CompTIA Network+ Certification Study Guide part 57 docx

ChApTEr 11: Network Troubleshooting Tools 548 Notice that you’re not returned to the command prompt, but instead you remain at the nslookup command’s interactive prompt which is represen

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ChApTEr 11: Network Troubleshooting Tools

546

the most up-to-date DNS information The command ipconfig /displaydns

will show you the contents of the local DNS cache After typing ipconfig /

displaydns at the command prompt, you’ll see output similar to what is

dis-played in Figure 11.19

The command ipconfig /registernds will refresh DHCP leases for all NICs

on the machine and will reregister the machine’s hostname and IP address with the DNS server The is a helpful switch to use when you’ve made changes to the local machine’s IP address configuration and need to quickly reregister the new information with the DNS server

utilizing the nslookup Command

The nslookup utility allows you to test and query the records stored in your DNS server You can use nslookup in command mode to perform a single query, or in interactive mode to perform multiple queries For example, if you enter the command nslookup www.smeekers.com, you’ll see the output

depicted in Figure 11.20 You can use any hostname in place of the one listed

in this example to see nslookup in action.

As you can see, you get returned to the command prompt once you receive

the information – this is typical of using nslookup in command mode To

enter interactive mode, just type nslookup at the command prompt, and

your output will look like the example in Figure 11.21

FIGurE 11.18 Ipconfig /all Output.

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FIGurE 11.19 Ipconfig /displaydns Output.

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548

Notice that you’re not returned to the command prompt, but instead you

remain at the nslookup command’s interactive prompt which is represented

with a “>” symbol Once you enter interactive mode, you can use the set commands to control the information that’s returned by your queries The

set commands available in nslookup are as follows:

■

■ set all Prints options, current server, and host.

■

■ set [no]debug Prints debugging information set debug turns debug-ging on, set no debug turns debugdebug-ging off.

■

■ set [no]d2 Prints the most exhaustive debugging information possible.

■

■ set domain=NAME Sets the default domain name for any queries.

■

■ set root=NAME Sets the root server being used.

■

■ set srchlist=N1[/N2/ /N6] Sets the DNS domains used to search

for a particular host

■

■ set retry=X Sets the number of retries.

■

■ set timeout=X Sets the initial timeout interval for DNS queries FIGurE 11.20 Nslookup Command Mode Example.

FIGurE 11.21 Nslookup Interactive Mode Example.

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When you’re ready to leave the interactive mode and return to the

command prompt, just type exit.

utilizing the route Command

You can use the route command to manipulate and display the routing table

for the local computer The route print command produces the same results

as the netstat –r command – it produces a copy of the local computer’s

rout-ing table, like the one shown in Figure 11.22

You can also use the route command to add, change, or delete routes that

are defined on the local computer To specify the route to a remote host using

the route command, you need to configure the following information:

■

■ Destination The destination that this route statement is designed

to reach This can be a single host, a network address, or the

default route for this computer, indicated by the syntax 0.0.0.0

The default route dictates where network packets will be sent if the

routing table doesn’t have a more specific entry for them

■

■ Mask This indicates the subnet mask for the route’s destination If the

destination is a single IP address, the mask will be 255.255.255.255

For a network address, this will be the subnet mask of the destination

network The mask for the default route is 0.0.0.0

■

■ Gateway The IP address that packets will be forwarded to for this

route This has to be an IP address that’s on the same subnet as the

local computer

■

■ Metric This specifies the metric or cost of a particular route, from

1 to 9999 The higher the metric, the less likely the computer is to

Test day Tip

Be sure that you’re familiar with the different query types that you can use with nslookup

In particular, be aware of how to search for mail server records using the MX option, and

DNS servers using the NS option.

