Category Archive Linux

Linux File System Table (/etc/fstab file) Explained

In this guide, we will learn what is fstab in Linux, and what is the usage of fstab file, the structure of /etc/fstab file and finally how to debug fstab issues. At the end, we will also discuss a brief introduction to mtab file, its usage and how mtab differs from fstab.

What is fstab?

As a server administrator or Linux desktop user or anyone who is new to Linux, understanding about /etc/fstab file in Linux and how to debug fstab issues are very important.

Fstab is a file system table used by the kernel during boot time to mount the file system. To put it in simple terms, you will create one or more partitions on your hard drive and you will make an entry for each partition in fstab which will be read by the kernel during boot time and the file system will be automatically mounted.

By default, any partitions you create during the OS installation will be automatically added to the fstab file. Let’s dive in and see how fstab is structured and how to use fstab.

NOTE : Before editing the fstab file, it is best practice to take a backup of the file. If in case something is messed up you can revert the backup copy.

Structure of /etc/fstab file

Fstab is located in the /etc/ directory and owned by the root user. So you should edit the file using sudo privilege or as the root user.

Display the fstab file by running the following command to understand its structure.

$ cat /etc/fstab
$ cat /etc/fstab | grep -i -v "^#" | column -t # Skip comments and format
Contents of fstab file
Contents of fstab file

Fstab file consists of six columns (sections).

  1. File system – Block device which is partitioned and created a file system on it.
  2. Mount Point – Where the file system is mounted in your system.
  3. Type – File system type (Ext4, Ext3, swap, Xfs, etc.)
  4. Options – This decides what mount parameters to be considered when mounting the file system
  5. Dump – This is for the backup purposes used by the dump utility. Setting it to zero means disabling the backup and one means enabling the backup to a given device/medium.
  6. Pass – File system check at boot time by the fsck utility. Zero will disable the file system check and for the root file system it should be set to 1 and for other partitions, it should be set to 2.

Each filesystem is described on a separate line. Fields on each line are separated by tabs or spaces.

Let’s see about these six parameters in detail.

1. File System

In the first column of fstab, you need to point to which partition you are going to mount. Either you can use a block device name like /dev/sda* or use the UUID.

When you create a file system on top of any partitioned drive using the mkfs command there will be a UUID created for that particular partition.

Let’s say you created a new partition and it is sdb1. In this case, your fstab will be as follows.

/dev/sdb1 /home ext4 defaults 0 0
Mount using names
Mount using names

To get the block id for any file system run the following command.

$ blkid                # Print info about all fs.
$ blkid /dev/sdb1      # Print info about given fs alone.

Now using this block id you can mount the file system as shown in the below image.

Mount using UUID
Mount using UUID

2. Mount Point

In the second column, you have to specify the mount point directory. Mount point refers to the directory in the file system where your partitioned drive will be mounted.

As you see in the below image, each partition is mounted under a different directory. For example, the partition /dev/sdb1 is dedicated to the home directory and mounted under /home.

Mount point
Mount point

Let’s say you created a new partition with 100GB size and with EXT4 as partition type. The fstab entry will be as follows. Here /mnt/data1 is the mount point.

UUID=xxxx-xxx-xxx /mnt/data1/ ext4 defaults 0 0


/dev/sdb2 /mnt/data1/ ext4 defaults 0 0

3. Filesystem Type

Here you will mention the file system type(EXT2, EXT3, EXT4, XFS, VFAT, SWAP, etc.). If you want to know the file system type for a block device, use blkid or lsblk command.

$ lsblk -f /dev/sdb1
NAME FSTYPE LABEL UUID                                 MOUNTPOINT
sdb1 ext4         365c64d7-4d65-4cdf-8ce1-0def8bb41997 /home
$ blkid /dev/sdb1
/dev/sdb1: UUID="365c64d7-4d65-4cdf-8ce1-0def8bb41997" BLOCK_SIZE="4096" TYPE="ext4" PARTUUID="8f174339-01"
Filesystem type
Filesystem type

4. Mount Options

This is an important column where you will pass mount options for the file system. There are a lot of mount options that can be used. For normal usage using the default option is enough. The option default points to the combination of seven (rwsuiddevexecautonouser, and async) options.

Let’s see what the default option does.

  • rw – This tells the file system to be mounted in read-write mode. When an error occurs you can make the file system to be mounted in read-only mode by using the option errors=remount-ro.
  • suid – Any operation related to suid and sgid will be allowed through this option.
  • dev – It will interpret character and block devices on the file system. If you do not wish to interpret characters and block devices you can use nodev option explicitly.
  • exec – From the file system you can execute the binaries by setting the exec option. Opposite to this, you can use the noexec option to not execute binaries from the file system.
  • auto – This option is responsible for auto mounting the file system during the boot time or whenever you run the mount -a command from the terminal. Opposite to this, there is noauto option that will only allow to mount the file system individually.
  • nouser – Only a superuser can mount the file system. You can also use “user” and “users” options which can set the ordinary user to mount and unmount the file system.
  • async – All the IO operations should be done asynchronously. If you use the option “sync” the IO operation will be done synchronously.

Depending upon the requirement you may need to use different options. If you have any existing production server in your environment, I suggest you take a look at the fstab file which will give you a good idea about what are the mount options used.

Mount options
Mount options

5. Dump

This column is specific for backup purposes. When it is set to 1, the dump utility will take backups. This is an obsolete feature now as it was originally designed to take backups in tapes for older file systems.

It is recommended to set it to zero.

Dump column in fstab
Dump column in fstab

6. Pass

During boot time the fsck utility will do a file system check based on the value we provide in the sixth column. If you set the value to zero or leave it empty, the file system check will be disabled.

