DNS INTERVIEW QUESTIONS AND ANSWERS

What is DNS? In brief ?

DNS stands for Domain Name System. It is a hierarchical system for identifying hosts on the Internet or on a private, corporate TCP/IP internetwork.
It resolves the IP addresses to host names (or friendly internet names) and Host names to IP addresses.

How to install DNS?

We can install DNS in 3 different ways:

·         While installing Operating System: While installing Operating System, It asks at Network Settings whether you want Typical settings or Custom Settings. Select Custom Settings–>Select Network Services–>click on Details–>Select DNS–>ok

·         While installing through Active Directory (DCPROMO): (During installation it asks for CD)

·         Independently: Programs –>Settings –>Control Panel –Add/Remove Programs –>Add/Remove Windows Components –>Select the Network Services–>Click on properties –>Select DNS –>OK (During the installation it asks for CD)

What is the main purpose of a DNS server?
DNS servers are used to resolve FQDN hostnames into IP addresses and vice versa.

What is the port no of DNS ?
53.

What is the structure of DNS?

The structure of DNS starts with root domain. Then it (root domain) braches to TOP level domains, then second level domains, and so on to the individual host names.
Root Domain –> Top level Domains–> Second level Domains–> So on so forth up to individual host systems

What is a Forward Lookup?
Resolving Host Names to IP Addresses.

What is Reverse Lookup?
Reverse DNS turns an IP address into a hostname. Example, it might turn 192.1.2.25 into host.example.com.

What is a Resource Record?
It is a record provides the information about the resources available in the N/W infrastructure.

What is a Zone?
A DNS zone is the contiguous portion of the DNS domain name space over which a DNS server has authority. A zone is a portion of a namespace. A zone contains the resource records for all of the names within the particular zone. Zone files are used if DNS data if not integrated with Active Directory. The zone files contain the DNS database resource records that define the zone. If DNS and Active Directory are integrated, then DNS data is stored in Active Directory.

What are the different types of Zones in DNS ?
The DNS Server service provides for three types of zones:
1. Primary Zone
2. Secondary Zone
3. Stub Zone
4. Active Directory-integrated Zone

What is caching only DNS server?

Just install the DNS service into your windows 2000 server. Then it is called caching only DNS server, because it starts working for your clients to resolve the Internet names to IP address.

When you are installing DNS service at that time it is preconfigured with root DNS servers IP addresses.
If anybody wants to resolve the records that belong to your domain then you need to configure your DNS server appropriately.

 Suppose under the forward lookup zone _msdcs, _tcp, _upd, _sites are not there (or you have deleted unexpectedly). How to get them back?

To get them back, simply just stop and start the netlogon service.

Type the following commands to stop and start the net logon service.

·         Net stop netlogon

·         Net start netlogon

Whenever you create a root zone (i.e., “.”) in a DNS server, the forwards, and root hints tabs will be disabled on that DNS server.

Explain Primary zone?
A primary zone is the only zone type that can be edited or updated because the data in the zone is the original source of the data for all domains in the zone. Updates made to the primary zone are made by the DNS server that is authoritative for the specific primary zone.

Explain Secondary zone?
A secondary zone is a read-only copy of the zone that was copied from the master server during zone transfer. In fact, a secondary zone can only be updated through zone transfer.

Explain Stub zone?
Stub zones only contain those resource records necessary to identify the authoritative DNS servers for the master zone. Stub zones therefore contain only a copy of a zone, and are used to resolve recursive and iterative queries.

Explain Active Directory-integrated zone?
An Active Directory-integrated zone is a zone that stores its data in Active Directory. DNS zone files are not needed. This type of zone is an authoritative primary zone. An Active Directory-integrated zone’s zone data is replicated during the Active Directory replication process. Active Directory-integrated zones also enjoy the Active Directory’s security features.

By default, if the name is not found in the cache or local hosts file, what is the first step the client takes to resolve the FQDN name into an IP address?
Performs a recursive search through the primary DNS server based on the network interface configuration.

What is round robin DNS?
Round robin DNS is usually used for balancing the load of geographically distributed Web servers. For example, a company has one domain name and three identical home pages residing on three servers with three different IP addresses. When one user accesses the home page it will be sent to the first IP address.

