The EPEL repository (Extra Packages for Enterprise Linux) is an open source and free community based repository project from Fedora team, which provides 100% high quality add-on software packages for Linux distributions.
The following article describes how to install and enable the EPEL repository on CentOS-8/7/6, how to verify the EPEL installation and how to remove it, if needed. (more…)
Nginx is an open source web server and a reverse proxy server for HTTP, SMTP, POP3 and IMAP protocols, with a strong focus on high concurrency, performance and low memory usage.
This guide explains how to install and perform the initial configuration of Nginx Web Server on CentOS/RHEL based systems.
If you want the latest version of nginx with all features and bugfixes, it’s recommended to use packages directly from the nginx yum repository.
Packages from the usual repositories are often somewhat out-of-date.
To add the nginx yum repository, create a file named /etc/yum.repos.d/nginx.repo, and paste one of the configurations below.
For CentOS:
[nginx]
name=nginx repo
baseurl=http://nginx.org/packages/centos/$releasever/$basearch/
gpgcheck=0
enabled=1
For RHEL:
[nginx]
name=nginx repo
baseurl=http://nginx.org/packages/rhel/$releasever/$basearch/
gpgcheck=0
enabled=1
Manually replace $releasever with either “5” (for 5.x) or “6” (for 6.x), depending upon your OS version.
Install the nginx web server with yum:
# yum install nginx
Set the nginx to start at boot:
# chkconfig nginx on
Backup the configuration files:
# cp /etc/nginx/nginx.conf /etc/nginx/nginx.conf.back # cp /etc/nginx/conf.d/default.conf /etc/nginx/conf.d/default.conf.back
Edit the nginx configuration file /etc/nginx/nginx.conf. Change the worker_processes value. It should be equal to the number of CPUs in your server.
worker_processes 1;
To list the number of CPUs, you can use the following command:
# lscpu | grep '^CPU(s)'
Enable compression by uncommenting the following line:
gzip on;
Save and close the file.
Edit the file /etc/nginx/conf.d/default.conf. Set the server’s name:
server_name example.com;
Save and close the file. Start Nginx:
# service nginx start
Edit the file /etc/sysconfig/iptables, if you use the firewall. Append the following line before the REJECT line, to open http port 80:
-A INPUT -m state --state NEW -m tcp -p tcp --dport 80 -j ACCEPT
Save and close the file. Restart the firewall.
# service iptables restart
To confirm that the Nginx has been successfully installed, you can enter your server’s name or IP address in your web-browser.
You should see “Welcome to Nginx!” page.
If you see this page, the Nginx Web Server is successfully installed and working.
# /etc/init.d/nginx start # /etc/init.d/nginx stop # /etc/init.d/nginx restart # /etc/init.d/nginx condrestart # /etc/init.d/nginx try-restart # /etc/init.d/nginx force-reload # /etc/init.d/nginx upgrade # /etc/init.d/nginx reload # /etc/init.d/nginx status # /etc/init.d/nginx help # /etc/init.d/nginx configtest
Reverse DNS is an IP address to domain name mapping – the opposite of Forward DNS which maps domain names to IP addresses.
Comparison:
## FORWARD LOOKUP ##
# host centos.org centos.org has address 72.232.194.162
## REVERSE LOOKUP ##
# host 72.232.194.162 162.194.232.72.in-addr.arpa domain name pointer www.centos.org.
The Reverse DNS is one of the basic requirements for running some Internet protocols. It is also often used as a spam filter to determine whether the IP address of the incoming message matches an authenticated domain name and to block the message if it doesn’t.
Reverse DNS is controlled by the owner of the IP address (probably your SIP).
Using one of the following commands, you can get the reverse DNS.
Using the ‘host’ command:
# host 72.232.194.162 162.194.232.72.in-addr.arpa domain name pointer www.centos.org.
Using the ‘nslookup’ command:
# nslookup 208.80.152.201 162.194.232.72.in-addr.arpa name = www.centos.org.
Using the ‘dig’ command:
# dig -x 208.80.152.201 162.194.232.72.in-addr.arpa. 859 IN PTR www.centos.org.
Lets setup PTR record for the IP address ‘192.168.0.5’. It should be pointed to ‘your.site.name’.
Add the new zone to the ‘named.conf’ file as follows:
zone "0.168.192.in-addr.arpa" {
type master;
file "/var/named/0.168.192.in-addr.arpa";
};
Create the zone file ‘/var/named/0.168.192.in-addr.arpa’, and add the the following content:
$TTL 3600
@ IN SOA ns0.name.server. root.ns0.name.server. (
2012020801 ; Serial
21600 ; refresh
3600 ; retry
3600000 ; expire
86400 ) ; minimum
IN NS ns0.name.server.
IN NS ns1.name.server.
