RTB parking server

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This document is only collection of author’s notes, experiences and point of views. It is not, in any meaning, either complete description of the topic nor official RTB documentation. It may be inaccurate, incomplete, obsolete, misleading or completely wrong. It may even cause loss of data or damage system integrity. It may not comply with company codex, values, presentation style or economic interests or may reveal company secrets.

As such, it is absolutely prohibited to distribute this document outside of RTB & Co. GmbH. Such an action may result into legal acts against both the sender and the company. It is only intended, after review of technical and presentational correctness and accuracy, to be used as an information source for official documentation.

Always contact documentation department for information about current presentation style and allowed formats before creating customer’s documentation.

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Date	2024-10-25 13:41:51 +0200
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Revision	052050ac12f1eafa4499ff7d62921fce5cd20697

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2024-10-25 13:41:51 +0200

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Creator’s guide

This document describes necessary steps how to install PDM.control in production environment. Used operating system is Ubuntu Linux 20.04. Installations steps are similar for different Linux distributions. Nevertheless there might be slight differences, which need to by solved by system administrator.

In case, that you want to run PDM.control under Windows even in production environment please contact our experienced server administrators at parking-helpdesk@rtb-bl.de and request appropriate documents.

1. Server environment

There are two ways possible. Either you install OS by your self(manual) or you let install it in hosting center. Preferred way is the second one to use hosting center. We recommend Digital Ocean (DO) because of variety of offered services and relative low prices.

1.1. Server environment

1.1.1. Setup Digital Ocean account

Go to menu Account Security SSH keys and insert you SSH key (see Generate key (PuTTY)).

1.1.2. Create server

Go to <your project> and click on Create button at the top of the screen. Select Droplets. From to option on new screen select flowing options:

Distributions: Ubuntu 20.04
Plan: Standard, 1CPU, 1GB RAM
Region: according your needs.
Additional options: Monitoring
Authentication: SSH keys
Host name, Tags … according your needs
Enable backup: Recommended

Click create droplet and you are good to go.

1.1.3. Check server

Swap

We need to be sure that the server has configured swap space. Otherwise can our application be terminated if there is not enough memory. Connect to the server and verify if there is swap space enabled. You should see something like this:

# swapon -s
Filename                Type        Size    Used    Priority
/swapfile               file        581564  577876  -2

If the output of swapon -s command produces empty table go to chapter Create swap space and create 1GB swap space according instructions found there.

1.2. Time zone

Check is time zone is set correctly for your server.

$ timedatectl
                      Local time: Wed 2020-02-12 11:58:43 CET
                  Universal time: Wed 2020-02-12 10:58:43 UTC
                        RTC time: Wed 2020-02-12 10:58:44
                       Time zone: Europe/Berlin (CET, +0100) (1)
       System clock synchronized: yes (2)
systemd-timesyncd.service active: yes
                 RTC in local TZ: no

1	Verify if the time zone is correct
2	Verify is time is synchronized If there is something which need to be fixed continue to chapter Time zone.

1.3. SSH

In order to access the server from remote location it is necessary to configure SSH server.

1.3.1. Use certificate

We strongly recommend key authentication. I you do not have any key yet refer to chapter Generate key (PuTTY) to create one.

Put key to the system

Create .ssh directory (if it does not exist yet) in your home directory:

$ mkdir -m 700 .ssh

Insert public key to file .ssh/authorized_keys. If file does not exist create it with correct permissions. Then insert public key as you see it in PuTTYgen’s text field at the top of the window.

$ touch .ssh/ authorized_keys
$ chmod 600 .ssh/ authorized_keys
$ nano .ssh/ authorized_keys

Test connection with key. Password shouldn’t be required.

Disable password login

To disable password authentication, edit /etc/ssh/sshd_config file.

# nano /etc/ssh/sshd_config

Find this line:

#PasswordAuthentication yes

Change it to:

PasswordAuthentication no

Then find the ChallengeResponseAuthentication line. Make sure it’s value is set to no like below. If it’s set to yes, you can still use password to login.

ChallengeResponseAuthentication no

Save the file and restart ssh service.

# sudo systemctl restart ssh

Now if you (remotely) login without key, then login will be immediately rejected without asking password.

