Network Configuration

An Introductory Tutorial

Network Parameters

The following are common parameters that need to be set in a networked computer: DNS Servers (e.g. 137.207.32.32)

The logical (e.g. IP) address of the DNS servers to be used to resolve domain names

These can be local (e.g. frankie.csfac8.uwindsor.ca) or remote (e.g. www.google.ca)

Host and domain information This includes the computer’s name (e.g. elvis) and the

domain on which it resides (e.g. csfac8.uwindsor.ca)

Network Parameters

Logical address (e.g. IP address: 137.207.132.21) This address is required to participate on a WAN, such as

the Internet

Subnet mask (e.g. 255.255.0.0) This is the binary mask that should be used to extract the

network and subnet information from the host’s address

Default gateway (e.g. 137.207.32.2) This is the logical (e.g. IP) address of the router which

should receive the host’s Internet-bound packets

All of these values would be given to a computer’s user by a network administrator

DNS Servers

DNS servers use the DNS protocol to determine the logical (e.g. IP) address of a host, given its domain name e.g. www.google.ca: 64.233.167.99

Many servers are given (in a specific order) so that local DNS servers are first This is because the main DNS servers (Internic)

would not know local domain names e.g. An Internic server would know about uwindsor.ca, but

not frankie.csfac8.uwindsor.ca) These server addresses would usually be well-

known

Host and Domain Info

Host name: This is the name of the host on the network

e.g. elvis Domain name:

This is the name of the domain on which the host resides A domain is a named group of computers, which may correspond

to a subnet of addresses e.g. csfac8.uwindsor.ca refers to the LAN on the 8th floor of

Lambton Tower Given these two pieces of information, the host’s DNS

name would be elvis.csfac8.uwindsor.ca Depending on the operating system, the domain may require

that you log on first The network admin will create one or more domains, but each host’s

user can determine the host name, provided it is unique

Logical Address

This is typically an IP addressThis address is necessary for all WAN

(including Internet) communicationRouting uses the logical address for finding

the path for packetsThese addresses are normally assigned

automatically by a DHCP/BOOTP serverThey can be random addresses on the

network, or sequentially assigned

Subnet Mask

The subnet mask that it used by the host is the same subnet mask used by the nearest routerse.g. If using 2-level subnetting of 2 and 3

bits, this mask would be the last subnet mask used (137.207.248.0)

A network admin would know the subnet masks he/she assigned to each network

Default Gateway

The default gateway (router) is the router to which all Internet-bound traffic should be sent Typically, a LAN will have exactly one router, and

this will be its address If there is more than one router, and only one leads

to the Internet, the Internet-connected router’s address is used here

If there is more than one router that connects to the Internet, either address could be used

The network admin would know the address of the routers

Troubleshooting Networks

How to Determine Network Configuration or Hardware Problems

Troubleshooting

A network admin’s job is not just to set up a network, but maintain itThus, if anything goes wrong, the admin

should track down the problem somehowThe problem may be a configuration error, which

the admin should fixThe problem may be a hardware failure, which

can only be fixed by repairing or replacing the hardware

Nevertheless, the admin must determine which hardware has failed, so this can happen

A Typical Scenario

A user calls up the network admin, saying their network connection is downFor most non-technical users, even this

clear of a statement of the problem might not be possible

A network admin will eventually figure at least this much out from the user’s comments

Step 1: Test Local Connectivity

First, an admin will try to connect to a local computercmd% ping frankie.csfac8.uwindsor.caping: unknown host frankie.csfac8.uwindsor.ca This could mean DNS servers are unavailable

The admin should check the DNS server addresses, and the servers

Next, the admin will make sure the IP address workscmd% ping 137.207.140.93Destination host unreachable.Destination host unreachable.Destination host unreachable. This could mean no connectivity

The admin should check the configuration settings (ipconfig, netconfig, network-admin)

However, if the ping command responds from a router that a host is unreachable, the node may actually be unreachable, or the address may not be bound

Obviously, we have connectivity since the router sent a message to us

How Ping Works

Ping works by sending an ICMP Echo Reqest to the destination

The destination sends back an ICMP Echo Reply to the host

The host records the round trip time, and repeats the process a few times

A Problem With Ping

ICMP Echo Requests are often disabled in some networksThis is due to denial of service attacks that

use ICMP Echo Request/ReplyIn these networks, ping will not work

Check Configuration Settings

In Windows (2000/XP):C:\> ipconfig(success)Windows IP ConfigurationEthernet adapter Wireless Network Connection: Connection-specific DNS Suffix . : csfac8.uwindsor.ca IP Address. . . . . . . . . . . . : 192.168.137.71 Subnet Mask . . . . . . . . . . . : 255.255.255.0 Default Gateway . . . . . . . . . : 192.168.137.2

