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Recover lost / forgotten root password for CentOS 7 Linux / Boot CentOS 6 into Single User mode to reset admin pass

Friday, September 27th, 2024

If you have some old CentOS 7 Virtual machine hanging for a long time and you don't remember the root password or you don't remember where you have stored it, but you have something important as data left over, you might need to recover root password for your CentOS 7 Virtual Machine.

I recently had to resolve that issue and here is the few easy steps to take to recover the lost root password.

Assuming you have tried to boot the VM and the VM boots fine and your few attempts to input manually some default passwords of yours failed, next

1. Reboot the Virtual Machine to the GRUB boot menu

The GRUB boot screen should appear and be there for few secs

2. Edit the boot loader kernel options ( add add rd.break enforcing=0 )

How to reset root password on CentOS Linux - Clouvider

Press 'e' to Edit the boot loader and modify the boot commands options passed to the linux kernel.

In GRUB edit mode:

add rd.break enforcing=0

to the end of the line starting with linux at the end of passed parameters list as shown in the picture.

When done editing, press Ctrl-x (Control button x key simultaneously) to boot with changed parameters.

ALTERNATIVE WAY TO BOOT THE SYSTEM INTO ROOT WITHOUT PASSWORD PROMPT:

Alternative options to use instead of add rd.break.enforcing=0 are to substitute the rhgb quiet kernel option with init=/bin/bash

As you might wonder for the meaning of the passed 2 parameters:

rd.break breaks the boot process at initramfs while
enforcing=0 disables the SELinux (which often enabled by default on CentOS).

Another way is to

3. Boot in CentOS emergency mode and Reset the root password

When done editing, press Ctrl-x to boot with changed parameters.

As you might wonder for the meaning of the passed parameters:

rd.break breaks the boot process at initramfs while
enforcing=0 disables the SELinux (which often enabled by default on CentOS).

Whence system boots up with the modified kernel options cmd, the switch_root prompt will appear.
As the emerency mode boots the filesystem into read-only mode under /sysroot default directory, in order to be able to
modify the MD5 root password stored hash inside RO mounted /sysroot/etc/shadow you need to remount the Filesystme
in read-write mode.

To Remount the read-only file system /sysroot in write mode:

# mount -o remount,rw /sysroot

As the /sysroot is not the root directory to be able to use a standard passwd command you need to make /sysroot
as the default root folder for the booted linux by chrooting into it.

Generate MD5 password manually (for Hardcore masochistic admins 🙂 )

If you're a hard core linux sysadmin of course, generate your own new md5 password and directly modify /etc/shadow copy pasting the md5 string.

If you want to manually generate the md5 string, you can do it depending on the required encryption algorithm with:

For (md5, sha256, sha512) encrypted pass

# openssl passwd -6 -salt xyz yourpass

For (md5, sha256, sha512) encrypted pwd

# mkpasswd –method=SHA-512 –stdin

For (des, md5, sha256, sha512) encrypted pw

# perl -e 'print crypt("YourPasswd", "salt", "sha512"),"\n"'

Once the string is generated;

# vim /etc/shadow

and exchange the old with new string for MD5

Change password with chroot (the easy common way)

remount read write the filesystem in emergency single user mode CentOS LINUX

# chroot /sysroot

That should drop you into another shell bash-4.x

Reset root user password in CentOS 7

# passwd
Changing password for user root.
New password:
Retype new password:

We need have to sync the entire filesystem we have to use the sync command, for novice sys admins who never heard about this command, below
short description:

The Linux sync command synchronizes cached data to permanent storage.
This data includes modified superblocks, modified inodes, delayed reads and writes, and others. sync uses several system calls:

sync()
syncfs()
fsync()
fdatasync()

For example, the sync command utilizes the sync() system call to write all buffered modifications to file data and metadata to an underlying storage device.

As a Linux systems administrator or developer, understanding the sync command can be crucial for efficient file synchronization. Additionally, sync can be helpful after crashes or when the file system becomes corrupted.

In this tutorial, we’ll explore the various aspects of the sync command. Also, we’ll see how we can use sync in different scenarios.

# sync

# exec /sbin/init

Try out the root password after booting normally into CentOS and the new set administrator pass should work.

Resetting forgotten (lost) root password on CentOS 6

The process is absolutely the same except on the Step 1 (in the modification of GRUB boot menu by pressing e key), add to

rhgb quiet

at the end one 'S'

This S character means 'boot CentOS into Single user mode'

rhgb quiet S

Then, press ENTER key and press b key to boot CentOS 6 into to single user mode.