1.4. PostgreSQL 12

1.4.1. Prepare repository

The latest PostgreSQL is not available in standard repository of Ubuntu 20.04. We have to add a repository managed by PostgreSQL manually. The steps are taken from official documentation and Wiki. I just repeat (and clarify them) here.

Create the file repository configuration:

# sh -c 'echo "deb http://apt.postgresql.org/pub/repos/apt $(lsb_release -cs)-pgdg main" > /etc/apt/sources.list.d/pgdg.list'

Import the repository signing key

# wget --quiet -O - https://www.postgresql.org/media/keys/ACCC4CF8.asc | apt-key add -

Update the package lists

# apt update

1.4.2. Install PostgreSQL

Install the PostgreSQL version 12 using following command

# apt install postgresql-{postgresql-ver}

1.4.3. Configure database password

If you’re working from a default PostgreSQL installation, then PostgreSQL will be configured with the user postgres. You need to set password to be able to connect from PDM.control

# su postgres

then attempt a connection to PostgreSQL:

$ psql

In the PostgreSQL console set password by command

\password

Set new password and close console by \q command. Then exit postgres user shell by exit command.

$ exit

1.5. Java

We recommend to use java from AdoptOpenJDK provider. In order to get always the latest version it is preferred to setup repository. Go to official page and make setup according to the description there.

Install java

# apt install adoptopenjdk-11-hotspot-jre

1.6. PDM.control

Crate database. First connect database via console.

# su postgres
$ psql

Create new database

CREATE DATABASE pdm_control;

You may verify that the database is created by command \l. Then quit console by \q. In order to improve security we will create new system user and group especially to run RTB applications. Let call the user rtb and the group rtb.

# addgroup --system rtb
# adduser --system --ingroup rtb --no-create-home --shell /bin/bash rtb

Crete home directory for PDM control and set rights

# mkdir /opt/pcon
# chown rtb:rtb /opt/pcon
# chmod 700 /opt/pcon

Copy war file provided by RTB to this directory new directory. Add configuration file named application-linux-prod.yml. To test application with console logging you may start it by command.

# runuser -l rtb -c 'java -Dspring.profiles.active=ubuntu-prod -Dlogging.config=classpath:logback-console-only.xml -jar pdm_control.war'

PDM.control should be available on http://your-domain:8080. You should see login screen.

Put configuration to file application-ubuntu-prod.yml. Either adapt provided sample or kontakt our helpdesk.

1.7. Front end server

In order to improve security and management of multiple applications running on the same server, it is recommended to install front end server. It is possible to choose between Nginx and Appach. We have chosen Nginx.

1.7.1. Server

The necessary steps are described here: How To Install Nginx on Ubuntu 18.04. Brief summary follows. As example domain is e1.rtp-parking.com taken. Replace it by yours in all listings below.

# apt install nginx

Test if it runs

# systemctl status nginx

Put configuration for your domain. File name can be derived from you domain.

# nano /etc/nginx/sites-available/e1.rtp-parking.com.conf

For PDM.control start with this configuration. It may be adjusted later.

server {
	server_name e1.rtp-parking.com;
	# Increase max file size to upload. Default 1MB
	client_max_body_size 8M;
	location / {
		# Default PDM.control port is 8080.
		proxy_pass http://127.0.0.1:8080;
		proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
		proxy_set_header X-Forwarded-Proto $scheme;
		proxy_set_header X-Forwarded-Port $server_port;
		proxy_set_header Host $http_host;
		proxy_headers_hash_max_size 512;
		proxy_headers_hash_bucket_size 128;
	}
}

Activate configuration

# cd /etc/nginx/sites-enabled
# ln -s /etc/nginx/sites-available/e1.rtp-parking.com.cfg
# nginx -t
# systemctl reload nginx

PDM.control should be available on http://e1.rtp-parking.com. You should see login screen.

1.7.2. HTTPS with certbot

Certbot is program which automatically install and re-new certificate with Let’s Encrypt services. Full installation steps are explained in article How To Secure Nginx with Let’s Encrypt on Ubuntu 20.04. For us important steps are summarized below.