The root file system should always be specified to the value 1. For other file systems, you can set the value to 2. Since I am using VM for the demonstration, I have set all the values to zero.

Pass column in fstab
Pass column in fstab

If you made any mistake in the fstab file and when you reboot the machine it will go into emergency mode. In emergency mode, there will be no network interface so you have to debug directly in the console.

Emergency mode
Boot into Emergency mode

You have to type the root password to enter into the emergency mode. Run the following command and scroll slowly to see for error messages. In my case, I intentionally removed the separator (/) in /dev/sdb1.

Fix Boot problems related to Fstab file
Fix Boot problems related to Fstab file

In emergency mode, if the file system is mounted in read-only mode then you have to mount in rw mode and edit the fstab file to fix the problem. To remount the file system in rw mode run the following command.

$ mount -o rw,remount /

Now edit the fstab to fix the error and reboot the machine. Now your machine should be able to boot properly.

$ reboot

To avoid the issues during boot time, you can run the mount command after adding the fstab entries.

Take a look at the below image where I have added a new entry for /dev/sdb2. I have not specified the file system type so it will throw me an error when I run the mount command.

Filesystem type error
Filesystem type error
$ mount -a
mount: /mnt: unknown filesystem type 'defaults'.

What is a Mtab File?

When you run the mount command without passing any arguments it prints the list of mounted file systems.

$ mount

There is also a file called mtab in the /etc/ directory. When you look at /etc/mtab, it will be exactly the same as the output of the mount command.

Let’s see what the man page has to say about the mtab file.

What is mtab file
What is mtab file

The /etc/mtab file is symlinked to mounts in the proc file system. If you run the mount or umount command the result will be updated in mounts.

$ ls -ln /etc/mtab
lrwxrwxrwx 1 0 0 19 Jan 9 17:45 /etc/mtab -> ../proc/self/mounts

To summarize the difference between fstab and mtab, fstab is used to mount the file system during the boot time and mtab is used to display the list of file systems mounted.

If /etc/mtab file is corrupted or deleted by accident, you can regenerate it by using the following command.

$ sudo sh -c 'grep -v rootfs /proc/mounts > /etc/mtab'


In this article we have discussed what is fstab and mtab and how it is important for the system to store and read the partition information during boot time. We also discussed what happens when there is a mistake made in the fstab file.

Samba & Bonjour with Avahi

Okay, so you have a shiny new Linux box, and it’s running Samba, all nice and configured to share your files.  You have a Mac, and you want to use it with your nifty new Windows shares.  You can connect with Command-K in the finder, but it doesn’t show up in Finder under the Shared section.

You need Avahi.

I won’t bother going into the details of configuring Samba.  If you’ve not gotten that far, there are some pretty good resources out on the ‘net that will tell you how.  Where interacting with Bonjour is concerned, however, most of the references I found were flat wrong with modern OS X and Samba.

To make this work, the steps are simple (I’m running Ubuntu 12.04, so you may have to adjust accordingly for your Linux distro of choice).  The first step is to install Avahi:

root@core:/# apt-get install avahi-daemon avahi-utils

When this command completes, you’ll essentially have Bonjour running on your Linux box.  This has a number of advantages, most notably that you can now log into the thing by hostname (eg. core.local for my machine) without having to configure DNS.  But it still won’t allow you to browse shares in Finder; for that, you need a bit of configuration.

And so we move to step 2: create a file in /etc/avahi/services called smb.service, and place the following content in it:

<?xml version="1.0" standalone='no'?>
<!DOCTYPE service-group SYSTEM "avahi-service.dtd">
 <name replace-wildcards="yes">%h</name>

Upon saving it, your new Linux box will happily appear in the Finder sidebar, and should all work.  You don’t even need to restart Avahi; it’ll pick up the new service file automagically.

There are other references out there for how to do this, but they all use port 139 – which doesn’t work.  I haven’t a clue when Microsoft changed the port number, but whatever; I don’t really care.  I have finder browsing goodness, so I’m happy.

How to Check Open Ports in Linux?

Which ports are occupied by which service? How many open ports are there? Learn to scan for open ports on your Linux system or any remote system.Table of Contents

  1. Method 1: Checking open ports in the currently logged in Linux system using lsof command
  2. Method 2: Checking ports on any remote Linux server using the netcat command
  3. Conclusion

Whether you are using Linux as a server or desktop, knowing open ports or ports in use can be helpful in a variety of situations.

For example, if you are running an Apache or Ngnix based web server, the port in use should be 80 or 443. Checking the ports will confirm that. Similarly, you can check which port is being used by SMTP or SSH or some other services. Knowing which ports are in use can be helpful while allocating the ports to a new service.

You may also check if there are open ports for intrusion detection.

There are various ways for checking ports in Linux. I’ll share two of my favorite methods in this quick tip.

Method 1: Checking open ports in the currently logged in Linux system using lsof command

If you are logged into a system, either directly or via SSH, you can use the lsof command to check its ports.

sudo lsof -i -P -n

This lsof command is used to find the files and processes used by a user. The options user here are:

  • -i: If no IP address is specified, this option selects the listing of all network files
  • -P: inhibits the conversion of port numbers to port names for network files
  • -n: inhibits the conversion of network numbers to host names for network files
Checking open ports in Linux

But, this also shows us a lot of extra ports that the computer does not actually listen to.

You can just pipe this output to the grep command and match the pattern “LISTEN”, like this:

sudo lsof -i -P -n | grep LISTEN

This will only show the ports that our computer is actively listening to and also which service is using said open port.

Method 2: Checking ports on any remote Linux server using the netcat command

nc (Netcat) is a command line utility that read and writes data between computers over network using the TCP and UDP protocols.

