Chapter 16 The Domain Name System

1. Chapter 16 The Domain Name System Yun Wan June 29, 1999

2. 16.1 Introduction 1. DNS is a distributed database that holds information about computers on the Internet, which includes the host’s name, IP address, mail routing information and so on. 2. Primary job : map between hostnames and IP addresses. Users refer to machines by names, while low-level network software understands only numbers.

3. 16.2 The History of DNS 1. Mapping between hostnames and addresses was kept in a single text file that was managed centrally and distributed to all the hosts. 2. It is reasonable for small network, but inadequate for the large and growing network. Solution : hierarchical hostnames and distributed responsibility 3. BIND, Berkeley Internet Name Domain system, was produced by Kevin Dunlap in 1985.

4. 16.3 Who Needs DNS • 1. A site on the Internet needs DNS to participate fully • 2. DNS defines : • A hierarchical namespace for hosts • A host table implemented as a distributed database • Library routines for querying this database • Improved routing for email • A protocol for exchangeing naming information

5. 16.4 The DNS Namespace 1. DNS namespace is a tree of domains with ascending authority. 2. The root of the tree is called “.”, followed by the top-level domains. 3. Two sorts of domain:domains in the US : describe organizational and political structures, which are given three-letter names such as com, edu, and so on.domains outside the US : two-letter ISO country codes such as AU(Australia),CA(Canada).

6. 16.4 The DNS Namespace (Cont’d) 4. A fully qualified hostname is formed by appending the domain name to the hostname such as boulder.colorado.edu. 5. The structure of the naming tree is illustrated as following : . Root Domain Top Level EDU COM FI Second Level colorado berkeley sun funet Subdomains cs boulder cs east central and Machines anchor vangogh mammoth

7. 16.4 The DNS Namespace (Cont’d) 6. Selecting a Domain NameCertain names are taboo such as names that are already taken, the keyword AT, combinations of top-level domains (edu.com) repeating names like x.x.com. 7. Registering a Second-level Domain NamesIn US, sent your application electronically or via US mail to Registration Services group of the Network Information Center 8. Creating your own Subdomainschoose a name that is unique in the local contextIdentify two or more hosts to be severs for your new domainCoordinate with the administrator of the parent domain

8. 16.5 Components of BIND 1. Three components: A deamon called named that answers queriesLibrary routines that resolve host queries using DNSCommnand-line interfaces to DNS: nslookup, dig and host 2. What does the named do ? Answers queries about hostnames and IP addresses If it doesn’t know the ansewer, asks other severs and cashes their responses performing “zone transfers” to copy data among the servers of a domain

9. 16.5 Components of BIND (cont’d) 3. Three modes of Name Severs :Primary, Secondary and Cashing-only 4. How to distinquish the three modes :where the data comes from and whether the sever is authoritative for the domain. 5. Primary name server keeps the master copy of the domain’s data on disk and is an authoritative name server. 6. Secondary name server copies its domain’s data from the primary server via a “zone transfer” operation and it is also an authoritative name server

10. 16.5 Components of BIND (cont’d) 7. A caching-only name server loads the addresses of a few important machines (servers for the root domain) from a startup file and gets all the rest of its data by caching answers to the queries it resolves. 8. A caching-only name server is a non-authoritative name server. 9. An authoritative answer is “guaranteed” to be accurate, while a non-authoritative answer may be out of date. 10. Caching-only servers can reduce the amount of DNS traffic on the networks 11. Even “authoritative” answers can be inconsistent if a sysadmin changes the primary name server’s data and forgets to update the secondaries.

11. 16.5 Components of BIND (cont’d) 12. The Resolver Library:Before DNS, hostname mappings were looked up in /etc/hosts by two library routines : gethostbyname and gethostbyaddrIn order for this information to be provided by DNS, these routines must change, either integrated into the system’s C library, libc.a or put in their own library, libresolv.a 13. Shell Interface :The nslookup command allows you to access DNS from a shell. It is much as the grep command used to look up a hostname in /etc/hosts

12. 16.6 How DNS Works 1. Name servers are either “recursive” or “non-recursive” 2. A non-recursive server is a lazy, half-hearted server.In the case that a non-recursive server has no answer cached froma previous query and it is not the authoritative server, it will provide a referral to authritative servers of another domain that are more likely to know the answer. The client will follow the referral to get the answer. 3. A recursive server returns only real answers and error messages. It follows referral itself.

13. 16.6 How DNS Works (cont’d) 4. Here is a real example Suppose we look up the address for mammoth.cs.berkeley.edu from the machine lair.cs.colorado.edu 1-Q 2-Q lair ns.cs.colorado edu 8-A 3-R 7-A 4-Q START 6-A cs.berkeley berkeley 5-Q Recursive Non-Recursive

14. 16.7 Caching and Efficiency 1. Caching increases the efficiency of lookups 2. A cached answer is almost free and usually correct because mappings change infrequently 3. Most queries are for local hosts and can be resolved quickly 4. Many queries are repeated.

15. 16.8 BIND Client Issues 1. Configuring the Resolver :Each BIND client must have a file called /etc/resolv.conf that lists the name servers to query. The format of resolv.conf is :; Commentsearch domainname …nameserver ipaddrthis is an example: search cs.colorado.edu colorado.edu; ns, piper, bouldernameserver 128.138.243.151nameserver 128.138.204.4nameserver 128.138.240.1

16. 16.8 BIND Client Issues (cont’d) 2. Testing the ResolverAfter configuring /etc/resolv.conf, we can test name lookups by either nslookup or dig. 3. Impact on the Rest of the System Changing from static host tables to DNS creates some dependencies that we need to protect against. Three particular areas are affected : booting, configuration files and mail delivery

17. 16.9 Setting Up a Name Server 1. Complete configuration for named consists of the boot file, the cach file and for primary servers, the data file or files containing address /etc/named.boot : the boot file The format of the named.boot file is as followings: directory dirnamecache . filenameprimary zone filenamesecondary zone ipaddr […] filename

18. 16.9 Setting Up a Name Server (cont’d) The directory keyword specifies that all subsequent filenames are relative to the listed directory the cache keyword specifies the name of the file that contains root name servers and their addresses. The primary keyword indicates that this host is the primary server for specified zone and that data for the zone is in filename The secondary keyword says that this host is secondary server for zone

19. 16.10 The DNS Database A domain’s DNS database is a set of text file maintained by the system administrator on the domain’s primary name server. Items stored in the database are called resource records The basic format of a resource record is [name] [ttl] [class] type data The name field identifies the entity that the record describesThe ttl (time to live) field specifies the time, in seconds, that the data item can be cached and still be considered validClass specifies the network type such as IN, CH, HS

20. 16.10 The DNS Database The type field specify the type of the resource records, which is grouped into three groups:Zone records : identify domains and their name serversBasic records : map names to addresses and route mailOptional records : provide extra information about hosts The content of the data field depend on the record type. Record types are listed in Table 16.5 on page 333.