The second user who accesses the home page will be sent to the next IP address, and the third user will be sent to the third IP address. In each case, once the IP address is given out, it goes to the end of the list. The fourth user, therefore, will be sent to the first IP address, and so forth

What is DDNS?

Dynamic DNS or DDNS is a method of updating, in real time, a Domain Name System to point to a changing IP address on the Internet. This is used to provide a persistent domain name for a resource that may change location on the network.

What are the resource records in DNS?

·         A (Address) Maps a host name to an IP address. When a computer has multiple adapter cards and IP addresses, it should have multiple address records.

·         CNAME (Canonical Name) Sets an alias for a host name. For example, using this record, zeta.tvpress.com can have an alias as www.tvpress.com.

·         MX (Mail Exchange) Specifies a mail exchange server for the domain, which allows mail to be delivered to the correct mail servers in the domain.

·         NS (Name Server) Specifies a name server for the domain, which allows DNS lookups within various zones. Each primary and secondary name server should be declared through this record.

·         PTR (Pointer) Creates a pointer that maps an IP address to a host name for reverse lookups.

·         SOA (Start of Authority) Declares the host that is the most authoritative for the zone and, as such, is the best source of DNS information for the zone. Each zone file must have an SOA record (which is created automatically when you add a zone).

What are a Forward and Reverse Lookup?

• Forward Lookup: When a name query is send to the DNS server against to IP address, it is generally said a forward lookup.
• Reverse Lookup: DNS also provides a reverse lookup process, enabling clients to use a known IP address during a name query and look up a computer name based on its address.

What is Caching Only Server?

Caching-only servers are those DNS servers that only perform name resolution queries, cache the answers, and return the results to the client. Once the query is stored in cache, next time the query in resolved locally from cached instead of going to the actual site.

What is Aging and Scavenging?
DNS servers running Windows Server support aging and scavenging features. These features are provided as a mechanism to perform cleanup and removal of stale resource records from the server and zone. This feature removes the dynamically created records when they are stamped as stale.

By default, the aging and scavenging mechanism for the DNS Server service is disabled.

Scavenging and aging must be enabled both at the DNS server and on the zone

What is forwarding in DNS?
A forwarder is a feature in DNS server that is used to forward DNS queries for external DNS names to DNS servers outside of that network. We ca configure a DNS server as a forwarder to forward the name query to other DNS servers in the network when they cannot resolve locally to that DNS server.

What is Conditional Forwarding in DNS?

We can configure the DNS server to forward queries according to specific domain names using conditional forwarders. In this case query is forward to an IP addressagainst a DNS domain name.

What are Queries types in DNS?

·         Recursive Query: This name queries are generally made by a DNS client to a DNS server or by a DNS server that is configured to pass unresolved name queries to another DNS server, in the case of a DNS server configured to use a forwarder.

·         Iterative Query: An iterative name query is one in which a DNS client allows the DNS server to return the best answer it can give based on its cache or zone data. If the queried DNS server does not have an exact match for the queried name, the best possible information it can return is a referral. The DNS client can then query the DNS server for which it obtained a referral. It continues this process until it locates a DNS server that is authoritative for the queried name, or until an error or time-out condition is met.

What are Tools for troubleshooting of DNS?
DNS Console, NSLOOKUP, DNSCMD, IPCONFIG, DNS Logs.

How to check DNS health?
Using the DCdiag.

 i.e. (dcdiag /test:dns /v /e)

What is NBNS?

 NetBIOS Naming System, ex. - WINS, 2k access resources using DNS namingConventions

What is the diff. DNS Roles?

Standard Primary, Standard Secondary, & AD Integrated.

What is primary, Secondary, stub & AD Integrated Zone?Primary Zone: - zone which is saved as normal text file with filename (.dns) in DBSfolder. Maintains a read, write copy of zone databaseSecondary Zone: - maintains a read only copy of zone database on another DNS server.Provides fault tolerance and load balancing by acting as backup server to primary server.Stub zone: - contains a copy of name server and SOA records used for reducing the DNSsearch orders. Provides fault tolerance and load balancing.

What does a zone consist of & why do we require a zone. ?

