Linux

5 Linux commands to check free disk space
6 tcpdump network traffic filter options
DHCP on Linux
dmidecode -t memory
How to Change Passwords in Linux
How To Configure Firewall with UFW on Ubuntu 20.04 LTS
How to Install NTP Server and Client(s) on Ubuntu 22.04 LTS
How to Set Time, Timezone and Synchronize System Clock in Linux
How to Symlink a Directory in Linux
LN Command - Create Links
Sysadmins Networking Cheat Sheet
Using Cert Bot to get a Valid SSL certificate
Using Netplan to set a static IP address
Using S.M.A.R.T to check disk health
Show used disk space on Linux
Clean Archive Cache
Expand LVM Volume
Ubuntu 20.04: How to Extend Partition (For Noobs)
Live resize root partition on Linux Server (VM)
Mount SMB Share on boot
How to enable Swap on Linux
How To Check Swap Usage Size and Utilization in Linux
Mount an SMB Share in Linux

5 Linux commands to check free disk space

Linux df command

The df command stands for "disk-free," and shows available and used disk space on the Linux system.

df -h shows disk space in human-readable format

df -a shows the file system's complete disk usage even if the Available field is 0

df -T shows the disk usage along with each block's filesystem type (e.g., xfs, ext2, ext3, btrfs, etc.)

df -i shows used and free inodes

You can get this information in a graphical view using the Disks (gnome-disk-utility) in the GNOME desktop. Launch it to see all disks detected by your computer, and click a partition to see details about it, including space used and space remaining.

opensource.com

Linux du command

du shows the disk usage of files, folders, etc. in the default kilobyte size

du -h shows disk usage in human-readable format for all directories and subdirectories

du -a shows disk usage for all files

du -s provides total disk space used by a particular file or directory

The following commands will check your total space and your utilized space.

This information can be represented visually in GNOME with the Disk Usage application, or with Filelight in the KDE Plasma desktop. In both applications, disk usage is mapped to concentric circles, with the middle being the base folder (usually your /home directory, but it's up to you) with each outer ring representing one directory level deeper. Roll your mouse over any segment for detailed information about what's taking up space.

opensource.com

Linux ls -al command

ls -al lists the entire contents, along with their size, of a particular directory

Linux stat command

stat <file/directory> displays the size and other stats of a file/directory or a filesystem.

Linux fdisk -l command

fdisk -l shows disk size along with disk partitioning information

6 tcpdump network traffic filter options

https://www.redhat.com/sysadmin/tcpdump-part-one

1. Option -D

tcpdump with -D provides a list of devices from which you can capture traffic. This option identifies what devices tcpdump knows about. Once you see this list, you can decide which interface you want to capture the traffic on. It also tells you if the interface is Up, Running, and whether it is a Loopback interface, as you can see below:

# tcpdump -D 
1.tun0 [Up, Running] 
2.wlp0s20f3 [Up, Running] 
3.lo [Up, Running, Loopback] 
4.any (Pseudo-device that captures on all interfaces) [Up, Running] 
5.virbr0 [Up]
 6.docker0 [Up] 
7.enp0s31f6 [Up]

2. Option -c X

The -c option captures X number of packets and then stops. Otherwise, tcpdump will keep running indefinitely. So when you want to capture only a small sample set of packets, you can use this option. However, if there is no activity on the interface, tcpdump keeps waiting.

# tcpdump -c 5 -i any 
dropped privs to tcpdump 
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
 listening on any, link-type LINUX_SLL (Linux cooked v1), capture size 262144 bytes 
16:19:22.128996 ARP, Request who-has _gateway tell 192.168.86.81, length 28
 16:19:22.130560 IP 172.217.222.189.https > kkulkarni.58810: Flags [P.], seq 3506342975:3506343029, ack 2537104576, win 377, options [nop,nop,TS val 4137065873 ecr 75405758], length 54 
16:19:22.130642 IP kkulkarni.58810 > 172.217.222.189.https: Flags [.], ack 54, win 501, options [nop,nop,TS val 75422756 ecr 4137065873], length 0 16:19:22.131198 IP ovpn-3-80.rdu2.redhat.com.36380 > infoblox-trust01.intranet.prod.int.rdu2.redhat.com.domain: 53320+ PTR? 1.86.168.192.in-addr.arpa. (43) 
16:19:22.131395 IP kkulkarni.53013 > ovpn-rdu2-alt.redhat.com.https: UDP, length 95 
5 packets captured 
49 packets received by filter 
37 packets dropped by kernel

3. Option -n

It is usually easier to work if you use IP addresses instead of names, such as kkulkarni.53013 as shown in the above output. You can use -n for this.

# tcpdump -c 5 -i any -n 
dropped privs to tcpdump 
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
 listening on any, link-type LINUX_SLL (Linux cooked v1), capture size 262144 bytes 
16:20:21.523375 IP 172.217.9.206.https > 192.168.86.31.34288: Flags [P.], seq 723352132:723352349, ack 2124268216, win 1059, options [nop,nop,TS val 2934032467 ecr 824781066], length 217 
16:20:21.563992 IP 192.168.86.31.34288 > 172.217.9.206.https: Flags [.], ack 217, win 12654, options [nop,nop,TS val 824783221 ecr 2934032467], length 0 
16:20:22.956717 IP 192.168.86.83.mdns > 224.0.0.251.mdns: 0 [2q] [1au] PTR (QU)? _companion-link._tcp.local. PTR (QU)? _homekit._tcp.local. (88) 16:20:22.956839 IP 192.168.86.83.mdns > 224.0.0.251.mdns: 0*- [0q] 2/0/3 (Cache flush)
 16:20:22.956932 IP6 fe80::2:8c40:fdea:5a16.mdns > ff02::fb.mdns: 0*- [0q] 2/0/3 (Cache flush) PTR local., (Cache flush) PTR local. (214) 
5 packets captured
 5 packets received by filter 
0 packets dropped by kernel

4. Option -s

tcpdump with -sXXX helps you control the capture size. On the second line in the previous output you can see it says capture size 262144 bytes, which is much larger than the packet. You can use -s to change the capture size. If you just want to inspect the packet headers, then you can use a smaller size for the capture. See the example below:

# tcpdump -c 5 -i any -n -s64
dropped privs to tcpdump
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on any, link-type LINUX_SLL (Linux cooked v1), capture size 64 bytes
16:24:39.909994 IP 10.22.3.80.46368 > 10.11.200.20.ldap: Flags [.], ack 2583785634, win 502, options [nop,nop,TS[|tcp]>
16:24:39.910118 IP 192.168.86.31.53013 > 66.187.232.72.https: UDP, length 76
16:24:39.981646 IP 192.168.86.111.mdns > 224.0.0.251.mdns: 0 [5a] [28q] [1n] [1au][|domain]
16:24:39.983954 IP 192.168.86.111.mdns > 224.0.0.251.mdns: 0*- [0q] 2/0/1[|domain]
16:24:40.186150 IP 192.168.86.111.mdns > 224.0.0.251.mdns: 0 [1n] [1au][|domain]
5 packets captured
6 packets received by filter
0 packets dropped by kernel

5. Port captures

tcpdump allows you to specify network packets that are either using some port X as source or destination. For example, to capture DNS traffic, you can use port 53. You could prefix the port keyword with src/dst as src port 53 or dst port 53 and filter it even further.

# tcpdump -i any port 53 -n
16:49:58.979410 IP 10.22.3.80.46391 > 10.11.5.19.domain: 31741+ A? youtube.com. (29)
16:49:58.979450 IP 10.22.3.80.46391 > 10.11.5.19.domain: 4579+ AAAA? youtube.com. (29)
16:49:58.985835 IP 10.11.5.19.domain > 10.22.3.80.44202: 8898 NXDomain 0/1/0 (154)
16:49:58.986761 IP 10.22.3.80.38074 > 10.11.5.19.domain: 43241+ PTR? 31.86.168.192.in-addr.arpa. (44)
16:49:59.015164 IP 10.11.5.19.domain > 10.22.3.80.38074: 43241 NXDomain 0/1/0 (122)
16:49:59.015209 IP 10.11.5.19.domain > 10.22.3.80.46391: 4579 1/0/0 AAAA 2607:f8b0:4004:810::200e (57)
16:49:59.015231 IP 10.11.5.19.domain > 10.22.3.80.46391: 31741 1/0/0 A 172.217.15.78 (45)
16:49:59.015831 IP 10.22.3.80.51955 > 10.11.5.19.domain: 2503+ PTR? 1.122.168.192.in-addr.arpa. (44)
16:49:59.041490 IP 10.11.5.19.domain > 10.22.3.80.51955: 2503 NXDomain 0/1/0 (122)

6. Option -w

If you want to write the output of tcpdump to a file, use the option -w .pcap to write to a file. If you want to see how many packages were written, you can add -v.