Install certbot

Not needed for Ubuntu 20.04. Continue to next chapter.

Following steps are necessary only for Ubuntu 18.04

# add-apt-repository ppa:certbot/certbot

Accept by pressing Enter.

# apt install python-certbot-nginx

Request certificate for your domain(s)

# certbot --nginx -d e1.rtp-parking.com

Continue according steps from wizard. At the end choose to redirect from HTTP to HTTPS; option 2. Now should be PDM.control accessible by HTTPS on https://e1.rtp-parking.com/.

Post configuration

Post configuration is probably not needed in Ubuntu 20.04. It seems that TLSv1 is not supported by default. Continue to next chapter.

TLS

Test encryption strength by SSLlabs tool or just TLSTest. A list of tools related to SSL is also here. You should disable TLS 1.0 to improve security if you do not care about very old devices. Open file /etc/nginx/nginx.conf and change line

ssl_protocols TLSv1 TLSv1.1 TLSv1.2;

ssl_protocols TLSv1.2 TLSv1.3;

TLS + Certbot

Certbot overwrites nginx’s protocol configuration in site configuration by line

include /etc/letsencrypt/options-ssl-nginx.conf; # managed by Certbot

It is necessary to change protocol definition also in this file. Change line ssl_protocols also to TLSv1.2 TLSv1.3.

ssl_protocols TLSv1.2 TLSv1.3;

Then restart ngingx and test again with SSLlabs above.

Cipher selection

Use Mozilla’s Security/Server Side TLS and SSL configuration generator sites to fine tune configuration based on your security and compatibility needs.

Test certificate renewal

Test if certbot’s key renewal still works.

# certbot renew --dry-run

However, this check may be insufficient in some cases. You may also test real key renewal.

# certbot renew --force-renewal

Further info How to remove TLSv1.0 / 1.1 and enable TLS 1.3 in Nginx.

If there is a need to remove certificate use command below (source)

# certbot delete

and the select certificate or specify certificate directly like this

# certbot delete --cert-name example.com

HTTP2

In site configuration file modify line

listen 443 ssl;

listen 443 ssl http2;.

See article: How To Set Up Nginx with HTTP/2 Support on Ubuntu 18.04.

Compression

Compression is enabled by default. It can be fine tuned in file /etc/nginx/nginx.conf. For example it can be restricted for some content types or responses up to some size.

gzip on;
# gzip_vary on;
# gzip_proxied any;
# gzip_comp_level 6;
# gzip_buffers 16 8k;
# gzip_http_version 1.1;
gzip_types text/plain text/css application/json application/javascript text/xml application/xml application/xml+rss text/javascript;
gzip_min_length 1000;

Hide nginx version

To make the work of attakers a bit harder it is recommend to hide nginx version. It makes more difficult to miss-use known errors. Open file /etc/nginx/nginx.conf and put following line into http directive

server_tokens off;

1.8. Firewall

Before you start to use SSH you need to configure firewall to let pass SSH. Use following command to configure firewall

# ufw allow OpenSSH

Enable firewall for nginx. It is necessary to open ‘Nginx Full’ profile, because for certbot’s key renewal is required open port 80.

# ufw allow 'Nginx Full'

Allow also connections on port for PDM by command below:

# ufw allow 55200 comment 'PDM.control / PDMs'

Real port number may be in your case different. Check your configuration.

1.9. Logs

Logs are very useful to investigate problems. They are store just next to application. There is very good log reading utility called lnav.

2. Maintanace tasks

2.1. Database logging

Not necessary for customer installations until required. First it is necessary to configure logging to be able to analyze something.

2.1.1. Create directory for log files

2.1.2. Database setup

What to log

First it is necessary to define what to log. The configuration below is recommend for further analysis with pgBadger (see chapter 4.1.3). See also pgBadger documentation.

# How slow query should be logged (ms)
log_min_duration_statement = 100
log_checkpoints = on
log_connections = on
log_disconnections = on
log_error_verbosity = default
log_line_prefix = '%t [%p]: [%l-1] user=%u,db=%d,app=%a,client=%h '
log_lock_waits = on
log_statement = 'none'
log_temp_files = 0
log_autovacuum_min_duration = 0

Where to log

Wee need extra directory for log files to avoid rotation by logrotate utility. It is installed by PostgreSQL and takes care of files in path /var/log/postgresql/*.log. We want to user logging collector provided by PostgreSQL which would collide with logrotate. Therefore we will setup logging to different directory, /var/log/postgresql/sql.

