Category: Linux

How to Reset root password in Proxmox VE 7

First, boot the system in the rescue kernel. I am assuming your boot partition is separated then the home partition, like in my case. Here are the list of commands I have used:

# mount /dev/md126 /mnt
# mount /dev/md127 /mnt/boot
# mount --bind /dev /mnt/dev
# mount --bind /dev/pts /mnt/dev/pts
# mount --bind /proc /mnt/proc
# mount --bind /sys /mnt/sys
# chroot /mnt
# passwd

Now, make sure to reset the password properly. Once done. Umount all the partition and boot in regular mode:

# umount /mnt/boot
# umount /mnt/sys
# umount /mnt/proc
# umount /mnt/dev/pts
# umount /mnt/dev
# umount /mnt
# reboot

It should be it.

How to mount raw (.img) images in Linux

You may have raw images in your system coming from some bare metal VM, like KVM/Vmware. You may mount them directly to the host machine without attaching them to a VM. To do so, you need two things:

  1. Partition to mount
  2. Sector size of the partition and the first position of the partition.

Both of these, can be derived using a command called fdisk with attribute l as the following:

[root@vps5 vpsextra]# fdisk -l mhdevelopment.img

Disk mhdevelopment.img: 268.4 GB, 268435456000 bytes, 524288000 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk label type: dos
Disk identifier: 0x000f1752

            Device Boot      Start         End      Blocks   Id  System
mhdevelopment.img1            2048   523241471   261619712   83  Linux
mhdevelopment.img2       523241472   524287999      523264   82  Linux swap / Solaris
You have new mail in /var/spool/mail/root
[root@vps5 vpsextra]#

Here, we are trying to mount an old image of mine, called ‘mhdevelopment.img. I would like to mount the partition that is not swap, which is namely ‘mhdevelopment.img2’ here. This partition starts from 2048 and the sector size is 512 bytes.

So to mount the partition, you can now use the following command:

sudo mount -o loop,offset=$((2048 * 512)) mhdevelopment.img /mnt

This will mount the the partition mhdevelopment.img1 to /mnt. If you want to change the mount destination, you may do this at the end in replace of /mnt to wherever you want this to be.

Happy shooting.

The GPG keys listed for the “MySQL 5.7 Community Server” repository are already installed but they are not correct for this package.

When you try to run yum update, you see an error like the following:

The GPG keys listed for the "MySQL 5.7 Community Server" repository are already installed but they are not correct for this package.

This is coming because the GPG key for Mysql 5.7 community repo has changed. To update the GPG key, you may run the following:

rpm --import https://repo.mysql.com/RPM-GPG-KEY-mysql-2022

This shall resolves the issue for you. Good luck.

How to Set Default Route Interface when there are multiple Interfaces in CentOS

Consider the following situation, you have two interfaces in your system, eth0, and eth1 (or more). Both the interfaces have a default gateway. But how can you choose which interface to follow as the default outbound requests by the server? This could be a situation like adding secondary alias IP addresses to an interface (Like eth0, eth0:0, eth0:1, eth0:2, etc).

In these cases, Linux can decide based on its own algorithm. The problem with this is in the case of Licensing. In most licensing systems these days, you need to provide an outbound IP address to your server. If this IP changes over time, you would have trouble managing the license. For these cases, you need to define your default interface to Linux.

You may set this configuration under /etc/sysconfig/network file. The ‘key’ for the settings is ‘GATEWAYDEV’.

So for example if your gatewaydev would be eth0:1, the file would look like something following:

NETWORKING=yes
HOSTNAME=hemonto.ethii.com
DOMAINNAME=ethii.com
GATEWAYDEV=eth0:1

or if it is eth1, the following:

NETWORKING=yes
HOSTNAME=hemonto.ethii.com
DOMAINNAME=ethii.com
GATEWAYDEV=eth1

I Hope, this helps somebody. Good luck

How to create Software RAID 1 on Fresh NVMe Drives in CentOS/RHEL

Let’s say, you just installed two NVMe drives. That means, you currently have the following devices on your system:

/dev/nvme0n1
/dev/nvme0n2

Now, to use Raid 1 on these devices, you need to first partition them. If your devices are less than 2TB, you can use label msdos with fdisk. But I prefer gpt with parted. I will partition the disks using parted.

Open the disk nvme0n1 using parted

parted /dev/nvme0n1

Now, set the label to gpt

mklabel gpt

Now, create the primary partition

mkpart primary 0TB 1.9TB

Assuming 1.9TB is the size of your drive.