Tags: about, absolutely, administrator, Admins, after, algorithm, ALL, com, command, default passwords, editing, filesystem, linux kernel, New, password, Reboot, Recover, root password, sync, www
Posted in CentOS, Linux, RedHat, System Administration | No Comments »

yum search file in all installable RPM, find out which rpm package provides binary file or missing library dependency on CentOS / RHEL / Fedora

Friday, August 23rd, 2024

Sometimes if you have a missing library or a file you know should be available via an rpm but you're not sure which RPM you have to install you have to look up for library or binary file amongs all available installable r[ms on Redhat Linux / CentOS / Fedora or other RPM based distro.

It is really annoying especially, if you try to install an rpm binary and the package does not install due to missing dependency library. Having a missing dependency package could happen, if you use some custom internal prepared repository that is mirroring from original rpm repositories and the RPM Repositories are situated behind a DMZ firewall network (such scenarios are common for corporations and IT companies).

Finding out which file is provided by which package on Debian / Ubuntu and other deb based linux distributions is easy and done via the

# apt-file search filename

Thus if you're a system administrator coming from a Debian GNU / Linux sysadmin realm into the wonderful world of redhats, you will want to have an alternative to apt-file tool. You will be happy to find out that that this tedious task is easily done in RPM based Linux and is integrated straight into yum package manager too.

The command to search which rpm package provides a file is:

# yum whatprovides 'nc'

[root@rhel-linux ~]# yum whatprovides nc
Loaded plugins: fastestmirror, versionlock
Loading mirror speeds from cached hostfile
2:nmap-ncat-6.40-19.el7.x86_64 : Nmap's Netcat replacement
Repo : base
Matched from:
Provides : nc

2:nmap-ncat-6.40-19.el7.x86_64 : Nmap's Netcat replacement
Repo : @base
Matched from:
Provides : nc

yum whatprovides search_file_name can be also invoked with its shortcut yum provides 'search_file_name'

[root@rhel-server ~]# yum provides '/bin/ls'
Loaded plugins: fastestmirror, versionlock
Loading mirror speeds from cached hostfile
coreutils-8.22-24.el7.x86_64 : A set of basic GNU tools commonly used in shell scripts
Repo : base
Matched from:
Filename : /bin/ls

coreutils-8.22-24.el7_9.2.x86_64 : A set of basic GNU tools commonly used in shell scripts
Repo : updates
Matched from:
Filename : /bin/ls

Here is another example:

[root@rhel-server ~]# yum -q provides '*lesspipe.sh*'
less-458-9.el7.x86_64 : A text file browser similar to more, but better
Repo : base
Matched from:
Filename : /usr/bin/lesspipe.sh

source-highlight-3.1.6-6.el7.i686 : Produces a document with syntax highlighting
Repo : base
Matched from:
Filename : /usr/bin/src-hilite-lesspipe.sh

source-highlight-3.1.6-6.el7.x86_64 : Produces a document with syntax highlighting
Repo : base
Matched from:
Filename : /usr/bin/src-hilite-lesspipe.sh

spirv-tools-2019.1-4.el7.x86_64 : API and commands for processing SPIR-V modules
Repo : epel
Matched from:
Filename : /usr/bin/spirv-lesspipe.sh

You can search for any file and if the RPm repository is defined under /etc/yum/repos.d/* and enabled, yum whatprovides command should be able to find it and tell you which RPM package you have to install to have the file installed Redhat way.

You can list all enabled RPM repositories with cmd:

[root@rhel-server ~]# yum repolist enabled
Loaded plugins: fastestmirror, versionlock
Loading mirror speeds from cached hostfile
repo id repo name status
3party Third party packages – x86_64 2,631
base/7/x86_64 CentOS-7 – Base 10,072
cr/7/x86_64 CentOS-7 – CR 0
epel/7/x86_64 EPEL packages for RedCent 7 – x86_64 13,791
extras/7/x86_64 CentOS-7 – Extras 526
updates/7/x86_64 CentOS-7 – Updates 5,802
zabbix-6.0 Zabbix 6.0 repo 429
repolist: 33,251

To list disable RPM repositories:

# yum repolist disabled
…

To list all present available repositories that could be enabled and are set via the /etc/yum.repos.d/* configs

# yum repolist all
…

Tags: API, available, cmd, dependency, distro, document, find, lib, list, loading, mirror, package, rpm, updates, yum
Posted in Linux, Linux Package Management, OS Update, RedHat, System Administration | No Comments »

How to Update / Migrate zabbix-agent 5 to zabbix-agent2 6 on Redhat / CentOS / Fedora Linux

Friday, August 9th, 2024

If you have servers reporting monitoring with Zabbix running still on Zabbix-Agent 1 version 5.0.X but already migrated the Zabbix-server to Zabbix 6, it is a good idea to update the Agent to Zabbix Agent 6 As sson as possible, as you know lacking behind in version makes updating harder and more complicated task.

