Extra Repositories for Ubuntu Server 18.04

/etc/apt/sources.list
deb http://archive.ubuntu.com/ubuntu bionic universe multiverse
deb-src http://archive.ubuntu.com/ubuntu bionic universe multiverse

deb http://us.archive.ubuntu.com/ubuntu/ bionic universe
deb-src http://us.archive.ubuntu.com/ubuntu/ bionic universe
deb http://us.archive.ubuntu.com/ubuntu/ bionic-updates universe
deb-src http://us.archive.ubuntu.com/ubuntu/ bionic-updates universe

deb http://us.archive.ubuntu.com/ubuntu/ bionic multiverse
deb-src http://us.archive.ubuntu.com/ubuntu/ bionic multiverse
deb http://us.archive.ubuntu.com/ubuntu/ bionic-updates multiverse
deb-src http://us.archive.ubuntu.com/ubuntu/ bionic-updates multiverse

deb http://security.ubuntu.com/ubuntu bionic-security universe
deb-src http://security.ubuntu.com/ubuntu bionic-security universe
deb http://security.ubuntu.com/ubuntu bionic-security multiverse
deb-src http://security.ubuntu.com/ubuntu bionic-security multiverse

References

https://help.ubuntu.com/lts/serverguide/configuration.html.en

Spring Boot: Configuring a Main Class

Spring Boot 1.x

jar {
    baseName = 'interface'
    version = '0.0.1-SNAPSHOT'
    manifest {
        attributes 'Main-Class': 'net.pupli.sobhan.SobhanApplication'
    }
}

Spring Boot 2.x

springBoot {
    mainClassName = 'org.baeldung.DemoApplication'
}

or

bootJar {
    mainClassName = 'org.baeldung.DemoApplication'
}
bootJar {
    manifest {
    attributes 'Start-Class': 'org.baeldung.DemoApplication'
    }
}

References

https://www.baeldung.com/spring-boot-main-class

Set Up SSH Keys on Ubuntu

Create the RSA Key Pair

ssh-keygen

Copy the Public Key to Ubuntu Server

cat ~/.ssh/id_rsa.pub | ssh username@remote_host "mkdir -p ~/.ssh && touch ~/.ssh/authorized_keys && chmod -R go= ~/.ssh && cat >> ~/.ssh/authorized_keys"

or Copying Public Key Manually

To display the content of your id_rsa.pub key, type this into your local computer:

cat ~/.ssh/id_rsa.pub

You will see the key’s content, which should look something like this:

ssh-rsa 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 demo@test

make sure the ~/.ssh directory exists.This command will create the directory if necessary, or do nothing if it already exists:

mkdir -p ~/.ssh

Now, you can create or modify the authorized_keys file within this directory. You can add the contents of your id_rsa.pub file to the end of the authorized_keys file, creating it if necessary, using this command:

echo public_key_string >> ~/.ssh/authorized_keys

Finally, we’ll ensure that the ~/.ssh directory and authorized_keys file have the appropriate permissions set:

chmod -R go= ~/.ssh

Authenticate to Ubuntu Server Using SSH Keys

ssh username@remote_host

Disable Password Authentication on your Server

sudo nano /etc/ssh/sshd_config
PasswordAuthentication no
sudo systemctl restart ssh

References

https://www.digitalocean.com/community/tutorials/how-to-set-up-ssh-keys-on-ubuntu-1604

Change the encryption cipher for server and client of OpenVPN Access Server

cd /usr/local/openvpn_as/scripts

View current cipher configuration keys

./sacli ConfigQuery | grep "cipher"

Set the cipher configuration key to the desired cipher:

./sacli --key "vpn.client.cipher" --value AES-128-CBC ConfigPut
./sacli --key "vpn.server.cipher" --value AES-128-CBC ConfigPut
./sacli start

References

https://docs.openvpn.net/command-line/additional-security-command-line-options/#Change_the_encryption_cipher_for_server_and_client

Padding Strings with format method on Java

public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    String s1 = sc.next();
    int x = sc.nextInt();

    PadLeft(s1);
    System.out.printf("%03d", x);
    System.out.println();

    PadRight(s1);
    System.out.printf("%03d", x);
}

private static void PadRight(String s)
{
    System.out.printf("%1$15s", s);
}

private static void PadLeft(String s)
{
    System.out.printf("%1$-15s", s);
}

15 represents the minimal width of the String

Input

Hello
12

Output

Hello          012
          Hello012

References
https://stackoverflow.com/questions/13475388/generate-fixed-length-strings-filled-with-whitespaces
http://www.rgagnon.com/javadetails/java-0448.html