Create new directory and set correct permissions:

# mkdir /var/log/postgresql/sql
# chown postgres:postgres /var/log/postgresql/sql

Enable log collector to be able to handle and rotate files by PostgreSQL background process. And configure logging according your needs. The example below keeps one week of logs.

logging_collector = on
# set logging directory
log_directory = '/var/log/postgresql/sql'
# long file name 'db-<day of week>. See https://www.tutorialspoint.com/c_standard_library/c_function_strftime.htm
log_filename = '%A.log'
# overwrite previous log file.
log_truncate_on_rotation = on
# enable rotation once a day.
log_rotation_age = 1d
# disable rotation on size.
log_rotation_size = 0

2.1.3. Result verification

For database analysis a tool named pgBadger (github) can be used. Install either version from package manager (apt install pgbadger or compile latest from source code).

To generate single report from available log use command below. Replace paths by yours.

$ pgbadger --exclude-query="^(COPY|COMMIT|BEGIN)" --outdir /var/log/postgresql/html /var/log/postgresql/sql*.log

It is also possible to generate reports incrementally to keep history.

$ pgbadger --exclude-query="^(COPY|COMMIT|BEGIN)" --incremental --outdir /var/log/postgresql/html /var/log/postgresql/sql/*.log

To keep generation running it is recommended to put command above to cron scheduler. Add following rule to editor opened by crontab -e command. Of course execution time has to be adjusted

19 3 * * * pgbadger --exclude-query="^(COPY|COMMIT|BEGIN)" --incremental --quiet --outdir /var/log/postgresql/html /var/log/postgresql/sql/*.log

2.1.4. Result on web

Result of database analyses can be also accessible via web for easier access. To achieve it, change path for generated output to /var/www/pgbadger like this.

19 14 * * * pgbadger --exclude-query="^(COPY|COMMIT|BEGIN)" --incremental --quiet --outdir /var/www/pgbadger /var/log/postgresql/sql/*.log

Then add new location to the PDM.control server like this.

location /pgbadger/ {
	root /var/www;
}

It is also recommend to secure the location by basic authentication to restrict access. See official nginx documentation.

Install package apache2-utils

# apt install apache2-utils

Create user with password

$ htpasswd -c /etc/nginx/rtb_htpasswd admin

Add security configuration into location directive. The whole directive looks like this.

location /pgbadger/ {
	root /var/www;
	auth_basic "PgBadger";
	auth_basic_user_file /etc/nginx/rtb_htpasswd;
}

2.2. Monitor java processes

In order to get better understanding how java processes works it is possible to connect VisualVM tool to observe internal behavior. Follow steps bellow to connect remote java process via SSH tunneling.

2.2.1. Install VisualVM

Download and extract VisualVM. In order to run it it is necessary to set JDK home like this:

<visualvm_install_dir>/bin/visualvm --jdkhome <path_to_jdk_home>

It is convenient to create a shortcut or startup script to avoid retyping.

2.2.2. Allow remote connections

Add flowing configuration to PDM.control startup script

 -Dcom.sun.management.jmxremote.ssl=false\
 -Dcom.sun.management.jmxremote.authenticate=false\
 -Dcom.sun.management.jmxremote.port=6380\
 -Dcom.sun.management.jmxremote.rmi.port=6390\
 -Djava.rmi.server.hostname=localhost\
 -Dcom.sun.management.jmxremote.local.only=false\

Port numbers can be changed based on your system.

2.2.3. Connect from VisualVM

Add new local connection to port 6380. You should see the VM properties and charts. For more info see tutorial.

2.3. RTB data backup

This service is not available for customer installations. It only describes how backup of our cloud services works. Customer may use our solution as an example to build his own.

The general idea is two step backup process.

Server it self creates regularly backup files and store all of them in one directory (in primary location).
Backup collector collects regularly created backup files and store them outside cloud environment (at secondary location).