# tcpdump -c 4 -i any port 53 -w dns.pcap -v
tcpdump: data link type LINUX_SLL2
dropped privs to tcpdump
tcpdump: listening on any, link-type LINUX_SLL2 (Linux cooked v2), snapshot length 262144 bytes
4 packets captured
24 packets received by filter
0 packets dropped by kernel

DHCP on Linux

renew ip address - sudo dhclient -r

dmidecode -t memory

dmidecode -t memory

Shows status of installed memory

How to Change Passwords in Linux

https://www.tomshardware.com/how-to/change-passwords-in-linux

How to Change your Password in Linux

Any user can change their password at any time. It is good practice to frequently change your password, and to not reuse or rotate passwords.

1. Open a terminal.

2. Type in the passwd command to start the password change process. Passwd may look like a spelling mistake, but it is the command to work with passwords on the terminal.

$ passwd

3. Type in your current password and press Enter.

4. Type in your new password, press Enter. Then type it in again to confirm that it is correct. Note that Linux will not echo (print) the password to the screen, nor will it show any asterisk indicating password length.

Change Passwords in Linux — (Image credit: Tom's Hardware)

Change Another User's Password

If you are an administrator of many users, there will come a time where a user forgets their password and it will fall to you, or someone with sudo privileges to reset their password.

To change another user's password we use the same command, but specify the user’s name.

1. Open a terminal.

2. Type in the passwd command along with the user name. To use this command you will either need to be root, or be part of the “sudo” group. In the code example we assume that you are in the sudo group.

3. Change the user’s password, and confirm the change.

Force a Password Reset

You’ve heard that one of your users has been reusing their passwords; this is bad. So let’s give them a chance to change their password. Using the -e argument we can enforce a password reset by expiring their password.

1. Open a terminal.

2. Type in the command and pass the -e argument to expire Tom’s password.

$ sudo passwd -e tom

When Tom next logs in, the system will force Tom to change their password. Here we have simulated Tom logging in by using the “su” command to switch our user to Tom.

There may come a time when a user account will need to be temporarily locked, preventing them from logging in. For this we can use the -l argument to lock the account. Here we are locking Tom out of their account.

1. Open a terminal.

2. Use the command with the -l argument to lock Tom’s account.

$ sudo passwd -l tom

When Tom next tries to login, their password is rejected and they receive an authentication error. Here we again simulate it using su to switch users.

3. To unlock Tom’s account we use the -u switch.

$ sudo passwd -u tom

Exploring the passwd Command

The passwd command has a number of useful arguments (parameters) that we can pass when using the command. Here are some examples.

Checking the status of a user’s password is a useful tool for system administrators when it comes to audits and housekeeping. Here our test account, Tom, is audited.

Open a terminal and run the command with the -S switch. This will show the status of the account.

$ sudo passwd -S tom

The output is formatted to show

The username

Password status Locked (L), No Password (NP), Password (P)

Date of last password change

Minimum password age

Maximum password age

Warning period (the number of days given to the user to change their password before it expires)

Inactivity period (number of days after a password expires before it is locked)

If we wanted to set the number of days for Tom’s warning period to 14 days, we would use this command.

sudo passwd -w 14 tom

If you want to list the password status for all users, then the -a switch is just the thing. This switch is used with -S to list the status of every user, even the users that you never knew existed. These extra users are used for specific tasks, such as printers and networking.

Here we can see all the users on our test machine, but the two “real” users are Les and Tom.

How To Configure Firewall with UFW on Ubuntu 20.04 LTS

https://www.cyberciti.biz/faq/how-to-configure-firewall-with-ufw-on-ubuntu-20-04-lts/

How do I set up and configure firewall with UFW on Ubuntu 20.04 LTS server?

UFW is an acronym for an uncomplicated firewall. Securing a network with an uncomplicated firewall is super easy and highly recommended. This page explains how to set up and secure your Ubuntu 20.04 LTS server with ufw.

Tutorial requirements
Requirements	Ubuntu Linux 20.04 LTS
Root privileges	Yes
Difficulty level	Easy
Category	Firewall
Prerequisites	ufw command
Est. reading time	7 minutes
Table of contents ↓ 1 Set up ufw policy 2 Open SSH port 3 Turn on ufw firewall 4 Open ports with ufw 5 Block ports with ufw 6 Get ufw firewall status 7 Delete ufw firewall rules 8 Firewall management commands 9 IP Masquerading 10 Egress filtering 11 Conclusion

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Step 1 – Set Up default UFW policies

To view status of ufw, type:
$ sudo ufw status
Sample outputs:

Status: inactive

The default policy firewall works out great for both the servers and desktop. It is always a good policy to closes all ports on the server and open only required ports one by one. Let us block all incoming connection and only allow outgoing connections from the Ubuntu 20.04 LTS box:
$ sudo ufw default allow outgoing $ sudo ufw default deny incoming

Enabling IPv6 support

Make sure the directive IPV6=yes do exists in /etc/default/ufw file. For instance:
$ cat /etc/default/ufw

Step 2 – Open SSH TCP port 22 connections

The next logical step is to allow incoming SSH ports. We can easily open SSH TCP port 22 using UFW as follows:
$ sudo ufw allow ssh
If you are running ssh on TCP port 2222 or TCP port 2323, enter:
$ sudo ufw allow 2222/tcp $ sudo ufw allow 2323/tcp
Some sysadmins have a static IP address (such as 202.54.2.5) at home or office location. In that case, only allow ssh access from the static IP address such as 202.54.2.5 to Ubuntu server IP address 172.24.13.45:
$ sudo ufw allow proto tcp from 202.54.2.5 to 172.24.13.45 port 22
But how do I find out my static IP 202.54.2.5 on Ubuntu server itself? Try the w command or lastlog command:
$ w $ lastlog -u {YOUR_ADMIN_LOGIN_NAME_HERE} $ lastlog -u vivek
And here is what I see:

Username         Port     From             Latest
vivek            pts/0    202.54.2.5       Thu Sep 29 15:19:21 +0000 2022

Next, let us limit ssh port, run:
$ sudo ufw limit ssh
See “How to limit SSH (TCP port 22) connections with ufw on Ubuntu Linux” for more information.

Step 3 – Turn on firewall

Now we got basic configuration enabled. In other words, the firewall will drop all incoming traffic except for ssh TCP port 22. Let us true it on the UFW, enter:
$ sudo ufw enable
Ubuntu Configure Firewall with UFW
Remember, once UFW enabled, it runs across system reboots too. We can verify that easily as follows using the systemctl command:
$ sudo systemctl status ufw.service
How to Configure Firewall with UFW on Ubuntu 20.04

Want to disable the UFW based firewall? Try

If you need to stop the firewall and disable on system startup, eenter:
$ sudo ufw disable
Sample outputs:

Firewall stopped and disabled on system startup

Step 4 – Open specific incoming connections/ports

Let us add more rules. Say you want to open ports and allow IP address with ufw. The syntax is as follows to open TCP port 80 and 443:
$ sudo ufw allow 80/tcp comment 'accept Apache' $ sudo ufw allow 443/tcp comment 'accept HTTPS connections'
Open UDP/1194 (OpenVPN) server:
$ sudo ufw allow 1194/udp comment 'OpenVPN server'

Allow port ranges via ufw

We can allow port ranges too say, tcp and udp 3000 to 4000:
$ sudo ufw allow 3000:4000/tcp $ sudo ufw allow 3000:4000/udp
In this example, you want to allow ALL connections from an IP address called 104.22.10.214, enter:
$ sudo ufw allow from 104.22.10.214
Let us allow connections from an IP address called 104.22.11.213 to our port 25, enter:
$ sudo ufw allow from 104.22.11.213 to any port 25 proto tcp
We can set dest IP 222.222.222.222 for port 25 too:
$ sudo ufw allow from 104.22.11.213 to 222.222.222.222 port 25 proto tcp

Allow connection on specific interface

Open port 22 for wg0 interface only:
$ sudo ufw allow in on wg0 to any port 22
Say you want to allow connection for TCP port 3306 on lxdbr0 interface from 10.105.28.22, then add:
$ sudo ufw allow in on lxdbr0 from 10.105.28.22 to any port 3306 proto tcp
Let us add sub/net instead of single IP address:
$ sudo ufw allow in on lxdbr0 from 10.105.28.0/24 to any port 3306 proto tcp

