I’ll be using CentOS for this tutorial but it’s the same on all distributions of Linux and Unix.

[root@localhost:~]# ln -s [target file/directory] [symbolic file]

Example

[root@localhost:~]# ln -s /var/www/cacti/index.php /home/index.php

If you want to see the link then you can just list the directory contents:

[root@localhost:~]# ll 
lrwxrwxrwx 1 root          root        46 Aug 27 15:41 index.php -> /var/www/cacti/index.php

mode=1 (active-backup) – Active-backup policy: Only one slave in the bond is active. A different slave becomes active if, and only if, the active slave fails. The bond’s MAC address is externally visible on only one port (network adapter) to avoid confusing the switch. This mode provides fault tolerance. The primary option affects the behavior of this mode.

mode=2 (balance-xor) – XOR policy: Transmit based on [(source MAC address XOR'd with destination MAC address) modulo slave count]. This selects the same slave for each destination MAC address. This mode provides load balancing and fault tolerance.

mode=3 (broadcast) – Broadcast policy: transmits everything on all slave interfaces. This mode provides fault tolerance.

mode=4 (802.3ad) – IEEE 802.3ad Dynamic link aggregation. Creates aggregation groups that share the same speed and duplex settings. Utilizes all slaves in the active aggregator according to the 802.3ad specification.

Pre-requisites:
Ethtool support in the base drivers for retrieving the speed and duplex of each slave.
A switch that supports IEEE 802.3ad Dynamic link aggregation. Most switches will require some type of configuration to enable 802.3ad mode.

mode=5 (balance-tlb) – Adaptive transmit load balancing: channel bonding that does not require any special switch support. The outgoing traffic is distributed according to the current load (computed relative to the speed) on each slave. Incoming traffic is received by the current slave. If the receiving slave fails, another slave takes over the MAC address of the failed receiving slave.

Prerequisite: Ethtool support in the base drivers for retrieving the speed of each slave.

mode=6 (balance-alb) - Adaptive load balancing: includes balance-tlb plus receive load balancing (rlb) for IPV4 traffic, and does not require any special switch support. The receive load balancing is achieved by ARP negotiation. The bonding driver intercepts the ARP Replies sent by the local system on their way out and overwrites the source hardware address with the unique hardware address of one of the slaves in the bond such that different peers use different hardware addresses for the server.

First we need to create the master bond file. (Replace the information provided with the relevant information pertaining to your network setup)
*Note: Make a backup of all files you edit incase you need to rollback.

[root@localhost:~]# vi /etc/sysconfig/network-scripts/ifcfg-bond0

Paste the following into the newly created file:

DEVICE=bond0
USERCTL=no
BOOTPROTO=none
ONBOOT=yes
IPADDR=10.0.0.10
NETMASK=255.255.255.0
NETWORK=10.0.0.0
GATEWAY=10.0.0.2
DNS1=10.0.0.11
BONDING_OPTS="miimon=100 mode=balance-alb"
TYPE=unknown
IPV6INIT=no

Next we edit the existing interface config files:

[root@localhost:~]# vi /etc/sysconfig/network-scripts/ifcfg-eth1

Paste the following into the ifcfg-eth1 file:

DEVICE=eth1
BOOTPROTO=none
ONBOOT=yes
MASTER=bond0
SLAVE=yes
USERCTL=no

[root@localhost:~]# vi /etc/sysconfig/network-scripts/ifcfg-eth2

Paste the following into the ifcfg-eth2 file:

DEVICE=eth2
BOOTPROTO=none
ONBOOT=yes
MASTER=bond0
SLAVE=yes
USERCTL=no

I’ll only be bonding two network interfaces for the purpose of this example.
Next you need to define the bond:

[root@localhost:~]# vi /etc/modprobe.d/bonding.conf

Insert the following text:

alias bond0 bonding

Restart the network service

[root@localhost:~]# service network restart

Source: CentOS Wiki

The following command will tell you the release name and version:

[root@localhost:~]# cat /etc/*-release
CentOS Linux release 6.0 (Final)

If you want to see the kernel version and if it’s 32-bit or 64-bit:

[root@localhost:~]# uname -mrs
Linux 2.6.32-71.el6.x86_64 x86_64

x86_64 = 64-bit
Kernel = 2.6.32-71.el6.x86_64
2 = Kernel Version
6 = Major revision of the kernel
32 = Minor revision of the kernel
71 = Immediate fixing / bug fixing for critical error
el6 = This is specific to the distribution, Redhat appends el and the major kernel version

CentOS 6.2

Yum (Yellowdog Updater, Modified)

IP Address: 192.168.0.10

You’d generally use the following config for an office or home network. I’ve setup a few of these BIND servers for office networks and also in the Amazon cloud platform. You don’t need the Webmin interface to work with BIND but I’ve found that making quick changes using Webmin is easier.

