Tag: 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 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 Update PATH Variable in Linux

A PATH variable is a system variable that stores the information about the binary files location that you may run for commands. When you log in as an user, or use a custom control panel like Plesk/Cyberpanel/Cpanel, you might want to add some custom paths as a user to take binary commands. One of the example, could be to change the default php path, or a laravel command location from vendor folder. To do this, you need to extend/update the PATH variable for a specific user.

PATH variable extends with the “:”. If you type the following, in your shell, you may see the existing paths in the PATH variable:

[elastic-keldysh@pl1 ~]$ echo $PATH
/usr/share/Modules/bin:/usr/local/bin:/usr/bin:/usr/local/sbin:/usr/sbin

Now, if I want to extend this to take the php binary available in /opt/plesk/php/7.2/bin/php, then we can extend the PATH variable using the following:

PATH=$PATH:/opt/plesk/php/7.2/bin/

Now, if you check, the PATH variable again, you can see it is added:

[elastic-keldysh@pl1 ~]$ echo $PATH
/usr/share/Modules/bin:/usr/local/bin:/usr/bin:/usr/local/sbin:/usr/sbin:/opt/plesk/php/7.2/bin/
[elastic-keldysh@pl1 ~]$

We have successfully modified the PATH variable, but only for the existing session. If you want to persist the changes, then, you need to add the command in .bashrc/.profile/.bash_profile file depending on your shell type and OS. You can add to either of the file and test with the following command:

[elastic-keldysh@pl1 ~]$ echo "PATH=$PATH:/opt/plesk/php/7.2/bin/" >> .profile

Replace .profile with .bashrc or .bash_profile depending on the file that works for you. You may logout and relogin, and then run the echo command again to see if the $PATH is persisting or not.

How To: Start a Screen Session and Run a Command at a Time

Sometimes, you may want to run a screen command in a remote server. That makes it necessary to run the command inside the screen session while starting it.

How to start a screen session and run a command in one line

# screen -d -m sh -c "yourcommand"

From the man page of Screen:

-d -m : Start screen in “detached” mode. This creates a new session but doesn’t attach to it. This is useful for system startup scripts.

sh -c: Starts a shell and runs a command for you.

 

How To: Get Username from UID & Vice Versa in Linux

Question:

I have an UID, how do I get the username belongs to this UID in Linux?

How to Get UID from USERNAME in Linux

We usually know, there is a dedicated command called ‘id’ in linux given to find UID from username is called ‘id‘

You can use that to get the UID from the username in linux:

# id -u root
0

How to get Username from UID in Linux

Although, there is no built in command get fetch the username from the UID. We can use a pipe and regular expression match on getent to do that.

getent is a unix command that helps a user get entries in a number of important text files called databases. This includes the passwd and group databases which store user information – hence getent is a common way to look up user details on Unix.

You can use the following command to find username of the UID 752 for example in a system:

# getent passwd "752"|cut -d: -f1
texstard

getent can take group database too, although, we have used passwd database as that contains the UID of the respective linux user.

Linux: Assertion failed on job for iptables.service.

If you are using Centos 7 or RHEL 7 or any of it’s variant, you are probably using ‘Firewalld’ by default. Although, if you are a iptables fan like me, who likes it’s simplicity and manipulative nature instead of a full form firewall, then you probably have disabled firewalld from your CentOS 7 instance and using iptables. There are couple of servers, where I use runtime iptables rules for postrouting and masquerading. These rules are dynamically generated by my scripts instead of the sysconfig file under:

/etc/sysconfig/iptables

This file is generated upon running the iptables save command:

service iptables save

which I rarely do so.

Error Details

Which is why, I don’t have a /etc/sysconfig/iptables file in those servers and a common error I see while restarting iptables in those system is the following:

# systemctl restart iptables.service
Assertion failed on job for iptables.service.

How to Fix The Error

The error appears because you don’t have any rule in /etc/sysconfig/iptables or the file doesn’t exist either. You can ignore the error as iptables would still run. To eradicate the error, simply make sure you have some iptables rules loaded on your system using the status command:

iptables -S

And then, run:

service iptables save

Once done, restarting iptables shouldn’t show the error any longer.

How to: Find dm number of a LVM logical volume

Sometimes, you will see the error thrown in dmesg or /var/log/messages are mentioned in dm-number format, while you manage the disk using lvm logical volume name. This is because lvm logical volumes are designed through kernel device mapper technique and kernel recognizes volumes using dm numbers. There is a tool to list all the device mappers used for block devices under Linux. Simply type the following to list the maps:

# lsblk

It shall show something like the following:

There you can see the dm number for each lvm volume is listed under first bracket. For example the swap in this server is created with LVM with the name vg_iof442/swap and has the dm-1 mapping.

Why does Your New Site Take Ages to Load?

I was trying to track down a couple of website slow down reports lately. There is an interesting change of slow down behaviour these days in web application. From a conventional standpoint, people firmly believes that their static contents are not going to affect the performance of their websites other than images being heavy.
 
In reality, they are ignoring the fact that they are using jQuery plugins of many kinds from multiple developers. Hence cumulative number & sizes of JS files are pretty large these days comparing with all the plugins were coming from a single developer. Once the number of static file increases and goes beyond 100 per page, a cookie domain can hit some serious performance penalty. Geolocation for these small files and accessing them from single source can also increase the time geometrically. There is undoubtedly a large market of CDN due to the nature of development in web application.
 
I have seen, people these days are more aware about handling large data wisely than before. If you are using a Cloud from any provider, you are possibly using an E5 core or multiple (Mellowhost uses only E5 nodes right at this moment), that usually comes with access to a 16/24/32MB cache. Your static handling going to be more important in performance on these type of resources than your database, as threading is more of a concern than a single process handling in these virtualised resources.

Misconception about Server Specification!

“Oh! Damn! That server seems pretty cheap, giving me 12GB RAM, dual intel xeon 5430, so, lets go and purchase it” – a  big misconception to judge a server specification looking at their cpu and the ram size in current web hosting industry. I have been playing a lot in Webhostingtalk and some other hosting forums these days and found people are asking the same question everyday, which server is going to be right for me. Here I will go through some basic idea why the idea of users are biasing everyday and how to judge a proper server specification.

Continue reading “Misconception about Server Specification!”