Windows Server 2008 R2 networking : Planning and Deploying DNS (part 4) - Monitoring and troubleshooting DNS

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Dynamic DNS records

Dynamic DNS (DDNS) allows dynamic creation and updates to DNS records. By allowing DDNS, hosts can automatically update their own records within the DNS zone. Using DDNS raises some obvious security questions. For this reason, it is best practice not to enable DDNS for any zones that are facing the public Internet. You should also consider using Secure Dynamic Updates on your LAN when the DNS zones are AD integrated. 

DNS and Active Directory

We have primarily covered traditional DNS systems using primary and secondary DNS zones. When using DNS in an AD environment, you have the option to integrate zones into AD instead of using the primary/secondary model. There are some inherit benefits of using AD integrated zones. 

Securing DNS

Due to the critical nature of DNS services, it is important that you make sure your DNS Servers are as secure as possible. This is especially true for DNS Servers that are connected to the Internet. Consider implementing some of the following to secure your DNS Servers:

  • Open only necessary firewall ports required to perform name resolution (53 TCP/UDP).

  • Restrict log-on to DNS Servers to DNS admins.

  • Turn off recursive lookups if the DNS Servers will be used only for responding to queries for zones they host. If you plan to allow clients to use the DNS Servers for name resolution, you will need to leave this on.

  • Do not allow DDNS Updates for non-AD-based zones. Be sure to use only Secure Dynamic Updates for AD-integrated zones.

  • Ensure that zone transfers can occur only to authorized secondary servers.

Taking the preceding steps and following other security best practices can help ensure that your DNS Servers remain secure and reliable.

Monitoring and troubleshooting DNS

To ensure that you have reliable DNS services, you need to monitor your DNS Servers and ensure that they perform name resolution properly. In this section, we will take a look at some of the tools provided in Windows to monitor DNS and troubleshoot problems.

Event log and debug logging

You will want to review the DNS event log on a regular basis to ensure that services are online and available. Search the event log for any error events and correct any issues that appear in the event log. You should also keep an eye on warning events. These can point to configuration issues that may not currently be causing an outage, but could do so at a future time.

Debug logging can really help you home in on the root cause of DNS problems (see Figure 17) where the solution may not be apparent using other monitoring methods and event logs. You will not want to leave debug logging enabled all the time. Turn it on when you need details on DNS packets sent and received from and to the server. After resolving the problem, be sure to disable debug logging to prevent the hard drive from filling up. The debug log settings can be found in the properties of the DNS Server.

Figure 17. Debug Logging.

DNS Monitoring tab

You can test basic query functionality of the DNS Server by going to the server properties and selecting the Monitoring tab. Here you can manually run both simple and recursive queries against the DNS Server manually and on a scheduled basis.

NSLookup and DNScmd

NSLookup and DNScmd are two very important command line tools that can assist in troubleshooting DNS problems. You should have both of these tools as part of your admin toolkit. Luckily, they are already installed on the server as a part of the operating system.

  • NSLookup is a tool used to test queries against DNS Servers. You can run this command line tool from your workstation and point it to a DNS Server that you wish to test. You can then run various queries against the server to see detailed information on the data returned.

  • DNScmd is a tool now included as a part of the Windows operating system. DNScmd includes an array of options that allow you to perform DNS administrative actions from the command line. These actions include creating/deleting DNS zones, adding and deleting records, and managing the DNS windows services. Table 1 lists some commonly used DNScmd commands.

    Table 1. Common DNScmd Commands
    DNScmd/zoneadd zonename/primaryCreate a new primary DNS zoneDNScmd/zoneadd
    DNScmd/zoneadd zonename/secondary IP Address of PrimaryCreate a new secondary DNS zoneDNScmd/zoneadd
    DNScmd/zonedelete zonenameDelete a DNS zoneDNScmd/zonedelete
    DNScmd/enumzonesList DNS zones on a serverDNScmd/enumzones
    DNScmd/zoneprint zonenameList all the DNS records in a zoneDNScmd/zoneprint
    DNScmd/recordadd zonename hostname A IP AddressCreate a new host (A) recordDNScmd/recordadd www A
    DNScmd/recordadd zonename @ MX priority FQDN of mail serverCreate a new mail exchanger (MX) recordDNScmd/recordadd @ MX 100

Viewing cache

If your DNS Server does recursive queries against other DNS Servers, it will begin building a cache of lookups it has performed. The next time the same lookup is requested, the DNS Server simply pulls the query result from the cache. You can view the cache by going to the DNS Server, and then going to the View menu and selecting the Advanced option. You will see the cache folder appear in the management console. You can open the zone to review records or right click and the option Clear Cache to delete all cached copies of the records.

Aging and scavenging

The aging and scavenging process allows DNS to perform basic automated administration by deleting old DNS records that are no longer in use. This feature will be more important for AD-integrated zones but can also be helpful for standard primary/secondary DNS zones.

The aging and scavenging process can be set up on the server level, zone level, or both. Server level settings apply to all the zones on the server. Zone-level settings can be set on individual zones to override the server-level settings. Aging and scavenging are set up either in the server properties or in the zone properties (see Figure 18).

Figure 18. Server Level Scavenging.

After providing your preferred scavenging settings, you have to enable a DNS Server to actually run the scavenge process. To do this, open the server properties window, and then select the Advanced tab. Click the option Enable automatic scavenging of stale records (see Figure 19).

Figure 19. Enabling a DNS Server to run the scavenge process.

Using the default settings, the scavenge process will run every seven days and will purge records that have not been updated in fourteen days.

Overview of WINS

The WINS provides name resolution services for NETBIOS names on Windows networks. WINS was originally developed to provide NETBIOS name resolution before Windows networks relied so heavily on DNS. WINS works much like DNS in the sense that DNS maps FQDNs to IP addresses while WINS maps NETBIOS names to IP addresses.

You should probably try to avoid using WINS if you are building a new network. Microsoft is deemphasizing WINS in current operating systems and may decide to remove support in future Windows versions. With that being said, it is possible that at some point you may end up needing to administer an existing network that still uses WINS for legacy applications or operating systems. You should understand how WINS works prior to taking ownership of that network.

Notes From the Field

WINS and IPv6

WINS is considered as a legacy name resolution service; thus it does not support IPv6 addresses. You need to keep this in mind if you have WINS deployed and plan on moving to IPv6. You may want to consider GNZs in DNS instead of WINS.

WINS is deemphasized to the point that it is not considered a role in Windows Server 2008 R2. To set up a WINS server, you will need to install the service from the Features node in Server Manager (see Figure 20).

Figure 20. Install WINS Feature.

Once the WINS feature is installed, it can be used immediately. You can configure clients and servers to register with the WINS server, and they will begin creating records within the WINS database.

When planning for WINS, you may want to consider placing a WINS server on larger network segments to limit the amount of traffic being sent over WAN links. Like DNS, you can place multiple WINS servers on your network that replicate with each other. WINS servers can be set up for push replication, pull replication, or both. During push replication, the server “pushes” changes out to replication partner. During pull replication, a WINS server “pulls” changes from a replication partner.

If mission-critical applications rely on WINS, you should also consider deploying multiple WINS servers for redundancy. Clients can then be pointed to multiple WINS servers for failover in the event that the primary server fails.

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