Given below is the syntax for nc command:

nc [options] host port

This utility has a nifty -z flag. When used, it will make nc scan for listening daemons without actually sending any data to the port.

Combine this with the -v flag, enabling verbosity, you can get a detailed output.

Below is the command you can use to scan for open ports using the nc command:

nc -z -v <IP-ADDRESS> 1-65535 2>&1 | grep -v 'Connection refused'

Replace IP-ADDRESS with the IP address of the Linux system you are checking the ports for.

As for why I selected values 1 to 65535, that is because the port range starts from 1 and ends at 65535.

Finally, pipe the output to the grep command. Using the -v option, it excludes any line that has “Connection refused” as a matched pattern.

This will show all the ports that are open on the computer which are accessible by another machine on the network.


Of the two methods, I prefer the lsof command. It’s quicker than nc command. However, you need to be logged into the system and have sudo access for that. In other words, lsof is more suitable a choice if you are managing a system.

The nc command has the flexibility of scanning ports without being logged in.

Both commands can be used for checking open ports in Linux based on the scenario you are in. Enjoy.TipsSHARE

What is APIPA (Automatic Private IP Addressing)?

APIPA stands for Automatic Private IP Addressing (APIPA). It is a feature or characteristic in operating systems (eg. Windows) which enables computers to self-configure an IP address and subnet mask automatically when their DHCP(Dynamic Host Configuration Protocol) server isn’t reachable. The IP address range for APIPA is ( to having 65, 534 usable IP addresses, with the subnet mask of


Initially, the Internet Engineering Task Force (IETF) has reserved the IPv4 address block ( – for link-local addressing. Due to the simultaneous use of IPv4 addresses of different scopes, traffic overload becomes high. The link-local addresses are allocated to interface i.e., stateless in nature such that communication will be established when not getting a response from DHCP Server. After that Microsoft refers to this address autoconfiguration method as “Automatic Private IP Addressing (APIPA)”.

Automatic Configuration and Service Checks

It starts with when the user(client) is unable to find the data/information, then uses APIPA to configure the system with an IP address automatically(ipconfig). The APIPA provides the configuration to check for the presence of a DHCP server(in every five minutes, stated by Microsoft). If APIPA detects a DHCP server on the network configuration area, it stops, and let run the DHCP server that replaces APIPA with dynamically allocated addresses.

Note: To Know the given IP address is provided by which addressing, just run the following command:



  • Communication can be established properly if not getting response from DHCP Server.
  • APIPA regulates the service, by which always checking response and status of the main DHCP server in a specific period of time.


  • It can be used as a backup of DHCP because when DHCP stops working then APIPA has the ability to assign IP to the networking hosts.
  • It stops unwanted broadcasting.
  • It uses ARP(Address Resolution Protocol) to confirm the address isn’t currently in use.


  • APIPA ip addresses can slow you network.
  • APIPA doesnot provide network gateway as DHCP does.


  • APIPA addresses are restricted for use in local area network.
  • APIPA configured devices follow the peer to peer communication rule.

Learn to Use CURL Command in Linux With These Examples

Page Contents

What is CURL ?

CURL is a tool for data transfer. It is also available as a library for developers and as a CLI for terminal-based use cases. Both have the same engine inside (Truth is that CLI tool is just the program that uses the library under the hood).

CURL works with every protocol you might have used. Head over this site to check whether CURL works with your target protocol or not.

What CURL can do?

Hmm… Everything that is related to data transfer. Everyone must have used a browser. Even now, you are reading this article through your browser. What browser does, it requests a page and gets it as a response. It can write and read cookies. And then it renders(displaying the content, images and executing JS scripts) it. 

CURL can do everything a browser except for the last part rendering because it is not related to data transfer.

As wrap up, CURL can download HTML pages, fill HTML forms and submit them, download files from a FTP/HTTP server and upload files to the same and read/write cookies.

This makes it an excellent tool to be used in scripting, debugging and forensic analysis etc.

Curl command examples

curl command examples in Linux

Let’s see what can you do with Curl.

1. Get a response from a server

Everything from server is a response to the request. So getting a HTML page is same as downloading a file.

To get a HTML response from,


To get the list of posts as a response from a server (, 


Since we know how to get a response from a server, you can download a file ( say Google logo ).


Above command will dump binary image data which you can’t view in the terminal. You need to save them and then use a photo viewer to see them.  Continue reading to find out how to do so.
READ  9 Useful Examples of Touch Command in Linux

Note that various option flags can be placed anywhere on the command instead of the strict ordering. So no worry if you placed any option in the last while the examples had the flag in the beginning.

2. Save the file with a default file name

Every file that is served on the internet has a filename. To use the same filename as the downloaded filename use -O flag.

curl -O

3. Save the file with custom name

To save the filename with your own custom name, use -o flag followed (strictly) by a custom name.

curl -O googleRobots.txt

4. Download multiple files

To download multiple files, separate them with a white space.

curl url1 url2 url3

If you want to use -O flag for all the URL’s, use

curl url1 url2 url3 -O -O -O 

The same workaround should be done for any flag. This is because the first occurrence of a certain flag is for the first URL, the second flag is for the second URL and so on.

5. Download a range of files

curl has the in-built ability to download a range of files from the server. This can be illustrated from the following example.


Above command downloads files from logo1.png, logo2.png, logo3.png and up to logo9.png.

