Me in IT UNIX/Linux Consultancy is based in Utrecht, The Netherlands and specialized in UNIX and Linux consultancy. Experience with Red Hat Enterprise Linux (Red Hat Certified Architect), Fedora Project, CentOS, OpenBSD and related released Open Source products makes Me in IT UNIX/Linux Consultancy a great partner in implementing, maintaining and upgrading your environment.

Open Source software is an important aspect of any Linux distribution. Me in IT UNIX/Linux Consultancy tries to use Open Source software where possible and tries to share experiences actively. In the articles section you will find many UNIX/Linux adventures shared for others to benefit.

Determining the quality of a Dutch contracting company

As a freelancer in Dutch IT, you will almost always use a contracting company. This company makes an initial contact with the end-customer and receives and pays your invoices.

When you are in contact with a new contracting company, you can check IT Bemiddelaars to see what a company has done before, compare rates and cost.

When you would like to review an "IT bemiddelaar", please go to IT Bemiddelaars to enter the information you'd like to share.

Setting up an OpenVPN server and clients on Fedora Core

Besides the extensive OpenVPN documentation I couldn't really find a step by step guide on how to setup an OpenVPN server and client.

I want to be able to connect my Fedora Core 10 laptop to the home-network via a Fedora Core 10 server.

Overview of the network setup

The server is 192.168.0.1, running Fedora Core 10 with openvpn 2.1. Newer versions are likely to work as well.
The laptop has a dynamic IP address.

Setting up the server

Please read the OpenVPN documentation if you run into troubles.

First setup the Certificate Authority keys.

cd /usr/share/openvpn/easy-rsa/2.0/
vi vars
# Fill KEY_COUNTRY, KEY_PROVINCE, KEY_CITY, KEY_ORG, and KEY_EMAIL, all at the bottom.
. ./vars
./clean-all
./build-ca
# Press "enter" on each item.

Now setup the Server key and the Diffie Hellman key.

./build-key-server server
# Press enter on each item.
./build-dh

And build the client keys, each client needs its own key, with a unique Common Name. The IP address assigned to the client is related to the Common Name, so if you use non-unique Common Names, you will have conflicting IP addresses.

./build-key client1
./build-key client2
./build-key client3
mv keys /etc/openvpn

cp /usr/share/doc/openvpn-2.1/sample-config-files/roadwarrior-server.conf /etc/openvpn/server.conf
vi /etc/openvpn/server.conf
# Check the ca, cert, key and dh variables.

Start the server by issuing:

chkconfig openvpn on
service openvpn start

Setting up the client

Move the ca.cert, client1.crt and client1.key to the client, in /etc/openvpn/keys and copy the configuration. You will need to modify the client.conf a little bit.

/usr/share/doc/openvpn-2.1/sample-config-files/roadwarrior-client.conf /etc/openvpn/client.conf
vi /etc/openvpn/client.conf
# Find "remote" and set it to the internet address of your VPN server.

Start the client:

chkconfig openvpn on
service openvpn start

Checking if it worked

On both server and client issue ifconfig tun0 to see what IP-address is assigned to the entry of the tunnel. From the either end of the tunnel you should be able to ping the other end. Also tcpdump -n -i tun0 should work.

Hope this works for you as well, check out /var/log/messages for information. /etc/init.d/openvpn status dump a status of all connected clients to /var/log/messages.

Price of an average webserver on Amazon Elastic Compute Cloud (EC2)

It's very difficult to get grip on the estimates price of a "simple" LAMP server. Here are some numbers to help you get an accurate estimate of a LAMP server.

The setup

  • One small Linux (Fedora) instance in Europe.
  • One 30 Gb volume in Europe.
  • Two 1 Gb volumes in Europe.
  • Apache, Mysql on the same machine and PHP.
  • Average daily visitors: 8000.

The price and numbers

  • 730 small instance hours = $ 80.30
  • 32 Gb of provisioned storage = $ 3.84
  • 95 Gb traffic in = $ 9.50
  • 27 Gb traffic out = $ 4.59
  • 5 million IO requests = $ 0.55
  • TOTAL $ 98.78 (Without tax)

So as a conclusion: One average webserver costs around $ 120,- (with tax) per month to run on the EC2. There are cheaper solutions to host your website(s), but Amazon EC2 provides the option to add a machine in a couple of minutes or even automated. This flexibility is not found in many other products.

Howto use Amazon Elastic Compute Cloud. (EC2)

While experimenting with Amazons interpretation of cloud computing, here is what I did to create persistent storage, create an instance and attach the storage to the instance.

Install and configure the local tools.

Go to Amazons developer section and download, unpack and install the files. Now edit ~/.bashrc (or any other file that is executed at login) and add this:

export EC2_HOME=~/.ec2
export PATH=$PATH:$EC2_HOME/bin
export EC2_PRIVATE_KEY=$(ls $EC2_HOME/pk-*.pem)
export EC2_CERT=$(ls $EC2_HOME/cert-*.pem)
export JAVA_HOME=/System/Library/Frameworks/JavaVM.framework/Home/
export EC2_URL=https://eu-west-1.ec2.amazonaws.com

As you can see, I have the tools installed in ~/.ec2, the JAVA_HOME is set for a Mac OS X machine and I use the European Amazon infrastructure. Get a list of these regions with ec2-describe-regions.

