1. Linux Operations and Administration
Chapter Eleven
Domain Name System

2. Objectives
Describe Domain Name System and how it relates to the Internet
Install and configure Berkeley Internet Name Daemon (BIND)
Check the status of a DNS server via the command line
Identify zones and describe the elements in a zone file
Explain how the different types of resource records affect zones

3. Introduction to Domain Name System
Name server
A central database that translates names to IP addresses (or IP addresses to names)
Simplified network management dramatically
This method worked for local networks, but not for resources on the Internet
Resource record
A file containing resource information or characteristics about a zone or domain

4. DNS on the Internet Internet’s Domain Name System (DNS)
Associates an IP address (such as ) with an actual name (such as server1)
Network resources can be accessed by easy-to remember names
Berkeley Internet Name Daemon (BIND)
Usually carries out translation for DNS
Open-source software containing DNS protocols needed to resolve hostnames to IP addresses

5. DNS on the Internet (cont’d.)
Goal of DNS is to decentralize administration
DNS is a distributed and hierarchical database
Allows controlling DNS management locally
Local DNS server
Can handle cached hostname requests
Doesn’t contain information on every hostname on the Internet
Forwards requests for unknown hostnames to a forwarder

6. DNS on the Internet (cont’d.)
Forwarder
A type of DNS server
Forwarder’s job is to handle off-site requests generated at the system known as “localhost”
Caching
A feature that stores DNS queries on the local site for fast hostname–to–IP address resolution
Local administrators can manage local DNS servers

7. DNS on the Internet (cont’d.)
Root DNS servers
Centrally controlled public DNS servers
Control the Internet’s top-level domains (TLDs)
Table 11-1
Describes some of the most recognizable top-level domains

8. DNS on the Internet (cont’d.)
Table 11-1 Common top-level domains

9. DNS on the Internet (cont’d.)
DNS database is structured as an inverted tree
Also known as domain namespace
Root at top of the tree
TLDs beneath it
Nodes (leaves) of the tree are called domains and have labels
Such as .gov for U.S. government domain
Domain names are derived from node labels
Each level of hierarchy is separated by dots in domain name

10. DNS on the Internet (cont’d.)
Figure 11-1 The DNS structure
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11. Installing BIND
Installing BIND is similar to installing Apache Web Server
YaST Software Management is used to install the DHCP and DNS Server pattern
Installs:
All the necessary DNS modules, such as BIND
BIND documentation files
DNS Server Configuration utility

12. Installing BIND (cont’d.)
BIND documentation files
Found in /usr/share/doc/packages/bind/arm/ directory
Consist of eight chapters
Activity 11-1: Installing BIND
Install BIND and other DNS-related packages

13. Configuring BIND BIND named.conf
Requires no further configuration after being installed in openSUSE
Runs as a daemon known as named
Pronounced “name-dee,” not “named”
named.conf
Main BIND DNS configuration file
Found in /etc directory

14. Configuring BIND (cont’d.)
To start the named daemon (BIND):
Use rcnamed start command
Table 11-2
Describes common commands for determining the daemon’s status
Options to edit named.conf file
Use a text editor, such as vim
Use YaST DNS module
Available after you install BIND

15. Configuring BIND (cont’d.)
Table 11-2 Common named commands

16. Configuring BIND (cont’d.)
To start YaST DNS module
Use yast2 dns-server command
Requires root privileges
DNS Server Installation Wizard
Opens the first time you start the module
Enables you to add forwarders
Expert mode
Available after you step through the basic settings
For more advanced configuration tasks

17. Configuring BIND (cont’d.)
Figure 11-2 The DNS Server Installation Wizard
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18. Forwarder Can have up to three forwarders in DNS configuration
These servers enable you to:
Forward unresolved queries to an off-site DNS server
Cache results on your local DNS server

19. Forwarder (cont’d.)
First time you configure a DNS server, the forwarder list is empty
Forwarder declaration in /etc/named.conf file is, by default, a commented line; for example:
#forwarders { ; ; };
Deleting # symbol adds the two servers ( and in example) to forwarder list

20. DNS Zones Zone Zone file Master name server
A portion of the DNS namespace
Zone file
Stores the data for a zone
Master name server
An authoritative name server that stores the primary copies of zone records

21. DNS Zones (cont’d.) Slave name server
Also known as a secondary name server
Maintains a copy of master zone file that’s used as a backup
Provides redundancy if master name server is unavailable

