removing the default kernel in CentOS 8 – remove elrepo kernel

Removing the default kernel aka the loaded kernel in CentOS 8 maybe challenging because the package is protected and cannot be removed by the yum or dnf.
Here is the case: an elrepo kernel-ml loaded and the dnf prints it cannot remove the package, because it is protected:

[root@srv ~]# dnf remove kernel-ml kernel-ml-core kernel-ml-modules
 Problem: The operation would result in removing the following protected packages: kernel-ml-core
(try to add '--skip-broken' to skip uninstallable packages or '--nobest' to use not only best candidate packages)
[root@srv ~]# uname -a
Linux srv.localhost 5.10.4-1.el8.elrepo.x86_64 #1 SMP Tue Dec 29 11:04:23 EST 2020 x86_64 x86_64 x86_64 GNU/Linux
[root@srv ~]# grubby --default-kernel

The system is booted up with the kernel we are trying to remove, which is impossible.

The solution is to set a new default kernel and load it. Then dnf will be able to remove the first kernel.

For CentOS 7, just use the yum instead of dnf command.
Using grubby is really easy and straightforward:

STEP 1) List all installed and available to boot kernels

[root@srv ~]# grubby --info=ALL |grep ^kernel

STEP 2) Select the kernel to load the next time

[root@srv ~]# grubby --set-default "/boot/vmlinuz-4.18.0-259.el8.x86_64"
The default is /boot/loader/entries/45e12f0814fd4947b99cbdcb88950361-4.18.0-259.el8.x86_64.conf with index 1 and kernel /boot/vmlinuz-4.18.0-259.el8.x86_64

Keep on reading!

Booting network installation from ipxe disk using IPMI KVM

There is a project for extended PXE Boot features This article is not for describing what this project may offer, but to show how to boot any Linux distribution (in fact, Windows 10, too) network installation wizard using the virtual CD/DVD of an IPMI KVM, DELL’s DRAC, HP iLO, IBM RSA/IMM and in general, KVM over IP.
Using the iPXE CD bootable disk mounted in the virtual CD/DVD of the server’s remote console (IPMI KVM and so on) will allow:

  • Booting from a CD/DVD with only a 1M size.
  • Extends the PXE features of the server’s network card.
  • Manual set IP address, i.e. not relying on DHCP server. In addition of the DHCP feature, but DHCP feature requires DHCP server, which is not always the case.
  • Load a Linux kernel and initramfs from a URL.
  • Boot a Linux live or installation CD/DVD from an URL. The server could load the instllation wizard from an official mirror in the Internet.
  • Manual install – boot from 1M CD and continue with multi gigabyte installation from an URL. For comparision the CentOS 8 network installation disk is more than 600M versus 1M iPXE CD disk. Booting directly from a 600M CentOS 8 network installation disk is unstable and really slow when the disk is mounted in user’s KVM. And not alwyas is possible to mount a disk next to the server location (or in the same co-location).
  • Automated install – simple unattended installation with kickstart files without the need of speacial features of the dedicated service provider.
  • No software installation or code writting needed.

This article uses the iPXE CD to boot and manually set an IP and then load the Linux kernel and initramfs of the CentOS 8 installation disk using an official URL mirror on the Internet. All types of server’s KVM, which supports CD/DVD virtual device, can be used.

Just 1 Mbytes of CD/DVD is required to boot an installation of a (connected to the Internet) server/machine.

Here are the steps and correct (all lines are tested) command lines to boot an installation wizard. The server is a SUPERMICRO server with IPMI KVM for remote management.
The iPXE ISO file is located here

SCREENSHOT 1) Open the IPMI KVM and click on “Virtual Storage” menu to open the image mount dialog.

main menu
Virtual Storage menu

Keep on reading!

CentOS 8 add a storage driver (megaraid_sas) when booting the installation disk

Installing CentOS 8 in relatively old hardware maybe a real challenge because of an old hardware device like storage, network, or both.
This article shows how to make the CentOS 8 Installation wizard detect the storage – a hardware controller AOC-USAS2LP-H8iR (smc2108 with LSI 2108). Unfortunately, the CentOS 8 (in fact, RHEL 8 removed the support, too) team decided to remove support for the LSI SAS2008/2108/2116 storage controllers by removing the “megaraid_sas” kernel driver. There are still servers in production with similar controllers, which were sold 4-5 years ago from the big vendors such as DELL, HP, and so on.

The method here is to boot the installation CD/USB with modified kernel boot parameters to include an URL link to the installation driver iso (where the megaraid_sas driver is included).

