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Introduction to Linux - A Hands on Guide
This guide was created as an overview of the Linux Operating System, geared toward new users as an exploration tour and getting started guide, with exercises at the end of each chapter.
For more advanced trainees it can be a desktop reference, and a collection of the base knowledge needed to proceed with system and network administration. This book contains many real life examples derived from the author's experience as a Linux system and network administrator, trainer and consultant. They hope these examples will help you to get a better understanding of the Linux system and that you feel encouraged to try out things on your own.
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By Mara at 2003-07-20 01:58
When you're browsing LinuxQuestions, your computer doesn't send data to www.linuxquestions.org. It sends to 126.96.36.199. You don't have to remember the numerical address - DNS servers do it for you.
This text was written to tell you what DNS is, how it works and how you can use it. It doesn't describe configuration of your own DNS server.
Before I describe how the DNS system works, you need to know about IP adresses and domains.
188.8.131.52 is an example. IP (Internet protocol) address has a form of four numbers (between 0 and 255), separated by dots. Each machine in the Internet must have an unique IP address. It works as a postal address - no message can reach the machine without a correct address.
Numbers are friendly for machines, but hard to remember for humans. That's why we have hostnames and domains (there are more reasons, for example names allow you to easily change IP addresses of servers - only the DNS entry must be changed).
Hostnames, domains, zones
An example of a hostname: www.linuxquestions.org. It's a part of linuxquestions.org domain. Last portion of the address (.org) refers to a toplevel domain where the host belongs. Other toplevel domains are: .com, .net, .gov etc. Every country has its own toplevel domain. For example: .fr (France), .de (Germany), .jp (Japan) or .br (Brazilia).
Zone and domain mean nearly the same. There's one difference: zone is delegated to one server. A domain can be diveded into different zones.
How it works?
When you connect to LQ, you send a query to your DNS resolver (more about it later). The server checks if it has needed information cached. If it does, it sends it back to you. The server that stores data from previous queries is called a caching nameserver.
If it doesn't have the information you're searching for, it asks one of the root servers. Root servers are on the top of DNS servers tree (the structure looks like a tree). They contain pointers to name servers for each top level domain.
Now the toplevel domain server for .org is asked about linuxquestions.org domain, it knows the address of the server containing detailed information about all hosts in the linuxquestions.org domain.
The last server is asked about the host www.linuxquestions.org hosts. When your resolver server gets the reply, it sends the result back to you.
Types of DNS servers:
A caching nameserver finds answers to queries and then remembers them. This not only speeds up your response time but helps to keep the load on the root nameservers (which handle thousands of queries per second) as low as possible.
An Authoritative server is a server that can answer queries about a zone without help from any other server. Both primary and secondary servers are authoritative. Primary server(in terms of specific zone) is a server that reads zone data from its local configuration. Secondary server(in terms of a specific zone) gets zone data from outside sources (usually from master server).
How to use DNS servers?
The file with IP addresses of your nameservers is /etc/resolv.conf. When the machine is correctly configured, it should look like this:
DNS servers are listed in lines beginning with 'nameserver' (there are more things to configure in that file). In the example above there are two DNS servers: 192.168.1.1 and 192.168.1.3 (don't copy them, local addresses are used and it may not work in your network).
There's one more important file - /etc/host.conf. It should look like below:
When the file is filled properly, everything should work fine.
Where to get the addresses from?
Your ISP is the best source. Most ISPs have their own DNS servers. They're closest to you (so should be the fastest).
If you're using DHCP (many ADSL connections) or modem, your ISP may pass the information to you when you connect. In such case, you don't need to edit /etc/resolv.conf manually.
How to test it?
You can use 'nslookup' or 'dig'.
First example, using nslookup:
sh-2.05b$ nslookup www.linuxquestions.org
Note: nslookup is deprecated and may be removed from future releases.
Consider using the `dig' or `host' programs instead. Run nslookup with
the `-sil[ent]' option to prevent this message from appearing.
As you can see it gives the same answer as we got when ping was used.
Next example, using 'dig'. The command format is:
dig @DNS_server host
It returns more information, so may be harder to read. I'm asking the server 192.168.1.1 about www.linuxquestions.org. You don't need to give the DNS server. If you simply use 'dig.linuxquestions.org', your default DNS server will be used. In the 'ANSWER SECTION' you can see that www.linuxquestions.org is 184.108.40.206. In the line 'Query time' you can see how much time it takes to get an answer. In the example below the time is very short, because I use caching nameserver.
sh-2.05b$ dig @192.168.1.1 www.linuxquestions.org
; <<>> DiG 9.2.2 <<>> @192.168.1.1 www.linuxquestions.org
;; global options: printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 28312
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 4, ADDITIONAL: 0
;; QUESTION SECTION:
;www.linuxquestions.org. IN A
;; ANSWER SECTION:
www.linuxquestions.org. 16 IN A 220.127.116.11
;; AUTHORITY SECTION:
linuxquestions.org. 16 IN NS ns2.choiceone.net.
linuxquestions.org. 16 IN NS ns3.choiceone.net.
linuxquestions.org. 16 IN NS ns1.choiceone.net.
linuxquestions.org. 16 IN NS ns1.linuxquestions.org.
;; Query time: 6 msec
;; SERVER: 192.168.1.1#53(192.168.1.1)
;; WHEN: Fri Jul 11 17:00:39 2003
;; MSG SIZE rcvd: 141