6. Download a file only if latest

To download a file only if the file’s modification time is latest than the given time.

curl url -z "DD MMM YY MM:HH:SS"

7. Resume Downloading

If you have already partially transferred a file, you can resume the transfer by using the -C flag. Offset from which transfer needs to be continued should be passed as a parameter to the -C flag.

curl -C 1024 -O

8. Upload a file

To upload a file to the server, one needs to use -T flag followed by the file path on your local system.

curl -T uploadFile.txt

9. Delete a file

To delete a file named deleteFile.txt in a server, one can use -X flag which is intended for any HTTP verb/method(like GET, POST, PUT, DELETE, PATCH). Most of the FTP servers will have configured DELETE method if not all advanced HTTP methods.

curl -X DELETE

You can also modify the above command for any HTTP method to do the corresponding task. For Example, if your server allows TRUNCATE method ( this is made-up HTTP method, not a standard one) which removes only the content in the file and not the file, one can use the command similar to the below one.


Above mentioned are the main uses of curl. But there might be difficulties which needed to be fought such as redirects, user authentication, SSL certificates, etc., We can call them add-ons as they are only optional but still remain crucial for certain purposes. Let’s see some of those addons and how to handle it with curl in the next section.READ  5 Practical Examples of chgrp command in Linux

10. Avoid redirects

When you request htttp:// , you will be served only the regional page such as This is done with the help of redirects (HTTP packets with status codes in the range 300-399).

You can avoid redirects with the option L.

curl -L  htttp://

11. Authentication

When the server is configured to serve for only certain individuals with credentials, they will be provided with username and password. One can make login with the help of -u flag.

curl -u username:password

12. Limit data transfer

If you want to impose a data transfer limit use –limit-rate flag. Following command tries to download the file with rate limit as 10K.

curl --limit-rate 10K

13. Show/Hide transfer Status

If the response is redirected from the terminal such as downloading, uploading then curl automatically shows the status/progress meter for the transfer.

If you do not want to see the progress meter, just append the command with -s flag. Progress will not be shown for response directed for the terminal.

14. Ignore SSL certificates

Do you remember the situations in which you need to give security certificate exception to visit some websites? If you trust the sources and you want to do a data transfer, you can ignore SSL certificate validation by using -k flag.

curl -k

15. Get Header Information also

To display the header information along with transferred data, use the -i flag.

curl -i

16. Get Header information Only

If you want only the headers and not the data, use the -I flag

curl -I

17. Change User Agent

Some websites and servers don’t allow certain kinds of devices to access their systems. But how do they know that we are using a specific kind of device? This is due to the User-Agent HTTP header field. We can change this User Agent with -A flag.

curl -A "Mozilla FireFox(42.0)"

18. Sending data to the Server

If the server needs some data such as token or API key, use -d flag to send the data. Data that needs to be sent should follow the flag in the command. One can use “&” to combine multiple data. This is usually done by GET and POST requests in browsers. This is one of the ways by which you can send your form information.

curl -d "token=34343abvfgh&name='seeni'"

19. Write Cookies to a File

Cookies are some small information that allows maintaining a session with a stateless HTTP protocol. If you want to know more about Cookies, refer to this great resource.READ  10 Practical Grep Command Examples for Developers

To write cookies to a file, -c flag followed by the cookie filename should be used.

curl -c googleCookie.txt

20. Reading Cookies from a File

To read a cookie from the file, -b flag followed by cookie filename can be used.

curl -b googleCookie.txt

Note that -b flag only reads the cookie from the file. So if the server resends another cookie, you might need to use -c option to write them.

21. Start a new Session

If you want to initiate a new session by discarding the cookies, use -j flag. It starts a new session even if you have provided the cookie file to read with -b flag.

curl -b googleCookie.txt -


Centos Install Server

Omdat CentOS nu eenmaal één van mijn favoriete server-besturingssystemen is, gaan we uit van een installatieserver met CentOS. Dat maakt overigens voor het principe niet heel veel uit, want een installatieserver maakt gebruik van dezelfde componenten, welke distributie je ook gebruikt. De server die momenteel voor me aan het werk is, staat gelijktijdig zowel OpenSUSE als CentOS te installeren. Met een goed ingerichte server installeer je dus moeiteloos elke distributie.



Een volledig geautomatiseerde installatieomgeving heeft een aantal dingen nodig. We gaan ervan uit dat je wilt opstarten vanaf het netwerk. Om dit succesvol te laten verlopen, heb je een DHCP-server nodig die IP-adressen uitdeelt aan de te installeren machines. De volgende stap bestaat eruit dat die machines ook een boot image nodig hebben. Hiervoor wordt gebruik gemaakt van een TFTP-server. De DHCP-server zorgt ervoor dat de client doorgestuurd wordt naar de TFTP-server, zodat daar de juiste bestanden aangeleverd worden om van op te starten.

Om op te kunnen starten, heeft een Linux-client 3 bestanden nodig: een boot loader, een kernel en een initramfs. De boot loader vervangt de lokale boot loader (meestal GRUB) en geeft aan welke kernel gestart moet worden. Die kernel laadt vervolgens het initramfs. Daarbij wordt ook aangegeven waarvandaan de rest van de installatie uitgevoerd moet worden. Voor dat laatste deel is een repository nodig. Om een flexibele installatieserver aan te bieden waarop je zelf bepaalt welke packages precies aangeboden worden, is het aan te raden zelf de repositories aan te bieden (en deze niet van internet te halen). Je kan daarvoor in principe elke service gebruiken, in dit artikel laten we zien hoe je voor dit doel een webserver in kan zetten.

Als je alles tot op dit punt werkend hebt, heb je een omgeving waar de te installeren servers op kunnen booten, een boot image krijgen, en vervolgens ook toegang krijgen tot een repository om de installatie verder uit te voeren. Als het hierbij blijft, moet die installatie echter nog wel steeds handmatig uitgevoerd worden. Leuk als je één of twee servers wilt installeren, minder leuk als je er 200 wilt installeren. Om dit gedeelte ook nog te automatiseren, heb je een installatiescript nodig.