The offered way to load the megaraid_sas (or any other driver) includes:

  1. Use assisted driver update to load an elrepo driver ISO during the first stage of the CentOS 8 Installation Wizard. elrepo is a famous community efford – More on the assited diver update here –
  2. Configure the network of the server to be able to download the driver ISO in the early stage of the CentOS 8 Installation Wizard. Add boot parameters to set up a valid network configuration.

The installation CD/USB can download an iso with kernel drivers. And of course, to download a file from the Internet a network should be set in the earliest stage of the CentOS 8 installation wizard.
The added string to the boot CD/USB CentOS 8 Installation disk is:

 inst.dd= ip= nameserver=

SCREENSHOT 1) Select with the arrows “Install CentOS Linux 8” and hit “TAB” button to edit the boot parameters.

As shown in the picture just add ” inst.dd= ip= nameserver=″. The “inst.dd” instructs the installation wizard where are the driver ISO located. The “ip” and “nameserver” command just sets a proper network in the early stage of the CentOS 8 Installation wizard to be able to download the driver ISO. Setting the network by these parameters is really important, because the download of the driver iso happens in this early stage of loading the installation wizard. Replace the IP and the whole network configuration if needed.

main menu
Installation wizard edit boot parameters

Keep on reading!

Configure Bond (802.3ad LACP) device in CentOS 8 – configuration files

Upgrading to a bond device is a common step when the server exhausts its current network port bandwidth.
The hardware setup of the bond example here is:

  • two 10G network cards – ens1f0 and ens1f0
  • bond name – bond0
  • bond mode – 802.3ad – Link Aggregation Control Protocol (LACP)

The systemd reconfiguration procedure consists of:

  • Stop the network target
    systemctl stop network
  • Set several configuration files – network device files for the network interfaces, bonding interface – master and slave devices.
  • Start the network target
    systemctl start network

*Note: the 802.3ad bonding mode needs aditional configuration in the switch of which the server is connected.

The example here is using CentOS 8 configuration file to make a permanent (i.e. persistent over reboots using the CentOS 8 network configuration files) bonding configuration.
Check out the official bonding documentation for all modes and options –

CONF 1) Configure the network interfaces.

The interface should be in down state in the configuration file.
Interface 1 – /etc/sysconfig/network-scripts/ifcfg-ens1f0:


Interface 2 – /etc/sysconfig/network-scripts/ifcfg-ens1f1:


CONF 2) Configure bonding master device – create a bonding group bond0

This device should be started up at boot.
Bonding device 1 – with name bond0 – /etc/sysconfig/network-scripts/ifcfg-Bond_connection_1:

BONDING_OPTS="downdelay=200 miimon=100 mode=802.3ad updelay=200"
NAME="Bond connection 1"

BONDING_OPTS are specific options for the bonding group with name bond0 and the bonding mode is set here, too.

CONF 3) Configure bonding slave devices – the two network cards

Adding the two network cards to the bonding group bond0. These devices should be started up at boot.
Interface 1 – /etc/sysconfig/network-scripts/ifcfg-bond0_slave_1:

NAME="bond0 slave 1"

Interface 2 – /etc/sysconfig/network-scripts/ifcfg-bond0_slave_2:

NAME="bond0 slave 2"

Set up (802.3ad LACP) bonding when installing CentOS 8

This article is to show how the user could install CentOS 8 (the steps are the same with CentOS 7) with a much complex network setup such as Bonding device in 802.3ad mode (LACP – Link Aggregation Control Protocol).
The whole installation procedure is not included here, but there are couple of other article on the subject “Install CentOS 7 or CentOS 8”:

Similar configuration files will be generated as in Configure Bond (802.3ad LACP) device in CentOS 8 – configuration files

SCREENSHOT 1) Click on “Network and Host Name” to configure the machine networking.

main menu
Installation Summary – Network and Host Name

Keep on reading!

collectd nginx plugin: curl_easy_perform failed because of selinux

Enabling the Nginx plugin for collectd under CentOS (or any other system using SELinux) might be confusing for a newbie. Most sources on the Internet would just install collectd-nginx:

yum install -y collectd-nginx

and configure it in the nginx.conf and collectd.conf. Still, the statistics might not work as expected, the collectd may not be able to gather statistics from the Nginx.

SELinux may prevent collectd (plugin) daemon to connect to Nginx and gather statistics from the Nginx stats page.

Checking the collectd log and it reports a problem:
Keep on reading!

Dracut boot failed with missing device – exit and continue normal booting!

