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Hostname
/etc/hostname file and communicated to the Linux kernel by initialization scripts through the hostname command. The current value is available in a virtual filesystem, and you can get it with the cat /proc/sys/kernel/hostname command./etc/hosts file; simply write a complete name for the machine there at the beginning of the list of names associated with the address of the machine, as in the following example:127.0.0.1 localhost 192.168.0.1 arrakis.falcot.com arrakis
Name Resolution
/etc/nsswitch.conf file. The entry that involves host name resolution is hosts. By default, it contains files dns, which means that the system consults the /etc/hosts file first, then DNS servers. NIS/NIS+ or LDAP servers are other possible sources.Configuring DNS Servers
www.eyrolles.com into the actual IP address, 213.244.11.247./etc/resolv.conf, one per line, with the nameserver keyword preceding an IP address, as in the following example:nameserver 212.27.32.176 nameserver 212.27.32.177 nameserver 8.8.8.8
/etc/resolv.conf file may be handled automatically (and overwritten) when the network is managed by NetworkManager or configured via DHCP.The /etc/hosts file
/etc/hosts file, usually reserved for local network stations. The syntax of this file is very simple: each line indicates a specific IP address followed by the list of any associated names (the first being “completely qualified”, meaning it includes the domain name)./etc/hosts generally only contains the most important entries.Before the advent of a distributed domain name system; networked computers used local files to map hostnames to IP addresses. On Unix systems this file was named /etc/hosts or “the hosts file”. In those days, networks were small and managing a file with a handful of hosts was easy. However as the networks grew so did the methods of mapping hostnames and IP addresses.
In modern days with the internet totaling at somewhere around 246 million domain names (as of 2012) the hosts file has been replaced with a more scalable distributed DNS service. While the hosts file is not used to map all hostnames to IP’s these days it can still be quite useful if used properly and it can also be a source of trouble if not understood or used properly.
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Times to use the /etc/hosts
A Local Network for Desktop Environments
Using the /etc/hosts file to give a human readable name to a local system within a desktop environment is perfectly reasonable. The hosts file is great to use in a home network or even in a small business environment where the machine count is only a handful of systems. As a side note the hosts file can be used to provide DNS for a local system but it can also be used to restrict accessto domains as well.
The best part of this is that the majority of Desktop Operating Systems have some sort of hosts file and they all follow roughly the same syntax, including Windows & OSX.
Critical services in a enterprise server environment
Let’s face it, sometimes local DNS systems can break. If you have servers that connect to critical systems via hostnames it may be wise to add the IP and hostname into the hosts file. This allows you to keep the connectivity to these services alive even during a local DNS outage.
A word of warning however, this extra security comes with a price. While it is easy to mistype an IP address into a DNS system it is just as easy to mistype that IP address into a hosts file. By managing these IP’s in hosts files it also adds the complexity of managing the hosts files and ensuring that they are 100% correct all of the time.
This method also only works if that critical system is always going to have the same IP address, if DNS load balancing is used to reroute traffic for this critical system than it is not advisable to add this to the hosts file as it would circumvent the DNS failover.
Specific Application Environments
Not every application environment has hundreds of servers that fill the same role, in enterprise environments it is quite common to see a smaller set of servers that are used for a specific application. That application may have a dedicated database server and/or dedicated web servers. In environments like this it is not uncommon to add each host in the environment into the hosts file. The same warnings apply in this environment as the one above, sometimes having hostnames go to DNS is the preferred method when the IP address of that system may change. However if the IP address is static and you do not trust your internal DNS system to be accurate or available, adding the IP into /etc/hosts is worth the effort.
As a note, when you choose to use the hosts file you also take the responsibility of having to manage this file and manage the accuracy of the file. This is easier with configuration management tools but even this can become troublesome if the IP’s of hosts change and the configuration is not kept up to date.
Times not to use the /etc/hosts file
Server environments at scale
Whenever managing a large set of servers thousands upon hundreds of thousands of systems. Using the /etc/hosts file to map systems can be quite difficult and should only be used to map to systems that are serviced from static IP addresses. In large scale environments however, a service is usually provided by multiple IP addresses and that load balancing is driven by an internal DNS system.
Adding a domain to /etc/hosts
Below I am going to show how to use the /etc/hosts file to locally point a domain to another IP. This can be used as a general example of how to add a domain or short hostname into the hosts file.