I’ll be working as root, use root if you feel comfortable enough that you wont break anything.

I’ve also disabled SELinux for this tutorial, I don’t recommend it but it’s just easier to work with CentOS if it’s disabled.


To start we’ll install BIND

[root@localhost ~]$ yum -y install bind bind-utils bind-libs

Make sure it starts at boot

[root@localhost ~]$ chkconfig named on

Then start it up

[root@localhost ~]$ service named start

Now configure named.conf

[root@localhost ~]$ vi /etc/named.conf

You can copy paste the following into there, it’s minimal config needed to get things going, just be sure to change the zone name to a domain name that you want. In this case I’m using windohs.co.za.

options {
        directory "/etc";
        pid-file "/var/run/named/named.pid";
        };

zone "." {
        type hint;
        file "/etc/db.cache";
        };

zone "windohs.co.za" {
        type master;
        file "/var/named/windohs.co.za.hosts";
        };

Above you can see where the process identification file is found, the hints file for BIND and the file that contains all the DNS entries.

Now we can create the zone file for the domain.

[root@localhost ~]$ vi /var/named/windohs.co.za.hosts

You can copy the following as is and just change the domain

$ttl 38400
windohs.co.za. IN SOA dns.windohs.co.za. lyle@windohs.co.za. (
1300270007
300
3600
604800
38400 )
windohs.co.za. IN NS dns.windohs.co.za.

Make sure that you have the server IP in /etc/resolv.conf

[root@localhost ~]$ vi /etc/resolv.conf

search windohs.co.za
nameserver 192.168.0.10

Restart the BIND service

[root@localhost ~]$ service named restart

Test your new BIND server

[root@localhost ~]$ nslookup www.windohs.co.za

Server:         192.168.0.10
Address:        192.168.0.10#53

Name:   www.windohs.co.za
Address: 192.168.0.20

Installing Webmin
First we need the Webmin repo.

[root@localhost ~]$ vi /etc/yum.repos.d/webmin.repo

Copy and paste the following into the webmin.repo file

[Webmin]
name=Webmin Distribution Neutral
#baseurl=http://download.webmin.com/download/yum
mirrorlist=http://download.webmin.com/download/yum/mirrorlist
enabled=1

Import the GPG Key, without it you wont be able to install Webmin using Yum

rpm --import http://www.webmin.com/jcameron-key.asc

Now install Webmin using Yum, the dependencies will resolve automatically.

[root@localhost ~]$ yum install webmin

Open your browser and browse to your new DNS server on port 10000.

http://192.168.0.10:10000

On the left under Servers you will find BIND DNS Server.

Below you can see a command that I sent to a backup server to stop the Bacula File Daemon service.

[lyle@localhost ~]$ ssh root@backup01 'service bacula-fd stop'
bacula-fd is stopped

As you can you just need to add the command in a single quote. After pressing enter to send the command you will be prompted for the password. You will then receive the output of the command.


Another handy option to add is -t, this will keep the connection open so that you can interact with a screen-based program like top. It would be very useful if editing a file using vi.

[lyle@localhost ~]$ ssh -t root@backup01 'top'

You can also send multiple commands in one line and receive the output. Using a semicolon to separate each command.

[lyle@localhost ~]$ ssh root@backup01 'ls -l; pwd; whoami'

The commands above will list the directories and files in the current directory, print the current working directory and tell you which account you’re logged in with.

The error that came up:

No Drives Found
An error occurred – no valid devices were found on which to create new file systems. Please check your hardware for the cause of this problem.

This particular batch of servers have the PERC H710 Mini RAID Controller. 300GB 15000RPM SAS drives installed in it.

It could be a different RAID controller in your server, so this will only work on the PERC H710 series.

