly coded applications can use a lot of memory, and cause memory leaks.
- Upgrade Memory (RAM). Adding more RAM to the server helps.
- Add Virtual Memory. You can add swap, this will use your disk as memory, it is slower than RAM, but help to avoid OOM when the server runs out of memory.
To see if your server had OOM error, run

Debian/Ubuntu
```
grep -i oom /var/log/syslog*
```
RHEL/AlmaLinux
```
grep -i oom /var/log/messages*
cat /var/log/messages* | grep 'Out of memory'
```
Back to Errors
July 30, 2022
odoo Unable to find Wkhtmltopdf on this system
On an odoo installation, I got the error “Unable to find Wkhtmltopdf on this system”, when generating reports.

This error was because server had old version of wkhtmltopdf installed.

You can download and install latest version of wkhtmltopdf from

https://wkhtmltopdf.org/downloads.html

On Ubuntu 20.04, the error is fixed by

Remove currently installed wkhtmltox
```
apt remove --purge wkhtmltox
```
Install newer version
```
wget https://github.com/wkhtmltopdf/packaging/releases/download/0.12.6.1-2/wkhtmltox_0.12.6.1-2.jammy_amd64.deb
dpkg -i wkhtmltox_0.12.6.1-2.jammy_amd64.deb
```
Create a symlink
```
rm -f /usr/bin/wkhtmltopdf
ln -s /usr/local/bin/wkhtmltopdf /usr/bin/wkhtmltopdf
```
Restart odoo
```
systemctl restart odoo
```
July 27, 2022

apt error doesn’t support architecture ‘i386’

On an Ubuntu server, when running “apt update” command, I got the following error message

root@s196379:~# apt update
Hit:1 http://mirror.fcix.net/ubuntu focal InRelease
Hit:2 http://mirror.fcix.net/ubuntu focal-updates InRelease                
Hit:3 http://security.ubuntu.com/ubuntu focal-security InRelease           
Hit:4 https://mirror.rackspace.com/mariadb/repo/10.5/ubuntu focal InRelease
Reading package lists... Done
Building dependency tree       
Reading state information... Done
All packages are up to date.
N: Skipping acquire of configured file 'main/binary-i386/Packages' as repository 'https://mirror.rackspace.com/mariadb/repo/10.5/ubuntu focal InRelease' doesn't support architecture 'i386'
root@s196379:~#

This error is because the repository do not support i386. To fix the error, edit the file

vi /etc/apt/sources.list

Find

deb https://mirror.rackspace.com/mariadb/repo/10.5/ubuntu focal main

Replace with

Find

deb [arch=amd64] https://mirror.rackspace.com/mariadb/repo/10.5/ubuntu focal main

Solution 2: Remove i386 support

Check your architecture

dpkg --print-architecture

If you are using 64 bit OS, you will see amd64.

Check if multiarch is enabled

dpkg --print-foreign-architectures

If you get “i386”, you have multiarch enabled.

If you don’t have any i386 applications running, you can remove i386 support with

dpkg --remove-architecture i386

If you want to enable i386 support, you can run

dpkg --add-architecture i386

Back to apt

July 25, 2022

Failed to mount /sysroot XFS Boot failure
On an Amazon EC2 server, the server won’t boot. On checking the serial console, the server was stuck with an error message
```
[FAILED] Failed to mount /sysroot
See 'systemctl status sysroot.mount' for details.
```
I created a new EC2 instance in the same region as the current server, detached the disk, and attached it to the new EC2 instance. When I try to mount the disk, it just hangs and i have to reboot the EC2 instance.

Used parted -l command to see the disk details.

The disk file system is XFS. Checked the disk with the command
```
xfs_repair -v DEVICE_NAME_HERE
```
In this case DEVICE_NAME is /dev/nvme1n1p1.

The disk has errors. Fixed it by running the command
```
xfs_repair -v -L /dev/nvme1n1p1
```
See HDD
July 18, 2022
How to install PPTP VPN on Rocky Linux 8
PPTP on Rocky Linux is provided by the EPEL repository, so enable EPEL repo by running
```
dnf install epel-release -y
```
Install the pptpd package
```
dnf install -y pptpd
```
Edit file
```
vi /etc/pptpd.conf
```
At end of the file, add
```
localip 10.0.0.1
remoteip 10.0.0.100-200
```
Edit file
```
vi /etc/ppp/options.pptpd
```
At the end, add
```
ms-dns 1.1.1.1
ms-dns 8.8.8.8
```
Start PPTPD VPN

To set PPTPD VPN server to start on boot, run
```
systemctl enable pptpd
```
To start VPN
```
systemctl start pptpd
```
To stop VPN
```
systemctl stop pptpd
```
To check status, run
```
systemctl status pptpd
```
Create a VPN user

To create a VPN user, edit the file
```
vi /etc/ppp/chap-secrets
```
Inside you can add your VPN user and password. For example
```
serverok   pptpd   ooyeegei8Ienai      *
```
This will create a VPN user with the username “serverok” and password “ooyeegei8Ienai”.

