Force Virtualbox Display in Fullscreen

I have a dual display setup with virtualbox.  In fullscreen I found that virtualbox would switched onto my smaller display.

This can be adjusted.

1) Switch to fullscreen with Virtualbox

2) Use the combination of your "host" key + "home" key. (right-ctrl + home) in my case.

3) Go to the "view" menu item -> "virtual screen"

4) Select the appropriate monitor.

5) If the size of your display varies,  you will have to go out of fullscreen and back into fullscreen to re-adjust the workspace to the correct size.

yum crashed with python import error - fixed corrupted rpm database

I ran into an interesting error while trying to find out, which repository one of my installed packages came from.  Before we proceed, let me explain that I had need to use the "repoquery" utility which is part of the "yum-utils" package.  I proceeded to install this one as I did not yet have it.  The installation worked perfectly well and did not install any dependencies.

# yum install yum-utils -y

Using the repoquery command, I attempted to query which repo my php53 package came from:

$ repoquery -i php53

Instead of getting the information I wanted, the script crashed with the following error:

File "/usr/bin/repoquery", line 38, in

from yum.i18n import to_unicode

cannot import name to_unicode

Googling around, many blog posts and site talked about the yum installation being broken.  That may well be, but I decided to see if maybe there was something a bit simpler at play here.  First I needed to find out which yum packages are already installed on this system:

# rpm -qa | grep -i yum

yum-fastestmirror-1.1.16-16.el5.centos
yum-3.2.22-37.el5.centos
yum-updatesd-0.9-2.el5
yum-utils-1.1.16-16.el5.centos
yum-metadata-parser-1.1.2-2.el5

NOTE: The listed version numbers do not reflect the original numbers I had on my system.  I went ahead and attempted to update all of the above listed packages:

# yum update yum yum-fastestmirror yum-updatesd yum-metadata-parser

Yum only found that yum and yum-fastestmirror needed to be updated.  I proceeded with the update.

After the update, the repoquery command started working perfectly well.  However, a completely unrelated problem occurred which I will discuss very briefly.

# repoquery -i php53

Instead of getting a nice listing of information from the RPM database, I received an error message saying the database was corrupted.  The next step then was to rebuild the dabase.  

The database files are located in "/var/lib/rpm" and are named __db.001 __db.002 etc... etc...

Delete those files:

# rm -f /var/lib/rpm/__db*

Rebuild the database:

# rpm -vv --rebuilddb

Once completed, I tried the repoquery command once again:

# repoquery -i php53

Name        : php53
Version     : 5.3.3
Release     : 1.el5_7.6
Architecture: x86_64
Size        : 3591477
Packager    : None
Group       : Development/Languages
URL         : http://www.php.net/
Repository  : updates
Summary     : PHP scripting language for creating dynamic web sites
Description :
PHP is an HTML-embedded scripting language. PHP attempts to make it
easy for developers to write dynamically generated webpages. PHP also
offers built-in database integration for several commercial and
non-commercial database management systems, so writing a
database-enabled webpage with PHP is fairly simple. The most common
use of PHP coding is probably as a replacement for CGI scripts.

The php package contains the module which adds support for the PHP
language to Apache HTTP Server.

Convert Blocks to Bytes

Very easily 

=========== CONVERT FROM BYTES TO BLOCKS =========== 

For example:

100 Megabytes = 1024 x 1024 x 100

---------------------------------

1 Megabyte = 1024 x 1024 x 1

1 Kilobyte = 1024 x 1

1 Byte = 1

A block is a set quantity of bytes. For example, a mounted partition could have blocks of 4096 bytes or 4k.

Typically however, quotas will have a block size of 1024 bytes ( 1k ).

