Editing your DNS in Linux/CentOS

There are a few ways to change your DNS Servers and this is specifically created with CentOS but can basically be applied to any Linux distro as they all use the /etc/resolv.conf file. To configure your DNS manually you will need to modify the /etc/resolv.conf file, if you are using DHCP this will reset when you reboot the system so you will need to do one of the below methods.

Option 1: Edit your /etc/resolv.conf (Static IP)

You will replace the DNS in the /etc/resolv.conf file. After replacing the IP your resolve.conf file should look similar to the one below.

[root@hostname]# cat /etc/resolv.conf
search dynamichosting.biz
nameserver 123.123.123.1
nameserver 123.123.123.2

Note: You do not need to restart the network as the changes are instant.
If you are using CentOS you can also change this from a text-gui while ssh’d into the server by typing system-config-network as root.

You will enter “Edit DNS configuration”

You will see something similar to the above photo.

These are the correct DNS Servers you should have now and you should be able to hit the tab button down to the OK button.

tab down to save and quit and that is it.

Option 2: Configure dhclient.conf

The /etc/dhclient.conf or /etc/dhcp/dhclient.conf file contains configuration information for dhclient. You can turn on or off DNS update and other options for specific interfaces or all interfaces using this file. You should add this to the file to prevent DHCP from resetting the DNS after reboot:

option domain-name-servers 123.123.123.1, 123.123.123.2

Save the file and restart your network and this should take effect.

Linux Set or Change User Password

Task: Set or Change User Password

Type passwd command as follows to change your own password:
$ passwd
Output:

Changing password for vivek
(current) UNIX password:
Enter new UNIX password:
Retype new UNIX password:
passwd: password updated successfully

The user is first prompted for his/her old password, if one is present. This password is then encrypted and compared against the stored password. The user has only one chance to enter the correct password. The super user is permitted to bypass this step so that forgotten passwords may be changed.

A new password is tested for complexity. As a general guideline, passwords should consist of 6 to 8 characters including one or more from each of following sets:

1. Lower case alphabetics
2. Upper case alphabetics
3. Digits 0 thru 9
4. Punctuation marks

Task: Change Password For Other User Account

You must login as root user, type the following command to change password for user vivek:
# passwd vivek
Output:

Enter new UNIX password:
Retype new UNIX password:
passwd: password updated successfully

Where,

• vivek - is username or account name.

Task: Change Group Password

When the -g option is used, the password for the named group is changed. In this example, change password for group sales:
# passwd -g sales

The current group password is not prompted for. The -r option is used with the -g option to remove the current password from the named group. This allows group access to all members. The -R option is used with the -g option to restrict the named group for all users.

Install Crontab in CentOS 6.4

Install the package and dependencies

#yum install vixie-cron

Enable the package on boot and start

# /sbin/chkconfig crond on

# /etc/init.d/crond start

Device eth0 does not seem to be present, delaying initialization: Linux Networking

PROBLEM

You receive random errors failing to initialize your network card at boot time or during manual restart that look like the one below:

$ifup eth0
via-rhine device eth0 does not seem to be present, delaying initialization. 
$

Or just a form of the

# service network restart
.
.
device does not seem to be present, delaying initialization
.
.
#

from any device such as bond0 or wlan0 etc.  Here's how to solve it.

SOLUTION

There are a couple of potential solutions to this issue:

• Missing 'HWADDR' variable from '/etc/sysconfig/network-scripts/ifcfg-eth0'.  Try adding "HWADDR=01:23:45:67:89:AB" to '/etc/sysconfig/network-scripts/ifcfg-eth0' and rerunning 'ifup eth0' command.
   
• You could try renaming the interface you have to the one the system is currently configured for and regunning service network restart.  An example of renaming a wireless LAN card is below:
  
  
  
  
  
  
  
  
  $ cd /sys/class/net
  $ ll
  lrwxrwxrwx 1 root root 0 Jun 14 06:17 wlan1 -> ../../devices/pci0000:00/0000:00:1d.7/usb1/1-2/1-2:1.0/net/wlan1
  $ ip link set dev wlan1 name wlan0
  $ ll
  lrwxrwxrwx 1 root root 0 Jun 14 06:17 wlan1 -> ../../devices/pci0000:00/0000:00:1d.7/usb1/1-2/1-2:1.0/net/wlan1

To make the above change persistent across reboots, please see the following post Linux Networking: Persistent naming rules based on MAC for eth0 and wlan0.  The persistent naming rules also solve this issue through the persistent naming rules in /etc/udev/rules.d/70-persistent-net.rules.  See the link for more on that.