Mine and I guess most system administrators experience points that Keeping at the same level of versioning on many applications historically has shown to reduce unexpected errors and bugs but nowadays, the rule of keeping local and remote application ( programs ) at the same version level is regularly broken.

Theoretically Zabbix-Agent (Client) and Zabbix (Server) has a compitability for a certain range of versions (Zabbix agents 2 from version 4.4 onwards are compatible with Zabbix 7.0; Zabbix agent 2 must not be newer than 7.0 – for more on zabbix agent – > server version compitability check here ) and having a slight version difference should not be really a problem but often you might have a third party proxies in between such as haproxy or zabbix-proxy or other network oddities and thus my personal opinion is that for interoperability it is better to keep the Zabbix Clients and Zabbix Servers across the DMZ-ed networks running at same version level.

Some would say I have an old fashion thinking as software and technology is moving forward, but as I see how programming code writing and even software is constantly degradating just a reflection of degradation of human element, I prefer to keep my old know how and always stick to same versioning whenever possible.

Some would wonder then why would I upgrade to Zabbix-agent2 ? , if have to keep the same versioning, the reason is zabbix-agent2 is written in GO Language and is much faster and supposably better piece of software than Zabbix Agent1 that is written in Python.

Moreover having Zabbix agent 2 instead of 1 gives also benefits as you can do a bit more with zabbix and on the other hand the machines are more ready for monitoring in terms of future. To know more about the Benefits of Zabbix Agent2 compared to Zabbix Agent 1 read the Agent vs Agent2 comparison on zabbix website.

With this little introduction, lets proceed with the exact steps to take to upgrade zabbix-agent1 to zabbix-agent2.

1. Check the current installed Zabbix-Agent version

[user@monitored-server ~]$ rpm -qa |grep -i zabb
zabbix-get-5.0.42-1.el8.x86_64
zabbix-sender-5.0.42-1.el8.x86_64
zabbix-agent-5.0.42-1.el8.x86_64
[user@server ~]$

2. Create backup copy of current system working zabbix_agentd.conf

Before messing up with the working zabbix-agent as usual create the necessery backup to prevent later suprises

[user@monitored-server ~]$ cp -vrpf /etc/zabbix/zabbix_agentd.conf /etc/zabbix/zabbix_agentd.conf.bak-$(date '+%Y-%m-%d_%H-%M-%S')
…

3. Check current configured Zabbix repos

[user@monitored-server ~]$ vim /etc/yum.repos.d/zabbix.repo

[zabbix-4.0]
name = zabbix-4.0 – 8
baseurl = http://zabbix-repo-server.com/external/zabbix-4.0/8/$basearch
enabled = 0
gpgkey = http://zabbix-repo-server.com/external/zabbix-4.0/zabbix-official-repo.key
gpgcheck = 1

[zabbix-4.4]
name = zabbix-4.4 – 8
baseurl = http://zabbix-repo-server.com/external/zabbix-4.4/8/$basearch
enabled = 0
gpgkey = http://zabbix-repo-server.com/external/zabbix-4.4/zabbix-official-repo.key
gpgcheck = 1

[zabbix-5.0]
name = zabbix-5.0 – 8
baseurl = http://zabbix-repo-server.com/external/zabbix-5.0/8/$basearch
enabled = 1
gpgkey = http://zabbix-repo-server.com/external/zabbix-5.0/zabbix-official-repo.key
gpgcheck = 1

[zabbix-5.4]
name = zabbix-5.4 – 8
baseurl = http://zabbix-repo-server.com/external/zabbix-5.4/8/$basearch
enabled = 0
gpgkey = http://zabbix-repo-server.com/external/zabbix-5.4/zabbix-official-repo.key
gpgcheck = 1

