Introduction to the Domain Name System
The Domain Name System (DNS) handles the growing number of Internet users. DNS translates names, such as www.cisco.com, into IP addresses, such as 192.168.40.0 (or the more extended IPv6 addresses), so that computers can communicate with each other. DNS makes using Internet applications, such as the World Wide Web, easy. The process is as if, when phoning your friends and relatives, you could autodial them based on their names instead of having to remember their phone numbers.
How DNS Works
How DNS Works
To understand how DNS works, imagine a typical user, John, logging in to his computer. He launches his web browser so that he can view the website at a company, ExampleCo (see Figure 14-1). He enters the name of their website—http://www.example.com. Then:
1. John's workstation sends a request to the DNS server about the IP address of www.example.com.
2. The DNS server checks its database to find that www.example.com corresponds to 192.168.1.4.
3. The server returns this address to John's browser.
4. The browser uses the address to locate the website.
5. The browser displays the website on John's monitor.
Figure 14-1 Domain Names and Addresses
John can access the ExampleCo website because his DNS server knows the www.example.com IP address. The server learned the address by searching through the domain namespace. DNS was designed as a tree structure, where each named domain is a node in the tree. The top-most node of the tree is the DNS root domain (.), under which there are subdomains, such as .com, .edu, .gov, and .mil (see Figure 14-2).
Figure 14-2 Domain Name System Hierarchy
The fully qualified domain name (FQDN) is a dot-separated string of all the network domains leading back to the root. This name is unique for each host on the Internet. The FQDN for the sample domain is example.com., with its domain example, parent domain .com, and root domain "." (dot).
Learning ExampleCo Address
Establishing a Domain
Difference Between Domains and Zones
Learning ExampleCo Address
When John's workstation requests the IP address of the website www.example.com (see Figure 14-3):
Figure 14-3 DNS Hierarchical Name Search
1. The local DNS server looks for the www.example.com domain in its database, but cannot find it, indicating that the server is not authoritative for this domain.
2. The server asks the authoritative root nameserver for the top-level (root) domain "." (dot).
3. The root nameserver directs the query to a nameserver for the .com domain that knows about its subdomains.
4. The .com nameserver determines that example.com is one of its subdomains and responds with its server address.
5. The local server asks the example.com nameserver for the www.example.com location.
6. The example.com nameserver replies that its address is 192.168.1.4.
7. The local server sends this address to John's Web browser.
Establishing a Domain
ExampleCo has a website that John could reach because it registered its domain with an accredited domain registry. ExampleCo also entered its domain name in the .com server database, and requested a network number, which defines a range of IP addresses.
In this case, the network number is 192.168.1.0, which includes all assignable hosts in the range 192.168.1.1 through 192.168.1.254. You can only have numbers 0 through 255 (28) in each of the address fields, known as octets. However, the numbers 0 and 255 are reserved for network and broadcast addresses, respectively, and are not used for hosts.
Difference Between Domains and Zones
The domain namespace is divided into areas called zones that are points of delegation in the DNS tree. A zone contains all domains from a certain point downward, except those for which other zones are authoritative.
A zone usually has an authoritative nameserver, often more than one. In an organization, you can have many nameservers, but Internet clients can query only those that the root nameservers know. The other nameservers answer internal queries only.
The ExampleCo company registered its domain, example.com. It established three zones—example.com, marketing.example.com, and finance.example.com. ExampleCo delegated authority for marketing.example.com and finance.example.com to the DNS servers in the Marketing and Finance groups in the company. If someone queries example.com about hosts in marketing.example.com, example.com directs the query to the marketing.example.com nameserver.
In Figure 14-4, the domain example.com includes three zones, with the example.com zone being authoritative only for itself.
Figure 14-4 Example.com With Delegated Subdomains
ExampleCo could choose not to delegate authority to its subdomains. In that situation, the example.com domain is a zone that is authoritative for the subdomains for marketing and finance. The example.com server answers all outside queries about marketing and finance.
As you begin to configure zones by using Cisco Network Registrar, you must configure a nameserver for each zone. Each zone has one primary server, which loads the zone contents from a local configuration database. Each zone can also have any number of secondary servers, which load the zone contents by fetching the data from the primary server. Figure 14-5 shows a configuration with one secondary server.
Figure 14-5 Primary and Secondary Servers for Zones
DNS is based on a client/server model. In this model, nameservers store data about a portion of the DNS database and provide it to clients that query the nameserver across the network. Nameservers are programs that run on a physical host and store zone data. As administrator for a domain, you set up a nameserver with the database of all the resource records (RRs) describing the hosts in your zone or zones (see Figure 14-6).