Zone consists of resource records and we require zone for representing sites

What is SRV record in DNS?
The SRV record is a resource record in DNS that is used to identify or point to a computer that host specific services i.e Active directory.

How to open DNS?

Start–>Programs–>Administrative Tools–>DNS

Or

Start–>Run–>dnsmgmt.msc

Or

Start–>Run –>cmd –>dnsmgmt.msc

How to configure the DNS?

Open the DNS Console. Then you will find there

·         DNS

·         Server name

·         Forward Lookup Zone

·         Reverse Lookup Zone

Note: If you have selected create automatically zones during the setup, then it creates the root zone and domain zone under forward lookup zone.

If no zones are there under forward lookup zone first create root zone then create domain zone.

What tabs are there on properties of Domain?

Domain properties contain the following tabs:

·         General

·         Start of Authority (SOA)

·         Named servers

·         WINS

·         Zone transfers

What tabs are there on properties of Sever?

Server properties contain the following tabs:

·         Interface

·         Forwarders

·         Advanced

·         Root hints

·         Logging

·         Monitoring

 Where to create the primary, secondary, Active Directory integrated zones?

If you want to create an Active Directory integrated zone, the server must be Domain Controller.
If you want to create the Primary DNS, you can create on Domain Controller or Member server. But if create on member you could not get 4 options under the domain which are meant for Active directory.
You can create Secondary zone on a Member Server or on a Domain Controller. There is no difference between them.

What commands do we use for DNS?

We use the following commands for DNS:

·         Nslookup (and all interactive mode commands)

·         Ipconfig /fulshdns

·         Ipconfig /registerdns

·          

What is the purpose of forward lookup?

Forward lookup resolves the Host names (Friendly Name) to IP addresses.

Secure services in your network require reverse name resolution to make it more difficult to launch successful attacks against the services. To set this up, you configure a reverse lookup zone and proceed to add records. Which record types do you need to create?
PTR Records

What is the purpose of Reverse lookup zone?

Reverse lookup resolves the IP addresses to Host names.

What is the difference between Primary zone and Secondary zone?

Primary zone has read and write permissions, whereas Secondary zone has read only permission.

Secondary zone is used for Backup and Load balancing.

 How to check whether DNS is working or not?

In order to check whether a DNS is working or not, type the command “nslookup” in command prompt. It will give the DNS server name and its IP address

What is Dynamic Updates in DNS?

Generally we need to create a host record for newly joined computer (either client or Member server or Domain controller). If you enable dynamic Update option, then DNS itself creates associated host record for newly joined computers.

How to get Dynamic Update option?

Right Click on any zone –>properties –>on General tab u will get

Allow Dynamic Updates? [_Yes/No/Secure Updates]

Note: Put always Dynamic Updates “YES”

Note: If it is Active Directory Integrated zone you will get above three options.

But if it is Primary or Secondary zone you will get only “YES/NO” (You won’t get secure updates)

SOA records must be included in every zone. What are they used for ?
SOA records contain a TTL value, used by default in all resource records in the zone. SOA records contain the e-mail address of the person who is responsible for maintaining the zone. SOA records contain the current serial number of the zone, which is used in zone transfers.

What is name Resolution?

The process of translating the name into some object or information that the name represents is called name resolution. A telephone book forms a namespace in which the names of telephone subscribers can be resolved to the phone numbers.

 What is a zone?

Also called a zone of authority, zone is a subset of the Domain Name System (DNS) namespace that is managed by a name server.

A database of records is called a zone.

What is an iterative query?

The query that has been sent to the DNS server from a Client is called iterative query.
(i. e., iterative query is nothing but gives the answer for my question, don’t ask to contact that person or this person or don’t say something else. Simply just answer to my question. That’s all)

What type of records do we find in DNS database?

The most general records found in DNS database are

·         Host Record (A record)

·         Mail Exchange Record (MX record)

·         Canonical name or CNAME record (CNAME)

What is PTR record?

PTR record is created in Reverse lookup zone. It is the record to main database for reverse lookup zone purposes (to convert from IP address to host names). Each host record in forward lookup should have an associated pointer record in reverse lookup zone.