To calculate the quantity of blocks for quotas you should allocate blocks using the following formula:

Allocate 100Mb of space to a user:

1024 x 1024 x 100 = 104,857,600 (in bytes) = 100Mb

Divide the number of bytes by the block size:

Block size of 4096: 104,857,600 / 4096 = 25,600 blocks

Block size of 1024: 104,857,600 / 1024 = 102,400 blocks

25,600  blocks = 100Mb if your block size is 4096 (unlikely for quotas)

102,400 blocks = 100mb if your block size is 1024

=========== CONVERT FROM BLOCKS TO BYTES ===========

Now determine how many Megabytes a quota of 262144 blocks equals:

1) 262144 multiplied by the size of the blocks:

262144 x 1024 = 268,435,456 (number of total bytes)

2) Since we are dealing with megabytes, divided by 1024 x 1024

( 268,435,456 / 1048576 ) = 256

Add date to Bash History

In order to add a date stamp to your bash history add the following two lines to your .bash_profile:

HISTTIMEFORMAT='%F %T '

export HISTTIMEFORMAT

Alternativelly, you can set this variable globally and have all history files keep the data by setting these two lines in a file under the /etc/profile.d directory.

echo "HISTTIMEFORMAT='%F %T '" > /etc/profile.d/histtimestamps.sh

# echo "export HISTTIMEFORMAT" >> /etc/profile.d/histtimestamps.sh

# chmod +x /etc/profile.d/histtimestamps.sh

Your history will look like this:

...

902 2012-02-16 09:50:33 cd /var/log

903 2012-02-16 09:50:33 ll

904 2012-02-16 09:50:33 ls -lat | sort -t

905 2012-02-16 09:50:33 ls -lat

...

Build an SELinux policy from an audit log

Often certain commands in linux will simply fail without any messages in /var/log/messages, or seemingly anywhere else... where we usually check. However, if you look at the selinux audit logs, sometimes the error messages are there. /var/log/audit/audit.log.

For example, every once in a while after a kernel update, I can't use the talk program. It simply says the connection is being refused by the other use. Since I already know Selinux is the culprit I grep the logs:

grep -i talkd /var/log/audit/audit.log

The result:

type=AVC msg=audit(1329155365.865:143): avc: denied { open } for pid=5631 comm="in.ntalkd" name="1" dev=devpts ino=4 scontext=system_u:system_r:ktalkd_t:s0-s0:c0.c1023 tcontext=unconfined_u:object_r:user_devpts_t:s0 tclass=chr_file

type=SYSCALL msg=audit(1329155365.865:143): arch=c000003e syscall=2 success=no exit=-13 a0=7fffc83c0eb8 a1=101 a2=7fffc83c0ec3 a3=7fffc83c0690 items=0 ppid=5630 pid=5631 auid=4294967295 uid=99 gid=5 euid=99 suid=99 fsuid=99 egid=5 sgid=5 fsgid=5 tty=(none) ses=4294967295 comm="in.ntalkd" exe="/usr/sbin/in.ntalkd" subj=system_u:system_r:ktalkd_t:s0-s0:c0.c1023 key=(null)

Two entries showing that talk is denied. If you really want to authorize this process grep the tail end of the file and use audit2allow to generate a policy file that will allow this.

tail /var/log/audit/audit.log | grep '1329155365.865:143' | audit2allow -M talkpolicy

audit2allow generates a talkpolicy.pp file and will also give you instructions on how to activate it. That would be:

semodule -i talkpolicy.pp

This will take a minute or two and has effectively authorized the blocked program to run.

Redhat / Centos / Fedora VM Clone - Nic gets the wrong eth name.

After creating a clone and changing the MAC address I reboot the machine and find that the eth0 doesn't show up when I run the ifconfig command. This is to be expected of course as I have changed the MAC on the VM guest, but not on the OS's configuration. So:

# vim /etc/sysconfig/network-scripts/ifcfg-eth0

Modify the MAC Address here. Reboot the VM guest.

During the reboot eth0 does not get configured. Running ifconfig still returns nothing except the Loopback device. If I run:

# service network restart

Bringing up interface eth0: Device eth0 does not seem to be present, delaying initialization.