Er zijn verschillende oplossingen voorhanden: SUSE gebruikt autoyast, Red Hat gebruikt Kickstart en ook op Ubuntu kun je Kickstart gebruiken. Zowel Kickstart als Autoyast gebruiken input files waarin alle parameters gedefinieerd zijn die tijdens de installatie gebruikt moeten worden. Dit script plaats je op de installatieserver, zodat het ook eenvoudig aangeroepen kan worden. Bij het aanroepen van de installatie-kernel geef je aan welk script gebruikt moet worden en daarmee maak je de procedure compleet. In de rest van dit artikel zullen we de belangrijkste delen van de configuratie bekijken die nodig zijn om dit allemaal te realiseren.


De DHCP-server

De DHCP-server heeft op zich niet veel configuratie nodig. In listing 1 zie je een voorbeeldconfiguratie, waarin de regel next-server ervoor zorgt dat DHCP de client doorstuurt naar een TFTP-server op dit adres. De filename pxelinux.0 is de PXE-boot loader die aan de client gestuurd wordt.


Listing 1: Voorbeeld DHCP-server configuratie


subnet netmask {

option routers ;

range ;


filename “pxelinux/pxelinux.0”;



De TFTP-server

TFTP is al heel lang een onderdeel van xinetd. Om TFTP te kunnen gebruiken, zet je het aan in xinetd en zorg je dat de xinetd service gestart is. Daarbij hoort een configuratiebestand waarin je het PXE-menu zet. Deze vind je standaard in /var/lib/tftptboot/pxelinux/pxelinux.cfg en heeft de naam default. Listing 2 laat een voorbeeld PXE menu zien waaruit verschillende configuraties aangeboden kunnen worden.


Listing 2: Voorbeeld PXE boot menu


default Linux

prompt 10

timeout 10

display boot.msg

label Linux

menu label ^Install RHEL

menu default

kernel vmlinuz

append initrd=initrd.img inst.repo= ks=

label Fedora

menu label Install ^Fedora Server

kernel vmlinuz-fedora

append initrd=initrd-fedora inst.repo= ks=

label Fedoraws

menu label Install Fedora ^Workstation

kernel vmlinuz-fedoraws

append initrd=initrd-fedoraws inst.repo=

label opensuse

menu label install opensuse

kernel vmlinuz-suse

append initrd=initrd-suse install=


Als je ooit een GRUB-menu gezien hebt, zie je dat een PXE boot-menu daar niet wezenlijk van verschilt. Het komt erop neer dat je voor elke distributie een kernel neerzet in de TFTP document root, daarnaast een initramfs en tot slot aanwijst welke repository gebruikt moet worden om de rest van de installatie uit te voeren. Je hebt dan in principe voldoende om de installatie op te starten en handmatig verder uit te voeren.


De Webserver

Het aanmaken van de repositories is overigens niet heel ingewikkeld. Een handige manier is om de ISO van de betreffende distributie te downloaden en via /etc/fstab te loop-mounten op de locatie waar de installatiebestanden beschikbaar moeten zijn. Handig, want je hoeft niets te kopiëren en het is eenvoudig te regelen als een nieuwe versie van je distributie beschikbaar komt.

Als je er dan ook nog voor zorgt dat die locatie beschikbaar is onder de documentroot van je webserver, hoef je helemaal niet veel meer te doen dan de webserver aan te zetten om te regelen dat repositories automatisch beschikbaar gesteld worden.


De installatie-instructies

Het laatste element van de installatie server bestaat uit de installatie instructies. Autoyast is de oplossing die door SUSE gebruikt wordt. Daarnaast is er Kickstart. De installatie-instructies worden aangeleverd in een kort tekstbestand waarin de antwoorden staan op de vragen die tijdens de installatie gesteld worden. Kickstart doet dat in een ASCII-tekstbestand, Autoyast doet het in een XML-bestand. In listing 3 zie je een voorbeeld van hoe dat eruitziet in een Kickstart bestand.


Listing 3: Voorbeeld Kickstart bestand


[root@server html]# cat anaconda-ks.cfg


# System authorization information

auth –enableshadow –passalgo=sha512


# Use network installation

url –url=””

# Run the Setup Agent on first boot

firstboot –enable

ignoredisk –only-use=sda

# Keyboard layouts

keyboard –vckeymap=us –xlayouts=’us’

# System language

lang en_US.UTF-8


# Network information

network  –bootproto=dhcp –device=p6p1 –ipv6=auto –activate

network  –hostname=localhost.localdomain

# Root password

rootpw –iscrypted $6$GUg.iLUId16gnydz$AHsdHvpPof2KmIYQH2nlF8H.lFFf9DM/J5tC91HsvZYvKpGSeTRo0oE9B8aR1KaZ5u5YK1NwXBOUhv1ZkbZVY.