Step 5 – Block and deny incoming connections/ports

Do you want to close ports and block certain IP addresses? The syntax is as follows to deny access. In other words, simply ignoring access to port 25:
$ sudo ufw deny 25/tcp
Make sure we deny all connections from an IP address called 203.5.1.43, enter:
$ sudo ufw deny from 203.5.1.43
Deny all connections from an IP/subnet called 103.13.42.13/29, enter:
$ sudo ufw deny from 103.13.42.13/29
Want to deny access to 1.1.1.2 (say bad guys IP) on port 22? Try:
$ sudo ufw deny from 1.1.1.2 to any port 22 proto tcp

Step 6 – Verify status of UFW

Use the status command as follows:
$ sudo ufw status

Status: active
 
To                         Action      From
--                         ------      ----
22/tcp                     ALLOW       Anywhere                  
80/tcp                     ALLOW       Anywhere                   # accept Apache
443/tcp                    ALLOW       Anywhere                   # accept HTTPS connections
1194/udp                   ALLOW       Anywhere                   # OpenVPN server
3000:4000/tcp              ALLOW       Anywhere                  
3000:4000/udp              ALLOW       Anywhere                  
Anywhere                   ALLOW       104.22.10.214             
25/tcp                     ALLOW       104.22.11.213             
222.222.222.222 25/tcp     ALLOW       104.22.11.213             
Anywhere                   DENY        203.5.1.43                
Anywhere                   DENY        103.13.42.8/29            
22/tcp                     DENY        1.1.1.2                   
22/tcp (v6)                ALLOW       Anywhere (v6)             
80/tcp (v6)                ALLOW       Anywhere (v6)              # accept Apache
443/tcp (v6)               ALLOW       Anywhere (v6)              # accept HTTPS connections
1194/udp (v6)              ALLOW       Anywhere (v6)              # OpenVPN server
3000:4000/tcp (v6)         ALLOW       Anywhere (v6)             
3000:4000/udp (v6)         ALLOW       Anywhere (v6)

Want verbose outputs? Try:
$ sudo ufw status verbose

Ubuntu 20.04 LTS UFW delete rules

So far we learned how to add, deny, and list the firewall rules. It is time to delete unwanted rules. The syntax is as follows to list all of the current rules in a numbered list format:
$ sudo ufw status numbered

Status: active
 
     To                         Action      From
     --                         ------      ----
[ 1] 22/tcp                     ALLOW IN    Anywhere                  
[ 2] 80/tcp                     ALLOW IN    Anywhere                   # accept Apache
[ 3] 443/tcp                    ALLOW IN    Anywhere                   # accept HTTPS connections
[ 4] 1194/udp                   ALLOW IN    Anywhere                   # OpenVPN server
[ 5] 3000:4000/tcp              ALLOW IN    Anywhere                  
[ 6] 3000:4000/udp              ALLOW IN    Anywhere

To delete 6th rule type the command:
$ sudo ufw delete 6 $ sudo ufw status numbered

How to Install NTP Server and Client(s) on Ubuntu 22.04 LTS

https://vitux.com/how-to-install-ntp-server-and-client-on-ubuntu/

NTP or Network Time Protocol is a protocol that is used to synchronize all system clocks in a network to use the same time. When we use the term NTP, we are referring to the protocol itself and also the client and server programs running on the networked computers. NTP belongs to the traditional TCP/IP protocol suite and can easily be classified as one of its oldest parts.

When you are initially setting up the clock to sync with NTP, it takes six exchanges within 5 to 10 minutes before the clock is set up. Once the clocks in a network are synchronized, the client(s) update their clocks with the server once every 10 minutes. This is usually done through a single exchange of messages (transaction). These transactions use port number 123 of your system.

In this article, we will describe a step-by-step procedure on how to:

Install and configure the NTP server on a Ubuntu machine.
Configure the NTP Client to be time synced with the server.

We have run the commands and procedures mentioned in this article on a Ubuntu 22.04 LTS system.

Install and configure NTP Server on the host computer

Follow these steps to install the NTP server on your Ubuntu server machine:

Note: We are using the Ubuntu command line, the Terminal, to install and configure NTP. You can open the Terminal application either through the application launcher search or the Ctrl+Alt+T shortcut.

Step 1: Update the package repository index

To install the latest software versions from the Internet repositories, your local repository index must align with them. Run the following command as sudo in order to update your local repository index:

$ sudo apt update

Step 2: Install NTP Server with apt-get

Please run the following command as sudo in order to install the NTP server daemon from the APT repositories:

$ sudo apt install ntp

Please note that only an authorized user can add, remove and configure software on Ubuntu.

The system might ask you for the sudo password and provide you with a Y/n option to continue the installation. Enter Y and then hit enter; the NTP server will then be installed on your system. The process may, however, take some time, depending on your Internet speed.

Step 3: Verify installation (optional)

You can verify your NTP installation and also check the version number by running the following command in your Terminal:

$ sntp --version

Step 4: Switch to an NTP server pool closest to your location

When you install the NTP server, it is mostly configured to fetch the proper time. However, you can switch the server pool to the ones closest to your location. This includes making some changes in the /etc/ntp.conf file.

Open the file in the nano editor as sudo by running the following command:

$ sudo nano /etc/ntp.conf

In this file, you will be able to see a pool list. We have highlighted this list in the above image. The task here is to replace this pool list by a pool of time servers closest to your location. The pol.ntp.org project provides reliable NTP service from a big cluster of time servers. To choose a pool list according to your location, visit the following page:

https://support.ntp.org/bin/view/Servers/NTPPoolServers

We have searched for a pool list for the US:

The page tells us to add the following lines to the ntp.conf file:

server 0.us.pool.ntp.org
server 1.us.pool.ntp.org
server 2.us.pool.ntp.org
server 3.us.pool.ntp.org

This is how my file looks after adding the above lines to it:

Quit the file by hitting Ctrl+X and entering y to save changes.

Step 5: Restart the NTP server

For the above changes to take effect, you need to restart the NTP server. Run the following command as sudo to do so:

$ sudo service ntp restart

Step 6: Verify that the NTP Server is running

Now, check the status of the NTP service through the following command:

$ sudo service ntp status

The Active status verifies that your NTP server is up and running.

Step 7: Configure Firewall so that client(s) can access the NTP server

Finally, it is time to configure your system’s UFW firewall so incoming connections can access the NTP server at UDP Port number 123.

Run the following command as sudo to open port 123 for incoming traffic:

$ sudo ufw allow from any to any port 123 proto udp

Your Ubuntu host machine is now configured as an NTP server.

Configure NTP Client to be Time Synced with the NTP Server

Let us now configure our Ubuntu client machine to be time-synchronized with the NTP server.

Step 1: Install ntpdate

The ntpdate command will let you manually check your connection configuration with the NTP-server. Open the Terminal application on the client machine and enter the following command as sudo:

$ sudo apt-get install ntpdate

Step 2: Specify IP and hostname of the NTP server in the hosts file

For your NTP server to be resolved by a hostname in your client machine, you need to configure your /etc/hosts file.

Open the hosts file as sudo in the nano editor by entering the following command:

$ sudo nano /etc/hosts

Now add your NTP server’s IP and specify a hostname as follows in this file:

Quit the file by hitting Ctrl+X and then save it by entering y.

Step 3: Check if the client machine’s time is synchronized with an NTP server

The following ntpdate command will let you manually check if time is synchronized between the client and server systems:

$ sudo ntpdate NTP-server-host

The output should ideally show a time offset between the two systems.

Step 4: Disable the systemd timesyncd service on the client

Because we want our client to sync time with the NTP server, let us disable the timesyncd service on the client machine.

Enter the following command to do so:

Step 5: Install NTP on your client

Run the following command as sudo in order to install NTP on your client machine:

$ sudo apt-get install ntp

Step 6: Configure the /etc/ntp.conf file to add your NTP server as the new time server

Now we want our client machine to use our own NTP host server to be used as the default time server. For this, we need to edit the /etc/ntp.conf file on the client machine.

Run the following command as sudo in order to open the file in the Nano editor:

$ sudo nano /etc/ntp.conf

Then, add the following line in the file, where NTP-server-host is the hostname you specified for your NTP server:

server NTP-server-host prefer iburst

This is how my file looks like after I have specified the time server:

Hit Ctrl+x to close the file and enter y to save the changes.

Step 7: Restart the NTP server

In order for the above changes to take effect, you need to restart the NTP service. Run the following command as sudo to do so:

$ sudo service ntp restart

Step 8: View the Time Synchronization Queue

Now your client and server machines are configured to be time-synced. You can view the time synchronization queue by running the following command:

$ ntpq -ps

You should be able to see NTP-server-host as the time synchronization host/source in the queue.

So this was all you needed to know about installing and configuring NTP to synchronize time on your networked Ubuntu machines. The process may seem a little cumbersome, but if you follow all of the above steps carefully, your machines will be synced in no time.