; ----------- ENREGISTREMENTS -----------
$ORIGIN 0.168.192.in-addr.arpa.
5 IN PTR your.site.name.
; ----------- ENREGISTREMENTS SPECIAUX -----------
Where ‘ns0.name.server’ and ‘ns1.name.server’ are your DNS servers.
Save the changes and reconfig ‘named’:
# rndc reconfig
Check the PTR:
# dig -x 192.168.0.5 @ns0.name.server +short your.site.name.
If you need to find out all the sub-domains of a given domain name, you can try AXFR request.
This hint works only if the NS that you are querying is configured to allow AXFR requests.
Not all name servers allow AXFR protocol queries. Most of them return “Transfer failed”.
The first thing that you have to do, is to find out the AUTHORITY NAME SERVER(s) for the required DOMAIN NAME e.g zonetransfer.me:
$ dig zonetransfer.me
Look under the AUTHORITY SECTION:
;; AUTHORITY SECTION: zonetransfer.me. 7122 IN NS ns16.zoneedit.com. zonetransfer.me. 7122 IN NS ns12.zoneedit.com.
In this case zonetransfer.me has 2 name servers: ns12.zoneedit.com and ns16.zoneedit.com.
Now you can query one of these servers for the sub-domains of zonetransfer.me:
$ dig @ns12.zoneedit.com zonetransfer.me AXFR
Here is the result:
; <<>> DiG 9.9.2-P1 <<>> @ns12.zoneedit.com zonetransfer.me AXFR ; (1 server found) ;; global options: +cmd *** zonetransfer.me. 7200 IN NS ns16.zoneedit.com. zonetransfer.me. 7200 IN NS ns12.zoneedit.com. zonetransfer.me. 7200 IN A 217.147.180.162 zonetransfer.me. 7200 IN MX 0 ASPMX.L.GOOGLE.COM. ***
If you need to troubleshoot an issue with slow Internet access on your Linux system, it would be a good idea to measure the current Internet speed.
In this article you’ll find how to test Internet speed from the Linux terminal, using speedtest_cli – a simple command-line client written in Python for measuring Internet bandwidth by using Speedtest.net infrastructure.
I’ll show how to simply check download/upload Internet speed to the geographically closest server, how to measure the network speed to specific server and how to share the results of speed test via Speedtest.net.
All these tasks can be performed using the Linux command line only.
Use the following command to Test Internet Download and Upload Speed from the Linux Command Line:
$ wget -O - https://raw.githubusercontent.com/sivel/speedtest-cli/master/speedtest.py | python
Sample output:
Retrieving speedtest.net configuration... Retrieving speedtest.net server list... Testing from ******* (xxx.xxx.xxx.xxx)... Selecting best server based on latency... Hosted by ******* [1.24 km]: 2.458 ms Testing download speed... Download: 84.35 Mbits/s Testing upload speed... Upload: 77.32 Mbits/s
If you are planing to measure Internet bandwidth often, you can install speedtest script on your system:
$ wget https://raw.github.com/sivel/speedtest-cli/master/speedtest.py $ chmod a+rx speedtest.py $ sudo mv speedtest.py /usr/local/bin/speedtest $ sudo chown root:root /usr/local/bin/speedtest
After installation, you will be able to test Internet speed from the terminal by simply running speedtest command, as follows:
$ speedtest Retrieving speedtest.net configuration... Retrieving speedtest.net server list... Testing from ******* (xxx.xxx.xxx.xxx)... Selecting best server based on latency... Hosted by ******* [1.24 km]: 2.458 ms Testing download speed... Download: 84.35 Mbits/s Testing upload speed... Upload: 77.32 Mbits/s
If you want to share the result of speed test, run speedtest command, as follows:
$ speedtest --share Share results: http://www.speedtest.net/result/3690948322.png
Run the following command to get the list of all available Speedtest.net servers around the world:
$ speedtest --list | more Retrieving speedtest.net configuration... Retrieving speedtest.net server list... 3464) Vodafone España (Alicante, Spain) [3047.78 km] 804) ServiHosting Networks (Elda, Spain) [3063.22 km] 4845) du (Dubai, United Arab Emirates) [3069.72 km] 4844) Etisalat (Dubai, United Arab Emirates) [3069.72 km]
You can manually specify server ID during testing, instead of using the geographically closest one.
Let’s test the link to “1746) Vodafone DE (Frankfurt, Germany) [1937.90 km]”:
$ speedtest --server 1746
You can make your BASH script more pretty, by colorizing its output.
Use ANSI escape sequences to set text properties like foreground and background colors.
