1. Linux+ Guide to Linux Certification, Third Edition
Chapter 13
Configuring Network Services

2. Objectives
Configure infrastructure network services, including DHCP, DNS, NTP, and NIS
Configure Web services using the Apache Web server
Configure files sharing services, including Samba, NFS, and FTP
Configure services, including Sendmail and Postfix
Configure database services using PostgreSQL
Linux+ Guide to Linux Certification, 3e

3. Infrastructure Services
Infrastructure services: provide network configuration and support for other computers on a network
Include:
DHCP
DNS
NTP
NIS
Linux+ Guide to Linux Certification, 3e

4. DHCP
Dynamic Host Configuration Protocol (DHCP): used for automatically configuring a network interface
Send DHCP broadcast on network
Request IP configuration information
DHCP server leases IP address to client computer for a period of time
Ensures each client has unique IP address
After expiration, must send another DHCP request
Linux+ Guide to Linux Certification, 3e

5. The DHCP Lease Process Involves several stages:
Client sends a request to all hosts on network
DHCP server sends offer containing potential IP configuration
Client selects (accepts) offer
DHCP server sends acknowledgement indicating the amount of time client can use IP configuration
Client configures itself with IP configuration
Linux+ Guide to Linux Certification, 3e

6. The DHCP Lease Process (continued)
Figure 13-1: The DHCP lease process
Linux+ Guide to Linux Certification, 3e

7. Configuring a Linux DHCP Server
Install DHCP daemon
Use yum install dhcp command
Edit DHCP daemon configuration file to list appropriate IP address range for the network and lease information
/etc/dhcp/dhcpd.conf stores IPv4 configuration
/etc/dhcp/dhcpd6.conf stores IPv6 configuration
service dhcpd start command: starts the DHCP daemon
Linux+ Guide to Linux Certification, 3e

8. DNS
Hierarchical namespace used to identify computers on large TCP/IP networks
Zone: portion of DNS administered by one or more DNS servers
Forward lookup: FQDN resolved to IP address
Reverse lookup: IP address resolved to FQDN
Linux+ Guide to Linux Certification, 3e

9. The DNS Lookup Process
Web browser performs a forward lookup of FQDN to contact IP of Web server
Performed by DNS server
Iterative query: resolved using DNS cache
Does not use top-level DNS servers
Recursive query: resolved with the use of top-level DNS servers
DNS cache file: contains IP addresses of top-level DNS servers
Linux+ Guide to Linux Certification, 3e

10. The DNS Lookup Process (continued)
Figure 13-2: The DNS lookup process
Linux+ Guide to Linux Certification, 3e

11. The DNS Lookup Process (continued)
Master or primary DNS server: contains read/write copy of zone
Slave or secondary DNS server: contains read-only copy of zone
Zone transfer: copying zone resource records from master to slave DNS server
Linux+ Guide to Linux Certification, 3e

12. Configuring a Linux DNS Server
Configure DNS name daemon for a specific zone
Add resource records that list FDQNs and associated IP addresses for computers in that zone
Configuration files have BIND format
Difficult to create manually
Use graphical utility such as BIND configuration utility
Start the DNS name daemon
Use service named start command
dig command: used to query records that exist on a specific DNS server
Linux+ Guide to Linux Certification, 3e

13. Configuring a Linux DNS Server (continued)
Table 13-1: Common zone configuration files
Linux+ Guide to Linux Certification, 3e

14. Configuring a Linux DNS Server (continued)
Table 13-1 (continued): Common zone configuration files
Linux+ Guide to Linux Certification, 3e

15. Configuring a Linux DNS Server (continued)
Figure 13-3: The BIND configuration utility
Linux+ Guide to Linux Certification, 3e

16. NTP
Network Time Protocol (NTP): used by OS to obtain time information BIOS system clock or from network servers
hwclock command: modifies BIOS date and time
Uses UDP port 123
Linux+ Guide to Linux Certification, 3e