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550

use a particular route This is particularly useful if you’re creating multiple routes to the same network as a fault tolerance measure; configure one route with a metric of 1 and another with a metric of

50 The local computer will use the first route unless it’s not avail-able, at which point it will revert to the second route

■

■ Interface This indicates the IP address of the NIC that should be

used to reach the destination specified in this route If you have two NICs installed that are attached to two different networks, you’ll

need to specify which NIC the route command should use to reach

its destination

To add a route to a remote network, issue the following command at the command line:

route add 172.16.1.0 255.255.0.0 192.168.1.101 1

FIGurE 11.22 Route print Command Example.

Exam warning

In most cases, you’ll only add routes in this fashion on a server that’s attached to multiple networks Workstations will simply rely on their default gateway to route network traffic.

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■ route –f will clear any routes to destination networks that are

cur-rently in the routing table If you use the –f switch while creating

or changing a route, the route command will clear the routing table

and then add the new route you specify

■

■ route –p will make the entry you’re adding to the routing table

persistent By default, any routes that you add manually will only

stay in the routing table until the computer reboots Using the

–p switch ensures that the route will remain in memory until you

manually delete it

Simple Network Management protocol

The Simple Network Management Protocol (SNMP) is a protocol used to

communicate status messages from computers and devices on your

net-work These messages are sent to machines configured to receive these

status messages Machines that send these messages run SNMP agent

software, and the machines that receive the status messages run SNMP

management software One way to remember how this works is to think

of the agent software as a “secret agent” that gets information about a

network device, and then reports the information to his “manager” at

headquarters

Although the name of the SNMP protocol would lead you to believe that

its primary function is to allow you to manage objects on your network, the

management that’s happening here has more to do with monitoring your

network, rather than actually making any changes to the devices themselves

SNMP allows you to audit the activities of servers, workstations, routers,

bridges, intelligent hubs, and just about any network-connected device that

supports SNMP agent software For example, the agent software available

with Windows 2003 allows you to monitor the Windows 2003 Server

operating system, as well as Windows services like DHCP, WINS, Routing

and Remote Access Service (RRAS), and others All of these Windows 2003

services can be monitored remotely by SNMP management software

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552

For the SNMP agent software to collect information regarding a particular service, a Management Information Base (MIB) must be created The MIB is

a database that describes which kinds of information should be gathered from

a particular device The MIBs included with Windows 2003 allow the agent software to communicate a wide range of information The agent software

is responsible for reporting the information gathered by the MIB However, agents rarely volunteer information spontaneously Rather, the SNMP agent

must be queried by an SNMP management system There is, however, an exception to this: a trap message A trap message is sent spontaneously by

an SNMP agent to the SNMP management system when an important event occurs For example, you could configure a trap message to indicate that the World Wide Web service has become unresponsive on a particular machine

We would then configure the agent to send a trap message to the IP address

of a computer running the SNMP management software so that an admin-istrator can quickly respond to this event SNMP trap messages are sent to UDP port 162

There are two other types of messages associated with SNMP A GET

message is a request that is sent from an SNMP management system that

requests information from an agent A SET message will allow the SNMP

management system to write changes to an MIB, and therefore extend its information-gathering abilities SNMP GET and SET messages communi-cate on UDP port 161

SNMP is a fairly open protocol that doesn’t have many security features

To control which machines receive SNMP trap messages, you will configure

an SNMP community name In addition, you’ll configure a trap destination,

which is the hostname or IP address of the computer running the SNMP management software For a system to report to an SNMP management system, you first need to install the agent software on the target computer

In Exercise 11.3, we’ll install and configure the SNMP agent software on a Windows XP machine

ExErcisE 11.3

Click

1 Start | Control Panel | Add/Remove Programs.

Click

2 Add/Remove Windows Components.

Scroll down to

3 Management and Monitoring Tools and select Details.

Enable the

4 Simple Network Management Protocol option.

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In the Community Name drop-down list, enter the name of the

9

SNMP community and click Add.

Under Trap Destinations, enter the

computer running the SNMP management software and click Add.