Figure 14-6 Client/Server Name Resolution
The DNS servers provide name-to-address translation, or name resolution. They interpret the information in a fully qualified domain name (FQDN) to find its address. If a local nameserver does not have the data requested in a query, it asks other nameservers until it finds it. For commonly requested names, this process can go quickly, because nameservers continuously cache the information they learn from queries about the domain namespace.
Each zone must have one primary nameserver that loads the zone contents from a local database, and a number of secondary servers, which load a copy of the data from the primary server (see Figure 14-7). This process of updating the secondary server from the primary server is called a zone transfer.
Even though a secondary nameserver acts as a kind of backup to a primary server, both types of servers can be authoritative for the zone. They both learn about hostnames in the zone from the zone authoritative database, not from information learned while answering queries. Clients can query both servers for name resolution.
As you configure the Cisco Network Registrar DNS nameserver, you specify what role you want the server to perform for a zone—primary, secondary, or caching-only. The type of server is meaningful only in context to its role. A server can be a primary for some zones and a secondary for others. It can be a primary or secondary only, or it can serve no zones and just answer queries by means of its cache.
Although all servers are caching servers, because they save the information until it expires, a caching-only server is one that is not authoritative for any zone. This server answers internal queries and asks other authoritative servers for the information. Sites create caching-only servers to unburden the authoritative servers so that they do not need to have every query directed to the authoritative servers.
Figure 14-7 DNS Zone Transfer
To configure the:
•Primary nameserver, see the "Managing Primary DNS Servers" section on page 15-5.
•Secondary server, see the "Managing Secondary Servers" section on page 15-15.
•Caching-only server, see the "Configuring Caching-Only DNS Servers" section on page 17-7.
The DNS servers described so far perform name-to-address resolution. They can do this easily by searching through their database for the correct address, because they index all the data by name. However, there are times when you need address-to-name resolution so that you can interpret certain output, such as computer log files.
Finding a domain name when you only know the address, however, would require searching the entire namespace. DNS solves this problem by supporting a domain namespace that uses addresses as names, known as the in-addr.arpa domain. This reverse zone contains subdomains for each network based on the network number. For consistency and natural grouping, the four octets of a host number are reversed.
The IP address as a domain name appears backward, because the name is in leaf-to-root order. For example, the ExampleCo example domain network number is 192.168.1.0. Its reverse zone is 1.168.192.in-addr.arpa. If you only know the DNS server address (192.168.1.1), the query to the reverse domain would find the host entry 220.127.116.11.in-addr.arpa that maps back to example.com.
Reverse domains are handled through Pointer (PTR) RRs, as indicated in Figure 14-8.
Figure 14-8 Reverse Domains
Because there can be only one primary DNS server per zone, you risk the failure of dynamic updates if the primary DNS server goes down. These updates can occur on the primary DNS server only; a secondary DNS server cannot record these changes, but must forward them to the primary. To solve this problem, a second primary server can become a hot standby that shadows the main primary. This is called High-Availability (HA) DNS (see Chapter 18, "Configuring High-Availability DNS Servers"). Both servers in this failover configuration must synchronize so that their primary zones and related attributes are identical. Cisco Network Registrar provides settings on the main server to identify the main and backup for synchronization, and the timeout period to go over into failover mode.
To send a DNS message above 512 bytes over UDP, you need to use an extension of the DNS protocol known as Extended DNS (EDNS). The EDNS protocol expands the number of flags, label types, and return codes available to the DNS protocol. A version of EDNS specified by RFC 2671 is known as EDNS0. EDNS uses a pseudo resource record known as OPT Resource Record (OPT RR). OPT RR differentiates conventional DNS from EDNS. OPT RRs appear only in the route transmission between DNS clients and servers, they do not appear in the zone files or caches. A DNS endpoint that marks a DNS packet as EDNS must insert an OPT RR in the additional data section of the DNS request or response.
The DNS server supports all EDNS0 extensions. You can modify the UDP payload size of the DNS server. The minimum UDP payload size of the DNS server is 512 bytes and the maximum is 4 KB, with the default set to 4 KB.
Note The DNS Server can handle requests from clients that do not support EDNS0, however, the DNS server is not permitted to use any extended capabilities, when it handles requests from clients that do not support EDNS0. The response to client requests are inserted into a default 512 byte message. To notify clients that the server supports EDNS0, an OPT RR is inserted into the additional section of the DNS message by the server.