This issue deserves a much more article, in fact, a straightforward tip:

You may be able to continue a normal boot only by typing “exit” and hitting enter in the “Dracut” console.

Most of the time this Dracut console entering is caused because the system administrator of the server/machine added, replaced or deleted a RAID or similar device and forgot to update the configuration (grub2 probably). And in most of these cases, the raid is not critical for machine normal boot from the root partition, but it may be critical for the services lately. Booting in normal mode, even without some devices, is the main goal because under the normal mode it easier to repair the system.
Check out the two articles on the topic (especially the first one):

SCREENSHOT 1) Just type “exit” and hit enter.

It’s worth noting that if you executed some commands in the console and/or mounted devices to test they are with healthy file system or for whatever reason you did it, the boot process may not continue after typeing exit and probablly a reboot is required. The server will go once more in this mode and then just typing will work.

main menu
type exit

Keep on reading!

podman – Error adding network: failed to allocate for range 0: has been allocated after server reboot

We’ve just stumbled on the following error with one of our podman CentOS 8 servers after restart:

[root@srv ~]# podman start mysql-slave
ERRO[0000] Error adding network: failed to allocate for range 0: has been allocated to c97823be46832ddebbce29f3f51e3091620188710cb7ace246e173a7a981baed, duplicate allocation is not allowed 
ERRO[0000] Error while adding pod to CNI network "podman": failed to allocate for range 0: has been allocated to c97823be46832ddebbce29f3f51e3091620188710cb7ace246e173a7a981baed, duplicate allocation is not allowed 
Error: unable to start container "mysql-slave": error configuring network namespace for container c97823be46832ddebbce29f3f51e3091620188710cb7ace246e173a7a981baed: failed to allocate for range 0: has been allocated to c97823be46832ddebbce29f3f51e3091620188710cb7ace246e173a7a981baed, duplicate allocation is not allowed

Apparently, something got wrong, because the two containers were fine before restarting and they were multiple times stopped, started and restarted.

The solution is to remove IP-named files in /var/lib/cni/networks/podman and start the podman containers again.

It resembles to a bug, which should have already been closed by the new minor CentOS 8 releases.

The interesting part is that the container we are trying to start mysql-slave has c97823be46832ddebbce29f3f51e3091620188710cb7ace246e173a7a981baed, but it reports it cannot allocate it, because it has already been allocated to a container with the same ID. That’s the problem:

The IP-named files in /var/lib/cni/networks/podman were not removed when the podman container had stopped.

Typically, when a podman container executes a stop command, the process should remove the files in /var/lib/cni/networks/podman. Before restarting the CentOS 8 server you may need to stop the podman containers for now.

[root@srv ~]# cd /var/lib/cni/networks/podman
[root@srv podman]# ls -altr
total 24
-rwxr-x---. 1 root root    0  3 Dec  0,43 lock
drwxr-xr-x. 3 root root 4096  3 Dec  0,43 ..
-rw-r--r--. 1 root root   64  9 Dec 18,34
-rw-r--r--. 1 root root   64 16 Dec 12,01
-rw-r--r--. 1 root root   10  1 Mar  9,28 last_reserved_ip.0
-rw-r--r--. 1 root root   70  1 Mar  9,28
drwxr-xr-x. 2 root root 4096  1 Mar  9,28 .
[root@srv podman]# rm
rm: remove regular file ''? y
[root@srv podman]# rm
rm: remove regular file ''? y
[root@srv podman]# podman start mysql-slave
[root@srv podman]# podman ps
CONTAINER ID  IMAGE                           COMMAND               CREATED       STATUS            PORTS  NAMES
c97823be4683  localhost/centos-mysql-5.6:0.9  / my...  2 months ago  Up 2 minutes ago         mysql-slave
e96134b31894        2 months ago  Up 6 minutes ago         example-client
[root@srv podman]# ls -altr
общо 20
-rwxr-x---. 1 root root    0  3 Dec  0,43 lock
drwxr-xr-x. 3 root root 4096  3 Dec  0,43 ..
-rw-r--r--. 1 root root   70  1 Mar  9,28
-rw-r--r--. 1 root root   10  1 Mar  9,32 last_reserved_ip.0
-rw-r--r--. 1 root root   70  1 Mar  9,32
drwxr-xr-x. 2 root root 4096  1 Mar  9,32 .
[root@srv podman]#

We’ve deleted the old IPs (old by date!) and and the mysql-slave container started successfully.

firewalld and podman (or docker) – no internet in the container and could not resolve host

