Which of the following records identifies server1 as an authoritative dns server?
IntroductionDNS, or the Domain Name System, is often a very difficult part of learning how to configure websites and servers. Understanding how DNS works will help you diagnose problems with configuring access to your websites and will allow you to broaden your understanding of what’s going on behind the scenes. Show
In this guide, we will discuss some fundamental DNS concepts that will help you hit the ground running with your DNS configuration. After tackling this guide, you should be ready to set up your domain name with DigitalOcean or set up your very own DNS server. Before we jump into setting up your own servers to resolve your domain or setting up our domains in the control panel, let’s go over some basic concepts about how all of this actually works. Domain TerminologyWe should start by defining our terms. While some of these topics are familiar from other contexts, there are many terms used when talking about domain names and DNS that aren’t used too often in other areas of computing. Let’s start easy: Domain Name SystemThe domain name system, more commonly known as “DNS” is the networking system in place that allows us to resolve human-friendly names to unique IP addresses. Domain NameA domain name is the human-friendly name that we are used to associating with an internet resource. For instance, “ The URL “ IP AddressAn IP address is what we call a network addressable location. Each IP address must be unique within its network. When we are talking about websites, this network is the entire internet. IPv4, the most common form of addresses, are written as four sets of numbers, each set having up to three digits, with each set separated by a dot.
For example, “ Top-Level DomainA top-level domain, or TLD, is the most general part of the domain. The top-level domain is the furthest portion to the right (as separated by a dot). Common top-level domains are “com”, “net”, “org”, “gov”, “edu”, and “io”. Top-level domains are at the top of the hierarchy in terms of domain names. Certain parties are given management control over top-level domains by ICANN (Internet Corporation for Assigned Names and Numbers). These parties can then distribute domain names under the TLD, usually through a domain registrar. HostsWithin a domain, the domain owner can define individual hosts, which refer to separate computers or services accessible through a domain. For instance, most domain
owners make their web servers accessible through the bare domain ( You can have other host definitions under the general domain. You could have API access through an “api” host ( SubDomainA subject related to hosts are subdomains. DNS works in a hierarchy. TLDs can have many domains under them. For instance, the “com” TLD has both “ Likewise, each domain can control “subdomains” that are located under it. This is
usually what we mean by subdomains. For instance you could have a subdomain for the history department of your school at “ The difference between a host name and a subdomain is that a host defines a computer or resource, while a subdomain extends the parent domain. It is a method of subdividing the domain itself. Whether talking about subdomains or hosts, you can begin to see that the left-most portions of a domain are the most specific. This is how DNS works: from most to least specific as you read from left-to-right. Fully Qualified Domain NameA fully qualified domain name, often called FQDN, is what we call an absolute domain name. Domains in the DNS system can be given relative to one another, and as such, can be somewhat ambiguous. A FQDN is an absolute name that specifies its location in relation to the absolute root of the domain name system. This means
that it specifies each parent domain including the TLD. A proper FQDN ends with a dot, indicating the root of the DNS hierarchy. An example of a FQDN is “ Name ServerA name server is a computer designated to translate domain names into IP addresses. These servers do most of the work in the DNS system. Since the total number of domain translations is too much for any one server, each server may redirect request to other name servers or delegate responsibility for a subset of subdomains they are responsible for. Name servers can be “authoritative”, meaning that they give answers to queries about domains under their control. Otherwise, they may point to other servers, or serve cached copies of other name servers’ data. Zone FileA zone file is a simple text file that contains the mappings between domain names and IP addresses. This is how the DNS system finally finds out which IP address should be contacted when a user requests a certain domain name. Zone files reside in name servers and generally define the resources available under a specific domain, or the place that one can go to get that information. RecordsWithin a zone file, records are kept. In its simplest form, a record is basically a single mapping between a resource and a name. These can map a domain name to an IP address, define the name servers for the domain, define the mail servers for the domain, etc. How DNS WorksNow that you are familiar with some of the terminology involved with DNS, how does the system actually work? The system is very simple at a high-level overview, but is very complex as you look at the details. Overall though, it is a very reliable infrastructure that has been essential to the adoption of the internet as we know it today. Root ServersAs we said above, DNS is, at its core, a hierarchical system. At the top of this system is what are known as “root servers”. These servers are controlled by various organizations and are delegated authority by ICANN (Internet Corporation for Assigned Names and Numbers). There are currently 13 root servers in operation. However, as there are an incredible number of names to resolve every minute, each of these servers is actually mirrored. The interesting thing about this set up is that each of the mirrors for a single root server share the same IP address. When requests are made for