7 understanding DNS
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DNS (Domain Name System) is a hierarchical distributed database that maps hostnames to IP addresses. It allows easy-to-remember hostnames to be used instead of hard-to-remember IP addresses. The document discusses DNS servers, clients, resolvers, zones, and record types including SOA, NS, A, CNAME, PTR, MX, and SRV records. It also introduces tools for working with DNS like Nslookup, Ipconfig, and DNSCmd.
7 understanding DNS
- 1. Configuring DNS PREPARED BY HAMEDA HURMAT
- 2. Overview Determine DNS Server; Understanding Servers, Clients and resolvers Understanding DNS Process Learn DNS Zones Learn DNS records
- 3. Introducing DNS The Domain Name System (DNS) is a service that allows you to resolve a hostname to an Internet Protocol (IP) address. DNS is a hierarchically distributed database. In other words, its layers are arranged in a definite order, and its data is distributed across a wide range of machines, each of which can exert control over a portion of the database. DNS is a standard set of protocols that defines the following: ■ A mechanism for querying and updating address information in the database ■ A mechanism for replicating the information in the database among servers ■ A schema of the database
- 4. In Computer 1. Open the HOSTS file: C:WindowsSystems32driversetc . 2. Add the IP-address-to-hostname mapping. 3. Try to ping the server using the hostname to verify that you can reach it using an easy-to- remember name.
- 5. Understanding Servers, Clients, and Resolvers DNS Server Any computer providing domain name services is a DNS name server . DNS Client A DNS client is any machine that issues queries to a DNS server. The client hostname may or may not be registered in a DNS database. Clients issue DNS requests through processes called resolvers . Resolver Resolvers are software processes, sometimes implemented in software libraries, which handles the actual process of finding the answers to queries for DNS data. Query A query is a request for information sent to a DNS server. Three types of queries can be made to a DNS server: recursive, inverse, and iterative.
- 6. Understanding the DNS Process Dynamic DNS (DDNS) standard, described in RFC 2136, allows DNS clients to update information in the DNS database files. For example, a Windows Server 2016 DHCP server can automatically tell a DDNS server which IP addresses it has assigned to what machines. Non-Dynamic DNS (NDDNS) does not automatically populate the DNS database. The client systems do not have the ability to update to DNS. If you decide to use Non-Dynamic DNS, an administrator will need to populate the DNS database manually. Non-Dynamic DNS is a reasonable choice if your organization is small to midsize and you do not want extra network traffic (clients updating to the DNS server) or if you need to enter the computer’s TCP/IP information manually because of strict security measures.
- 8. None This means your DNS server is Non-Dynamic. Nonsecure and Secure This means that any machine (even if it does not have a domain account) can register with DNS. Using this setting could allow rogue systems to enter records into your DNS server. Secure Only This means that only machines with accounts in Active Directory can register with DNS. Before DNS registers any account in its database, it checks Active Directory to make sure that account is an authorized domain computer.
- 9. Introducing DNS Database Zones Primary Zones Secondary Zones Stub Zones
- 10. Understanding Primary Zones Primary zone Primary zone with Active Directory Integration (Active Directory DNS) Local Database Primary DNS zones get stored locally in a file (with the suffix .dns) on the server. This allows you to store a primary zone on a domain controller or a member server. local database has many disadvantages: Lack of Fault Tolerance Think of a primary zone as a contact list on your smartphone. All of the contacts in the list are the records in your database. The problem is that if you lose your phone or the phone breaks, Additional Network Traffic Let’s imagine that you are looking for a contact number for John Smith. John Smith is not listed in your cell phone directory, but he is listed in your partner’s cell phone. You have to contact your partner to get the listing. You cannot directly access your partner’s cell contacts. No Security Staying with the cell phone example, let’s say that you call your partner looking for John Smith’s phone number. When your partner gives you the phone number over your wireless phone, someone with a scanner can pick up your conversation. Unfortunately, wireless telephone calls are not very secure.
- 11. Understanding Secondary Zones Secondary zones are noneditable copies of the DNS database. You use them for load balancing which is a way of managing network overloads on a single server. A secondary zone gets its database from a primary zone. Secondary zones have the following advantages: A secondary zone provides fault tolerance, so if the primary zone server becomes unavailable, name resolution can still occur using the secondary zone server. Secondary DNS servers can also increase network performance by offloading some of the traffic that would otherwise go to the primary server.
- 12. Understanding Stub Zones Stub zones work a lot like secondary zones—the database is a noneditable copy of a primary zone. The difference is that the stub zone’s database contains only the information necessary (three record types) to identify the authoritative DNS servers for a zone. You should not use stub zones to replace secondary zones, nor should you use them for redundancy and load balancing.
