All posts by Don Matteo

lebt in der Schweiz, ist System Engineer MCP bei A-Enterprise GmbH. Mitglied des UNBLOG Knowledge Network. Author und Blogger zu den Themen, Tutorials für Linux und Open Source.

IPv6 disable on Fedora Linux

IPv6 may be undesirable in certain situations, disable IPv6 on Fedora Linux for systems that can by only with the IPv4 protocol stack

Fedora and RHEL disable IPv6 stack as follows

vi /etc/sysctl.conf

net.ipv6.conf.all.disable_ipv6 = 1
net.ipv6.conf.default.disable_ipv6 = 1

If one interface should run without IPv6, e.g. ens160.

net.ipv6.conf.ens160.disable_ipv6 = 1

Save the sysctl.conf file and enable the setting.

sysctl -p

If you want to Disable IPv6 on Fedora or CentOS interface during operation, the following command applies.

echo 1 > /proc/sys/net/ipv6/conf/all/disable_ipv6
echo 1 > /proc/sys/net/ipv6/conf/default/disable_ipv6

Alternatively you can run this command in terminal shell.

sysctl -w net.ipv6.conf.all.disable_ipv6=1
sysctl -w net.ipv6.conf.default.disable_ipv6=1

IPv6 on Fedora is now disabled, a reboot is not necessary.

SSH Server AddressFamily configuration

If the removal of IPv6 for running daemons is a disaster, e.g. SSH Server, the configuration on Fedora or RHEL can be change as follows.

vi /etc/ssh/sshd_config

#AddressFamily any
AddressFamily inet
#or remove hash(#)

Then restart the SSH daemon.

systemctl restart sshd

Postfix ipv4 loopback interface

Postfix may also require to customize the ipv4 loopback interface.

vi /etc/postfix/

#inet_interfaces = localhost
inet_interfaces =

With CentOS 6, the kernel module can be customized.

vi /etc/modprobe.d/ipv6.conf

options ipv6 disable=1

Disable the IPv6 stack.

chkconfig ip6tables off

Then reboot the system.


Checking for IPv6 addresses.

ifconfig | grep inet6
ip a | grep inet6


Internet Protocol version 6 (IPv6) is the most recent version of the Internet Protocol (IP). The communications protocol that provides an identification and location system for computers on networks and routes traffic across the Internet. IPv6 was developed by the Internet Engineering Task Force (IETF) to deal with the long-anticipated problem of IPv4 address exhaustion, and was intended to replace IPv4. In December 1998, IPv6 became a Draft Standard for the IETF, which subsequently ratified it as an Internet Standard on 14 July 2017.

IPv6 features

IPv6 is an Internet Layer protocol for packet-switched internetworking and provides end-to-end datagram transmission across multiple IP networks. Closely adhering to the design principles developed in the previous version of the protocol, Internet Protocol Version 4 (IPv4).

Nagios Monitoring on Raspberry Pi

How to Install Nagios on Raspberry Pi

Nagios Open-Source IT infrastructure Monitoring

Nagios consists of a collection of modules for monitoring networks, hosts and their specific services, as well as a web interface to display queries of the collected data. It is under the GNU GPL, so it is free software and runs on numerous Unixoid operating systems. Because of its widespread use, Nagios has become a quasi-standard in professional use.

Nagios Monitoring with Raspberry Pi

Raspberry Pi with its fanless design, minimal dimensions and low power consumption , well suited as a single-board computer for a Nagios monitoring server that can even monitor itself.


The installation of Nagios Core 4 on the Raspberry’s own OS Raspbian, which is based on Debian, is unspectacular. Here in these instructions the procedure for a Raspberry Pi 3 Model B is shown, on a 32 GB microSD card type Class 10, a 16 GB microSD card would also suffice.

SanDisk Ultra SDHC I 16 GB – 80 /Sek, Class 10 microSD Card.

The provision of Raspbian on a microSD card is not discussed here in more detail.  After booting a Raspbian desktop image, the LXTerminal is opened on the Raspbian X desktop and the root shell is started, in headless operation a VNC session can be started with VNCViewer, with the login as user pi and the default password raspberry. If you want to use the Raspbian Minimal Image, authentication via SSH to the Raspberry Pi is recommended.