How to Set Time, Timezone and Synchronize System Clock in Linux

The ‘timedatectl‘ command is a new utility for RHEL-based and Debian-based distributions. It is a part of the ‘systemd‘ system and service manager, serving as a replacement for the old traditional date command used in ‘sysvinit‘ daemon-based Linux distributions.

The timedatectl command allows you to query and change the configuration of the system clock and its settings, you can use this command to set or change the current date, time, and timezone or enable automatic system clock synchronization with a remote NTP server.

In this tutorial, am going to take you through the ways you can manage time on your Linux system by setting the date, time, timezone, and synchronize time with NTP from the terminal using the new timedatectl command.

It is always a good practice to maintain the correct time on your Linux server or system and it can have the following advantages:

maintain a timely operation of system tasks since most tasks in Linux are controlled by time.
the correct time for logging events and other information on the system and many more.

How to Find and Set Local Timezone in Linux

1. To display the current time and date on your system, use the timedatectl command from the command line as follows:

# timedatectl  status

In the screencast above, RTC time is the hardware clock time.

2. The time on your Linux system is always managed through the timezone set on the system, to view your current timezone, do it as follows:

# timedatectl 
OR
# timedatectl | grep Time

3. To view all available timezones, run the command below:

# timedatectl list-timezones

4. To find the local timezone according to your location, run the following command:

# timedatectl list-timezones |  egrep  -o "Asia/B.*"
# timedatectl list-timezones |  egrep  -o "Europe/L.*"
# timedatectl list-timezones |  egrep  -o "America/N.*"

5. To set your local timezone in Linux, we will use the set-timezone switch as shown below.

# timedatectl set-timezone "Asia/Kolkata"

It is always recommended to use and set the coordinated universal time, UTC.

# timedatectl set-timezone UTC

You need to type the correct name timezone otherwise you may get errors when changing the timezone, in the following example, the timezone “Asia/Kolkata” is not correct therefore causing the error.

How to Set Time and Date in Linux

6. You can set the date and time on your system, using the timedatectl command as follows:

Set Time in Linux

To set time only, we can use a set-time switch along with the format of time in HH:MM:SS (Hour, Minute, and Seconds).

# timedatectl set-time 15:58:30

You may get the below error when setting the date as shown above:

Failed to set time: NTP unit is active

7. The error says that the NTP service is active. You need to disable it using the below command.

# systemctl disable --now chronyd

Set Date and Time in Linux

8. To set date and time, we can use a set-time switch along with the format of date in YY:MM:DD (Year, Month, Day) and time in HH:MM:SS (Hour, Minute, and Seconds).

# timedatectl set-time '2015-11-20 16:14:50'

How to Find and Set Hardware Clock in Linux

9. To set your hardware clock to coordinated universal time, UTC, use the set-local-rtc boolean-value option as follows:

First Find out if your hardware clock is set to the local timezone:

# timedatectl | grep local

Set your hardware clock to the local timezone:

# timedatectl set-local-rtc 1

Set your hardware clock to coordinated universal time (UTC):

# timedatectl set-local-rtc 0

Set Hardware Clock to UTC — Set the Hardware Clock to UTC

Synchronizing Linux System Clock with a Remote NTP Server

NTP stands for Network Time Protocol is an internet protocol, which is used to synchronize the system clock between computers. The timedatectl utility enables you to automatically sync your Linux system clock with a remote group of servers using NTP.

Please note that you must have NTP installed on the system to enable automatic time synchronization with NTP servers.

To start automatic time synchronization with a remote NTP server, type the following command at the terminal.

# timedatectl set-ntp true

To disable NTP time synchronization, type the following command at the terminal.

# timedatectl set-ntp false

How to Symlink a Directory in Linux

Symlink, also known as a symbolic link in Linux, creates a link to a file or a directory for easier access. To put it in another way, symlinks are links that points to another file or folder in your system, quite similar to the shortcuts in Windows. Some users refer to symlinks as soft-links. Before moving forward, let’s elaborate soft-links and hard-links.

Hard-links: Hard-links are the links that mirror or copy the original file. Hard-links have the same inode numbers.

Soft-links: Soft-links are simple links that points to the original file. You can access the original file through soft links. Soft-links can point to a file or folder in any partition and have different inode numbers.

Learning about creating symlink in Linux is a great way to improve your grip on the Linux terminal. So, let’s learn the steps involved in making the soft-links in Linux.

How to Create Symlink (soft-link) in Linux

To make symlink or soft link, we use the “ln” command. The syntax to follow to create symlink is mentioned below:

$ ln -s [path of the target file/directory] [symbolic name]

In the first argument after the “-s” option, you will be giving the path of the file of a folder you want to create the symlink of. While in the second argument, pass the name you want to give that symlink. To check the created links, use the following command:

$ ls -l

To check inode numbers, use the command mentioned below:

$ ls -i

How to Create a Symlink (soft link) to a File

Creating a soft link to a file is simple; use the syntax mentioned below:

$ ln -s [path of the target file] [symbolic name]

Important to note that if you do not specify the “[symbolic name]”, then the command will create a symlink by the original file’s name. Let’s understand it through an example.

I have created a directory “my_folder” that contains a text file “my_doc.txt”. Now, to create symlink to “my_doc.txt” file, I will use:

$ ln -s my_folder/my_doc.txt my_document

To verify it, use:

$ ls -l

As it can be seen in the above output, “my_document” is pointing to “my_folder/my_doc.txt” file. Both the symlink and the original file would have different inode number. To check inode numbers used:

$ ls -i

Hard links will always have same inode numbers. To verify, I created a hard link of “my_doc.txt” file and name it “my_document_2”:

It can be seen in the output that the original file and the hard link have same inode numbers.

How to Create a Symlink (Soft Link) of the Folder/Directory

To create a soft-link or symlink to a directory is quite similar to creating a symlink to a file. For instance, I am creating the symlink of the “my_folder” directory using:

$ ln -s my_folder my_doc_folder

The above command will create a symlinked folder in the current directory. To verify it, use:

$ ls -l

Now, check inode numbers:

$ ls -i

How to Overwrite the Symlink (Soft Link) in Linux:

If you try to update a symlink with the same name that already exist, then you will get an error:

$ ln -s my_folder_2/my_doc_2.txt my_document

We will have to use the force flag “-f” to overwrite the new path to the existing symlink.

$ ln -sf my_folder_2/my_doc_2.txt my_document

How to Remove Symlink (Soft Link) in Linux:

In many situation, you need to remove the unnecessary symlinks from your system. To delete symlink, we use the “unlink” command, and the syntax is given below:

$ unlink [symlink name]

Let’s remove the symlinks we created in the above examples. To unlink a symlink of a file, use:

$ unlink my_document

And to unlink the symlink of a directory:

$ unlink my_doc_folder

We can also use the “rm” command to remove symlinks.

$ rm my_document my_doc_folder

The advantage of “rm” over “unlink” is that you can remove multiple symlinks with the “rm” command, which is not possible with the “unlink” command as shown in the following image:

Note that whether you use the “unlink” or “rm” command, do not use trailing slash “/” even if it is a directory.

LN Command - Create Links

https://linuxize.com/post/how-to-create-symbolic-links-in-linux-using-the-ln-command/

Links Types

There are two types of links in Linux/UNIX systems:

Hard links. You can think a hard link as an additional name for an existing file. Hard links are associating two or more file names with the same inode . You can create one or more hard links for a single file. Hard links cannot be created for directories and files on a different filesystem or partition.
Soft links. A soft link is something like a shortcut in Windows. It is an indirect pointer to a file or directory. Unlike a hard link, a symbolic link can point to a file or a directory on a different filesystem or partition.

How to Use the `ln` Command

ln is a command-line utility for creating links between files. By default, the ln command creates hard links. To create a symbolic link, use the -s (--symbolic) option.

The ln command syntax for creating symbolic links is as follows:

ln -s [OPTIONS] FILE LINK

If both the FILE and LINK are given, ln will create a link from the file specified as the first argument (FILE) to the file specified as the second argument (LINK).
If only one file is given as an argument or the second argument is a dot (.), ln will create a link to that file in the current working directory . The name of the symlink will be the same as the name of the file it points to.

By default, on success, ln doesn’t produce any output and returns zero.

Creating Symlink To a File

To create a symbolic link to a given file, open your terminal and type:

ln -s source_file symbolic_link

Replace source_file with the name of the existing file for which you want to create the symbolic link and symbolic_link with the name of the symbolic link.