What is going on? I know the ethernet device is there, it's a VM. Let's see what dmesg says.

# dmesg | grep -i eth

e1000: eth0: e1000_probe: Intel(R) PRO/1000 Network Connection
udev: renamed network interface eth0 to eth1

Huh? Why? Well as it turns out, udev had also assigned the MAC address to it's own configuration before the clone took place. So now that the clone is done and we have a new MAC, udev knows about it, but it thinks its a new NIC entirely and doesn't have any configuration for it. Take a look at the udev file for the nic.

vim /etc/udev/rules.d/70-persisten-net.rules

You will notice probably 3 entries. Normally there is only one entry in this file. The first time the system booted up, udev detected a conflict with the MAC so it created a new entry. It was named NAME="eth1" with the new guest VM's real MAC.

The first entry is the original entry. And the last entry exists because we changed the MAC address in the ifcfg-eth0 file. The only valid entry now is the last one, so all we have to do is delete entry 1 and 2. Reboot the system.

Try ifconfig and automagically eth0 is there with an IP address. Provided you configured DHCP or a static IP that is.

Linux / Unix file system permissions. How to memorize the bits.

A very, very brief note on Linux / Unix file system permission bits.

Note: I purposely changed the font on this blog post to ensure the below table would appear ligned up properly.

Numeric Bit Value Table:

-------------------------------------

num bit   rep   meaning

-------------------------------------

0   000   ---   no permissions

1   001   --x   execute

2   010   -w-   write

3   011   -wx   write, execute

4   100   r--   read

5   101   r-x   read, execute

6   110   rw-   read, write

7   111   rwx   read, write, execute

-------------------------------------

Mnemonics:

Using  the above bits is simple, but it's not always obvious remembering which value represents which permissions. Eg: Read and Write is 5 or 6? It's easy if you have the chart in front of you, and if you can visualize the chart in your mind.

Here are 4 simple tricks to keep in mind to successfully memorize permission bits.

1) Always remember the order in which permissions are assigned (RWX). From left to right, "Read, Write, Execute." - Never "Write, Execute, Read" or any other combination.

2) Think of 4 as what cuts the permissions table in half. All read permissions are values 4 or above. Binary works from right to left, but the highest values will have a new position digit on the left side. Hence, think of "read" as in the highest values since it's on the left.

3) All write permissions are two values above 0 and 4. So: 2,3 and 6,7. Write access is probably the most important permission to remember due to it's sensitive nature.

4) Execute permissions have the easiest trick. Any odd bit is executable. How simple is that?

Linux - Backtik Operator - Return the result of a command as a parameter for another.

Linux has many useful features which make work easier. While managing packages I often find myself trying to run commands like yum remove but can't remember the exact version numbers, character case, or exact spelling. Usually the solution is to run rpm -qa | grep -i . For example:

$ rpm -qa | grep -i virtualbox

This would yield:

VirtualBox-4.1-4.1.0_73009_fedora14-1.x86_64

Try remembering how to type that package name every time you want to upgrade an RPM with yum.

Wouldn't it be nice to be able to combine both "yum remove" and "rpm -qa..." Linux offers this capability through the back-tick operator. ``. What it does is simply returns the output of the command executed within it. This enables the output to be passed as a parameter to some other command.

# yum remove `rpm -qa | grep -i virtualbox`

Yum will then nicely ask if you wish to remove the above mentioned program. Imagine the possibilities.  

WARNING: Be careful if you use this to remove packages from your system.  I only one package called virtualbox installed on the system, but if you type in some other package name like "pl" you could end up in a lot of trouble.  Double check what your system is trying to remove.

For example:

# yum remove `rpm -qa | grep -i pl`

My system tells me it will remove 693 packages using 4.5GB of space, and is asking me if I want to continue.  That's nearly all my supporting packages.

Basically, use the backtick operator to your heart's content, but be very wary when cross scripting with commands such as 'yum remove'.