Common network diagnostic
and configuration utilities
A ‘toolkit’ for network users and
managers when ‘troubleshooting’
is needed on your network
Kudlick Classroom
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lectern
Each student workstation has two Intel PRO1000 Gigabit Ethernet Controllers;
one is named ‘eth0’ and it is connected to a LAN switch that has a ‘gateway’
to the campus network and the Internet; the other is named ‘eth1’ and it is
connected to a different switch which does not offer an Internet ‘gateway’
‘hostname’
• This command will quickly show you the
network name for your host machine (or
will allow you to change that name if you
possess the necessary authorization)
• Example: $ hostname
• This command works by calling a standard
library function (e.g., ‘gethostname()’ ) that
we’ll often be using in our own programs
‘ping’
• A standard UNIX tool for testing whether a
particular host is reachable across an IP
network – and if so, then how quickly?
• Example: $ ping 138.202.171.14
• It also confirms that a Network Interface
Controller is operating properly, and that a
hostname can be ‘resolved’ to its address
• Example: $ ping stargate.cs.usfca.edu
‘traceroute’
• A tool for determining what route is being
taken by packets sent over an IP network
• Example: $ traceroute cs.stanford.edu
• This tool will report on each of the ‘hops’
between network ‘routers’ along a path to
the specified destination, and the elapsed
times for each ‘hop’ (in milliseconds)
• (A similar tool for Linux is ‘tracepath’)
‘route’
• It lets users display current information in
a host’s ‘routing table’ database -- and it
lets a privileged user manipulate that info
• Example: $ /sbin/route -n
• The routing table’s entries tell the kernel’s
networking subsystem how to deliver, or
forward, or else discard, any IP-network
packets that are being received by a NIC
‘tcpdump’
• This is a ‘packet decoding’ tool – it shows
where a packet is coming from, where it is
going to, as well as its size and its purpose
• But it’s only available to a ‘privileged’ user
• Example: $ sudo /usr/sbin/tcpdump –i eth0
• For an active interface on the internet, the
volume of output normally produced is so
intense as to overwhelm a human’s view!
‘netstat’
• It can show a lot of information about the
current state of your host’s networking
subsystem – such as routing-table entries,
active connections, ports and protocols, or
statistics for the active Network Interfaces
• Example: $ netstat -i
• It’s not a ‘privileged’ command – any user
can execute Linux’s ‘netstat’ utility 
‘whois’
• It’s queries an official Internet database to
determine the current owner of a network
domain name or host name
• Example: $ whois 138.202.0.0
• It allows users to find out who they can
contact in order to report any abuses or
malfunctions that have been identified as
originating from a particular IP-address
‘ifconfig’
• This program allows a user to view status
information and statistics for any network
interfaces which exist in your machine, or
to modify the state of a network interface if
you possess the necessary authorizations
• Example: $ /sbin/ifconfig -a
• With ‘sudo’ privileges, we will be able to
make changes to our interfaces’ states
‘ftp’
• This offers a convenient command-line
interface to the File-Transfer Protocol
which is implemented on the Internet
• Example: $ ftp hopper.usfca.edu
• You will usually need to enter a password
and a sequence of ftp-specific commands
in order to ‘download’ (or ‘upload’) a file
using this standard UNIX ‘ftp’ application
‘telnet’
• This program lets users communicate with
remote servers using the ‘telnet’ protocol
• Example: $ telnet rainmaker.wunderground.com 3000
• Of course, the remote host must allow the
connection for specified port-number and
for the user who is attempting to connect
• System administrators often configure a
network’s ‘firewall’ to reject telnet-requests
Thanks, Alex!
• Our System Administrator, Alex Fedosov,
has kindly provided a list of several other
networking utilities that he finds are very
useful in managing the CS Department’s
various local networks and its ‘gateways’
• Most of those tools are not mentioned by
our textbook because they are mainly of
use for specialized management tasks
‘nmap’
• This is a networking security port-scanner
with lots of options and capabilities that a
Network Administrator would find helpful
for doing security-related troubleshooting
• Example: $ nmap localhost
‘lsof’
• This utility lists a host’s open files and its
network connections (i.e., ‘sockets’), and
shows which processes are using them
• Example: $ /usr/sbin/lsof
• There are lots of command-line options
(see the online manual: $ man lsof ) that
can make the displayed information be
more helpful for your specific purpose
‘host’
• This is a Domain Name Service ‘resolver’
allowing a user to find out the hostname
for a particular Internet Protocol address
• Example: $ host 138.202.1.15
• Notice that the order in which the numeric
components of the host’s IP-address are
shown in this program’s output has been
reversed (bringing up the ‘endian’ topic)
‘arp’
• This utility displays information in a host’s
ARP cache (Address Resolution Protocol)
and allows a user to manipulate that info
• Example: $ /sbin/arp
Demo: ‘nicwatch.cpp’
• This is ‘packet sniffer’ that your Instructor
has created for your use in studying how
the network interfaces in our classroom
machines are working, and for assistance
in ‘debugging’ your own network programs
• Example: $ nicwatch eth0
• You can redirect its screen-output to a file
if you want to examine it more leisurely!
In-class exercise #1
• Find out what IP-address is being used by your
classroom workstation: $ ping hrn235xx
• Now look at your workstation’s ‘routing table’ to
see which interfaces are currently being used:
$ /sbin/route -n
• Then use the ‘ifconfig’ utility to assign an IPaddress to your machine’s secondary NIC (i.e.,
to the ‘eth1’ network interface)
$ sudo /sbin/ifconfig eth1 192.168.1.xxx up
• Did this cause any routing-table changes?
In-class exercise #2
• Now that your secondary NIC is “up” and
is using an IP-address on the 192.168.1.0
local area network, you can use ‘ping’ to
see if other host are reachable on the LAN
$ ping 192.168.1.yyy
• In a separate window you can execute our
‘nicwatch’ utility while ‘ping’ is running:
$ nicwatch eth1