# System timezone

timezone America/New_York –isUtc

user –name=user –password=$6$yMRAMUievKP2EYT5$JmwC3j.jo9ySsuo6ogUNsI.5sQvW51SgtCLtlGDD/6/dLlz.XLj2dvTXVbfTaeDSLKPfgEDkVqxvbstjpYZt9. –iscrypted –gecos=”user”

# X Window System configuration information

xconfig  –startxonboot

# System bootloader configuration

bootloader –location=mbr –boot-drive=sda

# Partition clearing information

clearpart –drives=sda –all

# Disk partitioning information

part /boot –fstype=”xfs” –ondisk=sda –size=1000

part pv.11 –fstype=”lvmpv” –ondisk=sda –size=31008

volgroup centos –pesize=4096 pv.11

logvol /  –fstype=”xfs” –size=30000 –name=root –vgname=centos

logvol swap  –fstype=”swap” –size=1000 –name=swap –vgname=centos
























mkdir /files

logger now running post

wget -O /files/centos73.iso

wget -O /files/

wget -O /files/how-to-use-labipa.pdf




De enige uitdaging is hoe je aan zo’n bestand komt. Welnu, dat is niet moeilijk. Na installatie van een Red Hat of Fedora systeem wordt het standaard aangemaakt in de home directory van de gebruiker root en op SUSE kan je het vanuit Yast genereren met behulp van de autoyast module. Het enige dat dan nog rest, is in het PXE-boot bestand aan te geven waar de te installeren server dit bestand kan vinden en je installatieserver is klaar voor gebruik.

BIOS and UEFI explained, all you need to know about.

BIOS and UEFI are two of a kind, but completely different from each other. They serve one major purpose: booting the machine and they do it in different ways and with different options. Without them, all your hardware and the very machine you’re reading this article on, wouldn’t even start. But what are the differences? And why are they mutually exclusive?

What is BIOS?

The Basic Input Output System is the older standard and dates back to old IBM-compatible computers. For almost twenty years, the BIOS has been a de facto standard in common computer implementations. The BIOS is a special software called firmware that is stored in a special chip soldered on the motherboard called ROM (usually EEPROM these days). When you press the power button, the BIOS is the first software that is run on the machine. This software is mostly responsible for three things:

  1. Performing POST: (Power-On Self-Test) in this phase the BIOS checks if the component installed on the motherboard are functioning (mostly CPUs and RAMs).
  2. Providing Basic IO: so that essential peripherals such as the keyboard, the monitor and serial ports can operate to perform basic tasks.
  3. Booting: this step is where all the magic happens, the BIOS tries to boot from the devices connected (SSDs, HDDs, PXE, whatever) in order to provide a better-suited interface (usually an Operating System) to fully make use of the hardware components.

As you can see the BIOS is pretty much a fundamental brick of the boot process and without it you wouldn’t be able to “start” the computer. A BIOS is usually associated with the motherboard and is mostly visible during the first seconds after powering the computer. When you see a great logo from the motherboard/computer manufacturer and (usually) hear beeps, the BIOS is at work.

What the BIOS can and can’t do

BIOS perform quite a strict role and it might appear to you that they always do the same thing. In the past, BIOS were written on plain ROMs (or difficult-to-erase ROMs), without the possibility to write or to erase the ROM, the software couldn’t be programmed or upgraded. Nowadays, BIOS can be updated to support newer hardware/features and can be programmed to perform specific tasks such as:

  • Turning on/off USB portsSerial ports or IDE/SATA ports;
  • Over/Underclocking CPUs/RAMs frequencies;
  • Regulate motherboard fan controllers;

Although BIOS can perform these task well, they still operate in the 16-bit realm and as such they are limited. The most prominent limitation can be observed when using 2TB+ disks. Most BIOS can only boot from an MBR-partitioned disks, but MBR itself supports up to 2TB partitions meaning it won’t recognize the disk past that. Well there’s GPT that solves the problem of disks bigger than 2TB, but wait… most BIOS can’t boot from GPT. This means that if you have a 3TB disk you have two choices:

  • Use MBR partitioning: you will be able to boot an Operating System but the system will be presented with 2TB only.
  • Use GPT partitioning: you will not be able to boot.

The choice is obvious. But how can a disk larger than 2TB be used as a boot disk?

UEFI the BIOS successor

The Unified Extensible Firmware Interface aims to resolve what BIOS could not. UEFI itself is the second version (2.*), the former being EFI (1.*). If you bought a computer after 2010, you will probably have a UEFI instead of a BIOS. You read correctly, BIOS and UEFI do the same thing, but they are pretty different in how and what they do. A UEFI can (in addition to what a BIOS can):

  • Boot from disks larger than 2TB using GPT (assuming the operating system supports both).
  • Provide the user with a graphical user interface which is easier to use than old terminal user interfaces of BIOS.
  • Provide support for mouse devices (BIOS can rarely do this).
  • Boot securely using a chain-of-trust. (More later on secure boot).
  • Network boot (although most BIOS can do that, that’s not a given).
  • Provide a modular interface which is independent from the CPU architecture.
  • Provide a modular interface for applications and devices based on EFI drivers (commonly called EBCsEFI Byte-Code).

Do I have a BIOS or a UEFI?

Unless you read your motherboard’s manual, there is no precise way to tell if you’re using a BIOS or a UEFI. But there are a few signs:

  • UEFIs usually have pretty, coloured interfaces.
  • In UEFI you can usually use your mouse.
  • If you bought the computer/motherboard after 2010, chances are you have a UEFI system.

UEFI and boot modes

With the inception of UEFI a new boot mode was born, leaving us with two modes:

  • UEFI mode: the newer boot mode, requires a separate partition (called EFI partition) where bootloaders are stored.
  • BIOS mode: the old way used by the BIOS, the bootloader would be stored on the disks (usually at the beginning of the disk).

This created a lot of confusion, especially among tech enthusiasts. Before UEFI the only way to install an operating system was the BIOS mode, but with UEFI, the UEFI mode was the new standard and the selected default. This, however, messed with Operating Systems: Operating System installed in BIOS mode can’t be booted using UEFI mode and vice versa. This means that if you have installed an operating system in BIOS mode you can’t boot in UEFI mode without modifying the installation or reinstalling the whole system, the same applies with a UEFI installation and a BIOS boot. That’s why many UEFI now support the so-called Legacy Mode.