Create a keypair.

Check out the wonderful section on Paul Stamatiou's website at "Getting Started". It describes how to create and use the keys.

Open ports 22 and 80.

You will have to authorize access from the internet to port 22 and 80, or any other. Here is how it's done:

ec2-authorize default -p 22
ec2-authorize default -p 80

default refers to all machines unless specified differently.
The option -p refers to a port number.

Create an instance.

An instance can be seen as an individual machine. It's virtual, but who cares about that? I use the image "ami-2a0f275e", but see other images can be used as well. Use ec2-describe-images -o amazon to get a list of available images owned (-o) by amazon.

ec2-run-instances -z eu-west-1a -k ec2-keypair ami-2a0f275e

The option -z specifies an "availability zone". Get a list of these zones with ec2-describe-availability-zones.
The option -k specifies what key to use. If you don't use this option, you will not be able to login using SSH.

Create a volume.

To allocate some space on the S3 infrastructure of Amazon, use this command:

ec2-create-volume -s 1 -z eu-west-1a

The option -s 1 specifies that the size is 1 Gib.
The option -z is used to determine the availability zone, it needs to match the instance's zone.

Attach the volume to the instance.

When the volume is created; assign it to an instance with this command:

ec2-attach-volume vol-38a24751 -i i-c2f2c5b6 -d sdb

This only makes the device available, you will have to partition, format and mount it to use it.
The argument vol-38a24751 describes the volume to attach. Use ec2-describe-volumes to get a list of available volumes.
The option -i specifies the instance. Use ec2-describe-instances to get a list of available instances.
The option -d specifies the name under which Linux will recognize the volume. Login to you machine, type dmesg to see if attaching has worked. This is the ouput I got:
dmesg | tail -n 1
sdb: unknown partition table

Logging into your instance

Login to your machine using SSH:

ssh -i .ec2-keypair root@MACHINE

The option -i specifies the identity to use.
The argument MACHINE needs to be replaces with the public DNS name of you instance. Get a list of the named using describe-instances.
If you like to type less; add this to your ~/.ssh/config file:
Host *.compute.amazonaws.com
IdentityFile ~/.ec2/ec2-keypair
User root

From the moment on that you have added this configuration, you can simply login to your instance without any options, just the DNS name of the instance.

Formatting and mounting the volume.

Now that you are ready, login and type:

mkfs.ext3 /dev/sdb

Mount the volume (once) by issuing:

mount /dev/sdb /mnt

There is 924 Megabytes (Mb) available. so you'll lose some 80 Mb's for the filesystem.

Setting a static IP.

You can continue to use the instance with this "static" IP, but to associate one IP with this instance, follow these steps. First register an IP:

ec2-allocate-address

You will see the IP printed on your screen.

Now link the IP with an instance.

ec2-associate-address 79.125.5.49 -i i-0ca09678

Conclusion.

The Amazon elastic compute cloud and S3 facilities work great, I'm not sure about the availability of EC2, not about S3, but Amazon states that S3 should be more "secure" then storing stuff in the local storage of the instance.

Linux permission numberic table

Permissions in Linux (or UNIX) can be difficult to understand. Here is a step-plan to determine the right combination of permissions.

Either read in the "Explanation" field in the table below what you want to do, or do ls -l and see what it means. Each object (file, directory, sockets, device, etc) has 10 positions to indicate what's possible with the object. For example you could see -rwxr-x---. You can split the 10 positions up into these parts:

  • The 1st character: what kind of object is it; - for file, d for directory, s for socket.
  • The 2nd until and including the 4th character: the permissions for the owner of the object.
  • The 5th until and including the 7th character: the permissions for the group that owns the object.
  • The 8th until and including the 10th character: the permissions for others.
Numeric Readable Explanation
0 --- No access.
1 --x Execute access.*
2 -w- Write access.**
3 -wx Write and execute access.***
4 r-- Read access.
5 r-x Read and execute access.
6 rw- Read and write access.
7 rwx Read, write and execute access.

*= This is an odd combination, executing something that's not readable is not possible.
**= A strange combination; writing when you are not able to read.
***= This is an weird combination, you can't execute when you can't read the file, though you may write the file.

There are some special permission sets. When you see an "s" or an "S" on the location where you'd expect an "x", this means:

  • "s" for the owner - If somebody is allowed to execute the script (group or other) then it's executed as if it was executed by the user. This is called a "set user id bit" or "suid" and can be set by appending a 4 to a permission set. For example: chmod 4755 object.
  • S for the owner - The set user id bit was set, but no execute permissions were set in the first place. This is a broken set of permissions, but may be achieved by chmod 4650 object.
  • s for the group - When somebody is allowed to execute a script (user or other) then it's executed as if it was executed by the group. This is called a "set group id bit" or "sgid" and can be set by appending a 2 to a permission set. For example: chmod 2775 object. This bit on a directory means all files in that directory that will be created, will be owned by the group that owns the directory.
  • S for the group - The set group id bit was set, but no execute permissions were set in the first place. This is a broken set of permissions, but can be achieved by executing chmod 2745 object.

ksh(3) survival guide for bash(1) users

So you like bash, just like me. There are times though where you'd need to work on a machine where there is no bash, but ksh(3). Here is a list of commands and keys to help you:

Go back in history

Hit escape a few times, now hit "k" and "j" to move back and forward into the history.
Edit the command with the letters "h" and "l".

