- Topic: Describe DNS lookup operation
- Topic: Troubleshoot client connectivity issues involving DNS
- Topic: Configure and verify DHCP on a router (excluding static reservations)
- Topic: Troubleshoot client- and router-based DHCP connectivity issues
- Topic: Configure and verify NTP operating in client/server mode
- Topic: Configure, verify, and troubleshoot basic HSRP
- Review Questions
- Answers to Review Questions
- Additional Resources
This chapter is from the book
This chapter is from the book
This chapter is from the book
Topic: Troubleshoot client connectivity issues involving DNS
Ensuring your clients are properly configured to use DNS is important for full functionality on the Internet today.
On a Windows client system, you can check the DNS settings using ipconfig, as shown in Example 14.1.
EXAMPLE 14.1 Examining DNS Settings on a Windows Client
C:\Users\terry>ipconfig /all
Windows IP Configuration
Host Name . . . . . . . . . . . . : DESKTOP-ABC123
Primary Dns Suffix . . . . . . . :
Node Type . . . . . . . . . . . . : Hybrid
IP Routing Enabled. . . . . . . . : No
WINS Proxy Enabled. . . . . . . . : No
DNS Suffix Search List. . . . . . : my-router.home
Ethernet adapter Ethernet:
Connection-specific DNS Suffix . : my-router.home
Description . . . . . . . . . . . : Realtek PCIe GBE Family
Controller
Physical Address. . . . . . . . . : 84-8F-69-F5-5F-3D
DHCP Enabled. . . . . . . . . . . : Yes
Autoconfiguration Enabled . . . . : Yes
Link-local IPv6 Address . . . . . : fe80::bc5e:a448:8dcc:72ce%3
(Preferred)
IPv4 Address. . . . . . . . . . . : 192.168.1.191(Preferred)
Subnet Mask . . . . . . . . . . . : 255.255.255.0
Lease Obtained. . . . . . . . . . : Monday3:33:08 AM
Lease Expires . . . . . . . . . . : Friday 3:33:19 AM
Default Gateway . . . . . . . . . : 192.168.1.1
DHCP Server . . . . . . . . . . . : 192.168.1.1
DHCPv6 IAID . . . . . . . . . . . : 59019113
DHCPv6 Client DUID. . . . . . . . : 00-01-00-01-1E-72-89-C7-84-8F-
69-F5-5F-3D
DNS Servers . . . . . . . . . . . : 192.168.1.1
NetBIOS over Tcpip. . . . . . . . : Enabled
C:\Users\terry>
Notice from the output in Example 14.1 that this client will send DNS requests to 192.168.1.1. This is, of course, a private-use-only address inside our network. This router receives public DNS server addresses automatically from our ISP so that it can resolve public website names that we want to visit.
Figure 14.1 shows the actual configuration for this Windows client in the graphical user interface of the Control Panel. Notice that the DNS information of 192.168.1.1 is being learned by this client automatically.
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FIGURE 14.1 The DNS Settings Inside of Windows
What about verifying the Windows client is fine from a DNS perspective? One approach is to ping a known and reachable Web server using the friendly name. Example 14.2 demonstrates this approach.
EXAMPLE 14.2 Checking DNS Functionality by Using PING
C:\Users\terry>ping www.cisco.com
Pinging e144.dscb.akamaiedge.net [23.202.192.170] with 32 bytes of data:
Reply from 23.202.192.170: bytes=32 time=35ms TTL=54
Reply from 23.202.192.170: bytes=32 time=37ms TTL=54
Reply from 23.202.192.170: bytes=32 time=36ms TTL=54
Reply from 23.202.192.170: bytes=32 time=35ms TTL=54
Ping statistics for 23.202.192.170:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 35ms, Maximum = 37ms, Average = 35ms
C:\Users\terry>
If you would like to receive even more information, however, use the NSLOOKUP command. Example 14.3 demonstrates this powerful tool.
EXAMPLE 14.3 Using NSLOOKUP to Verify DNS
C:\Users\terry>nslookup www.cisco.com
Server: ACME_Quantum_Gateway.my-router.home
Address: 192.168.1.1
Non-authoritative answer:
Name: e144.dscb.akamaiedge.net
Addresses: 2600:1408:10:18c::90
2600:1408:10:181::90
23.202.192.170
Aliases: www.cisco.Com
www.cisco.com.akadns.net
wwwds.cisco.com.edgekey.net
wwwds.cisco.com.edgekey.net.globalredir.akadns.net
C:\Users\terry>
Just as it can be convenient for your Windows client to use DNS, it can also be beneficial for your Cisco routers and switches. Table 14.1 provides commands available on these devices.
TABLE 14.1 DNS Related Commands on Cisco Devices
Cisco Command |
Description |
ip domain-lookup |
This command enables DNS-based host name-to- address translation; note this command is enabled by default on many Cisco devices. |
ip name-server |
This command specifies the address of one or more name servers for the device to use for DNS resolution. |
ip domain-name |
This command defines a default domain name that the Cisco IOS software uses to complete unqualified host names (names without a dotted-decimal domain name). |
CramQuiz
What is a common Windows client setting for IPv4 DNS?
A. The use of only Google DNS public servers- B. To acquire the DNS settings automatically via DHCP
- C. To use the public IP address of the ISP’s router
- D. To use a local loopback address
What command enables DNS-based host name translations on a Cisco router and is enabled by default on many Cisco routers?
- A. ip domain-name
- B. ip name-server
- C. ip domain-list
- D. ip domain-lookup
CramQuiz Answers
B is correct. A very common approach for Windows client’s DNS is to acquire this information dynamically.
D is correct. The ip domain-lookup command enables DNS-based host name resolution. This command is a default setting.