22. DNS Zones (cont’d.) Entries in zone file Line 1 Line 2 Line 3 Line 4
Time-to-live (TTL) entry
Line 2
Start of Authority (SOA) resource record
Line 3
Serial number
Line 4
Refresh rate

23. DNS Zones (cont’d.) Line 5 Line 6 Line 7 Line 8 Retry rate
Expiration entry
Line 7
Negative TTL entry
Line 8
NS resource record

24. MX RR Identifies mail servers (mail exchangers) for a zone Table 11-3
Describes fields in MX RR
Example:

25. MX RR (cont’d.)
Table 11-3 MX resource record fields

26. A RR A (address) RR Table 11-4 Example: Most common resource record
Used to resolve a hostname to an IPv4 address for locating a resource
Table 11-4
Describes fields in the A RR
Example:

27. A RR (cont’d.)
Table 11-4 A resource record

28. PTR RR PTR (pointer) RR Table 11-5 Example:
Used to resolve an IPv4 address to its hostname
Performs the reverse of what an A RR does
Table 11-5
Describes fields in the PTR RR
Example:

29. PTR RR (cont’d.)
Table 11-5 PTR resource record

30. CNAME RR CNAME (canonical name) RR Table 11-6 Example:
Enables you to create an alias for a host
Table 11-6
Describes fields in the CNAME RR
Example:

31. CNAME RR (cont’d.)
Table 11-6 CNAME resource record

32. Expert Configuration Mode
Final window in DNS Server Installation Wizard
Option to open DNS port in your firewall
Option to start DNS server automatically at bootup
After completing DNS Server Installation Wizard
Whenever you open the DNS module, you’re in expert configuration mode

33. Start-Up Settings under Start-Up window
Whether DNS starts automatically when the server starts or must be started manually
Firewall settings, such as opening the firewall port to allow remote computers to access DNS service
Stop, start, and reload DNS server

34. Start-Up (cont’d.) Figure 11-3 The Start-Up window
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35. Forwarders Forwarders window
Used for the same task as in DNS Server Installation Wizard
Adding IP addresses to the list of forwarder servers

36. Basic Options Basic Options window Ways to add options: Table 11-7
Displays options that have already been defined for the zone
Ways to add options:
Use Basic Options window
If you use this window, information is updated in /etc/named.conf file automatically
Edit /etc/named.conf file
Table 11-7
Describes the options that can be added or modified

37. Basic Options (cont’d.)
Figure 11-4 The Basic Options window
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38. Basic Options (cont’d.)
Table 11-7 Settings in the Basic Options window

39. Basic Options (cont’d.)
Table 11-7 Settings in the Basic Options window (continued)

40. Logging Logging window
Can be used to configure logging options for DNS server
By default, DNS server sends log data to the systemwide log file /var/log/messages
Additional Logging section
Log All DNS Queries option
Log Zone Updates option
Log Zone Transfers option

41. Logging (cont’d.) Figure 11-5 The Logging window
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42. Access Control List (ACL)
Access Control List (ACL) window
Used by BIND administrators to control who can perform operations on the name server
When creating an ACL, you must:
Give it a unique name
Specify an IP address under Value heading

43. Access Control List (ACL) (cont’d.)
When creating an ACL, the IP address must be:
Enclosed by braces
End with a semicolon
Example: { ; }

44. TSIG Keys Transaction signatures
Used to secure communication between two servers
Usually between DHCP and DNS servers
Ways to generate keys for this signature:
TSIG Keys window
dnssec-keygen command

45. TSIG Keys (cont’d.) Activity 11-2: Configuring BIND
Review settings in the /etc/named.conf file and configure a BIND server with the DNS Server Installation Wizard
Activity 11-3: Creating a Resource Record in YaST
Use the YaST DNS Server module to create a resource record

46. TSIG Keys (cont’d.)
Figure 11-6 An A resource record added to the master zone file
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47. Summary Domain Name System (DNS) BIND
Translates domain names and hostnames to IP addresses
A distributed and hierarchical database
Allows controlling DNS management locally
BIND
Open-source DNS server software
Local DNS sites can forward DNS queries to a forwarder DNS server
It forwards the request to an off-site DNS server

48. Summary (cont’d.) DNS structure is viewed as an inverted tree
Main DNS configuration file is named.conf file in the /etc directory
YaST DNS module is included with DNS installation and is used to configure DNS servers
DNS zone file is a text file that stores resource records and other data for a zone
Expert configuration mode for YaST DNS module gives access to more advanced configuration tasks