17. Understanding NTP Strata
Strata: hierarchical series of time resources used by NTP
Stratum 0: Atomic clock or GPS clock
Stratum 1: Obtain time directly from stratum 0 device
Stratum 2: Obtain time directly from stratum 1 device
Stratum is not an indication of quality or reliability
NTP servers obtain time information from multiple sources and use algorithm to determine most reliable time information
Linux+ Guide to Linux Certification, 3e

18. Configuring a Linux NTP Client
NTP daemon installed and started by default
Can act as NTP client to obtain time from Internet time server or as NTP server
To configure NTP client
Edit /etc/ntp.conf to add lines for different NTP servers that can be queried
ntpdate command: manually synchronize the time
Offset: time difference between time on local computer and time on time server
Linux+ Guide to Linux Certification, 3e

19. Configuring a Linux NTP Client (continued)
ntpq command: see what actual time servers system is synchronizing with
Jitter Buffer: stores the difference between the same time measurements from different NTP servers
Used by NTP when determining the most reliable time
-q option: displays the offset and jitter
tzselect command: used to change the time zone
Linux+ Guide to Linux Certification, 3e

20. Configuring a Linux NTP Client (continued)
Figure 13-5: The Date/Time Properties screen
Linux+ Guide to Linux Certification, 3e

21. Configuring a Linux NTP Server
By default in Fedora 13, NTP daemon not configured as NTP server
To allow other computers to query NTP daemon:
Edit /etc/ntp.conf
Add line identifying specific computers or networks that are allowed to query the NTP daemon
Restart NTP daemon for changes to take effect
Linux+ Guide to Linux Certification, 3e

22. NIS
Network Information Service (NIS): coordinate common configuration files across several computers
Computers belong to an NIS domain, use NIS map to access configuration information
commonly used to coordinate database files
NIS master server: Sends all NIS map configuration to NIS slave servers
NIS slave servers: Distribute maps to NIS clients
Linux+ Guide to Linux Certification, 3e

23. Configuring an NIS Server
Install NIS server daemons via yum install ypserv command
Define the NIS domain name via domainname NIS_ domain_name command
Add NISDOMAIN=“NIS_domain” to /etc/sysconfig/network file
Configure NIS domain at boot time
In the /var/yp/Makefile file, edit the list of files to be made into maps
If no slave servers, ensure NOPUSH=true
Linux+ Guide to Linux Certification, 3e

24. Configuring an NIS Server (continued)
Add identification of allowed clients to /var/yp/securenets file
Allow the allowed clients to access the appropriate maps in /etc/ypserv.conf
Start NIS daemon by service ypserv start command
Ensure NIS password server daemon is started at runlevel 5 by chkconfig –level 5 yppasswdd on command
Linux+ Guide to Linux Certification, 3e

25. Configuring an NIS Server (continued)
Generate configuration file maps by the /usr/lib/yp/ypinit –m command
Allow clients to connect by service ypbind start command
Ensure NIS binding server is started at runlevel 5 by chkconfig –level 5 ypbind on command
Linux+ Guide to Linux Certification, 3e

26. Configuring an NIS Client
Define the NIS domain name via domainname NIS_ domain_name command
Add NISDOMAIN=“NIS_domain” to /etc/sysconfig/network file
Configure NIS domain at boot time
In the /etc/yp.conf file, add, for each specific NIS server: domain NIS_domain server NIS_server
Alternatively add domain NIS_domain broadcast
Linux+ Guide to Linux Certification, 3e

27. Configuring an NIS Client (continued)
Start NIS client program by service ypbind start command
Ensure NIS binding server daemon is started at runlevel 5 by chkconfig –level 5 ypbind on command
Locate NIS server by ypwhich command
Add +:*:0:0::: to /etc/passwd to redirect requests to NIS server
yppasswd command: used by NIS clients to change NIS password
Linux+ Guide to Linux Certification, 3e

28. Web Services Apache is the most common Web server
Started as http daemon
Document root directory: stores default HTML content for a Web server
/var/www/html on Fedora Linux
Default document is index.html
/etc/httpd/conf/httpd.conf: default configuration file
Directive: Line within a configuration file
Linux+ Guide to Linux Certification, 3e