The symbolic_link parameter is optional. If you do not specify the symbolic link, the ln command will create a new link in your current directory:

In the following example, we are creating a symbolic link named my_link.txt to a file named my_file.txt:

ln -s my_file.txt my_link.txt

To verify that the symlink was successfully created, use the ls command:

ls -l my_link.txt

The output will look something like this:

lrwxrwxrwx 1 linuxize users  4 Nov  2 23:03  my_link.txt -> my_file.txt

The l character is a file type flag that represents a symbolic link. The -> symbol shows the file the symlink points to.

Creating Symlinks To a Directory

The command for creating a symbolic link to a directory is the same as when creating a symbolic link to a file. Specify the directory name as the first parameter and the symlink as the second parameter.

For example, if you want to create a symbolic link from the /mnt/my_drive/movies directory to the ~/my_movies directory you would run:

ln -s /mnt/my_drive/movies ~/my_movies

Overwriting Symlinks

If you try to create a symbolic link that already exists , the ln command will print an error message.

ln -s my_file.txt my_link.txt

ln: failed to create symbolic link 'my_link.txt': File exists

To overwrite the destination path of the symlink, use the -f (--force) option.

ln -sf my_file.txt my_link.txt

Removing Symlinks

To delete/remove symbolic links use either the unlink or rm command.

The syntax of the unlink is very simple:

unlink symlink_to_remove

Removing a symbolic link using the rm command is the same as when removing a file:

rm symlink_to_remove

No matter which command you use, when removing a symbolic link not append the / trailing slash at the end of its name.

If you delete or move the source file to a different location, the symbolic file will be left dangling (broken) and should be removed.

Sysadmins Networking Cheat Sheet

Using Cert Bot to get a Valid SSL certificate

This guide will detail how to get a valid SSL cert using certbot on Linux operating systems

Certbot user guide

Commands for defining Key Type

https://www.onepagezen.com/letsencrypt-auto-renew-certbot-apache/

Configuration file

https://eff-certbot.readthedocs.io/en/stable/using.html#config-file

Follow this guide to force Cerbot to use Elliptical Curve Diffe-Hellman Curves for all certificates. Any existing certs will be updated upon the next reboot.

Certbot accepts a global configuration file that applies its options to all invocations of Certbot. Certificate specific configuration choices should be set in the .conf files that can be found in /etc/letsencrypt/renewal.

By default no cli.ini file is created (though it may exist already if you installed Certbot via a package manager, for instance). After creating one it is possible to specify the location of this configuration file with certbot --config cli.ini (or shorter -c cli.ini). An example configuration file is shown below:

# This is an example of the kind of things you can do in a configuration file.
# All flags used by the client can be configured here. Run Certbot with
# "--help" to learn more about the available options.
#
# Note that these options apply automatically to all use of Certbot for
# obtaining or renewing certificates, so options specific to a single
# certificate on a system with several certificates should not be placed
# here.

# Use ECC for the private key
key-type = ecdsa
elliptic-curve = secp384r1

# Use a 4096 bit RSA key instead of 2048
rsa-key-size = 4096

# Uncomment and update to register with the specified e-mail address
# email = foo@example.com

# Uncomment to use the standalone authenticator on port 443
# authenticator = standalone

# Uncomment to use the webroot authenticator. Replace webroot-path with the
# path to the public_html / webroot folder being served by your web server.
# authenticator = webroot
# webroot-path = /usr/share/nginx/html

# Uncomment to automatically agree to the terms of service of the ACME server
# agree-tos = true

# An example of using an alternate ACME server that uses EAB credentials
# server = https://acme.sectigo.com/v2/InCommonRSAOV
# eab-kid = somestringofstuffwithoutquotes
# eab-hmac-key = yaddayaddahexhexnotquoted

If on the internal network set the DNS on the machine to use CloudFlared or Google. DNS will not resolve properly if using the internal DNS servers

install the package certbot sudo apt install certbot -y
navigate to the /etc/letsencrypt directory
Two types on install Manual and Auto
1. sudo certbot certonly --manual --preferred-challenges dns -d "*.coltscomputer.services"
  1. this will pull a SSL cert from Let's Encrypt
2. Go to the DNS hosted domain on Route 53 and add the _acme-challenge text to the existing _acme-challenge TXT record
For the auto install
1. This guide shows how to configure a virtual host
  1. cd /etc/apache2/sites-enabled
  2. edit the .conf files there for the necessary domain name
    1. sudo nano *.conf
    2. edit the YOUR-DOMAIN-NAME line
2. sudo certbot certonly --apache --key-type ecdsa --preferred-challenges dns -d "*.coltscomputer.services"
  1. This should auto renew
  2. use sudo certbot renew --dry-run to test if the renewal will work automatically

Using Netplan to set a static IP address

https://www.linux.com/topic/distributions/how-use-netplan-network-configuration-tool-linux/

Match the netplan configuration to the below
run sudo chmod 600 /etc/netplan/your_config_file.yaml
run sudo netplan try
- if there are no errors accept the changes

Using S.M.A.R.T to check disk health

https://linuxconfig.org/how-to-check-an-hard-drive-health-from-the-command-line-using-smartctl

smartctl has to be run as sudo or root user

smartctl -a /dev/sda shows all smart data from a drive

Run the following tests in the scenarios

short test
- The short test is meant to quickly check the most common problems that could be found on a storage device. The test should take no more than 10 minutes: mechanical, electrical and read performances of a disk are checked.
long test
- The long test is basically a more accurate version of the “short” test. In can take a lot of time to complete: as stated in the the smartctl manual, it can last from tens of minutes to several hours.
conveyance test
- The conveyance test is meant to check for possible damages occurred during the transportation of the device. It usually takes minutes to complete a conveyance test. It is available only on ATA devices.
select test
- The select test, like the “conveyance” one, is available only on ATA devices, and is meant to check only the specified range of LBAs (Logical Block Addresses). The range of addresses is specified when launching the test. For example, to check addresses from 10 to 20 (inclusive), we would run:
- smartctl -t select,10-20

-t option is for running a test immediately and specifies the test type

short for --test

Show used disk space on Linux

sudo du -h -x -d1 /

Clean Archive Cache

Run this after doing an apt upgrade

sudo apt-get clean

Expand LVM Volume

Expanding the Logical Volume

Check Available Space: Before expanding, check how much free space is available in the volume group.
1. vgdisplay
Resize the Logical Volume: To extend the logical volume, use the following command. Replace <size> with the amount you want to add (e.g., 10G for 10 GB):
1. sudo lvextend -L +<size> /dev/mapper/ubuntu--vg-ubuntu--lv
Resize the Filesystem: After resizing the logical volume, you need to resize the filesystem to use the new space:
1. For ext4 filesystem:
  1. sudo resize2fs /dev/mapper/ubuntu--vg-ubuntu--lv
2. For XFS filesystem:
  1. sudo xfs_growfs /dev/mapper/ubuntu--vg-ubuntu--lv

Shrinking the Logical Volume

Check Filesystem Size: Before shrinking, ensure the filesystem is smaller than the logical volume you want to resize it to. You might need to reduce the filesystem size first.
1. For ext4 filesystem:
  1. sudo resize2fs /dev/mapper/ubuntu--vg-ubuntu--lv <new_size>
2. For example, to shrink to 20G:
  1. sudo resize2fs /dev/mapper/ubuntu--vg-ubuntu--lv 20G
Reduce the Logical Volume: Now you can safely reduce the logical volume:
1. sudo lvreduce -L <new_size> /dev/mapper/ubuntu--vg-ubuntu--lv
Resize the Filesystem Again (if necessary): If you haven’t already resized the filesystem in the first step, you can do it now.

Ubuntu 20.04: How to Extend Partition (For Noobs)

https://forum.cloudron.io/topic/6086/ubuntu-20-04-how-to-extend-partition-for-noobs/2

Our Ubuntu 20 is installed on Hyper-V. First, let’s check the size of the file system by running df -h (all commands are executed as root):

# df -h
Filesystem           Size  Used Avail Use% Mounted on
tmpfs            394M         1,4M  392M            1% /run
/dev/sda3         24G         7,4G   16G           33% /
tmpfs            2,0G            0  2,0G            0% /dev/shm
tmpfs            5,0M            0  5,0M            0% /run/lock
tmpfs            4,0M            0  4,0M            0% /sys/fs/cgroup
/dev/sda2        512M         7,8M  505M            2% /boot/efi
tmpfs            394M         144K  394M            1% /run/user/1000

Our system partition, mounted in /, is 24 GB in size.