Because the backup server is located in physically different location it should be possible to recover data also in case of natural disaster affecting primary location.

Because backup server is not permanently connected with primary location should be this solution also resistant again ransom ware and other crypto-viruses.

The secondary location is the backup-ed to tape and move to tertiary location.

2.3.1. Backup creation procedure

Currently we backup only PDM.control database. Server with its configuration can be easily re-created from image.

User account

Creation

First we need to create new user account trough which the server collector will access prepared backups. Let call it rtb-backup.

# useradd -m  --shell /bin/bash  'rtb-backup'

Add user rtb-backup to group postgres to be able to use backup utility.

# usermod -a -G postgres rtb-backup

Configuration

In order to call pg_dump utility as postgres user without password it is necessary to allow it in sudousers. Run visudo command and add following line (source) at the end of file before line #includedir /etc/sudoers.d.

rtb-backup ALL=(postgres) NOPASSWD: /usr/bin/pg_dump

Exit editor in order to apply change.

Security

Allow backup collector authorization using key

# su rtb-backup -
$ mkdir .ssh
$ chmod 700 .ssh/
$ touch .ssh/authorized_keys
$ chmod 600 .ssh/authorized_keys

Then add backup collector’s key to .ssh/authorized_key file. Backup collector will log in via this account and will collect prepared scripts.

Backup script

Below is example of script which can be used to make a backup.

#!/usr/bin/bash
# Configuration
# Name of database to backup
DB_NAME="pcon_staging"

# Environment setting
TIMESTAMP=$(date --iso-8601='minutes')
BACKUP_DIR_FINAL=bk_$TIMESTAMP
# Directory where backup is in progress is prefixed by '_' character to avoid to collect incomplete backup.
BACKUP_DIR="_$BACKUP_DIR_FINAL"

echo "Creating backup directory $BACKUP_DIR_FINAL"
mkdir $BACKUP_DIR


# Backup database
echo "Running database backup for $DB_NAME ..."
DB_BACKUP_FILE=$BACKUP_DIR/$DB_NAME.backup
touch $DB_BACKUP_FILE
chgrp postgres $DB_BACKUP_FILE
chmod 664 $DB_BACKUP_FILE
sudo -u postgres pg_dump --format custom --file $DB_BACKUP_FILE $DB_NAME

# Mark backup as completed.
mv $BACKUP_DIR $BACKUP_DIR_FINAL
echo "Backup finished"

It can be put to file /home/rtb-backup/backup.sh. To call it regularly create a cron job for it

# sudo -u rtb-backup crontab -e

Add flowing line to crontab file. It will run the scrip every day at 4:47

47 4 * * * backup.sh

2.4. RTB server administrator

Preface

There may be situation when I am not available and PDM.control stops to work. Unfortunately we do not have any real system administrator, which could fix the cause of the problem in such a case. Nevertheless production PDM.control changes its version not so oft and if there is a problem, it is mostly temporal. In such situations should be enough to restart the service. When I am back I can properly analyze problem.

For this reason there is crated new user rtb-admin with privilege to restart PDM.control service an the whole virtual machine (for the case that restarting the service does not help).

2.4.1. User configuration

Create new user for "administrative" purposes and add it to rtb group in order to be able to see application logs.

# useradd -m  --shell /bin/bash  'rtb-admin'
# usermod -a -G rtb 'rtb-admin'

Privilege the rtb-admin user to restart PDM.control service and computer. Run visudo command and add following line at the end of file.

rtb-admin ALL=(root) NOPASSWD: /usr/bin/systemctl restart pdm_control,/usr/bin/systemctl restart pdm_control.service,/usr/sbin/reboot

Now the user rtb-admin can run following commands as root user without password prompt.

sudo systemctl restart pdm_control
sudo systemctl restart pdm_control.service
sudo reboot

It is also recommended to setup key authorization for rtb-admin user.

2.4.2. Security configuration

Allow users from rtb group to see content of /opt/pcon especially log files.

# chmod 750 /opt/pcon
# chmod 750 /opt/pcon/logs
# chmod 750 /opt/pcon/logs/archived

Be sure to not allow this user to see content of configuration file. It contains credential to various services.