(failure)Windows IP ConfigurationEthernet adapter Wireless Network Connection: Media State . . . . . . . . . . . : Media disconnected

Edit Configuration Settings

In Windows (2000/XP), editing your network configuration settings can be done from TCP/IP properties Open control panel, select Network Connections Select your connection, click Properties Select Internet Protocol (TCP/IP), click Properties From here you can edit your configuration settings

manually, or specify that a DHCP should provide the values for you

Check/Edit Configuration Settings

In Linux:

Linux% network-adminIn Solaris:

Solaris% netconfig

These bring up a graphical dialog where configuration can be viewed and edited

Step 2: Test Remote Connectivity

First, an admin will try to connect to a local computer

cmd% ping www.google.caping: unknown host www.google.ca

This could be a routing problem Most likely the routing problem is within the

organization The network administrator should check the

route using the traceroute (traceroute, tracert) command

Testing Routes

The traceroute command can give the logical addresses of each router in a route It may also tell you if there is a problem with the

route

In Windows (2000/XP):

tracert www.google.ca

In Linux/Solaris:

traceroute www.google.ca

Windows Tracert Output

C:\>tracert www.google.caTracing route to www.l.google.com [66.102.7.104]over a maximum of 30 hops: 1 1 ms 1 ms 1 ms 192.168.1.1 2 23 ms 16 ms 20 ms 10.71.96.1 3 11 ms 13 ms 13 ms d226-4-1.home.cgocable.net [24.226.4.1] 4 30 ms 15 ms 18 ms cgowave-0-157.cgocable.net [24.226.0.157] 5 30 ms 17 ms 17 ms cgowave-0-101.cgocable.net [24.226.0.101] 6 29 ms 18 ms 17 ms gw-google.torontointernetxchange.net [198.32.245.6] 7 19 ms 37 ms 17 ms 66.249.94.75 8 44 ms 23 ms 31 ms 64.233.175.96 9 53 ms 40 ms * 64.233.175.96 10 98 ms 92 ms 86 ms 66.249.95.215 11 98 ms 89 ms 94 ms 216.239.49.142 12 88 ms 95 ms 90 ms 216.239.49.142 13 104 ms 98 ms 89 ms 216.239.49.142 14 99 ms 94 ms 88 ms 66.102.7.104Trace complete.

Windows Tracert Output

The output contains lines like the following:

5 30 ms 17 ms 17 ms cgowave-0-101.cgocable.net [24.226.0.101]

5: The 5th hop

30,17,17ms: Round trip times to/from the router

cgowave-0-101.cgocable.net [24.226.0.101]: The domain name and address of the router

Linux traceroute Output

rfortier@elvis:~ $ traceroute www.google.catraceroute: Warning: www.google.ca has multiple addresses; using 64.233.167.147traceroute to www.l.google.com (64.233.167.147), 30 hops max, 38 byte packets 1 ssr68 (137.207.140.65) 0.395 ms 0.302 ms 0.250 ms 2 ssr (137.207.140.66) 0.597 ms 0.508 ms 0.472 ms 3 cs-ssr-main.newcs.uwindsor.ca (137.207.76.254) 1.038 ms 0.901 ms 0.692 ms 4 passport1.uwindsor.ca (137.207.32.1) 1.701 ms 1.245 ms 1.225 ms 5 internet.uwindsor.ca (137.207.92.14) 1.434 ms 1.136 ms 0.967 ms 6 acs7204.uwindsor.ca (137.207.232.3) 2.494 ms 1.563 ms 1.852 ms 7 f0-1.na03.b011027-0.yyz01.atlas.cogentco.com (38.112.19.97) 10.358 ms 10.811 ms 9.377 ms 8 g9-1.core01.yyz02.atlas.cogentco.com (66.28.66.53) 10.628 ms 9.289 ms 10.427 ms 9 p15-0.core01.yyz01.atlas.cogentco.com (154.54.2.221) 10.265 ms 15.687 ms10.139 ms10 p13-0.core02.ord01.atlas.cogentco.com (66.28.4.213) 24.619 ms 24.274 ms 25.259 ms11 p15-0.core01.ord01.atlas.cogentco.com (66.28.4.61) 24.502 ms 23.292 ms 24.221 ms12 p6-0.core01.ord03.atlas.cogentco.com (154.54.2.238) 24.316 ms 24.378 ms 24.401 ms13 google.ord03.atlas.cogentco.com (154.54.11.186) 22.021 ms 22.070 ms 21.698 ms14 66.249.94.193 (66.249.94.193) 22.524 ms 22.307 ms 66.249.94.195 (66.249.94.195) 24.050

ms15 64.233.175.38 (64.233.175.38) 26.037 ms 64.233.175.42 (64.233.175.42) 64.185 ms