29. Web Services (continued)
Default settings sufficient for most Web servers
Copy appropriate HTML files into /var/www/html
Start Apache by service httpd start
Separate httpd daemon is started each time a client request is received by Apache Web server
Called a Web page hit
First daemon started as root user, others started as Apache user
curl command: used at BASH shell to obtain Web page
Linux+ Guide to Linux Certification, 3e

30. Table 13-2: Common httpd.conf directives
Linux+ Guide to Linux Certification, 3e

31. Sharing Services: Samba
Samba daemon: Emulates SMB protocol
Formats TCP/IP data like Windows computers
NetBIOS name daemon: to create and advertise NetBIOS name for Windows computers to connect to Linux server
nmblookup command: Tests NetBIOS name resolution in Linux
Linux+ Guide to Linux Certification, 3e

32. Configuring a Samba Server
Create Linux user for each Windows user
smbpasswd command: Generate Samba passwords
/etc/samba/smb.conf: Default Samba configuration file
Edit to include NetBIOS name
testparam command: checks syntax of /etc/samba/smb.conf
Start Samba and NetBIOS name daemons by service smb start and service nmb start commands
Linux+ Guide to Linux Certification, 3e

33. Connecting to a Samba Server
Test Samba functionality after configuration
From Windows client enter \\Samba_server_name in the Run dialog box
smbclient command: used to connect a Linux computer to a Samba server
Can also be used to display an FTP-like interface on Samba or Windows servers
Linux+ Guide to Linux Certification, 3e

34. NFS
Network File System (NFS): allows Unix, Linux, and Macintosh OS X computers to share files transparently
Export a directory by placing its name in the /etc/exports file
mount command: used by another computer to access an exported directory across the network by mounting the remote directory on the local computer
Linux+ Guide to Linux Certification, 3e

35. Configuring a Linux NFS Server
Create directory containing information to share
Edit /etc/exports file:
Add line listing directory to be shared and options
Run exportfs –a command
Update list of exported filesystems
Restart the NFS processes:
service nfs start
service nfslock start
Linux+ Guide to Linux Certification, 3e

36. Connecting to a Linux NFS Server
Mount directory from remote NFS server to a directory on local computer
Use mount command specifying nfs filesystem type, server name or IP address, remote directory, and local directory as arguments
Use the mounted directory as any other local directory, with operations being performed on the remote computer
Use umount command to dismount remote directory
Linux+ Guide to Linux Certification, 3e

37. FTP Protocol most commonly used to transfer files on public networks
Hosts files differently than NFS
In anonymous access special directory is available to any user who wants to connect to FTP server
User can log in, via an FTP client program, to a home directory on the FTP server
Linux+ Guide to Linux Certification, 3e

38. Configuring a Linux FTP Server
Very secure FTP daemon (vsftpd): used by most Linux systems
To configure (assuming logon as “user1”):
Create directory below user1’s home directory to host the files
Ensure user1 owns directory
Edit /etc/vsftpd/vsftpd.conf to modify appropriate commented options
Run service vsftpd start to start vsftpd daemon
Linux+ Guide to Linux Certification, 3e

39. Connecting to a Linux FTP Server
Most Web browsers have built-in FTP utility
Allows you to access files on remote computer
To connect through Web browser, specify the location by typing ftp://servername in the browser
To log in as particular user, type
Most OSs have command-line FTP utility
Use ftp command and specify host name as argument, log in as anonymous or as specific user
Receive prompt that accepts FTP commands
Linux+ Guide to Linux Certification, 3e

40. Connecting to a Linux FTP Server (continued)
Figure 13-7: Using a Web browser FTP client
Linux+ Guide to Linux Certification, 3e

41. Connecting to a Linux FTP Server (continued)
Table 13-3: Common FTP commands
Linux+ Guide to Linux Certification, 3e

42. Connecting to a Linux FTP Server (continued)
Table 13-3 (continued): Common FTP commands
Linux+ Guide to Linux Certification, 3e

43. Services
Various protocols exist, including SMTP, ESMTP, POP, and IMAP
server looks up the name of target server in domain’s MX records, stored on public DNS server
Resolves target server name to IP address using public DNS server
Daemons and system components rely on to send important information to the root user
Linux+ Guide to Linux Certification, 3e