After logging in as user pi, we want to become root.

pi@raspberrypi:~ $ sudo su

First, all required packages are installed from the repository as a prerequisite.

$ apt-get update
$ apt-get install -y autoconf gcc libc6 make wget unzip apache2 apache2-utils php5 libgd2-xpm-dev

Download and unzip the Nagios Core 4 source packages. The last release can be found here. The core release as well as the agents and plugins are available on Github.

$ cd /tmp
$ wget -O nagioscore.tar.gz
$ tar xzf nagioscore.tar.gz


$ cd /tmp/nagioscore-nagios-4.4.3
$ ./configure --with-httpd-conf=/etc/apache2/sites-enabled
$ make all

Create the user nagios and the group.  The Apache user www-data is also added to the nagios group.

$ useradd nagios
$ usermod -a -G nagios www-data

Install the binaries.

$ make install

Installing the service daemon files and configuring them for the boot process.

$ make install-init
$ systemctl enable nagios.service

Installs and configures the external command file.

$ make install-commandmode

Just now * SAMPLE * configuration files will be installed. These are necessary because Nagios needs some configuration files to start.

$ make install-config

Apache web server configuration files are installed and the Apache settings for Nagios are configured.

$ make install-webconf
$ a2enmod rewrite
$ a2enmod cgi

Port 80 must be permitted for incoming data traffic on the local firewall so that the Nagios Core web interface can be reached.

$ iptables -I INPUT -p tcp --destination-port 80 -j ACCEPT
$ apt-get install -y iptables-persistent

Answer yes to save the existing rules.

An Apache user account is created so that it can log into Nagios.

The following command creates a user account called nagiosadmin and a password is created for the account, now remember this password.

$ htpasswd -c /usr/local/nagios/etc/htpasswd.users nagiosadmin

The Apache web server must be restarted.

$ systemctl restart apache2.service

Nagios Core will now start.

$ systemctl start nagios.service

Nagios is now ready to be tested.

You will be asked to log in with your user name and password.  The username is nagiosadmin (you created it in a previous step) and the password is what you provided earlier.

After successfully logging in, the Nagios Core web interface appears. Congratulations, you did it.

Nagios Core is now installed, the Nagios plugins are still required for operation.  The error message appears: (No output on stdout) stderr: execvp(/usr/local/nagios/libexec/check_load .. this is normal, the standard plugins are installed in the following steps.

Plugin Installation

The following packages are installed from the repository as a prerequisite for installing the plugins.

$ apt-get install -y autoconf gcc libc6 libmcrypt-dev make libssl-dev wget bc gawk dc build-essential snmp libnet-snmp-perl gettext

Download and extract the source packages. The last plugin releases can get from

$ cd /tmp
$ wget --no-check-certificate -O nagios-plugins.tar.gz
$ tar zxf nagios-plugins.tar.gz

Compile and install packages.

$ cd /tmp/nagios-plugins-release-2.2.1/
$ ./tools/setup
$ ./configure
$ make
$ make install

Go to a host or service object and “Re-schedule the next check” in the Commands menu.  The error that appeared before should now disappear and the correct output is displayed on the screen .

The daemon commands for start / stop / restart / status.

$ systemctl start nagios.service
$ systemctl stop nagios.service
$ systemctl restart nagios.service
$ systemctl status nagios.service

Nagios configuration

Now that the Nagios Core Server is ready for operation, it is time to create the configuration of the host and services that are to be monitored.  Under /usr/local/nagios/etc the main configuration is nagios.cfg, here the paths to the configuration files are defined with cfg_file, the hosts to be monitored can be entered in a file hosts.cfg.

## Default Linux Host Template ##
define host{
name                            linux-box               ; Name of this template
use                             generic-host            ; Inherit default values
check_period                    24x7
check_interval                  5
retry_interval                  1
max_check_attempts              10
check_command                   check-host-alive
notification_period             24x7
notification_interval           30
notification_options            d,r
contact_groups                  admins
register                        0                       ; DONT REGISTER THIS - ITS A TEMPLATE
## Default
define host{
use                             linux-box               ; Inherit default values from a template
host_name                       Diskstation             ; The name we're giving to this server
alias                           Synology                ; A longer name for the server
address                            ; IP address of Remote Linux host

If it is to be more structured, there is the possibility to save the host and service configuration in the directories printers, routers, servers, switches, for this the file  nagios.cfg is  edited and the comment characters # (hash) are  removed  accordingly in  cfg_dir = .