UEFI and Legacy Mode

Put it simply, the Legacy Mode is UEFI operating as if it was a BIOS. You will lose most of the benefits the UEFI such as the Secure Boot or the Fast Boot, but will retain the graphical user interface. The only difference is that the UEFI will be able to boot from MBR disks (hence without the required EFI partition) and will be able to boot non-UEFI installations. Most motherboards support Legacy Mode nowadays.

I have a UEFI, was my operating system installed in UEFI or Legacy mode?

This can be determined using the Operating system capabilities:

  • Windows: Use the Disk Management tool to check if a “EFI System Partition” exists on the disk where Windows is. If there is one, the system was installed in UEFI mode, if not it was installed in Legacy mode.
  • Linux: check if /sys/firmware/efi exists, if it does the system is installed in UEFI mode.

UEFI and Secure Boot

One of the most discussed features is the so-called Secure Boot (sometimes called Trusted Boot), the secure boot was born to ensure a more secure boot than the past. By denying the execution of unsigned code, Secure Boot enforces protection against malwares that operate in the pre-bootenvironment. This feature, however, had a negative effect on Linux users and vendors. To be able to boot an operating system, the same (more precisely the bootloader) had to be signed by a known key, which had to be recognized by the UEFI. When the first UEFI implementations started shipping it became clear that most Linux vendors weren’t prepared for this inception. Only a few vendors (namely Canonical, SUSE and Red Hat) could sign their operating system to work with Secure Boot. For a short time before UEFI, a fear that hardware vendors tied to Microsoft would enforce Secure Boot without the possibility to turn it off started spreading. Nowadays most UEFIs (albeit not every one) allow turning off Secure Boot. This enables a less-secure boot but allows unsigned operating systems to be booted.

Fast Boot? Quick Boot? Ultra Fast Boot?

All these names are vendor-specific ways to say “boot Windows faster“. These technologies use cache and hibernation files in order to produce a faster boot. This is usually so fast that the user won’t even see the POST screen or be able to boot from USB. Fast boot is a mechanism supported by Windows only.

Tips & tricks : Iptables provides powerful capabilities to control traffic coming in and out of your system.

Modern Linux kernels come with a packet-filtering framework named Netfilter. Netfilter enables you to allow, drop, and modify traffic coming in and going out of a system. The iptables userspace command-line tool builds upon this functionality to provide a powerful firewall, which you can configure by adding rules to form a firewall policy. iptables can be very daunting with its rich set of capabilities and baroque command syntax. Let’s explore some of them and develop a set of iptables tips and tricks for many situations a system administrator might encounter.

Avoid locking yourself out

Scenario: You are going to make changes to the iptables policy rules on your company’s primary server. You want to avoid locking yourself—and potentially everybody else—out. (This costs time and money and causes your phone to ring off the wall.)

Tip #1: Take a backup of your iptables configuration before you start working on it.

Back up your configuration with the command:

/sbin/iptables-save > /root/iptables-works

Tip #2: Even better, include a timestamp in the filename.

Add the timestamp with the command:

/sbin/iptables-save > /root/iptables-works-`date +%F`

You get a file with a name like:


If you do something that prevents your system from working, you can quickly restore it:

/sbin/iptables-restore < /root/iptables-works-2018-09-11
ln –s /root/iptables-works-`date +%F` /root/iptables-works-latest

Tip #4: Put specific rules at the top of the policy and generic rules at the bottom.

Avoid generic rules like this at the top of the policy rules:

iptables -A INPUT -p tcp --dport 22 -j DROP

The more criteria you specify in the rule, the less chance you will have of locking yourself out. Instead of the very generic rule above, use something like this:

iptables -A INPUT -p tcp --dport 22 –s –d -j DROP

This rule appends (-A) to the INPUT chain a rule that will DROP any packets originating from the CIDR block on TCP (-p tcp) port 22 (–dport 22) destined for IP address (-d

There are plenty of ways you can be more specific. For example, using -i eth0 will limit the processing to a single NIC in your server. This way, the filtering actions will not apply the rule to eth1.

Tip #5: Whitelist your IP address at the top of your policy rules.

This is a very effective method of not locking yourself out. Everybody else, not so much.

iptables -I INPUT -s <your IP> -j ACCEPT

You need to put this as the first rule for it to work properly. Remember, -I inserts it as the first rule; -A appends it to the end of the list.

Tip #6: Know and understand all the rules in your current policy.

Not making a mistake in the first place is half the battle. If you understand the inner workings behind your iptables policy, it will make your life easier. Draw a flowchart if you must. Also remember: What the policy does and what it is supposed to do can be two different things.

Set up a workstation firewall policy

Scenario: You want to set up a workstation with a restrictive firewall policy.

Tip #1: Set the default policy as DROP.

# Set a default policy of DROP

Tip #2: Allow users the minimum amount of services needed to get their work done.

The iptables rules need to allow the workstation to get an IP address, netmask, and other important information via DHCP (-p udp –dport 67:68 –sport 67:68). For remote management, the rules need to allow inbound SSH (–dport 22), outbound mail (–dport 25), DNS (–dport 53), outbound ping (-p icmp), Network Time Protocol (–dport 123 –sport 123), and outbound HTTP (–dport 80) and HTTPS (–dport 443).