Let’s see the output fdisk -l:

# fdisk -l
Disk /dev/sda: 25 GiB, 26843545600 bytes, 52428800 sectors
Disk model: Virtual Disk
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disklabel type: gpt
Disk identifier: F5F02D9D-060D-422F-BA27-1981A6CA23F4

Device       Start      End  Sectors  Size Type
/dev/sda1     2048     4095     2048    1M BIOS boot
/dev/sda2     4096  1054719  1050624  513M EFI System
/dev/sda3  1054720 52426751 51372032 24.5G Linux filesystem

In this example, we have 1 disk /dev/sda with a size of 25 gigabytes, which is divided into 3 logical ones: /dev/sda1, /dev/sda2 and /dev/sda3 with the Linux filesystem type – we are interested in it.

Increasing disk size

In a virtualization environment, we increase the size of the hard disk of our virtual machine. Most likely, your virtualization system will ask you to turn off the VM first. I increased the disk to 30 gigabytes, start the machine:

#fdisk -l
Disk /dev/sda: 30 GiB, 32212254720 bytes, 62914560 sectors
Disk model: Virtual Disk
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disklabel type: gpt
Disk identifier: F5F02D9D-060D-422F-BA27-1981A6CA23F4

Device       Start      End  Sectors  Size Type
/dev/sda1     2048     4095     2048    1M BIOS boot
/dev/sda2     4096  1054719  1050624  513M EFI System
/dev/sda3  1054720 52426751 51372032 24.5G Linux filesystem

Ubuntu 20 extend partition
Attention! Before starting work on expanding the system partition, be sure to make a backup copy of your data!

After increasing the size of the disk, you need to increase the system partition itself. Let’s execute fdisk /dev/sda, where /dev/sda is the label of our disk (Disk /dev/sda):

# fdisk /dev/sda

Welcome to fdisk (util-linux 2.32.1).
Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.

Command (m for help):

We enter p to look at the list of partitions:

Command (m for help): p

Disk /dev/sda: 30 GiB, 32212254720 bytes, 62914560 sectors
Disk model: Virtual Disk
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disklabel type: gpt
Disk identifier: F5F02D9D-060D-422F-BA27-1981A6CA23F4

Device       Start      End  Sectors  Size Type
/dev/sda1     2048     4095     2048    1M BIOS boot
/dev/sda2     4096  1054719  1050624  513M EFI System
/dev/sda3  1054720 52426751 51372032 24.5G Linux filesystem

To expand a partition, you must first delete information about it. To do this, enter d and specify the partition (3 for /dev/sda3):

Command (m for help): d
Partition number (1-3, default 3): 3

Partition 3 has been deleted.

In this case, only the record about the partition is deleted, the data itself remains on the disk!

Enter n – creating a new partition

Command (m for help): n

Next, we indicate the number of the partition:

Partition number (3-128, default 3):3

Next, the starting and ending sectors are indicated. Be sure to check that they match the hyphenated values. This way we use all the unallocated space:

First sector (1054720-62914526, default 1054720): 1054720
Last sector, +/-sectors or +/-size{K,M,G,T,P} (1054720-62914526, default 62914526): 62914526

Created a new partition 3 of type 'Linux filesystem' and of size 29.5 GiB.

As you can see, a 29.5 gigabyte partition was created with the Linux filesystem type.

It will also ask if we want to delete the current filesystem type. We refuse:

Partition #3 contains a ext4 signature.

Do you want to remove the signature? [Y]es/[N]o: N

It remains only to save the partition table:

Command (m for help): w

Reboot the virtual machine:

# reboot

Now we will use the resize2fs utility (for ext4) to increase the size of the filesystem:

# resize2fs /dev/sda3

resize2fs 1.45.6 (20-Mar-2020)
open: Permission denied while opening /dev/sda3
n0mit@n0mit-vm:~$ sudo resize2fs /dev/sda3
resize2fs 1.45.6 (20-Mar-2020)
Filesystem at /dev/sda3 is mounted on /; on-line resizing required
old_desc_blocks = 4, new_desc_blocks = 4
The filesystem on /dev/sda3 is now 7732475 (4k) blocks long.

Checking the result:

# df -h
Filesystem      Size  Used Avail Use% Mounted on
tmpfs           394M  1.4M  392M   1% /run
/dev/sda3        29G  7.4G   21G  27% /
tmpfs           2.0G     0  2.0G   0% /dev/shm
tmpfs           5.0M     0  5.0M   0% /run/lock
tmpfs           4.0M     0  4.0M   0% /sys/fs/cgroup
/dev/sda2       512M  7.8M  505M   2% /boot/efi
tmpfs           394M   84K  394M   1% /run/user/126
tmpfs           394M   72K  394M   1% /run/user/1000

As you can see in Ubuntu 20 extend artition is not such a difficult task.

Live resize root partition on Linux Server (VM)

https://www.clouvider.com/knowledge_base/live-resize-root-partition-on-linux-server/

Most virtual hypervisors (like Proxmox or VMWare) allow resizing VM storage on the fly.
However, if you don’t want to reboot your server while resizing, you can resize your root partition “live”.

Assuming your drive has been already resized on the hypervisor side (e.g. from 20GB to 40GB), you need to follow the following steps to ensure the OS knows about the increased storage capacity.

Step 1

Check your current partition size and drive by using df -h and fdisk -l /dev/sda command (assuming your drive is sda):

As you can see, the root partition / and the drive total capacity are around 20GB.

Step 2

Now you need to rescan your block device sda with the following command:

$ echo 1 > /sys/block/sda/device/rescan

And check again drive capacity by using fdisk -l /dev/sda command:

Now your drive has 40GB capacity, but your root partition still uses only 20GB. To resize the root partition, you need to run fdisk /dev/sda:

$ fdisk /dev/sda

Now press “p” to print the current partition layout.

This operation can be dangerous, so please follow the following steps with caution!

Step 3

Now delete your primary partition /dev/sda1 by pressing the d key.

Step 4

Create a new partition by pressing the n key and pressing 1 for the partition number.

Step 5

Use the default value of Fist sector and Last sector

Step 6

Do not remove the existing signature! Press n to avoid removal.

Step 7

: Now press the w key to save the partition table.

Step 8

Type resize2fs / command and check your current root patition capacity by using df -h:

$ resize2fs /
$ df -h

Your root partition / has been now resized.

Mount SMB Share on boot

How to enable Swap on Linux

https://azdigi.com/blog/en/linux-server-en/linux-fundementals/how-to-enable-swap-on-linux/

Step 1: SSH into your Linux system

To create and enable SWAP on Linux, we first need to SSH or access your VPS or server with root privileges. If you don’t know how to SSH into your VPS/Server, you can refer to the following tutorial:

After successfully SSH, we continue with step 2 to check Swap .

Step 2: Check Swap

Before proceeding to create the Swap file, we need to check if the current system has Swap enabled or not by running the following command:

AZDIGI Tutorial

swapon -s

If after running the command no information is displayed, it means that our server has not enabled Swap.

how-to-enable-swap-on-linux — *The system has not enabled **Swap**.*

Step 3: Check the free disk space

Check the free disk space so we can consider creating Swap file space in the next step. To check the free disk space, we use the following command:

AZDIGI Tutorial

df -h

My free space is up to 31GB, so it’s quite comfortable to create Swap. Because after creating Swap, your server capacity will be lost corresponding to the amount you have allocated for Swap.

Step 4: Create the Swap file

For my server with only 4GB of RAM, I will create a Swap partition from 1GB to 2GB is suitable.

Here I will use the dd command to create a Swap file with a capacity of 2GB.

AZDIGI Tutorial

dd if=/dev/zero of=/mnt/swapfile bs=1024 count=2048k

The above command will create a 2GB Swap file. You can change count=2048k to count=1024k … to create a 1GB Swap. The maximum Swap capacity should only be twice the physical RAM.

After we have created the Swap file, we need to create a partition for it in step 5.

Step 5: Create Swap Partition and enable Swap on Linux

To create a Swap partition, run the following command:

AZDIGI Tutorial

mkswap /mnt/swapfile

Then we continue to enable Swap on Linux with the command below:

AZDIGI Tutorial

swapon /mnt/swapfile

You will get a warning that the permission should be0600 to make this partition more secure because by default when initializing the directory, it will have permission 0644 . Please ignore this because we will perform the decentralization in the next step.

Check the Swap status again as in step 2, you will see that everything is different now.

AZDIGI Tutorial

swapon -s

Next, we need to set up the Swap partition to automatically activate after rebooting the system.

AZDIGI Tutorial

echo /mnt/swapfile none swap defaults 0 0 >> /etc/fstab

Continue to perform security settings for the Swap partition with the following two commands:

AZDIGI Tutorial

chown root:root /mnt/swapfile 
chmod 0600 /mnt/swapfile

Explanation of the two commands above:

Command 1: Delegation of group owner permissions belong to root.
Command 2: Authorize only the root to access and read and write data.

So we have successfully enabled Swap on Linux. However, if you need to prioritize using Swap when the ram level reaches the limit, then follow the rest of the article.