# chmod 700 /opt/pcon/application-ubuntu-prod.yml

If you version configuration in git do not forget also to protect .git directory.

# chmod 700 /opt/pcon/.git

3. Manual tasks

This chapter describe how to perform some administration tasks in case that you running server on your own. Usually it is not needed in hosting environment.

3.1. Key authorization

In order to increase security it is strongly recommended to disable remote login with password. Only keys will be used.

3.1.1. Generate key (PuTTY)

Generate a private public key pair. On windows PuTTy Key Generator can be used. Generate RSA 4096 bit key.

3.2. Create swap space

Swap is a space on a disk that is used when the amount of physical RAM memory is full. When a Linux system runs out of RAM, inactive pages are moved from the RAM to the swap space.

Swap space can take the form of either a dedicated swap partition or a swap file. In most cases when running Linux on a virtual machine a swap partition is not present so our only option is to create a swap file.

This tutorial was tested on Linux systems with Ubuntu 18.04 and CentOS 7 but it should work with any other Linux distribution.

3.2.1. Add swap file

Become root and follow these steps to add 1GB of swap to your server. If you want to add 2GB instead of 1 GB, replace 1G with 2G.

Create a file which will be used for swap

# fallocate -l 1G /swapfile

If faillocate is not installed or if you get an error message saying fallocate failed: Operation not supported, then you can use the following command to create the swap file:

# dd if=/dev/zero of=/swapfile bs=1024 count=1048576

Set the correct permissions

Only the root user should be able to write and read the swap file. To set the correct permissions, type:

# chmod 600 /swapfile

Set up a Linux swap area

Use the mkswap utility to set up the file as Linux swap area:

# mkswap /swapfile

Enable the swap

Activate the swap file with the following command:

# swapon /swapfile

To make the change permanent open the /etc/fstab file and append the following line:

/swapfile swap swap sw 0 0

Verify the swap status

To verify that the swap is active we can use either the swapon or the free command as shown below:

# swapon --show
NAME		TYPE	SIZE	USED	PRIO
/swapfile	file	1024M	507.4M	-1

# free -h
		total	used	free	shared	buff/cache	available
Mem:	488M	158M	83M		2.3M	246M		217M
Swap:	1.0G	506M	517M

Sdjust the swappiness value

Swappiness is a Linux kernel property that defines how often the system will use the swap space. Swappiness can have a value between 0 and 100. A low value will make the kernel to try to avoid swapping whenever possible while a higher value will make the kernel to use the swap space more aggressively.

The default swappiness value is 60. You can check the current swappiness value by typing the following command:

# cat /proc/sys/vm/swappiness
60

While the swappiness value of 60 is OK for most Linux systems, for production servers you may need to set a lower value. For example, to set the swappiness value to 10, type:

# sysctl vm.swappiness=10

To make this parameter persistent across reboots append the following line to the /etc/sysctl.conf file:

vm.swappiness=10

The optimal swappiness value depends on your system workload and how the memory is being used. You should adjust this parameter in small increments to find an optimal value.

Remove swap file

If for any reason you want to deactivate and remove the swap file, follow these steps:

First, deactivate the swap by typing:

# swapoff -v /swapfile

Remove the swap file entry`/swapfile swap swap defaults 0 0` from the /etc/fstab file. Finally delete the actual swapfile file using the rm command:

# rm /swapfile

3.3. Time configuration

3.3.1. Time zone

To see list all available time zones, enter:

$ timedatectl list-timezones

$ timedatectl list-timezones | more

$ timedatectl list-timezones | grep -i europe

To set the time zone to Europe/Berlin, enter:

# timedatectl set-timezone 'Europe/Berlin'

Verify it:

$ timedatectl

3.3.2. Time

Verify current settings

$ timedatectl

How to change the current date

To change the current date, type the following command as root user:

# timedatectl set-time YYYY-MM-DD

For example set the current date to 2015-12-01 (1st, Dec, 2015):

# timedatectl set-time '2015-12-01'

To change both the date and time, use the following syntax:

# timedatectl set-time YYYY-MM-DD HH:MM:SS

Where,

HH: An hour.
MM: A minute.
SS: A second, all typed in two-digit form.
YYYY: A four-digit year.
MM: A two-digit month.
DD: A two-digit day of the month.