FIRST INVESTIGATION

For a while now I kept getting the error message " via-rhine device eth0 does not seem to be present, delaying initialization" on and off when I logged into my Linux distribution.  A reboot fixed this problem virtually everytime.  So what's wrong?  First thing I tried is to use:

$ifup eth0
via-rhine device eth0 does not seem to be present, delaying initialization.
$

Naturally this produced absolutely the same error I saw at boot time, which wasn't very helpfull at all.  I know my primary card (My link to my WRT54GL switch and therefore my way of accessing the WEB) was down and I couldn't connect anywhere.  What it didn't tell me is the root problem.  At this point I started at the obvious points by checking the log files:

/var/log/messages
/var/log/secure
/var/log/dmesg

Next thing I checked is '/etc/sysconfig/hwconf', searched for NETWORK where I found both eth1 and eth0 correctly configured.  I compared the two and saw nothing out of the ordinary.  Next thing I tried is:

$ lspci|grep Ethernet
00:0c.0 Ethernet controller: National Semiconductor Corporation DP83815 (MacPhyter) Ethernet Controller
00:0d.0 Ethernet controller: D-Link System Inc RTL8139 Ethernet (rev 10)
00:12.0 Ethernet controller: VIA Technologies, Inc. VT6102 [Rhine-II] (rev 78)
$

Which showed that all network cards were being seen by my system.  In particular I went through /var/log/dmesg with a fine toothed comb but nothing appeared that provided any hint as to the problem.  At least nothing I could understand.  I tried restarting but unlike other times, this time, the system came back up and still didn't detect eth0. Not good!  At this point I decided to try a basic programming trick:

edit '/sbin/ifup' and insert 'echo' statements at various points to show variables the BASH utility sets along the way to the error message above (WARNING: Don't do this unless you know a bit about KSH/BASH and scripting in general).  That is until I reach something that would tell me more about where the problem is.  This took me long so you shouldn't expect a 5 minute job here.  Long story short, ultimately as I went along inserting statements I ended up modifying a bunch of files and utilities:

/sbin/ifup
/etc/sysconfig/network-scripts/network-functions
/etc/sysconfig/network-scripts/ifup-eth

All of which were called from '/sbin/ifup'.  This resulted in the following output:

$ ifup eth0
Running script /etc/sysconfig/network-scripts/ifup …
/sbin/ifup[1]: Running '. /etc/init.d/functions' …
Running '/etc/sysconfig/network-scripts/network-functions' through script /etc/sysconfig/network-scripts/ifup … /sbin/ifup[2]: Checking if folder '../network' exists …
/sbin/ifup[3]: Setting CONFIG=eth0 …
/sbin/ifup[4]: Running eth0 …
/sbin/ifup[5]: Check if ${CONFIG} ifcfg-eth0 exists…
/sbin/ifup[6]: Check if ${UID} 0 != 0 …
/sbin/ifup[7]: Running 'source_config' network-functions
[-1](PROC: source_config):
RESULTS:
$DEVNAME=||
$PARENTCONFIG=||
$PARENTDEVNAME=||
$TYPE=|| $DEVICE=|eth0|
$REALDEVICE=|eth0|
$ISALIAS=|no|
$DEVICETYPE=|eth|
$HWADDR = ||
$REALDEVICE=|eth0|
/sbin/ifup[8]: Test if $2 was specified as 'boot' and ${ONBOOT} 'yes' is set to 'no/NO', check IN_HOTPLUG and as 'Bridge' …
/sbin/ifup[9]: Check if VLAN: $VLAN = ||
/sbin/ifup[9.0]: Non VLAN config:
$VLAN = ||,
$DEVICE = |eth0|
/sbin/ifup[9.1]: Non VLAN config:
$VLAN = ||,
$DEVICE = |eth0|,
$BOOTPROTO = |static|
/sbin/ifup[9.2]: Test if '/sbin/ifup-pre-local' exists.  Run with |ifcfg-eth0|| if does.
/sbin/ifup[9.2]: Test if '/etc/sysconfig/network-scripts/ifup-eth' exists…
/sbin/ifup[9.2]: Execute: /etc/sysconfig/network-scripts/ifup-eth ifcfg-eth0
Running script /etc/sysconfig/network-scripts/ifup-eth …
Running '/etc/sysconfig/network-scripts/network-functions' through script /etc/sysconfig/network-scripts/ifup-eth … network-functions [-1](PROC: source_config):
RESULTS:
$DEVNAME=||
$PARENTCONFIG=||
$PARENTDEVNAME=||
$TYPE=||
$DEVICE=|eth0|
$REALDEVICE=|eth0|
$ISALIAS=|no|
$DEVICETYPE=|eth|
$HWADDR = ||
$REALDEVICE=|eth0|
network-functions[0]: eth0 = |eth0|\n
network-functions[1]: Running ‘modprobe -c | awk “/^(alias|install)[[:space:]]+eth0[[:space:]]/ { print $3 }"' …\n
network-functions[1]: Above alias returned: |via-rhine|.  Now running 'modprobe eth0' …\n
network-functions[2]: Checking $HWADDR = ||\n
network-functions[2]: Running 'ip -o link | grep -q eth0' again and providing it's return value to caller.
/etc/sysconfig/network-scripts/ifup-eth[0]: ${REALDEVICE} = |eth0|
network-functions[0]: eth0 = |eth0|\n
network-functions[1]: Running ‘modprobe -c | awk “/^(alias|install)[[:space:]]+eth0[[:space:]]/ { print $3 }"' …\n
network-functions[1]: Above alias returned: |via-rhine|.  Now running 'modprobe eth0' …\n
network-functions[2]: Checking $HWADDR = ||\n
network-functions[2]: Running 'ip -o link | grep -q eth0' again and providing it's return value to caller.
via-rhine device eth0 does not seem to be present, delaying initialization.
$

You'll noticed error is thrown right below the red line.  I tried the command above  to see what it would produce:

$ ip -o link|grep -q eth0
$

Nothing.  Apparently '/sbin/ifup' checks using '/sbin/ip' to look for a card.  Next I typed:

$ ip -o link
1: lo: <LOOPBACK,UP,10000> mtu 16436 qdisc noqueue \   
     link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
2: eth1: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc pfifo_fast qlen 1000\   
     link/ether 00:02:e3:17:e1:47 brd ff:ff:ff:ff:ff:ff
3: dev1804289383: <BROADCAST,MULTICAST> mtu 1500 qdisc noop qlen 1000\   
     link/ether 00:13:d4:3d:3b:6a brd ff:ff:ff:ff:ff:ff
4: eth2: <BROADCAST,MULTICAST> mtu 1500 qdisc noop qlen 1000\   
     link/ether 00:0d:88:38:0a:34 brd ff:ff:ff:ff:ff:ff
$

Notice the differences.  One of the devices wasn't given an alias 'eth0'.  Now the question is why the other cards have proper aliases but this one, my main card, isn't given one.  I decided to try running '/sbin/ifup eth1' for my second card, which was picking up without issues:

$ ifup eth1
Running script /root/ifup …
/sbin/ifup[1]: Running '. /etc/init.d/functions' …
Running '/etc/sysconfig/network-scripts/network-functions' through script /etc/sysconfig/network-scripts/ifup …
/sbin/ifup[2]: Checking if folder '../network' exists …
/sbin/ifup[3]: Setting CONFIG=eth1 …
/sbin/ifup[4]: Running eth1 …
/sbin/ifup[5]: Check if ${CONFIG} ifcfg-eth1 exists…
/sbin/ifup[6]: Check if ${UID} 0 != 0 …
/sbin/ifup[7]: Running 'source_config' network-functions
[-1](PROC: source_config):
RESULTS:
$DEVNAME=||
$PARENTCONFIG=||
$PARENTDEVNAME=||
$TYPE=||
$DEVICE=|eth1|
$REALDEVICE=|eth1|
$ISALIAS=|no|
$DEVICETYPE=|eth|
$HWADDR = |00:02:E3:17:E1:47|
$REALDEVICE=|eth1|
/sbin/ifup[8]: Test if $2 was specified as 'boot' and ${ONBOOT} 'yes' is set to 'no/NO', check IN_HOTPLUG and as 'Bridge' …
/sbin/ifup[9]: Check if VLAN: $VLAN = ||
/sbin/ifup[9.0]: Non VLAN config:
$VLAN = ||,
$DEVICE = |eth1|
/sbin/ifup[9.1]: Non VLAN config:
$VLAN = ||,
$DEVICE = |eth1|,
$BOOTPROTO = |static|
/sbin/ifup[9.2]: Test if '/sbin/ifup-pre-local' exists.  Run with |ifcfg-eth1|| if does.
/sbin/ifup[9.2]: Test if '/etc/sysconfig/network-scripts/ifup-eth' exists…
/sbin/ifup[9.2]: Execute: /etc/sysconfig/network-scripts/ifup-eth ifcfg-eth1
Running script /etc/sysconfig/network-scripts/ifup-eth …
Running '/etc/sysconfig/network-scripts/network-functions' through script /etc/sysconfig/network-scripts/ifup-eth … network-functions[-1](PROC: source_config):
RESULTS:
$DEVNAME=||
$PARENTCONFIG=||
$PARENTDEVNAME=||
$TYPE=||
$DEVICE=|eth1|
$REALDEVICE=|eth1|
$ISALIAS=|no|
$DEVICETYPE=|eth|
$HWADDR = |00:02:E3:17:E1:47|
$REALDEVICE=|eth1|
network-functions[0]: eth1 = |eth1|\n
/etc/sysconfig/network-scripts/ifup-eth[0]: ${REALDEVICE} = |eth1|
network-functions[0]: eth1 = |eth1|\n
Running '/etc/sysconfig/network-scripts/network-functions' through script /etc/sysconfig/network-scripts/ifup-post … network-functions[-1](PROC: source_config):
RESULTS:
$DEVNAME=||
$PARENTCONFIG=||
$PARENTDEVNAME=||
$TYPE=||
$DEVICE=|eth1|
$REALDEVICE=|eth1|
$ISALIAS=|no|
$DEVICETYPE=|eth|
$HWADDR = |00:02:E3:17:E1:47|
$REALDEVICE=|eth1|
Running '/etc/sysconfig/network-scripts/network-functions' through script /etc/sysconfig/network-scripts/ifup-aliases …
$

Comparing, I noticed the HWADDR was not empty for 'eth1' whereas for 'eth0' it was empty.  This made sense since 'ip -o link' showed a random string for the ethernet alias I needed.  This led me to compare the two config files:

/etc/sysconfig/network-scripts/ifcfg-eth0
/etc/sysconfig/network-scripts/ifcfg-eth1

Which quickly showed eth1 had HDADDR="…" the other didn't.  Sure enough after the change, 'ip' also showed correct settings:

$ ip -o link
1: lo: <LOOPBACK,UP,10000> mtu 16436 qdisc noqueue \   
     link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00 2: eth1: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc pfifo_fast qlen 1000\   
     link/ether 00:02:e3:17:e1:47 brd ff:ff:ff:ff:ff:ff
3: eth0: <BROADCAST,MULTICAST,UP,10000> mtu 1500 qdisc pfifo_fast qlen 1000\   
     link/ether 00:13:d4:3d:3b:6a brd ff:ff:ff:ff:ff:ff
4: eth2: <BROADCAST,MULTICAST> mtu 1500 qdisc noop qlen 1000\   
     link/ether 00:0d:88:38:0a:34 brd ff:ff:ff:ff:ff:ff
$

It is noteworthy to say that I then removed the HWADDR from the ifcfg-eth0 config file, yet bringing eth0 up and down worked fine now while the system was on.  This could lead to a common mistake where removal of HWADDR from config file doesn't appear to cause issues, might cause one at next cold boot.