Run the above process for nvme1n1 as well. This will create one partition on each device which would be like the following:

/dev/nvme0n1p1
/dev/nvme1n1p1

Now, you may create the raid, using mdadm command as follows:

mdadm --create /dev/md201 --level=mirror --raid-devices=2 /dev/nvme0n1p1 /dev/nvme1n1p1

If you see, mdadm command not found, then you can install mdadm using the following:

yum install mdadm -y

Once done, you may now see your raid using the following command:

[root@bd3 ~]# cat /proc/mdstat
Personalities : [raid1]
md301 : active raid1 sdd1[1] sdc1[0]
      976628736 blocks super 1.2 [2/2] [UU]
      bitmap: 0/8 pages [0KB], 65536KB chunk

md201 : active raid1 nvme1n1p1[1] nvme0n1p1[0]
      1875240960 blocks super 1.2 [2/2] [UU]
      bitmap: 2/14 pages [8KB], 65536KB chunk

md124 : active raid1 sda5[0] sdb5[1]
      1843209216 blocks super 1.2 [2/2] [UU]
      bitmap: 4/14 pages [16KB], 65536KB chunk

md125 : active raid1 sda2[0] sdb2[1]
      4193280 blocks super 1.2 [2/2] [UU]

md126 : active raid1 sdb3[1] sda3[0]
      1047552 blocks super 1.2 [2/2] [UU]
      bitmap: 0/1 pages [0KB], 65536KB chunk

md127 : active raid1 sda1[0] sdb1[1]
      104856576 blocks super 1.2 [2/2] [UU]
      bitmap: 1/1 pages [4KB], 65536KB chunk

unused devices: <none>

Here are a few key pieces of information about software raid:

  1. It is better not to use Raid 10 with software raid. In case the raid configuration is lost, it is hard to know which drives were set as stripe and which like a mirror by the mdadm. It is a better practice to use raid 1 as a rule of thumb with software raid.
  2. Raid 1 in mdadm doubles the read request in parallel. In raid 1, one request reads from one device, while the other request in parallel would read from the next device. This gives double read throughput when there is a parallel thread running. It still suffers from the write cost for writing data in two devices.

Unable to connect to any wifi with NetworkManager due to error: Secrets were required, but not provided

I was trying to connect to wifi from a CentOS 7 minimal installation using the following article:

How to connect CentOS Server to the Internet using USB Wifi Adapter?

But I started seeing the following error:

Unable to connect to any wifi with NetworkManager due to error: Secrets were required, but not provided

The reason the above error is showing is that NetworkManager is trying to reuse an existing connection, which shouldn’t happen. You need to delete the connection first. 

nmcli con delete <SSID>

Now, you can connect with the following:

nmcli dev wifi connect <SSID> password <password>

Apache detected an error in the Rewrite config. httpd_ls_bak: Syntax error in -C/-c directive: Include/IncludeOptional: Could not open directory /usr/local/apache/conf.modules.d: No such file or directory Please try again. – Cpanel Error

You might see the following set of errors with Cpanel

When trying to remove the redirect from Cpanel

Apache detected an error in the Rewrite config. <pre>httpd_ls_bak: Syntax error in -C/-c directive: Include/IncludeOptional: Could not open directory /usr/local/apache/conf.modules.d: No such file or directory </pre> Please try again.

/scripts/rebuildhttpdconf generates an error like the following:

httpd: Syntax error in -C/-c directive: Include/IncludeOptional: Could not open directory /usr/local/apache/conf.modules.d: No such file or directory

Resolution

Previously, we reinstalled apache24 to solve the issue like the following:

yum reinstall ea-apache24

Although you may also reinstall the apache config runtime extension, that shall fix the issue as well. 

rpm -e --nodeps --justdb ea-apache24-config-runtime.noarch
yum install ea-apache24-config-runtime.noarch

Now you may rebuild the httpd conf or remove the redirect from cpanel without any problem.

Unable to register authentication agent: GDBus.Error:org.freedesktop.PolicyKit1.Error.Failed: Cannot determine user of subject – Cyberpanel – CentOS 7

Error details:

After installing cyberpanel, you see the below error from shell when running any command:

** (pkttyagent:8559): WARNING **: 02:28:34.621: Unable to register authentication agent: GDBus.Error:org.freedesktop.PolicyKit1.Error.Failed: Cannot determine user of subject
Error registering authentication agent: GDBus.Error:org.freedesktop.PolicyKit1.Error.Failed: Cannot determine user of subject (polkit-error-quark, 0)

Resolution:

The error is appearing because cybepranel adds the following line in your fstab:

proc    /proc        proc        defaults,hidepid=2    0 0

This is hiding the /proc from your shell that uses polkit. To fix this, keeping the above mount rule, you need to add a group for polkitd user and assign that group to to access /proc. You may do this using the following:

groupadd nohideproc
usermod -a -G nohideproc polkitd
mount -o remount,rw,hidepid=2,gid=nohideproc /proc
systemctl restart polkit
systemctl restart polkit (do this twice)

Once done, you may edit your fstab, and comment the old line, add the following line:

# Edit your fstab:
nano /etc/fstab

# comment the following line :
from : proc    /proc        proc        defaults,hidepid=2    0 0
to : # proc    /proc        proc        defaults,hidepid=2    0 0

# add the following line
proc /proc proc defaults,hidepid=2,gid=nohidproc 0 0

Good luck.

How to measure the sequential write/read speed of a Hard Disk or SSD?

Question: How to measure the sequential write speed of a hard disk or SSD?

Answer

We may use two ways. The most popular one is to use dd tools. For hard disks, we may not avoid using cache to measure the disk performance. As sequential writes can write the data to the cache, which is basically a DRAM and can perform excessively fast masking the original speed of the HDD. Let’s check how can we measure the HDD speed using dd bypassing HDD write cache:

dd if=/dev/zero of=/hddvz/testfile.img bs=1G count=1 oflag=direct

/hddvz is my HDD mount, oflag=direct instruct the dd to confirm writes to the disk, not just the cache before saying the write is completed.

If it’s a good quality hard disk alone or soft raid 1, you can get speed up to 157-161MBps:

[root@bd3 ~]# dd if=/dev/zero of=/hddvz/testfile.img bs=1G count=1 oflag=direct
1+0 records in
1+0 records out
1073741824 bytes (1.1 GB) copied, 6.82675 s, 157 MB/s
[root@bd3 ~]#

With a good hardware raid controller and raid 10, you may be able to reach rates like 320MBps.

There is something to remember, software raid mdadm is able to read simultaneously from two disks. You may have the evidence, with two dd one by one, both in background as following:

[root@bd3 ~]# dd if=/dev/zero of=/hddvz/testfile.img bs=1G count=1 oflag=direct &
[1] 12039
[root@bd3 ~]# dd if=/dev/zero of=/hddvz/testfile1.img bs=1G count=1 oflag=direct &
[2] 12040
[root@bd3 ~]# 1+0 records in
1+0 records out
1073741824 bytes (1.1 GB) copied, 11.1064 s, 96.7 MB/s
1+0 records in
1+0 records out
1073741824 bytes (1.1 GB) copied, 10.7155 s, 100 MB/s

[1]-  Done                    dd if=/dev/zero of=/hddvz/testfile.img bs=1G count=1 oflag=direct
[2]+  Done                    dd if=/dev/zero of=/hddvz/testfile1.img bs=1G count=1 oflag=direct

As you can see, both the request was able to reach 100MBps roughly while running in parallel.

Testing Read Speed Using hdparm

To check the read speed, you may use hdparm like the following:

[root@bd3 ~]# hdparm -Tt /dev/sda

/dev/sda:
 Timing cached reads:   40444 MB in  1.99 seconds = 20365.12 MB/sec
 Timing buffered disk reads: 396 MB in  3.06 seconds = 129.43 MB/sec
[root@bd3 ~]# hdparm -Tt /dev/sdb

/dev/sdb:
 Timing cached reads:   41006 MB in  1.99 seconds = 20649.43 MB/sec
 Timing buffered disk reads: 562 MB in  3.01 seconds = 186.81 MB/sec
[root@bd3 ~]#

How to connect CentOS Server to the Internet using USB Wifi Adapter?

I was recently working on an Asus RS300 server, installed with CentOS 7 minimal installation before placing it to the datacenter. I didn’t have any RJ45 cable to get this connected to the internet at home. I had a spare ‘Linksys wusb600n’ USB wifi adapter, that I wanted to try out. I connected it to the USB, and got the interface listed as wlp18s0b1 using ifconfig.

To connect this to wifi, we can use nmcli like the following:

Show the connection:

nmcli connection show

To connect to the wifi with the password, use the following:

nmcli dev wifi connect your-ssid password your-wifi-pass

Remember to replace ‘your-ssid’ with the wifi name of your and ‘your-wifi-pass’ with the password for your wifi.

Once you run the above command, this should get connected to the wifi. Now, you can see the connection details and up the device like the following:

nmcli connection show
nmcli connection up wlp18s0b1

Remember to replace ‘wlp18s0b1’ with the one you can see in ifconfig.

Good luck.