Quite a silly fix for this problem… Download and install CentOS 6.2. Problem solved. There have also been reports of it working on CentOS 5.8.

Download Mirrors

The one I use the most would be to direct standard output to a text file for reference later.

One of the most common uses for redirection is to send output to /dev/null, /dev/null is a device that is connected to nothing so it’s like a black hole for unwanted data. Most of the time it would be used to discard error messages.

The << redirection is not typically used on the command line but more in scripts. You wouldn’t use it to specify a file like most of the others mentioned previously, as an example << EOF. Everything before the EOF would be passed as input to a command.

There’s one more I’d like to mention before i get to the most useful, which is pretty useful. tee, is used to send standard output to the terminal and to a specified file so that you can refer to it later.

[root@localhost ~]# ifconfig | tee ifconfig.txt

I know that I haven’t mentioned the most obvious one yet | (pipe), which you can see in use above. Normally ifconfig would show us the network interfaces and their properties. By adding the pipe it sends the output to tee which will then write the output to the file I’ve specified which is ifconfig.txt and it will output the information to the terminal. If I had used the following then it would create a new file or overwrite the file if it exists and not show any output on the terminal.

[root@localhost ~]# ifconfig > ifconfig.txt

For the LPIC 101 exam, you’d need to know the table I’ve added above. Not all will be in the exam but they select a few.

The du or disk usage command will estimate the amount of space being used by the files and directories within the current directory. The individual file sizes will just be a cumulative size shown with the directory as a whole.

If you’d like the full write-up about the du command then you could check the manual pages.

[root@localhost:/home]# man du

I’ll list the most useful options to use with the du command in this post.

Firstly we would most likely be able to see the size of our current directory. We would use the -s (summarize) option here. I’m using the /etc/nagios directory for these examples because there’s quite a bit of files in there.

[root@localhost nagios]# du -s
413488  .

You’ll notice this tells you the amount of space used but it’s not really human readable, so we add -h (human readable) to the mix.

[root@localhost nagios]# du -sh
404M    .

This gives us the size in megabytes and tells us that its showing the size of the current directory and all it’s contents.

You’ll notice that if you ever use the du command without any options in a large directory that you’ll end up with a lot more output than you wanted to see. In this case we would use the –max-depth=N option. N is the number of directories deep you’d like to view. I’ve added -h so that we don’t have to workout the size of the folders.

[root@localhost nagios]# du --max-depth=1 -h
12K     ./var
88K     ./globalconfig
4.0K    ./.ssh
210M    ./archives
404M    .

There’s one more that I’ve found useful but it uses an additional command which is pretty useful. The less command. Particularly useful for directories with a large amount of sub-directories. For the next example, the /etc directory. Depending on the size of your SSH window you’ll get a result like I have below:

[root@localhost etc]# du -h | less
8.0K    ./snmp
24K     ./audit
16K     ./reader.conf.d
24K     ./lsb-release.d
24K     ./yum/pluginconf.d
40K     ./yum
8.0K    ./cups/ppd
:

Now you can move between the different pages using the F key to jump forward, B to go backwards and Q to quit.

I work with Dell a lot and dealing with their technical support is the least favourite part of my day when something goes wrong. You always need to supply them with a service tag number, but what if you need a part for a server that’s located in an internet data centre? Most times I run into this exact problem. Other times, I don’t feel like getting up and going to have a look.

Using Command Prompt:

C:\wmic bios get serialnumber
SerialNumber
123ABCD

The following command will give you the service tag, model number and vendor name:

C:\wmic csproduct get vendor,name,identifyingnumber
IdentifyingNumber  Name            Vendor
123ABCD            PowerEdge T710  Dell Inc.

The next one can be executed to get a service tag remotely if you find yourself without remote desktop access to the machine.

C:\wmic /user:administrator /node:remote.machine bios get serialnumber
Enter the password :************

SerialNumber
123ABCD

Using Linux CLI:

[root@remotehost ~]# dmidecode -s system-serial-number
123ABCD

After obtaining the Service tags I’d recommend keeping a spreadsheet with all of them, and also you can put the warranty expiry dates in there so that you don’t place a pointless call with Dell.

You could always download the ROM and install that manually but I wouldn’t recommend that. Best to just wait for the OTA update to come out, once the battery life issue is resolved.