SECOND INVESTIGATION

Nov 1 2012: Another potential location where this might occur is when bonding module isn't loaded.  In this case, all that is necessary is to load the bonding module and you're set:

# service network restart
Shutting down loopback interface:                          [  OK  ]
Bringing up loopback interface:                            [  OK  ]
Bringing up interface bond0:  bonding device bond0 does not seem to be present, delaying initialization.
                                                           [FAILED]
# modprobe bonding
# service network restart
Shutting down interface bond0:                             [  OK  ]
Shutting down loopback interface:                          [  OK  ]
Bringing up loopback interface:                            [  OK  ]
Bringing up interface bond0:  RTNETLINK answers: File exists
Error adding address 192.168.0.8 for bond0.
RTNETLINK answers: File exists
                                                           [  OK  ]
#

THIRD INVESTIGATION

The third investigation really has it's roots from the second investigation where I simply tried renaming the device name:

# service network restart
Shutting down loopback interface:                          [  OK  ]
Bringing up loopback interface:                            [  OK  ]
Bringing up interface eth0:  Device eth0 does not seem to be present, delaying
initialization.
                                                           [FAILED]
Bringing up interface wlan0:  Device wlan0 does not seem to be present, delaying
initialization.
                                                           [FAILED]
#

Checking the devices under /sys/class/net we see that wlan0 doesn't exist at which point I attempted a rename as above:

# cd /sys/class/net
# ll
total 0
lrwxrwxrwx 1 root root 0 Jun 14 06:17 eth2 ->
../../devices/pci0000:00/0000:00:1e.0/0000:02:05.0/net/eth2
lrwxrwxrwx 1 root root 0 Jun 14 06:17 lo -> ../../devices/virtual/net/lo
lrwxrwxrwx 1 root root 0 Jun 14 20:19 sit0 -> ../../devices/virtual/net/sit0
lrwxrwxrwx 1 root root 0 Jun 14 06:17 virbr0 -> ../../devices/virtual/net/virbr0
lrwxrwxrwx 1 root root 0 Jun 14 06:17 wlan1 ->
../../devices/pci0000:00/0000:00:1d.7/usb1/1-2/1-2:1.0/net/wlan1
# ip link set dev wlan1 name wlan0
# ll
total 0
lrwxrwxrwx 1 root root 0 Jun 14 06:17 eth2 ->
../../devices/pci0000:00/0000:00:1e.0/0000:02:05.0/net/eth2
lrwxrwxrwx 1 root root 0 Jun 14 06:17 lo -> ../../devices/virtual/net/lo
lrwxrwxrwx 1 root root 0 Jun 14 20:19 sit0 -> ../../devices/virtual/net/sit0
lrwxrwxrwx 1 root root 0 Jun 14 06:17 virbr0 -> ../../devices/virtual/net/virbr0
lrwxrwxrwx 1 root root 0 Jun 14 21:08 wlan0 ->
../../devices/pci0000:00/0000:00:1d.7/usb1/1-2/1-2:1.0/net/wlan0
#

Per above, the error message produced from restarting the network is identical however this time wlan0 failed to reconnect following asetup of a wireless network configuration earlier on my HTPC when I swapped my G wireless adapter for an N wireless adapter.  The most likely reason for that was the system probably did not unregister the previous wlan0 fully registering only the wlan1 instead. 

So the renaming worked temporarily but always reverted back to the wlan1 configuration after a reboot.  Instead of renaming the card, I created an /etc/sysconfig/network-scripts/fcfg-wlan1 configuration file as well and rebound my wpa_supplicant configuration to run off this new device.  I preferred this over yanking out any reference to wlan0 in case I had to reuse my G wireless adapter.  For more details on this please see the above link.

MORAL OF THE STORY

This is the sure fire way to identify an issue provided all files are scripts (not binaries) and can be modified.  Nonetheless, and to cut yourself a bunch of work, be sure to compare the not working file to a working one when the option exists.  I could have saved myself a bunch of work.    Nonetheless, it wasn't a total waste.  I did get to learn a bit more about the Linux networking and the zoo of functions available.   

Cheers,
TK

CentOS 6.3 Device eth0 does not seem to be present (Minimal non-cloned setup)

In my situation, I changed the network adapter configuration of my hypervisor after the installation of CentOS. What happened in this case is that the old interface eth0 no longer existed but a new one called eth1 did.

ls /sys/class/net
eth1 lo

I changed the network config /etc/sysconfig/network-scripts/ifcfg-eth0 to use DEVICE=eth1 and it worked again.