44. Working with Sendmail
Sendmail: one of oldest and most complex daemons
By default accepts on TCP port 25
Test using telnet port_num, EHLO, and HELO commands
mail command: checks local
/etc/aliases file: contains other names used to identify the users on the system
newaliases command: rebuilds the aliases database after modifications
Linux+ Guide to Linux Certification, 3e

45. Working with Postfix Postfix: easier to configure than Sendmail
Install using yum install postfix
Edit /etc/postfix/main.cf configuration file
Run service sendmail stop ; service postfix start command to stop Sendmail daemon and activate Postfix daemon
To make the change permanent, run the chkconfig -- level 5 sendmail off ; chkconfig -- level 5 postfix on command
Linux+ Guide to Linux Certification, 3e

46. Working with Postfix (continued)
Table 13-4: Lines in /etc/postfix/main.cf to uncomment or add when configuring Postfix
Linux+ Guide to Linux Certification, 3e

47. Database Services
Databases: large files that store information in the form of tables
Table: organizes information into a list
Record: set of information about a particular item within a list
Fields: categories of information within a record
Relational databases: databases in which information within one table is related to information within other tables
Tables are usually linked by a common field
Linux+ Guide to Linux Certification, 3e

48. Database Services (continued)
Figure 13-8: A simple relational database structure
Linux+ Guide to Linux Certification, 3e

49. Database Services (continued)
Structured Query Language (SQL): programming language used to store and access data in databases
The server programs that allow use of SQL are called SQL servers
Offer advanced backup, repair, replication, and recovery utilities for data
Allow programs to access databases from across the network
Linux+ Guide to Linux Certification, 3e

50. Database Services (continued)
Table 13-5: Common SQL statements
Linux+ Guide to Linux Certification, 3e

51. Database Services (continued)
Table 13-5 (continued): Common SQL statements
Linux+ Guide to Linux Certification, 3e

52. Configuring PostgreSQL
Powerful SQL server that provides large number of features
To install: yum install postgresql command
Prepare for use by:
Use passwd postgres command to assign user a password
Initialize internal databases using service postgresql initdb command
Modify PostgreSQL configuration files
Start the PostgreSQL engine
Linux+ Guide to Linux Certification, 3e

53. Configuring PostgreSQL Databases
Log in as postgres user
Execute PostgreSQL command-line utilities to create and manage databases
Can create tables and add records within the PostgreSQL utility using appropriate SQL statements
PostgreSQL utility has many built-in commands
Prefixed with a \ character
Can be used to obtain database information or perform functions within the utility
Linux+ Guide to Linux Certification, 3e

54. Configuring PostgreSQL Databases (continued)
Table 13-6: PostgreSQL command-line utilities
Linux+ Guide to Linux Certification, 3e

55. Configuring PostgreSQL Databases (continued)
Table 13-7: Common built-in PostgreSQL utility commands
Linux+ Guide to Linux Certification, 3e

56. Summary
DHCP, DNS, NTP, and NIS are infrastructure services since they provide network-related services to other computers
DHCP servers lease other computers an IPv4 or IPv6 configuration
DNS servers provide name resolution services for other computers on the network
Linux computers can use the system time stored within the computer BIOS or obtain time from an NTP server across the network
Linux+ Guide to Linux Certification, 3e

57. Summary (continued)
NIS servers provide key configuration files to other Linux computers that are configured as NIS clients
The Apache server shares Web pages from its document root directory to computers on the network using the HTTP protocol
Samba can be used to share files to Linux, UNIX, Macintosh, and Windows computers using the SMB protocol
NFS can be used to natively share files among Linux, UNIX, and Macintosh systems
Linux+ Guide to Linux Certification, 3e

58. Summary (continued)
FTP can be used to share files to any computer that has an FTP client utility
servers deliver s to users, accept new s from users and relay the new s to other servers on the Internet for delivery
Applications that store data in databases on database servers use SQL statements to manipulate information within a database
PostgreSQL provides advanced configuration and utilities
Linux+ Guide to Linux Certification, 3e