After adding a user you need to restart the PPTPD service with the command
```
systemctl restart pptpd
```
Confire firewall to allow VPN connection

Install firewalld with command
```
dnf install firewalld -y
```
Enable and start firewalld
```
systemctl enable firewalld
systemctl start firewalld
systemctl status firewalld
```
Open ports needed for PPTPD VPN on the firewall
```
firewall-cmd --permanent --zone=public --add-port=1723/tcp
firewall-cmd --permanent --add-rich-rule='rule protocol value="gre" accept'
firewall-cmd --permanent --zone=public --add-masquerade
firewall-cmd --reload
```
Enable IP forwarding by editing the file
```
vi /etc/sysctl.conf
```
In the file, add
```
net.ipv4.ip_forward = 1
```
Run
```
sysctl -p
```
See PPTPD VPN
July 2, 2022

Allow PPTP VPN connection on firewalld

To allow PPTP VPN connection when using firewalls, run the following commands.

Allow 1723/tcp

sudo firewall-cmd --permanent --zone=public --add-port=1723/tcp

Allow gre protocol

firewall-cmd --permanent --add-rich-rule='rule protocol value="gre" accept'

sudo firewall-cmd --permanent --zone=public --direct --add-rule ipv4 filter INPUT 0 -p gre -j ACCEPT
sudo firewall-cmd --permanent --zone=public --direct --add-rule ipv6 filter INPUT 0 -p gre -j ACCEPT

Enable IP masquerade

sudo firewall-cmd --permanent --zone=public --add-masquerade

Reload firewall.

sudo firewall-cmd --reload

See firewalld

July 2, 2022

DevOps

Autodeploy

https://www.cyclic.sh
https://tekton.dev/docs/getting-started/ – CI/CD

CircleCI

It is a cloud-based CI/CD service that can be used to build, test, and deploy applications. CircleCI supports multiple programming languages and platforms. Free plan allow up to 6,000 build minutes and 5 active users per month. Build for Docker, Windows, Linux, Arm, and macOS or on your own compute with self-hosted runners.

https://circleci.com

June 26, 2022
How to take screenshot with ffmpeg
ffmpeg provides x11grab driver that can be used to take screenshots.

You can use the following command to take screenshots, you can run it manually on the terminal or using cronjob
```
export DISPLAY=:1
ffmpeg -f x11grab -video_size 1920x1080 -i $DISPLAY -vframes 1 image.png
```
If you want to make a thumbnail from a video, you can use
```
ffmpeg -ss 00:01:00 -i video.mp4 -vframes 1 image.png
```
You can change 00:01:00 to whatever time you need to generate a thumbnail.

See ffmpeg
June 20, 2022

pvcreate Device /dev/sdb excluded by a filter

When creating LVM physical volume with the command “pvcreate”, I got the following error

root@sok1 ~ # pvcreate /dev/sdb
  Device /dev/sdb excluded by a filter.
root@sok1 ~ #

This disk was previously used as a software raid.

To fix the error message, run

wipefs -a /dev/sdX

Example

root@sok1 ~ # pvcreate /dev/sdb
  Device /dev/sdb excluded by a filter.
root@sok1 ~ # wipefs -a /dev/sdb
/dev/sdb: 8 bytes were erased at offset 0x00000200 (gpt): 45 46 49 20 50 41 52 54
/dev/sdb: 8 bytes were erased at offset 0xe8d7ffffe00 (gpt): 45 46 49 20 50 41 52 54
/dev/sdb: 2 bytes were erased at offset 0x000001fe (PMBR): 55 aa
/dev/sdb: calling ioctl to re-read partition table: Success
root@sok1 ~ # pvcreate /dev/sdb
  Physical volume "/dev/sdb" successfully created.
root@sok1 ~ #

See HDD, pvcreate

June 17, 2022

SSH Server refused our key
I got the following error when logging in to a server using SSH key authentication.
```
Server refused our key
```
I checked the log file
```
tail /var/log/secure
```
found following error
```
May 11 17:45:31 server58 sshd[18483]: Authentication refused: bad ownership or modes for file /home/video/.ssh/authorized_keys
```
The problem is fixed by setting proper permission for the .ssh folder and key file.
```
chmod 600 ~/.ssh
chmod 600 ~/.ssh/authorized_keys
```
See SSH
June 13, 2022

How to create public key from SSH private key?

Lost SSH public key, don’t worry, you can generate it from your private key file. If you have an SSH private key, you can generate the public key using the ssh-keygen command.

ssh-keygen -f PRIVATE_KEY_HERE -y

-y option will read a private OpenSSH format file and print an OpenSSH public key to stdout.
-f specifies the location of your private key file.

Example

If you have passphrase set, ssh-keygen will prompt for the password.

root@ee:~/my-key# ssh-keygen -f boby.pem -y
Enter passphrase: 
ssh-rsa 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 root@ee
root@ee:~/my-key#

See SSH, ssh-keygen

June 10, 2022

How to check if ImageMagick is installed?
How to find ImageMagick is installed on the server, run
```
which convert
```
Usually, it will be installed at – /usr/bin/convert

On Ubuntu/Debian, you can run the command
```
dpkg -l | grep imagemagick
```
On RHEL/CentOS/AlmaLinux, use the command
```
yum list | grep -i image 
```
To find the version of ImageMagick installed on your computer, run
```
convert -version
```
Back to ImageMagick
June 3, 2022

Category: Linux

How to avoid OOM Error

Solution 2: Remove i386 support

Start PPTPD VPN

Create a VPN user

Confire firewall to allow VPN connection

Autodeploy

CircleCI