V. Configure Swappiness

Swappness is the priority of using Swap of Linux system. When the amount of free RAM remaining equals the value of Swappiness (as a percentage), the Linux server will switch to use. For example, if your server has only 10% free RAM and Swappiness is set to 10, the server will switch to using Swap.

Because the speed of RAM is many times faster than the speed of the hard disk, when data is written/read from Swap, it will give a much slower speed when used on RAM. So you should consider tweaking the Swappiness range from 5 to 10 as appropriate.

Note: The higher the number, the more often the server uses Swap, which means that the hard disk will have to read and write more and degrade faster.

Swappiness will have a value between 0 – 100.

Here are some commonly used scenarios:

swappiness = 0: swap is only used when RAM is used up.
swappiness = 10: swap is used when 10% RAM is available.
swappiness = 60: swap is used when the RAM is 60% free.
swappiness = 100: swap takes precedence as RAM.

You can check the Swappiness parameter with the following command:

AZDIGI Tutorial

cat /proc/sys/vm/swappiness

Here are the results on my server.

And to change this value to 10, we use the following command:

AZDIGI Tutorial

sysctl vm.swappiness=10

Then check again with the test command you can see that the number Swappiness has been changed.

However, using this command is only a temporary configuration. And after restarting the server the parameters will be back to default.

To make sure this parameter stays the same every time you restart the server, you need to adjust the parametervm.swappiness at the end of the /etc/sysctl.conf file, in case your server does not have this line, please add it manually.

Then save this file and restart your server to get the new parameters. Please wait a moment for the server to restart, you can check again with the following commands.

AZDIGI Tutorial

swapon -s
cat /proc/sys/vm/swappiness

Below is the result after I restart the server.

In some cases CentOS 7 server even if you change the parameters vm.swappiness at the end of the /etc/sysctl.conf file also doesn’t work after restarting the server, maybe that server has a tune profile that will automatically overwrite the parameters vm.swappiness every time it reboots.

To check which profile is overriding, use the following command:

AZDIGI Tutorial

grep vm.swappiness /usr/lib/tuned/*/tuned.conf

The results will be as follows:

According to the result, please edit the parametersvm.swappiness in the /usr/lib/tuned/virtual-guest/tuned.conf file to 10 at your disposal. And finally, restart the server.

How To Check Swap Usage Size and Utilization in Linux

https://www.cyberciti.biz/faq/linux-check-swap-usage-command/

Check swap usage size and utilization in Linux

The procedure to check swap space usage and size in Linux is as follows:

Open a terminal application.
To see swap size in Linux, type the command: swapon -s.
You can also refer to the /proc/swaps file to see swap areas in use on Linux.
Type free -m to see both your ram and your swap space usage in Linux.
Finally, one can use the top or htop command to look for swap space Utilization on Linux too.

How to Check Swap Space in Linux using `/proc/swaps` file

Type the following cat command to see total and used swap size:

cat /proc/swaps
Sample outputs:

Filename				Type		Size	Used	Priority
/dev/sda3                               partition	6291448	65680	0

Another option is to type the grep command or egrep command as follows:

grep '^Swap' /proc/meminfo
Here is what I see:

SwapCached:            0 kB
SwapTotal:        524284 kB
SwapFree:         524284 kB

Look for swap space in Linux using `swapon` command

Type the following command to show swap usage summary by device

swapon -s
Sample outputs (you may see a filename such as /swap1 instead of a partition such as /dev/sda3):

Filename				Type		Size	Used	Priority
/dev/sda3                               partition	6291448	65680	0

Use `free` command to monitor swap space usage

Use the free command as follows:

free -gfree -k

free -m
Sample outputs (look for swap line):

             total       used       free     shared    buffers     cached
Mem:         11909      11645        264          0        324       8980
-/+ buffers/cache:       2341       9568
Swap:         6143         64       6079

See Linux Check Memory Usage Using the CLI and GUI tutorial page for more info.

See swap size in Linux using `vmstat` command

Type the following vmstat command:

vmstat

vmstat 1 5
Sample outputs:

procs -----------memory---------- ---swap-- -----io---- -system-- ----cpu----
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa
 1  9 1209512 101352   1504 127980    0    3    11    20   60   55  3  1 95  1
 2 11 1209640 101292   1508 134132  844  424  5608   964 23280 15012  2  8 20 70
 0 10 1210052 108132   1532 125764  648  660 10548   916 22237 18103  3 10 11 77
 1 13 1209892 106484   1500 128052  796  240 10484   980 24024 12692  2  8 24 67
 1  9 1209332 113412   1500 124028 1608  168  2472   620 28854 13761  2  8 20 70

Note down the following output from swap field:

si: Amount of memory swapped in from disk (/s).
so: Amount of memory swapped to disk (/s).

`top/atop/htop/glances` Linux commands

Prerequisite
By default, htop, atop, and glances command may not be installed on your system. Hence, use the apk command on Alpine Linux, dnf command/yum command on RHEL & co, apt command/apt-get command on Debian, Ubuntu & co, zypper command on SUSE/OpenSUSE, pacman command on Arch Linux to install the htop, atop, and glances.

Type any one of the following command at the CLI:
atophtoptopglances
Sample outputs from top command:

top - 02:54:24 up 15:24,  4 users,  load average: 0.45, 4.84, 6.75
Tasks: 266 total,   1 running, 264 sleeping,   0 stopped,   1 zombie
Cpu(s):  3.2%us,  1.4%sy,  0.0%ni, 94.4%id,  1.0%wa,  0.0%hi,  0.1%si,  0.0%st
Mem:   8120568k total,  7673584k used,   446984k free,     4516k buffers
Swap: 15859708k total,  1167408k used, 14692300k free,  1151972k cached

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND            
13491 vivek     20   0 1137m 279m 6692 S   10  3.5  19:17.47 firefox            
 5663 vivek     10 -10 1564m 1.1g  59m S    8 14.5   5:10.94 vmware-vmx         
 2661 root      20   0  352m 185m 8604 S    6  2.3  65:40.17 Xorg               
 3752 vivek     20   0 3566m 2.6g  12m S    6 33.6  63:44.35 compiz             
 4798 vivek     20   0  900m  50m 4992 S    2  0.6   0:11.04 chrome             
 5539 vivek     20   0 1388m 838m 780m S    2 10.6   1:45.78 VirtualBox         
 6297 root      20   0     0    0    0 S    2  0.0   0:00.15 kworker/2:0        
 6646 root      20   0 19252 1404  936 R    2  0.0   0:00.01 top                
    1 root      20   0  8404  644  608 S    0  0.0   0:03.32 init               
    2 root      20   0     0    0    0 S    0  0.0   0:00.03 kthreadd           
    3 root      20   0     0    0    0 S    0  0.0   0:02.30 ksoftirqd/0        
    6 root      RT   0     0    0    0 S    0  0.0   0:00.00 migration/0        
    7 root      RT   0     0    0    0 S    0  0.0   0:00.24 watchdog/0         
   37 root       0 -20     0    0    0 S    0  0.0   0:00.00 cpuset             
   38 root       0 -20     0    0    0 S    0  0.0   0:00.00 khelper            
   39 root      20   0     0    0    0 S    0  0.0   0:00.00 kdevtmpfs          
   40 root       0 -20     0    0    0 S    0  0.0   0:00.00 netns

Sample outputs from htop command:

Fig.01: Linux: Swap Memory Usage Command

Sample outputs from glances command:
Linux Check Swap Usage Size Using glances command
See how to keep an eye on your Linux system with glances monitor for more info.

Linux Find Out What Process Are Using Swap Space

Try the smem command:

smem
OR

top

Linux GUI tool to monitor swap space size and usage

Try Gnome or KDE system monitor tool. For example, the GNOME System Monitor shows you what programs are running and how much processor time, memory (including paging/swap space size), and disk space are being used.
Swap space on Linux displayed using System Monitor

Mount an SMB Share in Linux

https://www.linode.com/docs/guides/linux-mount-smb-share/

Determining how to share files and directories between computers is a common problem — one that has many different solutions. Some of these solutions include file transfer protocols (like SFTP), cloud storage services, and distributed file system protocols (like NFS and SMB). Figuring out what solution is right for your use case can be confusing, especially if you do not know the correct terminology, techniques, or the tools that are available. Sharing files can be made even more complicated if you intend to do so over the internet or use multiple operating systems (like Linux, Windows, and macOS).

This guide covers the Server Message Block (SMB) protocol. Specifically, it discusses using the SMB protocol to mount a Windows SMB share (a shared directory) to a Linux system. By following this guide, you will be able to access all of your files within a Windows folder (such as C:\My_Files) on your Linux system at whichever directory you choose as a mount point (such as /mnt/my_files). This method of file sharing is appropriate when you need to access entire Windows directories remotely as if they were local resources. In most cases, SMB is a native (or easily installed) file sharing solution for users that need access to the same directory and is commonly shared through a corporate intranet or the same private network.