Red Hat 6 Installation Guide – RHEL 6 Install Screenshots

This is Red Hat 6 RHEL (Red Hat Enterprise Linux) installation guide, step-by-step walkthrough with screenshots. Current version of Red Hat 6 is Beta, but also this RHEL 6 Beta version looks very stable and good OS, so that’s why I decided to write this RHEL 6 Installation guide.

The Red Hat Enterprise Linux 6 Beta is available on the following architectures:

• i386
• AMD64/Intel64
• System z
• IBM Power (64-bit)

Download Red Hat 6 Beta DVD images here.

Some of the many improvements and new features that are included in Red Hat Enterprise Linux 6

• Power management – tickless kernel and improvements through the application stack to reduce wakeups, power consumption measurement by Powertop, Power Management (ASPM, ALPM), and adaptive system tuning by Tuned, all enhance more efficient system power usage.
• Next generation networking – comprehensive IPv6 support (NFS 4, CIFS, mobile support [RFC 3775], ISATAP support), FCoE, iSCSI, and a new and improved mac 802.11 wireless stack.
• Scalable filesystems – ext4 file system provides support for larger file sizes and significantly reduces repair times over ext3. XFS® is a high-performance file system that supports extremely large files and is optimized for large data transfers.
• Virtualization – KVM includes performance improvements and new features, sVirt protects the guest and host from unauthorized access, SR-IOV and NPIV deliver high-performance virtual use of physical devices, and libvirt leverages kernel resource management functionality.
• Enterprise security enhancement – SELinux includes improved ease of use, application sandboxing, and significantly increased coverage of system services, while SSSD provides unified access to identity and authentication services as well as caching for off-line use.
• Development and runtime support – SystemTap improvements, ABRT is a new framework for simple collection and reporting of bug information, and improvements are made to GCC (version 4.4.3), glibc (version 2.11.1), and GDB (version 7.0.1).

On this guide I use Graphical Installer. There is also Kickstart automated installation method and Text-based Installer available. And I install this machine for Software Development Workstation and testing environment. So package selection is following: Desktop, Web server, Databases, Compiling tools, Java. Same method works also for servers, normal desktops and other setups, but of course different software selection. So let’s begin installation…

Burn Downloaded Image to DVD and Boot Computer Using Red Hat 6 Installation DVD

Check RHEL image MD5 sum and burn image to DVD with your favourite CD/DVD burner. And boot computer using Red Hat Installation DVD.

Red Hat 6 RHEL Installation

1. Select Install or upgrade an existing system option on Grub Menu

2. Choose a language

3. Choose a keyboard type

4. Choose a installation media

5. Skip DVD media test (or select media test, if you want to test installation media before installation)

6. Red Hat 6 graphical installer starts, select next

7. Accepct Pre-Release Installation

8. Select storage devices

9. Insert computer name

10. Select time zone

11. Enter a password for root user

12. Select type of installation

Read every options info carefully. And select encrypting if needed and option to review and modify partition layout.

13. Review partition layout

Modify if needed. Default setup with ext4 and LVM looks good for desktop machine.

14. Accept write changes to disc

15. Writing changes (creating partitions) to disc

16. Configure boot loader options

Select device to install bootloader and check/create boot loader operating system list.

17. Select softwares to install and enable repositories

This case we select Software Development Workstation and enable Red Hat Enterprise Linux 6.0 Beta Repository and select Customize now.

18. Customize package selection

Select PHP and Web Server to installation.

Select MySQL and PostgreSQL Databases.

Select set of Development tools like Eclipse IDE.

19. Checking dependencies for installation

20. Starting installation process

21. Installing packages

22. Installation is complete

Click reboot computer and remove installation media.

Red Hat 6 RHEL Finishing Installation

23. Selecting RHEL 6 from grub

24. Booting Red Hat 6

25. Red Hat 6 Welcome screen

26. Create normal user

27. Setup date and time and keep up-to-date with NTP

28. Login Red Hat 6 Gnome Desktop

29. Red Hat (RHEL) 6 Gnome Desktop, empty and default look