# Set a default policy of DROP

# Accept any related or established connections

# Allow all traffic on the loopback interface
-A INPUT -i lo -j ACCEPT

# Allow outbound DHCP request
-A OUTPUT –o eth0 -p udp –dport 67:68 –sport 67:68 -j ACCEPT

# Allow inbound SSH
-A INPUT -i eth0 -p tcp -m tcp –dport 22 -m state –state NEW  -j ACCEPT

# Allow outbound email
-A OUTPUT -i eth0 -p tcp -m tcp –dport 25 -m state –state NEW  -j ACCEPT

# Outbound DNS lookups
-A OUTPUT -o eth0 -p udp -m udp –dport 53 -j ACCEPT

# Outbound PING requests
-A OUTPUT –o eth0 -p icmp -j ACCEPT

# Outbound Network Time Protocol (NTP) requests
-A OUTPUT –o eth0 -p udp –dport 123 –sport 123 -j ACCEPT

# Outbound HTTP
-A OUTPUT -o eth0 -p tcp -m tcp –dport 80 -m state –state NEW -j ACCEPT
-A OUTPUT -o eth0 -p tcp -m tcp –dport 443 -m state –state NEW -j ACCEPT


Restrict an IP address range

Scenario: The CEO of your company thinks the employees are spending too much time on Facebook and not getting any work done. The CEO tells the CIO to do something about the employees wasting time on Facebook. The CIO tells the CISO to do something about employees wasting time on Facebook. Eventually, you are told the employees are wasting too much time on Facebook, and you have to do something about it. You decide to block all access to Facebook. First, find out Facebook’s IP address by using the host and whois commands.

host -t a is an alias for has address
whois | grep inetnum
inetnum: –

Then convert that range to CIDR notation by using the CIDR to IPv4 Conversionpage. You get To prevent outgoing access to, enter:

iptables -A OUTPUT -p tcp -i eth0 –o eth1 –d -j DROP

Regulate by time

Scenario: The backlash from the company’s employees over denying access to Facebook access causes the CEO to relent a little (that and his administrative assistant’s reminding him that she keeps HIS Facebook page up-to-date). The CEO decides to allow access to only at lunchtime (12PM to 1PM). Assuming the default policy is DROP, use iptables’ time features to open up access.

iptables –A OUTPUT -p tcp -m multiport –dport http,https -i eth0 -o eth1 -m time –timestart 12:00 –timestart 12:00 –timestop 13:00 –d  -j ACCEPT

This command sets the policy to allow (-j ACCEPT) http and https (-m multiport –dport http,https) between noon (–timestart 12:00) and 13PM (–timestop 13:00) to (–d

Regulate by time—Take 2

Scenario: During planned downtime for system maintenance, you need to deny all TCP and UDP traffic between the hours of 2AM and 3AM so maintenance tasks won’t be disrupted by incoming traffic. This will take two iptables rules:

iptables -A INPUT -p tcp -m time –timestart 02:00 –timestop 03:00 -j DROP
iptables -A INPUT -p udp -m time –timestart 02:00 –timestop 03:00 -j DROP

With these rules, TCP and UDP traffic (-p tcp and -p udp ) are denied (-j DROP) between the hours of 2AM (–timestart 02:00) and 3AM (–timestop 03:00) on input (-A INPUT).

Limit connections with iptables

Scenario: Your internet-connected web servers are under attack by bad actors from around the world attempting to DoS (Denial of Service) them. To mitigate these attacks, you restrict the number of connections a single IP address can have to your web server:

iptables –A INPUT –p tcp –syn -m multiport -–dport http,https –m connlimit -–connlimit-above 20 –j REJECT -–reject-with-tcp-reset

Let’s look at what this rule does. If a host makes more than 20 (-–connlimit-above 20) new connections (–p tcp –syn) in a minute to the web servers (-–dport http,https), reject the new connection (–j REJECT) and tell the connecting host you are rejecting the connection (-–reject-with-tcp-reset).

Monitor iptables rules

Scenario: Since iptables operates on a “first match wins” basis as packets traverse the rules in a chain, frequently matched rules should be near the top of the policy and less frequently matched rules should be near the bottom. How do you know which rules are traversed the most or the least so they can be ordered nearer the top or the bottom?

Tip #1: See how many times each rule has been hit.

Use this command:

iptables -L -v -n –line-numbers

The command will list all the rules in the chain (-L). Since no chain was specified, all the chains will be listed with verbose output (-v) showing packet and byte counters in numeric format (-n) with line numbers at the beginning of each rule corresponding to that rule’s position in the chain.

Using the packet and bytes counts, you can order the most frequently traversed rules to the top and the least frequently traversed rules towards the bottom.

Tip #2: Remove unnecessary rules.

Which rules aren’t getting any matches at all? These would be good candidates for removal from the policy. You can find that out with this command:

iptables -nvL | grep -v "0     0"

Note: that’s not a tab between the zeros; there are five spaces between the zeros.

Tip #3: Monitor what’s going on.

You would like to monitor what’s going on with iptables in real time, like with top. Use this command to monitor the activity of iptables activity dynamically and show only the rules that are actively being traversed:

watch --interval=5 'iptables -nvL | grep -v "0     0"'

watch runs ‘iptables -nvL | grep -v “0     0″‘ every five seconds and displays the first screen of its output. This allows you to watch the packet and byte counts change over time.

Report on iptables

Scenario: Your manager thinks this iptables firewall stuff is just great, but a daily activity report would be even better. Sometimes it’s more important to write a report than to do the work.

Use the packet filter/firewall/IDS log analyzer FWLogwatch to create reports based on the iptables firewall logs. FWLogwatch supports many log formats and offers many analysis options. It generates daily and monthly summaries of the log files, allowing the security administrator to free up substantial time, maintain better control over network security, and reduce unnoticed attacks.

Here is sample output from FWLogwatch:

More than just ACCEPT and DROP

We’ve covered many facets of iptables, all the way from making sure you don’t lock yourself out when working with iptables to monitoring iptables to visualizing the activity of an iptables firewall. These will get you started down the path to realizing even more iptables tips and tricks.

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