If you happen to use CentOS 8 you have already discovered that Red Hat (i.e. CentOS) switch to podman, which is a fork of docker. So probably the following fix might help to someone, which does not use CentOS 8 or podman. For now, podman and docker are 99.99% the same.
So creating and starting a container is easy and in most cases one command only, but you may stumble on the error your container could not resolve or could not connect to an IP even there is a ping to the IP!
The service in the container may live a happy life without Internet access but just the mapped ports from the outside world. Still, it may happen to need Internet access, let’s say if an update should be performed.
Here is how to fix podman (docker) missing the Internet access in the container:

  • No ping to the outside world. The chances you are missing
    sysctl -w net.ipv4.ip_forward=1

    And do not forget to make it permanent by adding the “net.ipv4.ip_forward=1” to /etc/sysctl.conf (or a file “.conf” in /etc/sysctl.d/).

  • ping to the outside IP of the container is available, but no connection to any service is available! Probably the NAT is not enabled in your podman docker configuration. In the case with firewalld, at least, you must enable the masquerade option of the public zone
    firewall-cmd --zone=public --add-masquerade
    firewall-cmd --permanent --zone=public --add-masquerade

    The second command with “–permanent” is to make the option permanent over reboots.

The error – Could not resolve host (Name or service not known) despite having servers in /etc/resolv.conf and ping to them!

One may think having IPs in /etc/resolv.conf and ping to them in the container should give the container access to the Internet. But the following error occurs:

[root@srv /]# yum install telnet
Loaded plugins: fastestmirror, ovl
Determining fastest mirrors
 * base:
 * extras:
 * updates: [Errno 14] curl#6 - "Could not resolve host:; Unknown error"
Trying other mirror. [Errno 14] curl#6 - "Could not resolve host:; Unknown error"
Trying other mirror.

Exiting on user cancel
[root@srv /]# ^C
[root@srv /]# ping
PING ( 56(84) bytes of data.
64 bytes from icmp_seq=1 ttl=56 time=5.05 ms
64 bytes from icmp_seq=2 ttl=56 time=5.06 ms
--- ping statistics ---
2 packets transmitted, 2 received, 0% packet loss, time 1000ms
rtt min/avg/max/mdev = 5.050/5.055/5.061/0.071 ms
[root@srv ~]# cat /etc/resolv.conf 
[root@srv /]# ping
ping: Name or service not known

The error 2 – Can’t connect to despite having ping to the IP!

[root@srv /]# ping
PING ( 56(84) bytes of data.
64 bytes from icmp_seq=1 ttl=56 time=9.15 ms
64 bytes from icmp_seq=2 ttl=56 time=9.16 ms
[root@srv2 /]# mysql -h2.2.2.2 -uroot -p
Enter password: 
ERROR 2003 (HY000): Can't connect to MySQL server on '' (113)
[root@srv2 /]#

Despite having ping the MySQL server on and despite the firewall on allows outside connections the container could not connect to it. And testing other services like HTTP, HTTPS, FTP and so on resulted in “unable to connect“, too. Simply because the NAT (aka masquerade is not enabled in the firewall).

rsync daemon in CentOS 8

Apparently, the team behind the CentOS 8 decided to split the rsync functionality to two packages – one for the client-side and for the server-side, despite the binary rsync is only one and offers the client-size and server-side.
So there two packages in CentOS 8:

  1. rsync – provides the client-side and server-side as usual
  2. rsync-daemon – provides configuration example file and the systemd to start it as a service.

So if you wonder where is your rsync service after installing the rsync package under CentOS 8 you must install additional package “rsync-daemon”.

Of course, you may just create anywhere “rsyncd.conf” (the best place for the configuration file is in /etc, but could be placed anywhere with “–config=PATH/FILE” option) file and start the daemon as usual with “–daemon” option included to have the rsync server-side up and running.

rsync --daemon --config=/etc/rsyncd.conf

Just create yourself the configuration “/etc/rsyncd.conf” file.

Install the rsync program – the client and the daemon

dnf install rsync

Install the configuration and systemd files

dnf install rsync-daemon

rsync and rsync-daemon files

The files included in the two packages:

[root@srv ~]# dnf repoquery -l rsync-daemon
Last metadata expiration check: 0:33:02 ago on Wed 22 Jan 2020 02:57:06 PM UTC.
[root@srv ~]# dnf repoquery -l rsync
Last metadata expiration check: 0:33:06 ago on Wed 22 Jan 2020 02:57:06 PM UTC.