64.233.175.38 (64.233.175.38) 25.547 ms16 64.233.167.147 (64.233.167.147) 23.303 ms 24.924 ms 22.002 ms

How To Interpret the Output

The round trip times can tell you if there is a problemA round trip time of * means a timeout has

occurred, which is a definite problemA round trip time > 500 ms usually means

there is a problem with that routerThe router could be dropping packets (due to

overloaded buffer or routing loops typically)

How Does Traceroute Work?

Traceroute sends either a UDP or ICMP Echo packet to the destination Initially, the hop limit is set to 1, which causes the

first router to drop the packet Routers, however, will respond with an ICMP Time

Exceeded error message, which contains the router’s IP address

The process is repeated a few times to get a few round trip times

The domain name of the IP address is found (for convenience of the network admin)

Next, the process is repeated with a hop limit of 2 Then 3, 4, etc. until the destination host is reached

A Problem with Traceroute

One problem with traceroute is that it could read a false path If the routers reconfigure (e.g. due to congestion) during its

execution, the readings could be inaccurate

S

A1 A2

D

A2

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A Problem with Traceroute

One problem with traceroute is that it could read a false path If the routers reconfigure (e.g. due to congestion) during its

execution, the readings could be inaccurate

S

A1 A2

D

A2

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A Problem with Traceroute

One problem with traceroute is that it could read a false path If the routers reconfigure (e.g. due to congestion) during its

execution, the readings could be inaccurate

S

A1 A2

D

A2

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A Problem with Traceroute

One problem with traceroute is that it could read a false path If the routers reconfigure (e.g. due to congestion) during its

execution, the readings could be inaccurate

S

A1 A2

D

A2

B2B2

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2 A2

3 D

A Problem with Traceroute

For networks with ICMP Echo Request/Reply disabled, the UDP format must be used Some traceroute commands do not support the

UDP message format, however

LAN Setup

10BaseT Ethernet

802.11g Wireless

10BaseT Ethernet Setup

To connect a 10BaseT Ethernet network: Install a network interface card (NIC) into each computer

(host) Connect each NIC to a hub or switch, using unshielded

twisted pair (UTP) cabling Hub:

Every frame received by a hub is reproduced on all of it’s ports

Switch (or Cable/DSL Router): Every frame received by a switch is examined, and the

correct port is determined for the frame A Cable/DSL also provides network address translation (NAT)

802.11g Wireless Setup

To setup a 802.11g wireless network: Install an 802.11g (or 802.11b) NIC into

each hostPlug a wireless router or base station into

your Internet connection (e.g. cable modem)Configure the SSID and security protocols

on the wireless router and each of the NICse.g. WPA encryption, and specify the key

Ad Hoc Wireless Networks

If you have only a few hosts (2-3), you can create an ad hoc networkThis type of network does not have a base

station (or wireless router)The hosts communicate directly with one

another

Wireless Networks Comment

A wireless network connection is often not useful for a desktop PCFor this reason, wireless routers typically

contain a few Ethernet ports It is typical to have desktop PCs wired, and

notebooks and PDAs will have wirelesse.g. Notebooks with 802.11g and PDAs with

802.11b

Wireless networks will be discussed in depth later

Packet Sniffers

Packet sniffers are a useful tool for exploration of networking They can also be used by network programmers to

diagnose packet format errors They typically examine many protocols

This includes the header information e.g. Wireless, Ethernet, TCP/IP, HTTP

Some common packet sniffers are: Ethereal (Linux/Mac/Windows, multi-protocol) TCPDump (Linux/Windows, TCP/IP) Kismet (Linux, wireless)

Ethereal Tutorial

Here is how to sniff packets using Ethereal: Start Ethereal Select the correct network interface Start capturing packets, stop when done Select a packet from the list Examine the packet hierarchy and header

information, such as: Ethernet or wireless frame headers IP datagram headers TCP datagram headers SMTP message