Click

11 OK to finish configuring the SNMP Agent.

lINux ToolS

Although you may be spending a great deal of your “real world”

troubleshoot-ing time worktroubleshoot-ing on Windows computers, you should also be aware of the

troubleshooting tools that are available for Linux- and UNIX-based

comput-ers This is especially important because large organizations often employ

these open-source operating systems as Web servers, firewalls, and other

components of the network infrastructure Many of the troubleshooting tools

available for Linux and UNIX are similar or even identical to tools we’ve

already covered for Windows For example, you can use the ping command

from both a Windows and a UNIX computer In this section, we’ll look at

some tools that are specific to Linux- and UNIX-based operating systems

utilizing the ifconfig Command

You’ll use the ifconfig command to configure the NICs installed in a Linux

computer, as well as to view information about any configured interfaces

Like most Linux tools, you’ll work with ifconfig from the command line

The most basic syntax and output for ifconfig is to simply type ifconfig,

which will produce the following output:

eth0 Link encap:Ethernet HWaddr 00:80:C8:F8:4A:51

inet addr:192.168.99.35 Bcast:192.168.99.255 Mask:255.255.255.0

UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1

RX packets:190312 errors:0 dropped:0 overruns:0 frame:0

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554

TX packets:86955 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:100

RX bytes:30701229 (29.2 Mb) TX bytes:7878951 (7.5 Mb) Interrupt:9 Base address:0x5000

Lo Link encap:Local Loopback inet addr:127.0.0.1 Mask:255.0.0.0

UP LOOPBACK RUNNING MTU:16436 Metric:1

RX bytes:29504 (28.8 Kb) TX bytes:29504 (28.8 Kb)

Even if you haven’t worked with Linux before, you should be able to recognize the two adapters that ifconfig is displaying information for: an Ethernet adapter called “eth0”, and the Loopback Adapter You can see the following information listed for the eth0 interface:

■

■ HWaddr 00:80:C8:F8:4A:50 is the MAC address of the Ethernet

adapter

■

■ addr:192.168.99.35 Bcast:192.168.99.255 Mask:255.255.255.0

indicates the IP address, broadcast address, and subnet mask

■

■ Interrupt:9 Base address:0x5000 give you information about the

hardware resources being used by the Ethernet adapter

■

■ RX packets indicate the number of packets received by this adapter

TX packets show the number of packets transmitted.

You can quickly disable the Ethernet NIC on this Linux computer by

typing ifconfig eth0 down If you reissue the ifconfig command after you’ve

brought the Ethernet adapter down, you’ll only see configuration informa-tion for the loopback adapter:

ifconfig

lo Link encap:Local Loopback inet addr:127.0.0.1 Mask:255.0.0.0

Bringing an adapter back online is slightly more complicated, as you need

to have the IP address and subnet mask ready to configure the adapter with

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eth0 Link encap:Ethernet HWaddr 00:80:C8:F8:4A:51

inet addr:192.168.99.35 Bcast:192.168.99.255

Mask:255.255.255.0

UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1

TX packets:86955 errors:0 dropped:0 overruns:0 carrier:0

collisions:0 txqueuelen:100

RX bytes:30701229 (29.2 Mb) TX bytes:7878951 (7.5 Mb)

Interrupt:9 Base address:0x5000

lo Link encap:Local Loopback

inet addr:127.0.0.1 Mask:255.0.0.0

TX packets:306 errors:0 dropped:0 overruns:0 carrier:0

collisions:0 txqueuelen:0

utilizing the dig Command

The dig command is the Linux equivalent to nslookup in Windows – it’s

used to send name resolution queries to DNS servers to troubleshoot name

resolution on a Linux-based client computer The syntax of the dig

com-mand is as follows:

Test day Tip

ifconfig will configure a NIC in a Linux machine at the command line in the same way

that the Network applet in Control Panel works on a Windows computer.

dig [@server] [-b address] [-c class] [-f filename] [-k filename] [ -n ][-p port#] [-t type] [-x addr] [-y name:key] [name] [type] [class] [queryopt ]

dig [-h]

dig [global-queryopt ] [query ]

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