- 13. Advantages of DNS in Windows Server 2016 DNS in Microsoft Windows Server 2016 has some great advantages over many other versions of Microsoft DNS. Here are some of the improvements of DNS in Windows Server 2016 (some of these became available in previous versions of Windows Server): ■■ Background zone loading ■■ Support for TCP/IP version 6 (IPv6) ■■ Read-only domain controllers ■■ GlobalName zone ■■ DNS socket pools ■■ DNS cache locking ■■ Response Rate Limiting (RRL) ■■ Unknown Record Support ■■ IPv6 Root Hints ■■ DNS Security Extensions (DNSSEC) ■■ DNS devolution ■■ Record weighting ■■ Netmask ordering ■■ DnsUpdateProxy group ■■ DNS Policies
- 14. Introducing DNS Record Types Start of Authority (SOA) Records The first record in a database file is the start of authority (SOA) record. The SOA defines the general parameters for the DNS zone, including the identity of the authoritative server for the zone. The SOA appears in the following format: @ IN SOA primary_mastercontact_e-mailserial_number refresh_timeretry_timeexpiration_timetime_to_live
- 15. Name Server Records Name server (NS) records list the name servers for a domain. This record allows other name servers to look up names in your domain. A zone file may contain more than one name server record. The format of these records is simple:
- 16. Host Record A host record (also called an A record for IPv4 and AAAA record for IPv6) is used to associate statically a host’s name to its IP addresses. The format is pretty simple: host_nameoptional_TTL IN A IP_Address Here’s an example from my DNS database: www IN A 192.168.0.204 SMTP IN A 192.168.3.144
- 17. Alias Record Closely related to the host record is the alias record, or canonical name (CNAME) record. The syntax of an alias record is as follows: Alias optional_TTL IN CNAME hostname Aliases are used to point more than one DNS record toward a host for which an A record already exists. For example, if the hostname of your web server was actually chaos, you would likely have an A record such as this: chaos IN A 192.168.1.10 Then you could make an alias or CNAME for the record so that www.example.com would point to chaos: www IN CNAME chaos.example.com. Note the trailing dot (.) on the end of the CNAME record. This means the root domain is not appended to the entry.
- 18. Pointer Record A or AAAA records are probably the most visible component of the DNS database because Internet users depend on them to turn FQDNs like www.microsoft.com into the IP addresses that browsers and other components require to find Internet resources. However, the host record has a lesser-known but still important twin: the pointer (PTR) record. The format of a PTR record appears as follows: reversed_address.in-addr.arpa. optional_TTL IN PTR targeted_domain_name
- 19. Mail Exchanger Record The mail exchanger (MX) record is used to specify which servers accept mail for this domain. Each MX record contains two parameters—a preference and a mail server, as shown in the following example: domain IN MX preference mailserver_host The MX record uses the preference value to specify which server should be used if more than one MX record is present. The preference value is a number. The lower the number, the more preferred the server. Here’s an example: example.com. IN MX 0 mail.example.com. example.com. IN MX 10 backupmail.example.com.
- 20. Service Record Windows Server 2016 depends on some other services, like the Lightweight Directory Access Protocol (LDAP) and Kerberos. Using a service record, which is another type of DNS record, a Windows 2000, XP, Vista, Windows 7, Windows 8 / 8.1, or Windows 10 client can query DNS servers for the location of a domain controller. This makes it much easier (for both the client and the administrator) to manage and distribute logon traffi c in large-scale networks. For this approach to work, Microsoft has to have some way to register the presence of a service in DNS. Enter the service (SRV) record. Service (SRV) records tie together the location of a service (like a domain controller) with information about how to contact the service. SRV records provide seven items of information. Let’s review an example to help clarify this powerful concept. ldap.tcp.example.com. 86400 IN SRV 10 100 389 hsv.example.com ldap.tcp.example.com. 86400 IN SRV 20 100 389 msy.example.com
- 21. Using Nslookup Nslookup is a standard command-line tool provided in most DNS server implementations Nslookup offers you the ability to perform query testing of DNS servers and to obtain detailed responses at the command prompt.
- 22. Using Ipconfig You can use the command-line tool ipconfig to view your DNS client settings, to view and reset cached information used locally for resolving DNS name queries, and to register the resource records for a dynamic update client. If you use the ipconfig command with no parameters, it displays DNS information for each adapter, including the domain name and DNS servers.
- 24. Using DNSCmd DNSCmd allows you to display and change the properties of DNS servers, zones, and resource records through the use of command-line commands. The DNSCmd utility allows you to modify, create, and delete resource records and/or zones manually, and it allows you to force replication between two DNS servers.