Complete commands

Type a part of your command, hit escape a few times, now hit "\" to complete the command.
You can't display what your options are, (in bash just tab a few times)

Get a decent prompt

Either run this command once, or add it to your ~/.profile:

export PS1="${USER}@$(hostname) ${PWD##*/} $ "

One major problem; the PWD variable does not change when changing directories!

A group password in Linux

Have you ever heard of (or used) a group password in Linux? For me this strange concept was new, but here's what you can use it for.

A group password in Linux allows a user to temporarily (in a subshell) gain extra permissions of a group, after successfully entering the group password.

To set a group password use gpasswd:

# gpasswd finance
New Password:
Re-enter new password:

To gain those extra permissions you can use newgrp:

$ newgrp finance

Some of the disadvantages are:

  1. Sharing a password is not good; a password should be personal.
  2. You can also solve this by adding the user to a secondary group.
  3. Another way to solve it is to use sudo.
  4. Usage of the group password is not accountable.

Why a umask of 0027 creates files like 0640

The User Mask (umask) can be managed with the command umask. A umask is the reverse value of the octal permission set that files and directories are created with.

So, a umask of 0777 creates files with an octal permission value of 0000; no permissions to read, write or execute.

But; there is a strange thing about the umask; it never allows to make files executable. Here is a demonstration of this "flaw"/"security feature".

$ umask 0027
$ touch me
$ ls -l me | awk '{print $1}'
-rw-r-----

I would have expected 750, instead 640 is produced. This is default security behaviour of UNIX/Linux.

The longest left hand typed word

On slashdot somebody desribes that you can find the longest left hand typed password with UNIX tools like grep. That's a simple solution for a difficult problem!

Here is a script that would print the longest words typable with the left hand side of a keyboard. The list shows the longest word on top.

#!/bin/sh

status="notfound"
cat /usr/share/dict/words | while read word ; do
  for letter in y u i o p h j k l n m ; do
   echo "$word" | grep "$letter" > /dev/null
   if [ "$?" = 0 ] ; then
    status="found"
   fi
  done

  if [ "$status" = "notfound" ] ; then
   echo "$word"
  fi
done | while read match ; do
  length=$(echo "$match" | wc -c)
  echo "$length $match"
done | sort -nr | awk '{print $2}'

Howto use NAT on Fedora Core 9 machine with iptables

Network Address Translation is a technique to masquerade IP addresses on your internal LAN to the outside world. In other words; the outside world will not be able to look into your network.

This technique is easy to setup and maintain, saves IP addresses and is likely more secure that pure routing. To set it up, you require:

  1. A Linux machine, for this example Fedora Core 9
  2. Two network cards, eth0 attached to you LAN, eth1 connected to the internet.
  3. A little bit of Linux/UNIX knowledge
  4. 30 minutes or so

Enable IP forwarding

To be able to use IP forwarding, you must tell the kernel that it's okay to forward traffic from one network card to another. This setting is found in /etc/sysctl.conf. Set net.ipv4.ip_forward to 1.
To do this, execute:

echo 1 > /proc/sys/net/ipv4/ip_forward
echo "net.ipv4.ip_forward = 1" >> /etc/sysctl.conf

The first command enables IP forwarding now, a reboot would undo this setting. To enable this setting for every reboot, also execute that second line.

Enable the firewall (IPtables)

Make sure the service IPtables is running now and is started at bootup:

# service iptables status
<output omitted>
# chkconfig --list iptables
iptables 0:off 1:off 2:on 3:on 4:on 5:on 6:off

Configure the firewall (IPtables)

Now that the kernel knows it's allowed to forward traffic from one NIC to another, configure the firewall. The firewall is the intelligent part of setting up NAT, IPtables actually 'does the work'. Here are the commands to set it up:

/sbin/iptables -A FORWARD -i eth0 -o eth1 -m state --state RELATED,ESTABLISHED -j ACCEPT
/sbin/iptables -A FORWARD -i eth1 -o eth0 -j ACCEPT
/sbin/iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE

Safe the firewall rules

You have only configured the firewall for now, a reboot would undo all settings. Run this command once you are happy with the setup:

# /sbin/service iptables save

Reboot to test the setup. Your LAN client will have to set the default route to the IP address of the NAT machine's LAN NIC.

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References Red Hat Certified Architect By Robert de Bock Robert de Bock
Curriculum Vitae By Fred Clausen +31 6 14 39 58 72
By Nelson Manning [email protected]
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