To check the listening ports and applications on Linux:
sudo lsof -i -P -n | grep LISTEN
sudo netstat -tulpn | grep LISTEN
sudo ss -tulpn | grep LISTEN
sudo lsof -i:22 ## see a specific port such as 22 ##
sudo nmap -sTU -O IP-address-Here
Compiling a custom kernel has its advantages and disadvantages. However, new Linux user/admin find it difficult to compile Linux kernel. Compiling kernel needs to understand few things and then type a couple of commands. This step by step howto covers compiling Linux kernel version 5.6.9 under an Ubuntu or Debian Linux. The following instructions successfully tested on an RHEL/CentOS 7/8 (and clones), Debian Linux, Ubuntu Linux and Fedora Linux 30/31. However, instructions remain the same for any other Linux distribution.
The procedure to build (compile) and install the latest Linux kernel from source is as follows:
Let us see all steps in details.
Visit the official project site and download the latest source code. Click on the big yellow button that read as “Latest Stable Kernel“:
The filename would be linux-x.y.z.tar.xz, where x.y.z is actual Linux kernel version number. For example file linux-5.6.9.tar.xz represents Linux kernel version 5.6.9. Use the wget command to download Linux kernel source code:$ wget https://cdn.kernel.org/pub/linux/kernel/v5.x/linux-5.6.9.tar.xz
You really don’t have to extract the source code in /usr/src. You can extract the source code in your $HOME directory or any other directory using the following unzx command or xz command:$ unxz -v linux-5.6.9.tar.xz
OR$ xz -d -v linux-5.6.9.tar.xz
First grab the PGP signature for linux-5.6.9.tar:$ wget https://cdn.kernel.org/pub/linux/kernel/v5.x/linux-5.6.9.tar.sign
Try to verify it:$ gpg --verify linux-5.6.9.tar.sign
Sample outputs:
gpg: assuming signed data in 'linux-5.6.9.tar' gpg: Signature made Sun 12 Aug 2018 04:00:28 PM CDT gpg: using RSA key 79BE3E4300411886 gpg: Can't check signature: No public key
Grab the public key from the PGP keyserver in order to verify the signature i.e. RSA key ID 79BE3E4300411886 (from the above outputs):$ gpg --recv-keys 79BE3E4300411886
Sample outputs:
gpg: key 79BE3E4300411886: 7 duplicate signatures removed gpg: key 79BE3E4300411886: 172 signatures not checked due to missing keys gpg: /home/vivek/.gnupg/trustdb.gpg: trustdb created gpg: key 79BE3E4300411886: public key "Linus Torvalds <torvalds@kernel.org>" imported gpg: no ultimately trusted keys found gpg: Total number processed: 1 gpg: imported: 1
Now verify gpg key again with the gpg command:$ gpg --verify linux-5.6.9.tar.sign
Sample outputs:
gpg: assuming signed data in 'linux-5.6.9.tar' gpg: Signature made Sun 12 Aug 2018 04:00:28 PM CDT gpg: using RSA key 79BE3E4300411886 gpg: Good signature from "Linus Torvalds <torvalds@kernel.org>" [unknown] gpg: aka "Linus Torvalds <torvalds@linux-foundation.org>" [unknown] gpg: WARNING: This key is not certified with a trusted signature! gpg: There is no indication that the signature belongs to the owner. Primary key fingerprint: ABAF 11C6 5A29 70B1 30AB E3C4 79BE 3E43 0041 1886
If you do not get “BAD signature” output from the “gpg –verify” command, untar/extract the Linux kernel tarball using the tar command, enter:$ tar xvf linux-5.6.9.tar
Before start building the kernel, one must configure Linux kernel features. You must also specify which kernel modules (drivers) needed for your system. The task can be overwhelming for a new user. I suggest that you copy existing config file using the cp command:$ cd linux-5.6.9
$ cp -v /boot/config-$(uname -r) .config
Sample outputs:
'/boot/config-4.15.0-30-generic' -> '.config'
You must have development tools such as GCC compilers and related tools installed to compile the Linux kernel.
Type the following apt command or apt-get command to install the same:$ sudo apt-get install build-essential libncurses-dev bison flex libssl-dev libelf-dev
See “Ubuntu Linux Install GNU GCC Compiler and Development Environment” for more info.
Try yum command:$ sudo yum group install "Development Tools"
OR$ sudo yum groupinstall "Development Tools"
Additional packages too:$ sudo yum install ncurses-devel bison flex elfutils-libelf-devel openssl-devel
Run the following dnf command:$ sudo dnf group install "Development Tools"
$ sudo dnf install ncurses-devel bison flex elfutils-libelf-devel openssl-devel
Now you can start the kernel configuration by typing any one of the following command in source code directory:
For example, run make menuconfig command launches following screen:$ make menuconfig
You have to select different options as per your need. Each configuration option has HELP button associated with it so select help button to get help. Please note that ‘make menuconfig’ is optional. I used it here to demonstration purpose only. You can enable or disable certain features or kernel driver with this option. It is easy to remove support for a device driver or option and end up with a broken kernel. For example, if the ext4 driver is removed from the kernel configuration file, a system may not boot. When in doubt, just leave support in the kernel.
Start compiling and tocreate a compressed kernel image, enter:$ make
To speed up compile time, pass the -j as follows:## use 4 core/thread ##
$ make -j 4
## get thread or cpu core count using nproc command ##
$ make -j $(nproc)
Compiling and building the Linux kernel going take a significant amount of time. The build time depends upon your system’s resources such as available CPU core and the current system load. So have some patience.
$ sudo make modules_install
So far we have compiled the Linux kernel and installed kernel modules. It is time to install the kernel itself:$ sudo make install
It will install three files into /boot directory as well as modification to your kernel grub configuration file:
You need to modify Grub 2 boot loader configurations. Type the following command at a shell prompt as per your Linux distro:
$ sudo grub2-mkconfig -o /boot/grub2/grub.cfg
$ sudo grubby --set-default /boot/vmlinuz-5.6.9
You can confirm the details with the following commands:grubby --info=ALL | more
grubby --default-index
grubby --default-kernel
The following commands are optional as make install does everything for your but included here for historical reasons only:$ sudo update-initramfs -c -k 5.6.9
$ sudo update-grub
You have compiled a Linux kernel. The process takes some time, however now you have a custom Linux kernel for your system. Let us reboot the system.
Just issue the reboot command or shutdown command:# reboot
Verify new Linux kernel version after reboot:$ uname -mrs
Sample outputs:
Linux 5.6.9 x86_64
Configurations! You completed various steps to build the Linux kernel from source code and compiled kernel should be running on your system. I strongly suggest that you always keep backup of essential data and visit the kernel.org page here for more info.
nano /etc/systemd/system/vnc.service [Unit] Description = X11VNC pre-login After = multi-user.target [Service] Type = simple ExecStart = /usr/bin/x11vnc -ncache 10 -auth guess -nap -forever -loop -repeat -rfbauth /root/.vnc/passwd -rfbport 5900 [Install] WantedBy = multi-user.target Then to activate the systemd service: systemctl daemon-reload Then to enable the service on boot: systemctl enable vnc Then reboot, and it should come up on its own. shutdown -r now
For example, if we wanted to have the system date written in a file called date.txt when Linux restarts, we would add the following string:
@reboot date >> ~/date.txtIf we wanted to run the backup shell at reboot, we would add:
@reboot /root/backup.shNote: In some cases, the crond service needs to be enabled on boot for the configuration to function.
To check if the crond service is enabled, use:
sudo systemctl status cron.serviceTo enable this service, use:
sudo systemctl enable cron.service