[zabbix-6.0]
name = zabbix-6.0 – 8
baseurl = http://zabbix-repo-server.com/external/zabbix-6.0/8/$basearch
enabled = 0
gpgkey = http://zabbix-repo-server.com/external/zabbix-6.0/zabbix-official-repo.key
gpgcheck = 1

4. Modify repositories and include the Zabbix Agent6 yum repos

[user@monitored-server ~]$ cp -rpf zabbix.repo zabbix.repo.5.0.rpmsave

As we want to keep only the 6.0 version, leave only the zabbix-6.0 section and enable the repo:

[user@monitored-server ~]$ vim /etc/yum.repos.d/zabbix.repo

[zabbix-6.0]
name = zabbix-6.0 – 8
baseurl = http://zabbix-repo-server.com/external/zabbix-6.0/8/$basearch
enabled = 1
gpgkey = http://zabbix-repo-server.com/external/zabbix-6.0/zabbix-official-repo.key
gpgcheck = 1

5. Update zabbix-agent to zabbix-agent2 and update zabbix-get zabbix-sender versions

To not disrupt reported monitoring for zabbix-agent, don't delete zabbix-agent1 but instead in pararallel install and configure
zabbix-agent2 and then once configuration is migrated from Agent 1 to 2, stop the old zabbix-agent and bring up the new one.

[user@monitored-server ~]$ yum check-update

[user@monitored-server ~]$ yum install zabbix-agent2 zabbix-get zabbix-sender -y

Note that if you want to have a precise version number of zabbix-agent that is lets say 6.0.31 to correspond to zabbix-server 6.0.31 (even though in the repositories newer RPM versions are available), run:

[user@monitored-server ~]$ yum upgrade zabbix-agent2-6.0.31-release1.el8

Check new zabbix_agent2 installed version

# zabbix_agent2 -V
zabbix_agent2 (Zabbix) 6.0.31
Revision b6d93755a1b 17 June 2024, compilation time: {undefined} {undefined}, built with: go1.21.3
Plugin communication protocol version is 6.0.13

Copyright (C) 2024 Zabbix SIA
License GPLv2+: GNU GPL version 2 or later <https://www.gnu.org/licenses/>.
This is free software: you are free to change and redistribute it according to
the license. There is NO WARRANTY, to the extent permitted by law.

This product includes software developed by the OpenSSL Project
for use in the OpenSSL Toolkit (http://www.openssl.org/).

Compiled with OpenSSL 1.1.1k FIPS 25 Mar 2021
Running with OpenSSL 1.1.1k FIPS 25 Mar 2021

We use the library Eclipse Paho (eclipse/paho.mqtt.golang), which is
distributed under the terms of the Eclipse Distribution License 1.0 (The 3-Clause BSD License)
available at https://www.eclipse.org/org/documents/edl-v10.php

We use the library go-modbus (goburrow/modbus), which is
distributed under the terms of the 3-Clause BSD License
available at https://github.com/goburrow/modbus/blob/master/LICENSE

6. Migrate old /etc/zabbix/zabbix_agentd.conf to /etc/zabbix/zabbix-agent2.conf

For readability to show the main configured variables for zabbix-agent without the tons of comments, to later include in agent2

[root@monitored-server ~]# cat /etc/zabbix/zabbix_agentd.conf | grep -v '\#' | sed '/^$/d'
PidFile=/var/run/zabbix/zabbix_agentd.pid
LogFile=/var/log/zabbix/zabbix_agentd.log
LogFileSize=0
Server=10.50.37.8,127.0.0.1
ServerActive=10.50.37.8,127.0.0.1
Hostname=fqdn-of-monitored-host.domain.com
Timeout=20
Include=/etc/zabbix/zabbix_agentd.d/*.conf

The default zabbix-agent2 installed config would like similar to:

[root@monitored-server ~]# cat /etc/zabbix/zabbix_agent2.conf | grep -v '\#' | sed '/^$/d'
PidFile=/run/zabbix/zabbix_agent2.pid
LogFile=/var/log/zabbix/zabbix_agent2.log
LogFileSize=0
Server=127.0.0.1
# Specify the location of the Zabbix server host.
ServerActive=127.0.0.1
Hostname=Zabbix server
Include=/etc/zabbix/zabbix_agent2.d/*.conf
PluginSocket=/run/zabbix/agent.plugin.sock
ControlSocket=/run/zabbix/agent.sock
Include=./zabbix_agent2.d/plugins.d/*.conf