Use the following template for writing colored text:
echo -e "\e[COLORmSample Text\e[0m"
| Option | Description |
|---|---|
| -e | Enable interpretation of backslash escapes |
| \e[ | Begin the color modifications |
| COLORm | Color Code + ‘m’ at the end |
| \e[0m | End the color modifications |
Examples:
$ echo -e "\e[31mRed Text\e[0m" Red Text $ echo -e "\e[42mGreen Background\e[0m" Green Background
Shell scripts commonly use ANSI escape codes for color output:
| Color | Foreground Code | Background Code | Sample |
|---|---|---|---|
| Black | 30 | 40 | |
| Red | 31 | 41 | |
| Green | 32 | 42 | |
| Brown | 33 | 43 | |
| Blue | 34 | 44 | |
| Purple | 35 | 45 | |
| Cyan | 36 | 46 | |
| Light Gray | 37 | 47 |
Escape sequence also allows to control the manner in which characters are displayed on the screen:
| ANSI Code | Description |
|---|---|
| 0 | Normal Characters |
| 1 | Bold Characters |
| 4 | Underlined Characters |
| 5 | Blinking Characters |
| 7 | Reverse video Characters |
Examples:
$ echo -e "\e[1mBold Text\e[0m"
Bold Text
$ echo -e "\e[3mUnderlined Text\e[0m"
Underlined Text
By combining all these escape sequences, we can get more fancy effect.
echo -e "\e[COLOR1;COLOR2mSample Text\e[0m"
There are some differences between colors when combining colors with bold text attribute:
| Color | Foreground Code | Background Code | Sample |
|---|---|---|---|
| Dark Gray | 1;30 | 1;40 | |
| Light Red | 1;31 | 1;41 | |
| Light Green | 1;32 | 1;42 | |
| Yellow | 1;33 | 1;43 | |
| Light Blue | 1;34 | 1;44 | |
| Light Purple | 1;35 | 1;45 | |
| Light Cyan | 1;36 | 1;46 | |
| White | 1;37 | 1;47 |
Examples:
$ echo -e "\e[1;34mLight Blue Text\e[0m" Light Blue Text $ echo -e "\e[1;33;4;44mYellow Underlined Text on Blue Background\e[0m" Yellow Underlined Text on Blue Background
Don’t Forget VMware Tools!
VMware Tools is a suite of utilities that enhances the performance of the virtual machine’s guest operating system and improves management of the virtual machine.
Without VMware Tools installed in your guest operating system, guest performance lacks important functionality.
Open Vmware Worstation or ESX console and Locate the VM you need to install VMware Tools to.
Right click the VM and choose “Tools”, then select “Install VMware Tools”.
It will connect the virtual CD-ROM device to the appropriate CD image containing the tools for your virtual machine.
Log into VMware Guest you need to install VMware Tools to.
Install the kernel-devel, gcc, dracut, make, perl and eject packages using yum:
$ yum -y install kernel-devel gcc dracut make perl eject
Mount CD image with VMware Tools:
$ mount /dev/cdrom /media
Uncompress the archive with VMware Tools:
$ tar -zxf /media/VMwareTools-*.tar.gz -C /tmp
Run the installer (exclude the --default option if you prefer to control the installation process):
$ /tmp/vmware-tools-distrib/vmware-install.pl --default
Remove the temporary files after installation:
$ rm -rf /tmp/vmware-tools-distrib
Use the command as follows to validate that VMware Tools is running:
$ initctl list | grep vmware-tools vmware-tools start/running
To determine the vmware-tools version, run the command:
$ vmware-toolbox-cmd -v
The grep, egrep, sed and awk are the most common Linux command line tools for parsing files.
From the following article you’ll learn how to match multiple patterns with the OR, AND, NOT operators, using grep, egrep, sed and awk commands from the Linux command line.
I’ll show the examples of how to find the lines, that match any of multiple patterns, how to print the lines of a file, that match each of provided patterns and how to find and print the lines, that do not match a pattern (negative matching). (more…)
The client options file is an editable file that identifies the server and communication method. It provides the configuration for backup, archiving, hierarchical storage management, and scheduling.
The configuration for the TSM client is defined in the following files depending on your OS:
| Client OS | Default configuration location | Purpose |
|---|---|---|
| Windows | C:\Program Files\Tivoli\TSM\baclient\dsm.opt | Main TSM configuration file |
| Unix/Linux | /opt/tivoli/tsm/client/ba/bin/dsm.sys | Main TSM configuration file |
| Unix/Linux | /opt/tivoli/tsm/client/ba/bin/dsm.opt | Secondary TSM configuration file (used for dsmcad) |
These are the minimum options required to get started using TSM:
$ cat /opt/tivoli/tsm/client/ba/bin/dsm.sys Servername server_a COMMMethod TCPip TCPPort 1500 TCPServeraddress node.domain.company.com
We mainly need it if we use several TSM servers:
$ cat /opt/tivoli/tsm/client/ba/bin/dsm.opt Servername server_a
Typical configuration options look like:
The SERVERNAME option specifies the name you want to use to identify a server.