For example, set the date ’23rd Nov 2015′ and time to ‘8:10:40 am’, enter:

# timedatectl set-time '2015-11-23 08:10:40'

Verify by by date command.

How to set the current time only

The syntax is:

# timedatectl set-time HH:MM:SS

E.g.

# timedatectl set-time '10:42:43'

3.3.3. NTP

How to synchronize the system clock with a remote server using NTP? Simply type the following command:

# timedatectl set-ntp yes

Verify it:

$ timedatectl

3.4. Increase virtual disk size

This manual is valid for Virtual box or custom hosting. Digital ocean resizes disk automatically.

Default root partition has size 10 GB. It should be enough for small to medium installation. If there is a need for more space in the future then the partition can be extended. This is two step process. The example below shows how to resize partition from 10 GB to 15 GB.

Virtual box

Resize the virtual disk by using command below. 15000 is new size in mega bytes.

c:\VMs\Server>"C:\Program Files\Oracle\VirtualBox\VBoxManage.exe" modifymedium "root.vdi" --resize 15000
0%...10%...20%...30%...40%...50%...60%...70%...80%...90%...100%

Find out which partition is mounted as root by df -h command. You can get following output:

$ df -h
Filesystem      Size  Used Avail Use% Mounted on
udev            966M     0  966M   0% /dev
tmpfs           200M  984K  199M   1% /run
/dev/sda2       9.8G  2.0G  7.4G  21% /
/dev/loop0       89M   89M     0 100% /snap/core/7270

In our case is the correct partition /dev/sda2. Review disk partitions and free space by parted command.

# parted /dev/sda

It will warn us about extra space. Choose to fix it.

Warning: Not all of the space available to /dev/sda appears to be used, you can fix the GPT to use all of the space (an extra 9748480 blocks) or continue with
the current setting?
Fix/Ignore? F

Write command print free and press enter. Then it can bee seen that there is 5GB free space.

(parted) print free
Model: ATA VBOX HARDDISK (scsi)
Disk /dev/sda: 15.7GB
Sector size (logical/physical): 512B/512B
Partition Table: gpt
Disk Flags:

Number  Start   End     Size    File system  Name  Flags
        17.4kB  1049kB  1031kB  Free Space
 1      1049kB  2097kB  1049kB                     bios_grub
 2      2097kB  10.7GB  10.7GB  ext4
        10.7GB  15.7GB  4992MB  Free Space

Insert command resizepart. Next prompt asks about partition number. In this case enter 2. Confirm warning that the partition is in use.

(parted) resizepart 2
Warning: Partition /dev/sda2 is being used. Are you sure you want to continue?
Yes/No? y
End?  [10.7GB]? 100%

Enter new end of partition in mega bytes or simply 100%.

Verify that there is no more free space by entering print free command. Free space should not be visible any more and the partition 2 should have size 15GB.

(parted) print free
Model: ATA VBOX HARDDISK (scsi)
Disk /dev/sda: 15.7GB
Sector size (logical/physical): 512B/512B
Partition Table: gpt
Disk Flags:

Number  Start   End     Size    File system  Name  Flags
        17.4kB  1049kB  1031kB  Free Space
 1      1049kB  2097kB  1049kB                     bios_grub
 2      2097kB  15.7GB  15.7GB  ext4

We are done with parted. You may leave it by entering quit command. Now is the partition extended. Wee need to extend also file system. This is done be command:

# resize2fs /dev/sda2
resize2fs 1.44.1 (24-Mar-2018)
Filesystem at /dev/sda2 is mounted on /; on-line resizing required
old_desc_blocks = 2, new_desc_blocks = 2
The filesystem on /dev/sda2 is now 3839483 (4k) blocks long.

You can check new partition size by command df.

$ df -h
Filesystem      Size  Used Avail Use% Mounted on
udev            966M     0  966M   0% /dev
tmpfs           200M  988K  199M   1% /run
/dev/sda2        15G  2.0G   12G  15% /
/dev/loop0       89M   89M     0 100% /snap/core/7270