The new migrate one, should be like:

[root@monitored-server ~]# vim /etc/zabbix/zabbix_agent2.conf
PidFile=/run/zabbix/zabbix_agent2.pid
LogFile=/var/log/zabbix/zabbix_agent2.log
LogFileSize=10
Server=10.34.89.7,127.0.0.1
ServerActive=10.34.89.7,127.0.0.1
Hostname=lqgblu02f.ffm.de.int.atosorigin.com
Timeout=20
Include=/etc/zabbix/zabbix_agent2.d/*.conf
PluginSocket=/run/zabbix/agent.plugin.sock
ControlSocket=/run/zabbix/agent.sock
Include=/etc/zabbix/zabbix_agent2.d/plugins.d/*.conf

7. Add few Optimization variables for better zabbix-server -> zabbix-proxy -> zabbix-server interactions

If you have sometimes a network delays between zabbix server -> zabbix client and vice versa (depending on whether Zabbix agent is configured as Active or Passive mode), it is often useful
to add those 2 variables:

# How often list of active checks is refreshed, in seconds
RefreshActiveChecks=60
# Refresh the active checks on start.ForceActiveChecksOnStart=1
ForceActiveChecksOnStart=1

Also it might be a good practice to add zabbix_agent2.log monitoring with the agent itself, if the log exceeds certain amount, instead of calling it via logrotate.

# Perform log file rotation at the 1 MB point for the specified filepath
LogFileSize=1

[root@monitored-server ~]# vim /etc/zabbix/zabbix_agent2.conf
PidFile=/run/zabbix/zabbix_agent2.pid
LogFile=/var/log/zabbix/zabbix_agent2.log
LogFileSize=10
Server=10.34.89.7,127.0.0.1
ServerActive=10.34.89.7,127.0.0.1
Hostname=lqgblu02f.ffm.de.int.atosorigin.com
RefreshActiveChecks=60
ForceActiveChecksOnStart=1
Timeout=20
Include=/etc/zabbix/zabbix_agent2.d/*.conf
PluginSocket=/run/zabbix/agent.plugin.sock
ControlSocket=/run/zabbix/agent.sock
Include=/etc/zabbix/zabbix_agent2.d/plugins.d/*.conf

8. Stop the old zabbix agent process and run the new one

# systemctl status –full zabbix-agent2
# systemctl stop zabbix-agent

Assuming that the configuratoin of zabbix-agent is correct, execute zabbix-agent2 via system control.and check its status

# systemctl start zabbix-agent2
# systemctl status –full zabbix-agent2

If no errors in the configuration, the zabbix_agent2 process should be up and running and the status of above systemctl cmd should report fine.
If you need concretics regarding exact Zabbix checks or whther current conigured Userparameter scripts errors, or any other warnings or errors
of zabbix_agent2 interacting to the server, check further the logs

[root@monitored-server ~]# tail -n 10 /var/log/zabbix/zabbix_agent2.log
2024/08/06 17:26:52.998749 using plugin 'WebPage' (built-in) providing following interfaces: exporter, configurator
2024/08/06 17:26:52.998760 using plugin 'ZabbixAsync' (built-in) providing following interfaces: exporter
2024/08/06 17:26:52.998794 using plugin 'ZabbixStats' (built-in) providing following interfaces: exporter, configurator
2024/08/06 17:26:52.998804 lowering the plugin ZabbixSync capacity to 1 as the configured capacity 100 exceeds limits
2024/08/06 17:26:52.998820 using plugin 'ZabbixSync' (built-in) providing following interfaces: exporter
2024/08/06 17:26:52.998993 Plugin communication protocol version is 6.0.13
2024/08/06 17:26:52.999018 Zabbix Agent2 hostname: [lqgblu02f.ffm.de.int.atosorigin.com]
2024/08/06 17:26:54.000667 [102] cannot connect to [127.0.0.1:10051]: dial tcp :0->127.0.0.1:10051: connect: connection refused
2024/08/06 17:26:54.000836 [102] active check configuration update from host [lqgblu02f.ffm.de.int.atosorigin.com] started to fail
2024/08/06 17:26:59.344837 Zabbix Agent 2 stopped. (6.0.31)

Tags: backup copy, checks, Hostname Zabbix, key, Modify, plugin, servers, Stop, version, zabbix-agent
Posted in CentOS, Linux, Monitoring, OS Update, RedHat, System Administration, System Optimization, Zabbix | No Comments »

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