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Chapter 7:
Using Network Clients
The Complete Guide To Linux System
Administration
Objectives
• Log in to a Linux system over a network
connection
• Use command-line tools for common network
services such as FTP and the Web
• Perform simple network diagnostic checks
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Remote Login
• Networking integrated into kernel
– Easily log in across network
– Perform virtually same work as if you were seated
in front of Linux system
• Server
– Program running on Linux system listens for
connections over network
– Serves up information to another program
• Client
– Program making requests
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Remote Login (continued)
• telnet
– Most basic remote login capability
– Set up by default on Red Hat Linux
– Completely unprotected
• r-utilities
– Designed to access remote computers in order to:
• Run programs
• Transfer files
• Perform other functions within trusted network
environment
– Designed for convenience, not security
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Remote Login (continued)
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Remote Login (continued)
• Secure shell (SSH)
– Similar to telnet
– Uses encryption so that everything transmitted
over network is unreadable to other users
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The Telnet Remote Login Utility
• Terminal emulation program
• Log in to remote computer as if you were sitting
at computer’s keyboard
• Must have:
– telnet client
– telnet server
• Command
– telnet paris.mydomain.org
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The Telnet Remote Login Utility
(continued)
• telnet session
– Login
– Execute commands on remote computer
• telnet designed for text only
• telnet program included with most versions of
windows
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The Telnet Remote Login Utility
(continued)
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The telnet Remote Login Utility
(continued)
• To use telnet, need to know:
–
–
–
–
Remote host
Port number
Type of terminal to emulate
User name and password
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Using r-Utilities for Remote
Execution
• r-utilities
– Not used much anymore
– Useful for working with multiple computers in
trusted network
– Most effective when you can set up configuration
file on multiple computers
• /etc/hosts.equiv file
– Contains hosts and user names permitted to
access system using r-utilities
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Using r-Utilities for Remote
Execution (continued)
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Using r-utilities for Remote
Execution (continued)
• rhosts file
– List of hosts and user names that you want to
permit to access account without password
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Secure Shell (SSH)
• Encrypted version of telnet
• Data encoded so that only authorized recipient
can read it
• Can also act as complete networking toolkit
• SSH consists of:
– Client utility―ssh or slogin
– Server program―sshd
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Secure Shell (SSH) (continued)
• SSH package also includes utilities
• Openssh
– Free version of SSH
– Installed by default on many Linux systems
• SSH can authenticate a connection using:
– r-utilities files
– User passwords
– Public key encryption (preferred method)
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Secure Shell (SSH) (continued)
• Public key encryption
– Method of encrypting information
– Establishes secure connection without sending
key over network connection
• In a way that compromises security of connection
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Data Services
• Tools for accessing
– Web
– E-mail
• Other network-based data services
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Accessing the Web
• Lynx
– Text-based browser
– Access Web sites when working on system that
doesn’t have graphical interface
– Very fast
– Can be run from any command-line window
– To use: lynx www.yahoo.com
– Can download pages automatically using -dump
option
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Accessing the Web (continued)
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Accessing the Web (continued)
• wget command
– Download Web pages
– Designed specifically for command-line
downloading of Web pages
– Example: wget
http//www.gnu.org/philosophy/philosophy.html
– Download recursively
• Download Web page you indicate
• Plus all pages referred to on page
• Plus all pages linked to from those pages
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Accessing the Web (continued)
• Mirroring
– Process of copying entire Web site or FTP site
– Mirror entire Web site
• wget -m -k -K -E http//www.gnu.org/ -o
/tmp/gnu_site_log &
• kget graphical utility
– Enter one or more urls
– Have them downloaded to system in background
while you do other work
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Accessing the Web (continued)
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Accessing the Web (continued)
• rsync utility
– Uses SSH to transport files between systems
– After first time files are copied, checks remote files
to see what changes need to be updated
– Example: rsync -avz stockholmdocs/ /data/docs
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Accessing FTP Servers
