NETWORK CONFIGURATIONTROUBLE SHOOTING NETWORKS

Dr. Zeeshan Bhatti

Lecture 2

Troubleshooting Networks How to Determine Network Configuration or Hardware

Problems

2

TROUBLESHOOTING

A network admin’s job is not just to set up a network, but maintain it

Thus, if anything goes wrong, the admin should track down the problem somehow

The problem may be a configuration error, which the admin should fix

The 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

3

A TYPICAL SCENARIO

A user calls up the network admin, saying their network connection is down For 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

4

STEP 1: TEST LOCAL CONNECTIVITYFirst, an admin will try to connect to a local computer

cmd% ping frankie.csfac8.uwindsor.ca

ping: 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 works

cmd% ping 137.207.140.93

Destination 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 5

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

6

A PROBLEM WITH PING

ICMP Echo Requests are often disabled in some networks

This is due to denial of service attacks that use ICMP Echo Request/Reply

In these networks, ping will not work

7

CHECK CONFIGURATION SETTINGS

In Windows (2000/XP):

C:\> ipconfig

(success)

Windows IP Configuration

Ethernet 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 Configuration

Ethernet adapter Wireless Network Connection:

Media State . . . . . . . . . . . : Media disconnected

8

EDIT CONFIGURATION SETTINGS

In Windows , editing your network configuration settings can be done from TCP/IP propertiesOpen 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

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STEP 2: TEST REMOTE CONNECTIVITY

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

cmd% ping www.google.ca

ping: unknown host www.google.ca

This could be a routing problemMost likely the routing problem is within the organization

The network administrator should check the route using the traceroute (traceroute, tracert) command

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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

11

WINDOWS TRACERT OUTPUT

C:\>tracert www.google.ca

Tracing 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 66.102.7.104

Trace complete.

12

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

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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

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A PROBLEM WITH TRACEROUTEOne 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

15

S

A1 A2

D

A2

B2B2

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

1 A1

16

S

A1 A2

D

A2

B2B2

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

1 A1

2 A2

17

S

A1 A2

D

A2

B2B2

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

1 A1

2 A2

3 D

18

S

A1 A2

D

A2

B2B2

THE WINDOWS NETWORK DIAGNOSTIC COMMANDS

Ipconfig

Ipconfig is a Console Command which can be issued to the Command Line Interpreter (or command prompt)

to display the network settings currently assigned to any or all network adapters in the machine. This

command can be utilized to verify a network connection as well as to verify your network settings.

19

Netstat

Displays active TCP connections, ports on which the computer is listening, Ethernet statistics,

the IP routing table, IPv4 statistics (for the IP, ICMP, TCP, and UDP protocols), and IPv6 statistics

(for the IPv6, ICMPv6, TCP over IPv6, and UDP over IPv6 protocols). Used without parameters,

netstat displays active TCP connections.

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Tracert

The tracert command is used to visually see a network packet being sent and received and the amount

of hops required for that packet to get to its destination.

Users with Microsoft Windows 2000 and Windows XP who need additional information network latency

and network loss should also consider using the patching command.

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Ping

Helps in determining TCP/IP Networks IP address as well as determine issues with the network and

assists in resolving them.

22

Pathping

Provides information about network latency and network loss at intermediate hops between a source

and destination. Pathping sends multiple Echo Request messages to each router between a source and

destination over a period of time and then computes results based on the packets returned from each

router.

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Nslookup

Displays information that you can use to diagnose Domain Name System (DNS) infrastructure. Before using this tool,

you should be familiar with how DNS works. The Nslookup command-line tool is available only if you have installed

the TCP/IP protocol.

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