Generally first you will create a Forward lookup zone. Then go for Reverse lookup zone. Now we have to create associated pointer records for host records in forward lookup zone. If you are creating a host record in forward lookup zone after creating reverse lookup zone then here we need not to create a pointer record by coming to reverse lookup zone. When creating new host, click on the check box of create associated pointer record. Then it automatically creates associated pointer record in reverse lookup zone.

 Is there any possibility to have two Primary DNS zones?

No, we cannot have two primary DNS zones. Why because if u have two primary DNS zones some clients contacts first one, some clients contacts second one according to their configuration in TCP/IP properties. Then you will get problems. Actually Primary DNS zone means Single master. i.e., master is only one that is only one primary DNS zone. But you can have as many as Secondary zones.
To overcome from above problem (i.e., single master problem) in Windows 2000 we have Active Directory Integrated zones, which are multi masters.

 What is the default time setting in primary zone to Refresh, Retry, Expire intervals for secondary zone?

The default settings are

·         To Refresh interval 10 minutes

·         To Retry interval, 15 minutes

·         To Expire after 1 day

Suppose the Secondary zone is Expired; now how to solve the problem?

First go to primary zone check primary zone is working or not.

IF primary zone is working then go to secondary zone, Right click on zone name select the “Transfer from Master” then it automatically contacts the primary DNS, if any updates are there then it takes the updates from the Primary.

How to know whether the recent changes in Primary zone are updated to secondary zone or not?

Compare the Serial Number on Start of Authority tab in both secondary on primary DNS zone properties.

If both are same then recent updates are made to secondary zone.

If not (i.e., secondary is less then primary) click on “Transfer from Master”

What is Recursive query?

Your DNS server requests the root level DNS server for specific IP address. Now DNS server says I don’t know but I can give the address other person who can help you in finding IP address.

How to pause the zone?

·         Click on the Pause button.

·         Click on General tab

·         Go to properties of a zone

Note: When you install a Windows 2000 DNS server, you immediately get all of the records of root DNS servers. So every windows 2000 DNS server installed on Internet has pre configured with the address of root DNS servers. So every single DNS server in the Internet can get root servers.

What is a forwarder?

(Open DNS console ==> Right click on Domain name ==> Click on forwarder tab)

A forwarder is server, which has more access than the present DNS server. May be our present DNS server is located in internal network and it cannot resolve the Internet names. Maybe it is behind a firewall or maybe it is using a proxy server or NAT server to get to the Internet. Then this server forwards the query to another DNS server that can resolve the Internet names.

Command prompt commands:

·         Type Nslookup to get into the Nslookup mode.

·         Type set type=SOA then press enter type domain name

·         Type set type=NS then press enter; and type domain name.

·         Type set type=ALL then press enter.

Note: To come out from Nslookup mode type exit.

What is name resolution?

Name resolution is nothing but the process of mapping a name to its corresponding IP address.

When developing naming strategy, which DNS server should we use?

Microsoft suggests that we use Windows 2000 DNS server. If you want to get 100% functionality you have to use 2000 DNS server. But you can also use BIND (Berkeley internet name domain) DNS servers, but if you want you use with Windows 2000 you should have at least Bind version 4.9.7. (Previous versions don’t support SRV records)

What is DNS ?
The Domain Name System (DNS) is a hierarchical distributed naming system for computers. The Domain Name System distributes the responsibility of assigning domain names and mapping those names to IP addresses by designating authoritative name servers for each domain. Authoritative name servers are assigned to be responsible for their particular domains, and in turn can assign other authoritative name servers for their sub-domains.

Command of DMS ?

Dnsmgmt.msc

What is NSLOOKUP Command?

nslookup is a network administration command-line tool available for many computer operating systems for querying the Domain Name System (DNS) to obtain domain name or IP address mapping or for any other specific DNS record.

What is Tracert Command?

TRACERT : Go to CMD   type   tracert google.com  ,   it  let you know how many router you have to trace to reach google.com.  it will show you the internal configuration 

What is Ipconfig and IP Config/all ?

IPCONFIG , IPCONFIG/all :  The ipconfig command display all current TCP/IP configuration  , it gives information to get your IP address , your routers IP , DNS IP , DHCP server IP etc..