Linux+ Guide to Linux Certification, Second Edition Chapter 15 Configuring Network Services and Security.

Linux+ Guide to Linux Certification, Second Edition

Chapter 15Configuring Network Services

and Security

Linux+ Guide to Linux Certification, 2e 2

Objectives

• Identify and configure common network services

• Configure routing and firewalls

• Describe the different facets of Linux security

• Increase the security of a Linux computer

• Outline measures that can be used to detect a Linux security breach


Network Services

• Processes that provide some type of valuable service for client computers on network

• Must identify types and features of network services before they can be configured

• Important to configure network-related services, such as routing and firewalls


Identifying Network Services

• Port: Number uniquely identifying a network service– Ensure that packets delivered to proper service– Range from 0 to 65534

• /etc/services file: Lists ports and associated protocol

• Well-known port: Ports from 0 to 1024 – Used by common networking services


Identifying Network Services (continued)

Table 15-1: Common well-known ports



• Internet Super Daemon (xinetd): Initializes and configures many networking services

• Standalone daemons: Daemons normally started at boot-up – e.g., Apache Web server– Configure themselves without assistance– ntsysv utility can configure most standalone

daemons to start in various runlevels



Figure 15-1: Interacting with network services


Configuring Common Network Services

Table 15-2: Common network services


Configuring Common Network Services (continued)

Table 15-2 (continued): Common network services











Configuring DNS

• 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


Configuring DNS (continued)

Figure 15-2: The DNS lookup process



• Iterative query: Resolved without use of 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



• 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



Table 15-3: Common zone configuration files



Table 15-3 (continued): Common zone configuration files


Configuring DHCP

• 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

• /etc/dhcpd.conf file: Configure computer as a DHCP server– List appropriate IP address range for network


Configuring Apache

• Most common Web server

• 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


Configuring Apache (continued)

Table 15-4: Common httpd.conf directives


Configuring Samba

• SaMBa daemon: Emulates SMB protocol

• Windows computers advertise computer names using NetBIOS protocol– Can use NetBIOS name daemon to create and

advertise NetBIOS name • Connect Windows computers to Linux server

• smbpasswd command: Generate Samba passwords


Configuring Samba (continued)

• /etc/samba/smb.conf: Default Samba configuration file

• /etc/rc.d/init.d/smb start: Start Samba and NetBIOS name daemons– Restart if smb.conf changed


Configuring NFS

• Create directory containing information to share

• Edit /etc/exports file:– Add line listing directory to be shared and options

• Run exportfs –a – Update list of exported filesystems

• Restart the NFS processes


Configuring FTP

• Very secure FTP daemon (vsftpd): Packaged with Fedora Linux

• To configure (assuming logon as “user1”):– Create directory below user1’s home directory to

host the files• Ensure user1 owns directory

– Run /etc/rc.d/init.d/vsftpd start• Start vsftpd daemon


Configuring NIS

• Network Information Service (NIS): Coordinate common configuration files across several computers– Computers belong to a NIS domain, use NIS map to

access configuration information– Most commonly used for password databases


Configuring NIS (continued)

• Define the NIS domain via domainname <NIS_ domain_name> command

• Add “ NISDOMAIN=‘NIS_domain’ ” to /etc/sysconfig/network file– Configure NIS domain at boot time

• Add “domain <NIS_domain> server <NIS_server>” to /etc/yp.conf file – Query specific NIS server


Configuring the Secure Shell Daemon

• Secure Shell daemon (sshd): Allows use of ssh utility to log in to network servers

• /etc/ssh/sshd_config file: Contains configuration options

• Uses challenge-response authentication by default


Configuring the Secure Shell Daemon (continued)

• Supported encryption standards:– Triple Data Encryption Standard (3DES)– Advanced Encryption Standard (AES)– Blowfish– Carlisle Adams Stafford Tavares (CAST)– ARCfour


Routing and Firewall Services

• Network services not provided entirely by network daemons– Provided by Linux kernel – Do not listen to a particular port


Routing

• Route table: Indicates which networks are connected to network interfaces

• Route command: Manipulate the route table

• Multihomed hosts: Computers with multiple network interfaces

• IP forwarding: Forwarding TCP/IP packets between networks

• Routing: Forwarding data packets between networks


Routing (continued)

• Enabling routing: – Place number 1 in /proc/sys/net/ipv4/ip_forward file– Place “net.ipv4.ip_forward = 1” in /etc/sysctl.conf file


Routing (continued)

• Large networks may have several routers

• route add <route> command: Add entries to route table

• route del <route> command: Remove entries from route table

• Can use ip command to add entries to route table


Routing (continued)

Figure 15-3: A sample routed network


Routing (continued)

• Contents of route table lost when computer powered off– Add to /etc/rc.d/rc.local file

• Most routers configured with a default gateway– For packets addressed to destinations not in route

table

• traceroute command: Troubleshoot routing– Displays routers between current and remote

computer


Firewall Services

• netfilter/iptables: Used to create a firewall– Discard network packets according to chains of rules

• Chains: Specify general type of network traffic to apply rules to

• Rules: Match network traffic to be allowed or dropped


Firewall Services (continued)

• Three chain types:– INPUT chain: Incoming packets – FORWARD chain: Packets passing through

computer– OUTPUT chain: Outgoing packets

• iptables command: Creates rules for a chain


Firewall Services (continued)

Table 15-5: Common iptables commands


Security

• Linux systems typically available across networks such as the Internet

• Should improve local and network security

• Understand how to detect intruders who breach the system


Securing the Local Computer

• Limit access to computer itself– Prevent malicious users from accessing files

• Server closet: Secured room to store servers

• Remove floppy and CD-ROM devices from workstations

• Ensure BIOS prevents booting from USB ports


Securing the Local Computer (continued)

• Ensure BIOS password is set

• Set boot loader password in LILO or GRUB configuration file

• Limit access to graphical desktops and shells

• Minimize root user’s time logged in


Securing the Local Computer (continued)

• nohup command: Prevents other commands from exiting when parent process killed

• su (switch user) command: Switch current user account to another

• sudo command: Perform commands as another user via entries in /etc/sudoers file


Protecting Against Network Attacks

• Always a possibility that crackers can manipulate a network service

• Buffer overrun: Network service altered in memory

• Minimize number of running network services

• nmap (network mapper) command: Scan ports on network computers– See what network services are running


Protecting Against Network Attacks (continued)

• Enable encryption on essential network services

• Ensure network service daemons not run as root user when possible

• New network service versions usually include fixes for known network attacks– Keep network services up-to-date


Protecting Against Network Attacks (continued)

• TCP wrapper: Run network daemon with additional security via /etc/hosts.allow and /etc/hosts.deny files

• Examine permissions for files and directories associated with system and network services


Detecting Intrusion

• Log files can contain information or irregularities indicating an intrusion

• Review system log files associated with authentication

• Pluggable Authentication Module (PAM): Handles authentication requests by daemons– Log file in /var/log/secure


Detecting Intrusion (continued)

• Check /var/log/wtmp log file– Lists users who receive BASH shells

• Tripwire: Monitors files and directories

• Intrusion Detection System (IDS): Detect unauthorized access


Detecting Intrusion (continued)

Table 15-6: Common Linux intrusion detection systems

Linux+ Guide to Linux Certification, Second Edition Chapter 15 Configuring Network Services and Security.

Documents

linux guide

linux certification

linux computer

common network services

network services port

features of network

networkrelated services

linux security breach