• File transfer protocol (FTP)
– Standard method used to move files between
computers on Internet
• Most publicly visible FTP servers support
anonymous access
– Log in using user name anonymous or FTP
– Enter e-mail address as password
– Browser provides login information automatically
in background
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Accessing FTP Servers
(continued)
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Accessing FTP Servers
(continued)
• User name and password access
– Administrator of FTP server must permit this type
of connection
– Access url: ftp//nwells@FTPXmissionCom/pub
– Web browser starts connection and prompts for
password
• Graphical FTP clients
– IglooFTP pro
– gFTP
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Accessing FTP Servers
(continued)
• FTP utility
–
–
–
–
Text-mode program
Example: ftp ftp.ibiblio.org
Prompt changes to ftp>
Can transfer files in text mode or binary mode
• ncftp program
– Similar to ftp
– Preferred by administrators
– Package includes other utilities (ncftpget, ncftpput)
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Accessing FTP Servers
(continued)
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Accessing FTP Servers
(continued)
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Accessing FTP Servers
(continued)
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Networking and Network
Diagnostic Tools
• Solve basic networking problems
– Understand basics of networking
– Use a few standard utilities
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Introduction to Networking
• Internet protocol (IP)
– Core of networking in Linux
– Sends packets of information across network
• Ethernet
– Most widely used type of networking hardware
• Other network hardware
– Token ring
– Wireless 80211b
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Introduction to Networking
(continued)
• IP address
– Allows computers to locate each other on network
– Each computer that uses IP must have address
– Represented as series of four numbers
• Each from 0 to 255
• Network mask helps system route packets
correctly between multiple networks
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Introduction to Networking
(continued)
• Routing
– Moving packets of information efficiently through
networks to reach correct destination
– Only necessary when you have multiple networks
that need to communicate
• Gateway or router
– Computer attached to multiple networks used by
computers on one network can reach other
networks
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Introduction to Networking
(continued)
• Transmission control protocol (TCP) provides
stable connection between two computers
• Domain name
– Used by humans to address computers
– Must be converted to IP address
• Domain name system (DNS) converts domain
names to IP addresses
• Dynamic host configuration protocol (DHCP)
issues IP addresses on request
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Introduction to Networking
(continued)
• ifconfig command
– View active networking interfaces
– Usually two network interfaces on Linux client
• Local interface
• Ethernet interface
• Can also use graphical tools to set up or alter
network configuration
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Using ping for System Testing
• ping utility
– Sends diagnostic data packet to specified
computer
– Verifies network connectivity
• Example: ping 1986022.20
• Use series of ping commands to test networking
– ping to hosts further and further away from your
system
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Using ping for System Testing
• Can also be used for malicious attacks
– “Flood ping”
– “Ping of death”
– Many computers on internet do not respond to
ping to prevent attacks
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Using traceroute to Examine
Routing Patterns
• traceroute command
– Carefully tracks each router between you and
destination host
– Lets you see
• Exactly where packets are going
• How long each hop takes
– Tries a maximum of 30 hops to reach destination
• Can change this value
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Using traceroute to Examine
Routing Patterns (continued)
• Useful tool for diagnosing problems such as
– Where packet stops
– Where packet slows down
• My traceroute program
– Graphical interface in Red Hat
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Using traceroute to Examine
Routing Patterns (continued)
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Summary
• Well-known methods of accessing Linux systems
remotely
– telnet
– r-utilities
– SSH suite of utilities
• Command-line utilities provided in Linux to
download Web pages and access FTP sites
• Networking in Linux is based on IP
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Summary (continued)
• Computers communicate across multiple
networks by:
– Relying on routing information
– Sending data packets through routers or gateways
• DNS servers convert domain names to IP
addresses
• ifconfig utility displays network interfaces in Linux
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Summary (continued)
• ping command sends test packet to network
location to determine if location is reachable
• traceroute command shows computers that data
packet traverses between your system and
system you want to reach
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