Category: Linux

nslookup
To find IP of a domain, use
```
nslookup serverok.in
```
To find MX record for a domain, use
```
nslookup -q=mx serverok.in
```
To see TXT records, run
```
nslookup -q=txt serverok.in
```
Asking Authritative Name Servers

When you run nslookup, it checks with caching name servers, generally provided by your ISP. But these are not authoritative name servers for your domain name. These caching name servers cache any DNS result for several hours depending on TTL (Time To Live) value in a domain’s DNS zone. If you are debugging some DNS problems, you need to ask domains authoritative name servers, that you can find by taking whois of a domain name.

Here is an example
```
nslookup -q=a DOMAIN_NAME_HEARE AUTHORITATIVE_NAME_SERVER_HERE
```
If you take whois of serverok.in, you will see the domain use name servers
```
Name Server: elle.ns.cloudflare.com
Name Server: carl.ns.cloudflare.com
```
To find MX record of the domain, you can use
```
nslookup -q=mx serverok.in
```
But what you get is the cached result from a non-authoritative name server. To get an authoritative result, you need to use the following command
```
nslookup -q=mx serverok.in elle.ns.cloudflare.com
```
Here elle.ns.cloudflare.com is one of the name servers used by serverok.in, that you find from whois. You can use any of the name servers with the nslookup command.

What it does is, ask the name server elle.ns.cloudflare.com (authoritative name server) what is the MX record for the domain serverok.in

Install nslookup on ArchLinux
Find Reverse-DNS/PTR using dig, nslookup, host
How to verify rDNS (reverse DNS)/PTR Record
July 21, 2018
Start a program after reboot using cronjob
To start a program using cronjob, create a cronjob like
```
@reboot /root/start.sh
```
On system boot, the command “/root/start.sh” will be executed.
July 20, 2018
Install memcached on CentOS 7
To install memcached on CentOS 7, run
```
yum -y install memcached
```
Set memcached to start on boot
```
systemctl enable memcached
```
Start memcached with
```
systemctl start memcached
```
Installing PHP Module

You need remi repository installed, which provide a memcached module for all versions of php they support.

For PHP 5.6, run
```
yum install php-memcache
```
Now phpinfo() will show
- Secure Memcached on CentOS/RHEL 7
- How to Install memcached on CentOS Plesk Server
See memcached
July 9, 2018

Ubuntu 18.04 Remove Trash Icon from Desktop

First find the Trash related settings with gsettings command.

boby@ok-pc-01:~$ gsettings  list-recursively | grep -i trash
org.gnome.gedit.plugins.filebrowser.nautilus confirm-trash true
org.gnome.desktop.privacy remove-old-trash-files false
org.gnome.eog.ui disable-trash-confirmation false
org.gnome.nautilus.preferences confirm-trash true
org.gnome.nautilus.preferences show-move-to-trash-shortcut-changed-dialog true
org.gnome.DejaDup exclude-list ['$TRASH', '$DOWNLOAD']
org.gnome.nautilus.desktop trash-icon-name 'Trash'
org.gnome.nautilus.desktop trash-icon-visible true
boby@ok-pc-01:~$

To disable the Trash Icon on desktop, run

gsettings set org.gnome.nautilus.desktop trash-icon-visible false

Method 2

If you have gnome tweaks tool installed, go to Desktop, you have option to disable Trash Icon.

See Ubuntu 18.04

July 7, 2018

grub

CentOS 7 Grub 2
grubby
OVH CentOS 7 server grub rescue prompt

RHEL Based – AlmaLinux, CentOS, RockeyLinux, Oracle Linux

Regenerate grub config

grub2-mkconfig -o /boot/grub2/grub.cfg
grub2-mkconfig -o /boot/efi/EFI/centos/grub.cfg
grub2-mkconfig -o /boot/efi/EFI/almalinux/grub.cfg

grub config on RHEL

/etc/default/grub

To install Grub, run

/sbin/grub2-install /dev/sda

To see all available kernels

awk -F\' '$1=="menuentry " {print i++ " =  "$2}' /etc/grub2.cfg

If all kernels not showing, edit

vi /etc/default/grub

Add

GRUB_DISABLE_SUBMENU=true
GRUB_DEFAULT=saved

Rebuild grub

grub2-mkconfig -o /etc/grub2.cfg

To set default Kernel

grub2-set-default 0

To see the current Kernel

grub2-editenv list

Reinstall kernel and set it to default

yum reinstall kernel
grub2-set-default 0
grubby --set-default 0

Ubuntu

sudo update-grub
/usr/sbin/grub-mkconfig -o /boot/grub/grub.cfg

July 4, 2018

Haproxy Site With SSL

To handle SSL/HTTPS traffic on haproxy, use following config in your /etc/haproxy/haproxy.cfg file.

frontend https-frontend-new
	bind :::443 ssl crt domain.pem

	acl https ssl_fc
	http-request set-header X-Forwarded-Proto http  if !https
	http-request set-header X-Forwarded-Proto https if https

	option forwardfor

	acl secured_cookie res.hdr(Set-Cookie),lower -m sub secure
	http-response replace-header Set-Cookie (.*) \1;\ secure if https !secured_cookie
	
	default_backend https-backend-new

backend https-backend-new
	balance static-rr
	option httpchk
	cookie SRV insert indirect nocache maxidle 30m maxlife 8h
	server web1 BACKEND_SERVER_IP:443 check ssl verify none

domain.pem

SSL certificate of your domain in PEM format. This is done by using combining your SSL cert, private key and ca bundle.

cat yourdomain.crt yourdomain.key yourdomain.ca-bundle > yourdomain.pem

If you have more sites with SSL, you can specify SSl certs like

bind :::443 ssl crt domain.pem crt domain-2.pem crt domain-3.pem

BACKEND_SERVER_IP

This is IP of your back end server.