SERVERNAME TSM-SERVER-1
The COMMMethod option specifies the communication method you use to provide connectivity for client-server communication.
COMMMethod TCPip
The TCPPort option specifies a TCP/IP port address for a Tivoli Storage Manager server.
TCPPort 1500
The TCPServeraddress option specifies the TCP/IP address for a Tivoli Storage Manager server.
TCPServeraddress 192.168.0.5
The TCPCLIENTAddress option specifies a TCP/IP address if your client node has more than one address, and you want the server to contact an address other than the one that was used to make the first server contact.
TCPCLIENTAddress 192.168.0.200
The TCPCLIENTPort option specifies a different TCP/IP port number for the server to contact than the one that was used to make the first server contact.
TCPCLIENTPort 1501
The PASSWORDAccess option specifies whether you want to generate your password automatically or set as a user prompt.
PASSWORDAccess generate
Use the NODename option in your client options file dsm.opt to identify your workstation to the server.
NODename CLIENT-SERV-123456
The SCHEDMODe option specifies whether you want to use the polling mode (your client node periodically queries the server for scheduled work), or the prompted mode (the server contacts your client node when it is time to start a scheduled operation).
SCHEDMODe PROMPTED
The INCLExcl option specifies the path and file name of an include-exclude options file.
INCLExcl /opt/tivoli/tsm/client/ba/bin/inclexcl.def
The MANAGEDServices option specifies whether the Tivoli Storage Manager Client Acceptor service (CAD) manages the scheduler, the Web client, or both.
MANAGEDServices schedule
The ERRORLOGName option specifies the fully qualified path and file name of the file in which to store information about errors that occur during processing.
ERRORLOGName /opt/tivoli/tsm/client/ba/bin/dsmerror.log
The ERRORLOGRetention option specifies how many days to maintain error log entries before pruning, and whether to save the pruned entries.
ERRORLOGRetention 7 D
The SCHEDLOGName option specifies the path and file name where you want to store schedule log information.
SCHEDLOGName /opt/tivoli/tsm/client/ba/bin/dsmsched.log
The SCHEDLOGRetention option specifies the number of days to keep entries in the schedule log, and whether to save the pruned entries.
SCHEDLOGRetention 7 D
The DEDUPLICATION option configures the client so that you can use data deduplication.
DEDUPLICATION YES
You can use the ‘query options’ command to display all or part of your options and their current settings relevant to the command-line client.
Display all options:
tsm> query options
Display only options beginning with sched:
tsm> query options sched*
Here are the typical configuration files that i use for deploying new TSM clients:
$ cat /opt/tivoli/tsm/client/ba/bin/dsm.sys SErvername TSM-SERVER-1 COMMMethod TCPip TCPPort 1500 TCPServeraddress 192.168.0.5 PASSWORDAccess generate NODename CLIENT-NODE-12345 SCHEDMODe PROMPTED MANAGEDServices schedule ERRORLOGName /opt/tivoli/tsm/client/ba/bin/dsmerror.log ERRORLOGRetention 7 D SCHEDLOGName /opt/tivoli/tsm/client/ba/bin/dsmsched.log SCHEDLOGRetention 7 D DEDUPLICATION yes $ cat /opt/tivoli/tsm/client/ba/bin/dsm.opt SErvername TSM-SERVER-1
There are several reasons why you may want to use alternative DNS servers:
Here is a list of Free public DNS servers.
Google Public DNS – Free DNS server list.
# For IPv4: nameserver 8.8.8.8 nameserver 8.8.4.4 # For IPv6: nameserver 2001:4860:4860::8888 nameserver 2001:4860:4860::8844
OpenDNS – Free Public DNS IP address.
nameserver 208.67.220.220 nameserver 208.67.222.222
Norton ConnectSafe – Free DNS server list.
nameserver 198.153.192.1 nameserver 198.153.194.1 # A - Security (malware, phishing sites and scam sites): nameserver 198.153.192.40 nameserver 198.153.194.40 # B - Security + Pornography: nameserver 198.153.192.50 nameserver 198.153.194.50 # C - Security + Pornography + Non-Family Friendly: nameserver 198.153.192.60 nameserver 198.153.194.60
Comodo Secure DNS – Free Public DNS IP address.
nameserver 8.26.56.26 nameserver 8.20.247.20
DNS Advantage – Free Public DNS IP address.
nameserver 156.154.70.1 nameserver 156.154.71.1
ScrubIT – Free Public DNS IP address.
nameserver 67.138.54.100 nameserver 207.225.209.66
Verizon (Level 3 Communications) – Free Public DNS IP address.
nameserver 4.2.2.1 nameserver 4.2.2.2 nameserver 4.2.2.3 nameserver 4.2.2.4 nameserver 4.2.2.5 nameserver 4.2.2.6