$ vi /usr/local/nagios/etc/nagios.cfg
# Definitions for monitoring a Windows machine

The .cfg files created in the directories are read out.

Example for a mail and web server with this IMAP and HTTPS is checked.

define host {
        use                             linux-server
        host_name                       mailserver
        alias                           Exchange
        max_check_attempts              5
        check_period                    24x7
        notification_interval           30
        notification_period             24x7

define service {
        use                             generic-service
        host_name                       mailserver
        service_description             PING
        check_command                   check_ping!100.0,20%!500.0,60%

define service {
        use                             generic-service
        host_name                       mailserver
        service_description             HTTPS
        check_command                   check_tcp!443 -H -u
        notifications_enabled           0

define service {
        use                             generic-service
        host_name                       mailserver
        service_description             IMAP
        check_command                   check_imap
        notifications_enabled           0

The Nagios server is restarted after each change.

$ systemctl restart nagios.service

A look at the Nagios log file can be worthwhile.

$ tail -30 /usr/local/nagios/var/nagios.log

Additional configuration examples for Linux, Windows, printer routers and switches can be found  under the  objects  directory .

Beispiel: Nagios Service Configuration

With remote agents such as NCPA, active checks can be carried out on Windows and Linux hosts; passive checks can be carried out using NRDP and NRPE, which provide values ​​on CPU load, memory usage, processes, user and disk usage.

Nagios Notification

In the file nagios.cfg and objects/contacts.cfg the recipient email  root@localhost can be left.

$ vi /usr/local/nagios/etc/objects/contacts.cfg

define contact{
        contact_name                    nagiosadmin             ; Short name of user
        use                             generic-contact         ; Inherit default values from generic-contact$
        alias                           Nagios Admin            ; Full name of user

        email                           root@localhost          ; <<***** CHANGE THIS TO YOUR EMAIL ADDRESS *$

In the file  nagios.cfg  at admin_email .

$ vi /usr/local/nagios/etc/nagios.cfg

Postfix is ​​used here as the mail transport agent for the Nagios email notification.  This is installed and configured as follows .

$ sudo apt-get -y install postfix

During the installation you will be asked to select a mail server configuration, here we select  Internet Site .

In order to be able to test the sending of emails later, the package mailutils is installed.

$ sudo apt-get -y install mailutils

The Postfix main configuration is adapted .

$ vi /etc/postfix/
relayhost =

At relayhost, the mail server is entered that allows Raspberry Pi to receive emails, if the Raspberry is behind a firewall with NAT, the public IP address of the mail server must be authorized for reception.

Set up an email address for root by editing the aliases file .

$ vi /etc/aliases

At the end a valid email address is entered so that mails from this host are delivered, here as an example it is, the colon for root: is mandatory.

The changes in the  aliases  file  must still generate the aliases.db file .

$ newaliases

The Postfix configuration also has to be read in and activated.

$ postfix reload

Now sending emails from Raspberry Pi, this can be done as follows.

$ echo "hello from raspberry pi" | mail -s hello root

An email should now be in the inbox of

Read the email log can also provide further information here.

$ tail /var/log/mail.log

If the attempt to  send  returns  the  status=bounced , receipt on the mailer is not yet authorized. With Exchange, the IP address of the Raspberry Pi must be entered in the receive connector in the frontend transport under area definition for email received from servers with these remote IP addresses.  For Postfix a smtpd_client_restrictions  directive must exist  in .

smtpd_client_restrictions =
smtpd_recipient_restrictions =
        check_client_access hash:/etc/postfix/client_access,

client_access  file  contains  the IP address of the Raspberry Pi .

# Raspberry Pi Nagios    ok

Postfix database still needs to be generated.

$ postmap /etc/postfix/client_access

If the SMTP requests are accepted by the mailer, the queue process and delivery can take place.