a certain root server, the request will be routed to the nearest mirror of that root server. What do these root servers do? Root servers handle requests for information about Top-level domains. So if a request comes in for something a lower-level name server cannot resolve, a query is made to the root server for the domain. The root servers won’t actually know where the domain is hosted. They will, however, be able to direct the requester to the name servers that handle the specifically requested top-level domain. So if a request for “ It will instead find a record for the “org” TLD and give the requesting entity the address of the name server responsible for “org” addresses. TLD ServersThe requester then sends a new request to the IP address (given to it by the root server) that is responsible for the top-level domain of the request. So, to continue our example, it would send a request to the name server responsible for knowing about “org” domains to see if it knows where “ Once again, the requester will
look for “ However, it will find a record listing the IP address of the name server responsible for “ Domain-Level Name ServersAt this point, the requester has the IP address of the name server that is responsible for knowing the actual IP address of the resource. It sends a new request to the name server
asking, once again, if it can resolve “ The name server checks its zone files and it finds that it has a zone file associated with “ What is a Resolving Name Server?In the above scenario, we referred to a “requester”. What is the requester in this situation? In almost all cases, the requester will be what we call a “resolving name server.” A resolving name server is one configured to ask other servers questions. It is basically an intermediary for a user which caches previous query results to improve speed and knows the addresses of the root servers to be able to “resolve” requests made for things it doesn’t already know about. Basically, a user will usually have a few resolving name servers configured on their computer system. The resolving name servers are usually provided by an ISP or other organizations. For instance Google provides resolving DNS servers that you can query. These can be either configured in your computer automatically or manually. When you type a URL in the address bar of your browser, your computer first looks to see if it can find out locally where the resource is located. It checks the “hosts” file on the computer and a few other locations. It then sends the request to the resolving name server and waits back to receive the IP address of the resource. The resolving name server then checks its cache for the answer. If it doesn’t find it, it goes through the steps outlined above. Resolving name servers basically compress the requesting process for the end user. The clients simply have to know to ask the resolving name servers where a resource is located and be confident that they will investigate and return the final answer. Zone FilesWe mentioned in the above process the idea of “zone files” and “records”. Zone files are the way that name servers store information about the domains they know about. Every domain that a name server knows about is stored in a zone file. Most requests coming to the average name server are not something that the server will have zone files for. If it is configured to handle recursive queries, like a resolving name server, it will find out the answer and return it. Otherwise, it will tell the requesting party where to look next. The more zone files that a name server has, the more requests it will be able to answer authoritatively. A zone file describes a DNS “zone”, which is basically a subset of the entire DNS naming system. It generally is used to configure just a single domain. It can contain a number of records which define where resources are for the domain in question. The zone’s So if a zone file is used to configure the “ This is either configured at the top of the zone file or it can be defined in the DNS server’s configuration file that references the zone file. Either way, this parameter describes what the zone is going to be authoritative for. Similarly, the Record TypesWithin the zone file, we can have many different record types. We will go over some of the more common (or mandatory types) here. SOA RecordsThe Start of Authority, or SOA, record is a mandatory record in all zone files. It must be the
first real record in a file (although The start of authority record looks something like this:
Let’s explain what each part is for:
A and AAAA RecordsBoth of these records map a host to an IP address. The “A” record is used to map a host to an IPv4 IP address, while “AAAA” records are used to map a host to an IPv6 address. The general format of these records is this:
So since our SOA record called out a primary server at “ The record could look something like this:
Notice that we don’t have to give the full name. We can just give the host, without the FQDN and the DNS server will fill in the rest with the
In most cases, this is where you’ll define your web server as “www”:
We should also tell where the base domain resolves to. We can do this like this:
We could have used the “
We also have the option of resolving anything that under this domain that is not defined explicitly to this server too. We can do this with the “
All of these work just as well with AAAA records for IPv6 addresses. CNAME RecordsCNAME records define an alias for canonical name for your server (one defined by an A or AAAA record). For instance, we could have an A name record defining the “server1” host and then use the “www” as an alias for this host:
Be aware that these aliases come with some performance losses because they require an additional query to the server. Most of the time, the same result could be achieved by using additional A or AAAA records. One case when a CNAME is recommended is to provide an alias for a resource outside of the current zone. MX RecordsMX records are used to define the mail exchanges that are used for the domain. This helps email messages arrive at your mail server correctly. Unlike many other record types, mail records generally don’t map a host to something, because they apply to the entire zone. As such, they usually look like this:
Note that there is no host name at the beginning. Also note that there is an extra number in there. This is the preference number that helps computers decide which server to send mail to if there are multiple mail servers defined. Lower numbers have a higher priority. The MX record should generally point to a host defined by an A or AAAA record, and not one defined by a CNAME. So, let’s say that we have two mail servers. There would have to be records that look something like this:
In this example, the “mail1” host is the preferred email exchange server. We could also write that like this:
NS RecordsThis record type defines the name servers that are used for this zone. You may be wondering, “if the zone file resides on the name server, why does it need to reference itself?”. Part of what makes DNS so successful is its multiple levels of caching. One reason for defining name servers within the zone file is that the zone file may be actually being served from a cached copy on another name server. There are other reasons for needing the name servers defined on the name server itself, but we won’t go into that here. Like the MX records, these are zone-wide parameters, so they do not take hosts either. In general, they look like this:
You should have at least two name servers defined in each zone file in order to operate correctly if there is a problem with one server. Most DNS server software considers a zone file to be invalid if there is only a single name server. As always, include the mapping for the hosts with A or AAAA records:
There are quite a few other record types you can use, but these are probably the most common types that you will come across. PTR RecordsThe PTR records are used define a name associated with an IP address. PTR records are the inverse of an A or AAAA record. PTR records are unique in that they begin at the Here is an example of a PTR record for
This example of a PTR record for an IPv6 address shows the nibble format of the reverse of Google’s IPv6 DNS Server
The command line tool Here is an example of a dig command. The
The output for the dig command above will be the domain name in the PTR record for the IP address:
Servers on the Internet use PTR records to place domain names within log entries, make informed spam handling decisions, and display easy-to-read details about other devices. Most commonly-used email servers will look up the PTR record of an IP address it receives email from. If the source IP address does not have a PTR record associated with it, the emails being sent may be treated as spam and rejected. It is not important that the FQDN in the PTR matches the domain name of the email being sent. What is important is that there is a valid PTR record with a corresponding and matching forward A record. Normally network routers on the Internet are given PTR records that correspond with their physical location. For example you may see references to ‘NYC’ or ‘CHI’ for a router in New York City or Chicago. This is helpful when running a traceroute or MTR and reviewing the path Internet traffic is taking. Most providers offering dedicated servers or VPS services will give customers the ability to set a PTR record for their IP address. DigitalOcean will automatically assign the PTR record of any Droplet when the Droplet is named with a domain name. The Droplet name is assigned during creation and can be edited later using the settings page of the Droplet control panel. Note: It is important that the FQDN in the PTR record has a corresponding and matching forward A record. Example: CAA RecordsCAA records are used to specify which Certificate Authorities (CAs) are allowed to issue SSL/TLS certificates for your domain. As of September 8, 2017 all CAs are required to check for these records before issuing a certificate. If no record is present, any CA may issue a certificate. Otherwise, only the specified CAs may issue certificates. CAA records can be applied to single hosts, or entire domains. An example CAA record follows:
The host,
You may use
For more detailed information about CAA records, you can read RFC 6844, or our tutorial How To Create and Manage CAA Records Using DigitalOcean DNS ConclusionYou should now have a pretty good grasp on how DNS works. While the general idea is relatively easy to grasp once you’re familiar with the strategy, this is still something that can be difficult for inexperienced administrators to put into practice. For an overview check out How To Set Up Domains within the DigitalOcean Control Panel. Which DNS record identifies the authoritative server for the domain?4. NS record. A nameserver (NS) record specifies the authoritative DNS server for a domain. In other words, the NS record helps point to where internet applications like a web browser can find the IP address for a domain name.
What is an authoritative DNS server?Authoritative DNS is the system that takes an address, like google.com, and provides an answer about the resources in that zone. The typical transaction looks something like this: User types an address into a web browser, or an application calls out to a given name of a resource on the Internet.
Which of the following are authoritative DNS server types?There are two types of Authoritative Name Servers:. Master server (primary name server) – A master server stores the original master copies of all zone records. A hostmaster only make changes to master server zone records. ... . Slave server (secondary name server) – A slave server is exact replica of master server.. What type of DNS resource record is used to identify a server that is authoritative for a zone?Every DNS zone requires a start of authority (SOA) record. RFC 1035 specifies SOA record formats. The SOA record stores important information about the zone, such as its primary authoritative name server and the administrator's email address.
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