Our example domain
In our example we are going to use example.com which today resolves to 93.184.216.119.
$ ping -c 1 example.com
PING example.com (93.184.216.119) 56(84) bytes of data.
Adding the host name & domain name to the /etc/hosts file
To add our example we will first need to open the hosts file with vi or your favorite editor.
$ sudo vi /etc/hosts
Append:
# IP Domain ShortHost
192.168.0.193 example.com example
Ensure nsswitch.conf is correct
Before we go off checking if the resolution is correct let’s ensure that the nsswitch.conf file is correct. The nsswitch.conf file controls in which order services will be consulted for name service lookups, in our case we are looking for the “hosts” service.
$ grep host /etc/nsswitch.conf
hosts: files dns
This setting is based on order, if files is before dns than the system will consult the /etc/hosts file before checking DNS for name service requests. However, if DNS was before files than the domain lookup would always go to DNS first.
Validating the new resolution
As you can see from the ping command example.com now resolves to 192.168.0.193 and not 93.184.216.119.
$ ping -c 1 example.com
PING example.com (192.168.0.193) 56(84) bytes of data.
Don’t use host or nslookup commands
Using the host or nslookup command to validate the change will not work and will simply cause confusion. Both of these commands will consult DNS and ignore the settings in hosts and nsswitch.conf.
$ host example.com
example.com has address 93.184.216.119
The fact that the host and nslookup command commands do not return the same result as the hosts file entry adds to the complication of using /etc/hosts. 3 months down the line when you are troubleshooting connectivity to example.com and the host command returns a valid result it will be easy to forget that you added it to the hosts file. My advice is, if you are going to use the hosts file make sure that the reason you are using it is valid and that it is the only way to accomplish what you need.
Whenever any applications performs DNS Lookup in Linux operating system it looks in both “/etc/hosts” and “/etc/resovl.conf” configuration files to resolve the DNS name. In Linux for DNS lookup order it use “/etc/nsswitch.conf” file. There are two library libc and glibc are used to perform this operations in Linux , by defining DNS Lookup orders.
What is DNS ?
DNS ( Domain Name System) is mechanism to make internet human friendly. Computer communicate with each other using their IP addresses. There are lots of IP address in internet world, and It is very difficult to remember all IP address . To solve this issue and make more human friendly DNS invented. IP address map with host-name in DNS server.
When we enter a domain name into our browser like http://www.goole.com computer find our nearest DNS server and ask what is the correct IP address for http://www.google.com. Than it returns the IP address to our system to communicate with http://www.google.com.
/etc/nsswitch.conf file
In /etc/nsswitch.conf file there is a database for DNS lookup called “hosts”.
#cat /etc/nsswitch.conf | grep "hosts" hosts: files dns
Above you can see DNS lookup refers to /etc/hosts/ and /etc/resolv.conf. First DNS lookup look into /etc/hosts file to resove name and than it will look into /etc/resolv.conf file to resolve the host-name.
Please note :- DNS lookup toots like nslookup and dig will ignore /etc/nsswitch.conf file and always refer to DNS server using /etc/resolv.conf.
/etc/hosts file
/etc/hosts file is the most important file in Linux operating system. It is a text file for name resolution.
# cat /etc/hosts 127.0.0.1 localhost localhost.localdomain localhost4 localhost4.localdomain4 ::1 localhost localhost.localdomain localhost6 localhost6.localdomain6 192.168.0.2 client01 192.168.0.3 client02 www.example.com
Both fields are separated by space or tab followed by IP and Host-name per line. Host-name contain only alphanumeric characters, minus sign (-) and period (.) .
Fields Explanations :
192.168.0.3 : IP address
client02 : Host-name
http://www.example.com : Aliases of IP address
You can also see other entries in /etc/hosts file like 127.0.0.1 is loopback addresses. Which is pointed to “localhost” host-name.
/etc/resolv.conf file
This is the another important file in Linux operating system. It contains information that help to computer to convert domain name to its IP address . All process called resolving.
# cat /etc/resolv.conf nameserver 173.204.4.5 nameserver 173.204.4.7
Above you can see “nameserver” directive which is pointed to the IP address of Name Server.
In my example 173.204.4.5 and 173.204.4.7 is my own Name Servers.
You can also use Public Name Server following below configuration.
# cat /etc/resolv.conf nameserver 8.8.8.8 nameserver 8.8.4.4 nameserver 4.2.2.2
weicode 