1 Internet Network Services

2 Module - Internet Network Services ♦ Overview This module focuses on configuring and customizing the servers on the network using Redhat Enterprise Linux. This will help the users to connect and use Internet services. ♦ Lessons covered in this module ► Introduction to Domain Name System (DNS) ► DNS Zones ► DNS Server Configuration

3 Internet Network Services ♦ Introduction DNS is a distributed database of host names and IP addresses used for Internet Communication. The DNS servers have a hierarchy and need to be resolved for effective communication. ♦ Topics covered in this lesson DNS and Berkeley Internet Name Domain (BIND) Internet DNS Hierarchy Name Server Hierarchy Configuring the DNS Resolver Lesson 1 - Introduction to Domain Name System (DNS)

4 Internet Network Services Topic 1 – DNS and BIND ♦ DNS has two main components : Name servers and Resolvers. Name servers maintain the database of host names and IP Addresses. DNS Name servers respond to requests from Resolvers, which are client computers. Resolvers may use more than one server to resolve a query. ♦ Berkeley Internet Name Domain service (BIND) is the common DNS server used in Red Hat Linux. The program named provides the BIND service. It loads a set of master files into its internal cache at start up and waits for queries from user processes. BIND can be set up in different ways.

5 Internet Network Services Topic 2 – Internet DNS Hierarchy ♦ On the Internet host names in DNS are organized in a hierarchy of domains. Each domain is a collection of sites that are related in some manner. These domains are arranged in a hierarchical structure of five levels: Root-level domain, Top-level domain, Second-level, domain, Subdomains and Host names. ♦ The Root-level is maintained by a set of 13 servers and indicated by a period (.). This level manages the Top-level domains. Some common Top-level domains are :.com - to register a business,.edu - educational institution,.gov - government agency,.mil – military,.net - network affiliated organization

6 Internet Network Services Topic 3 - Name Server Hierarchy ♦ DNS Name servers also have a hierarchy. They are of the following types : ► Root server provides information about the Top-level. There are 13 Root servers located in different countries. ► Primary server (Master server) is responsible for controlling and managing the information of a particular zone. ► Secondary servers (Slave servers) are backup servers for the Primary servers. ► Caching servers provide information to other servers on the local Networks and not to outside sources. ► Forwarding servers forward locally unresolved name resolutions to the Root servers.

7 Internet Network Services Topic 4 - Configuring the DNS Resolver ♦ Edit resolver configuration file as follows: vi /ete/resolv.conf search mylearning.com nameserver ♦ The first command defines the default domain to be appended to simple hostnames. The next command tells that the host (machine) should be used to resolve all DNS queries. Remove all statements other than the localhost hostname definitions from hostname configuration file: /etc/hosts.

8 Internet Network Services Lesson 2 - DNS Zones ♦ Introduction Host names and domains in DNS are maintained in different zones. Zone statements in zonal files indicate this. Record types indicate other attributes of DNS domains. BIND utilities help to check the configuration. ♦ Topics covered in this lesson ► Zonal Files ► Record Types ► Bind Utilities

9 Internet Network Services Topic 1 - Zonal Files ♦ Zone statements refer to zone files. They begin with the word zone followed by domain name and data class. Data classes are: in, hs, hesiod and chaos and the default data class is in meaning Internet. The start of authority (SOA) is the first line in the zone file. It identifies the name server for the domain. ♦ After semicolon the following comments are specified: ► Refresh- The time interval for the server to refresh its data. ► Retry - Contact the primary server again after this interval. ► Expire - If not contacted within specified period, the data expires and should be discarded. ► TTL - Lifetime for the data to be retained in the cache.

10 Internet Network Services Topic 2 - Record Types ♦ Resource Records (RR) provide additional server information after the SOA header information. These records comprise the bulk of resources in database files. ♦ Their wrong setup can cause problems. Therefore you should be careful before making changes. ♦ These are the Resource Records : NS, A, PTR, CNAME, MX, TXT, WKS, HINFO.

11 Internet Network Services Topic 3 - Bind Utilities ♦ If there are syntax errors BIND will not start. ♦ BIND utilities check the syntax and configuration ♦ Always use them before restarting BIND. ♦ The BIND utilities are: ► named-checkconf - Inspects /var/named/chroot/etc/named.conf by default. ► named-checkzone - Inspects a specific zone configuration named- checkzone redhat.com

12 Internet Network Services Lesson 3 – DNS Server Configuration ♦ Introduction For effective communication on the Internet the DNS Servers on network should be configured properly. This is done with BIND packages. The procedures for master, slave and stub servers are different. ♦ Topics covered in this lesson ► Configuring Master Server ► Configuring Slave Server ► Configuring Stub (Caching only) Server

13 Internet Network Services Topic 1 - Configuring Master Server ♦ BIND packages used for configuring servers are: bind, bind-utils, bind- chroot, caching-nameserver. ♦ The server configuration files are: named.conf, named.ca, named.local ♦ Configuration statements listed here contain commands for configuring the server options, include, acl, server, zone, key. Contd …

14 Internet Network Services Topic 1 - Configuring Master Server ♦ Configuration statements: ► options - for listing defaults and global configurations ► include - for including information from another file ► acl - for IP addresses in access control list ► Server - to specify remote server properties ► zone - to specify zone information ► key - to specify security keys for authentication Contd …

15 Internet Network Services Topic 1 - Configuring Master Server ♦ options statement lists the files to be used by named and their location. options { value “property” ; } ♦ Commonly used global options: directory, forwarders, allow-query, allow-transfer, cleaning-interval, dump-file, forward. ♦ Check the configuration with named-checkconfig command.

16 Internet Network Services Topic 2 - Configuring Slave Server ♦ Steps are similar to that of master server. ♦ Open the /etc/named.conf file. ♦ Add two more zones ► One for the forward lookup ► One for the reverse lookup ♦ Restart the service.

17 Internet Network Services Topic 3 - Configuring Stub (Caching only) Server ♦ Default installation of BIND in Red Hat Linux creates the configuration files required for a caching name server. You should ensure the files are correct. ♦ The files to be checked are ► /etc/named.conf, ► /var/named/named.local, ► /etc/nsswitch, ► /etc/resolv.conf, ► /etc/host.conf

18 Internet Network Services Lab Exercises ♦ Resolving DNS in different situations. ► When dig does not work ► When servers are down

19 Internet Network Services Conclusion ♦ Summary ► DNS is a distributed database of host names and their IP addresses. BIND service is the most common DNS server. ► Internet host names in DNS are organized in a hierarchy of domains in five levels. DNS servers also have a corresponding hierarchy. ► You can configure servers by editing appropriate files in /var/named/chroot and using these packages: bind, bind-utils, bind- chroot, and caching-nameserver. ♦ Question and Answer Session