Installation

The LinuxCIFS utils package provides the tools needed to connect to a share and manage mounts on a Linux system. You use it to help create and manage a connection to a Windows, macOS, or Linux share.

Update the list of available packages using the below command:
```
sudo apt update && sudo apt upgrade
```
Install the both the LinuxCIFS utils package (needed to mount SMB shares) and the psmisc package (needed to gain access to the fuser command, which shows you which users are using the various mounts on your server).
```
sudo apt install cifs-utils psmisc
```

Verify that LinuxCIFS is available using the following command:

mount -t cifs

No error or output message is expected as there are no CIFS connections set up yet.
Verify that you have access to the fuser command.
```
fuser
```

This command shows a list of the various command line switches that can be used with the fuser utility.

Usage: fuser [-fMuvw] [-a|-s] [-4|-6] [-c|-m|-n space] [-k [-i] [-s sig] | -SIGNAL] NAME...

All files in Linux are accessible on a single giant hierarchical directory tree, which starts at the root (/). The mount command (used in this tutorial) enables you to access other storage devices or file systems from that same tree. These other storage resources do not have to be physical disks and they do not have to be using the same file system. To learn more about the mount command, review the following guides:

The following sections detail how to mount an SMB share on Ubuntu, but the essential process is the same for other Linux distributions.

Create an empty directory to be used as the mount point. This directory can be located wherever you wish, though it’s common to use the /mnt directory.
```
mkdir /mnt/smb_share
```

Enter the following command to mount the SMB share, replacing [server-ip] with the IP address of your SMB server, [share-path] with the file path to your SMB share on that server, and [mount-point] with the new directory you just created.

mount -t cifs //[server-ip]/[share-path] /[mount-point]

In the example below, the SMB server’s IP is 192.0.2.17, the share’s path is SharedFiles, and the mount point is /mnt/smb_share.

mount -t cifs //192.0.2.17/SharedFiles /mnt/smb_share

When prompted, enter the password to connect to the remote share.
If the connection is successful, you should see the remote share mounted on the mount point directory you created. To verify this, type the following command:
```
mount -t cifs
```
The command above lists all mounted SMB shares. Among this list, you should see the share you just mounted.
You should now be able to access the files as if they were on a local drive. In the command below, replace [mount-point] with the directory you have created (such as /mnt/smb_share).
```
cd [mount-point]
```

From here, you can run the ls command to view your files and you can interact with the files as you would any other files on your system.

Create a Credentials File

You don’t want to have to type in your credentials every time you access a share. On the other hand, putting the credentials where everyone can see is not a good idea. The following steps help you create a credentials file to automate the process of logging in.

Use your preferred text editor such as vi or nano to create a file to store the credentials. You can name the file anything you want, but using a period before the filename will hide it from view. For example, you can create a file named .credentials using the following command:
```
nano ~/.credentials
```

Add the necessary credentials to the file in the following format:

File: .credentials

username=target_user_name password=target_user_password domain=domain
If the domain is not required (except on Windows systems), you can omit that entry. Replace the target_user_name and target_user_password with the actual credentials you need to use to access the SMB share. Save and close the file.
Set ownership of the credentials file to the current user by running the following command:
```
sudo chown <User Name[:Group Name]> <Credentials Filename>
```
Replace <User Name> with your username and <Credentials Filename> with the name of your credentials file.
Set the file permissions to 600 to ensure that only the owner has read and write access:
```
sudo chmod 600 <Credentials Filename>
```

sudo mount -t cifs -o credentials=<Credentials Filename> //<IP Address of Server>/<Share on Server> /<Mount Point>

Replace <IP Address of Server> with the IP address of the server hosting the share, <Share on Server> with the name of the share you want to mount, and <Mount Point> with the local mount point where you want to access the share. You aren’t asked for credentials this time because mount uses the credentials file instead.
Verify that the share has been successfully mounted using the following command:
```
mount -t cifs
```

This should show you the share information as output, confirming that the share has been successfully mounted using the credentials file.

Remounting the SMB share every time you restart the server can be tedious. You can instead set your server up to automatically remount the share every time you restart it using the following steps. Before starting these steps, make sure that the share is currently unmounted.

Open the /etc/fstab file in your preferred text editor. This file contains configurations that the server uses on reboot to reconnect to shares (among other things). There are columns for the file system, mount point, type, and options.
Enter the information below in each of the columns:

File: /etc/fstab

<file system>: //<IP Address of Server>/<Share on Server> <mount point>: <Mount Point> <type>: cifs <options>: credentials=<Credentials Filename>
From the file above, replace <IP Address of Server> with the IP address of the server hosting the share, <Share on Server> with the name of the share you want to mount, <Mount Point> with the local mount point where you want to access the share, <Credentials Filename> with the name of your credentials file,
Save the file so the share is available next time you reboot the server.
Verify that the share is mounted correctly using the <Mount Point> as an identifier because the mount is reading the /etc/fstab file.

You may need to unmount a share at some point. To unmount an SMB share that has been mounted using the mount command, you can use the umount command followed by the mount point of the share. The correct command is umount, not unmount.

So to unmount an SMB share at the mount point <Mount Point>, run the following command:

umount -t cifs /<Mount Point>

Linux

5 Linux commands to check free disk space

Linux df command

Linux du command

Linux ls -al command

Linux stat command

Linux fdisk -l command

6 tcpdump network traffic filter options

1. Option -D

2. Option -c X

3. Option -n

4. Option -s

5. Port captures

6. Option -w

DHCP on Linux

dmidecode -t memory

How to Change Passwords in Linux

How to Change your Password in Linux

Change Another User's Password

Force a Password Reset

Exploring the passwd Command

How To Configure Firewall with UFW on Ubuntu 20.04 LTS

Step 1 – Set Up default UFW policies

Enabling IPv6 support

Step 2 – Open SSH TCP port 22 connections

Step 3 – Turn on firewall

Want to disable the UFW based firewall? Try

Step 4 – Open specific incoming connections/ports

Allow port ranges via ufw

Allow connection on specific interface

Step 5 – Block and deny incoming connections/ports

Step 6 – Verify status of UFW

Ubuntu 20.04 LTS UFW delete rules

How to Install NTP Server and Client(s) on Ubuntu 22.04 LTS

Install and configure NTP Server on the host computer

Step 1: Update the package repository index

Step 2: Install NTP Server with apt-get

Step 3: Verify installation (optional)

Step 4: Switch to an NTP server pool closest to your location

Step 5: Restart the NTP server

Step 6: Verify that the NTP Server is running

Step 7: Configure Firewall so that client(s) can access the NTP server

Configure NTP Client to be Time Synced with the NTP Server

Step 1: Install ntpdate

Step 2: Specify IP and hostname of the NTP server in the hosts file

Step 3: Check if the client machine’s time is synchronized with an NTP server

Step 4: Disable the systemd timesyncd service on the client

Step 5: Install NTP on your client

Step 6: Configure the /etc/ntp.conf file to add your NTP server as the new time server

Step 7: Restart the NTP server

Step 8: View the Time Synchronization Queue

How to Set Time, Timezone and Synchronize System Clock in Linux

How to Find and Set Local Timezone in Linux

How to Set Time and Date in Linux

Set Time in Linux

Set Date and Time in Linux

How to Find and Set Hardware Clock in Linux

Synchronizing Linux System Clock with a Remote NTP Server

How to Symlink a Directory in Linux

How to Create Symlink (soft-link) in Linux

How to Create a Symlink (soft link) to a File

How to Create a Symlink (Soft Link) of the Folder/Directory

How to Overwrite the Symlink (Soft Link) in Linux:

How to Remove Symlink (Soft Link) in Linux:

LN Command - Create Links

Links Types

How to Use the ln Command

Creating Symlink To a File

Creating Symlinks To a Directory

Overwriting Symlinks

Removing Symlinks

Sysadmins Networking Cheat Sheet

Using Cert Bot to get a Valid SSL certificate

Using Netplan to set a static IP address

Using S.M.A.R.T to check disk health

Show used disk space on Linux

Clean Archive Cache

Expand LVM Volume

Expanding the Logical Volume

Ubuntu 20.04: How to Extend Partition (For Noobs)

How to Use the `ln` Command

How to Check Swap Space in Linux using `/proc/swaps` file

Look for swap space in Linux using `swapon` command

Use `free` command to monitor swap space usage

See swap size in Linux using `vmstat` command

`top/atop/htop/glances` Linux commands