Restart Haproxy with

systemctl restart haproxy

July 1, 2018

How to check if systemctl service is enabled

To check if a systemctl service is enabled, run

systemctl is-enabled SERVICE_NAME

Example

[root@mail ~]# systemctl is-enabled memcached
disabled
[root@mail ~]#

You can enable the service, so it run at boot with command

[root@mail ~]# systemctl enable memcached
Created symlink from /etc/systemd/system/multi-user.target.wants/memcached.service to /usr/lib/systemd/system/memcached.service.
[root@mail ~]#

To check if a service is enabled to start on boot, run

systemctl list-unit-files | grep SERVICE_NAME_HERE

Example

[root@ok ~]# systemctl list-unit-files | grep httpd
httpd.service                                 enabled 
[root@ok ~]#

June 29, 2018

Auto Start pm2 on boot
To auto start pm2 on boot, run
```
pm2 startup
```
This will generate auto start script for your system.

Once you started all applications, you can save it with
```
pm2 save
```
To uninstall, run
```
pm2 unstartup systemd
```
Running Application as Normal User

First login as the user, then start the application using pm2.
```
pm2 start app.js
```
Now save it.
```
pm2 save
```
Generate command for auto startup
```
pm2 startup
```
It will show a command. That you need to run as user root. If you have sudo privilage, you can run it with sudo as same user.

This will create a service with name pm2-USERNAME.

After reboot, you will be able to check status with command
```
systemctl status pm2-USERNAME
```
See pm2
June 25, 2018
sqlite

.database

Show location of the database file.

sqlite> .database main: /root/config.db sqlite>

.tables

List tables in a database

sqlite> .tables tree sqlite>

select

To see data from table, use select statement like you do in MySQL.

select * from TABLE_NAME;

.schema

To see how a table is created, use

.schema TABLE_NAME

This is equivalent to “show create table TABLE_NAME;” command in MySQL.

June 25, 2018

lftp Fatal error: Certificate verification: Not trusted

When i connect to an FTP server, i get following error

root@hestia-lnx:~# lftp -u sokftp 54.37.215.1
Password: 
lftp [email protected]:~> dir                   
ls: Fatal error: Certificate verification: Not trusted
lftp [email protected]:~> ls -la
ls: ls -la: Fatal error: Certificate verification: Not trusted
lftp [email protected]:~>

To fix this, run

set ssl:verify-certificate no

You can add this to /etc/lftp.conf to make this permanant

vi  /etc/lftp.conf

Add

set ssl:verify-certificate no

See lftp

June 23, 2018

SSH Remember Passphrase
When i connect to remote server using SSH using private key, it ask for passphrase every time.
```
root@admin:~# ssh [email protected]
Enter passphrase for key '/root/.ssh/id_rsa': 

root@admin:~# 
```
I want it only ask once, then remember it.

To do this, you need to run
```
eval `ssh-agent -s`
ssh-add
```
When you run “ssh-add” command, it ask for your passphrase. Now you will be able to use your SSH Key with out getting promoted for passphrase.

See ssh
June 22, 2018
Ubuntu Slow Boot
Last day my PC become very slow to boot.
```
boby@ok-pc-01:~$ systemd-analyze 
Startup finished in 15.777s (kernel) + 3min 1.400s (userspace) = 3min 17.178s
graphical.target reached after 1min 47.169s in userspace
boby@ok-pc-01:~$ 
```
It take 3 minutes and 17 seconds to boot up.

This happend after i did some changes to my 2nd hard disk and added ZFS partition. First thing i thought it was ZFS that make my PC slow.

systemd-analyze blame did not show any useful info.
```
systemd-analyze blame
```
To find the problem, i edited the file
```
sudo  vi /etc/defaults/grub
```
Find the line
```
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
```
Replace it with
```
GRUB_CMDLINE_LINUX_DEFAULT=""
```
Now rebuild grub.cfg with
```
update-grub2
```
Rebooted the PC. Now instead of showing Ubuntu splash screen, you get lot of text, that shows what actually your PC is doing.

From the boot message i found system waiting 1 minutes 30 seconds for mounting one of the disk partition. This is because i re-partitioned my 2nd hard disk and forget to remove the disk from /etc/fstab.

I removed the non existant disk entry from /etc/fstab and rebooted PC. Now it boot much faster.

See Ubuntu
June 22, 2018

Category: Linux

Asking Authritative Name Servers

RHEL Based – AlmaLinux, CentOS, RockeyLinux, Oracle Linux

Ubuntu

Running Application as Normal User