DNS Domain Name System

DNS Domain Name System

Domain names and IP addresses People prefer to use easy-to-remember names instead of IP addresses Domain names are alphanumeric names for IP addresses e.g., neon.ece.utoronto.ca, www.google.com, ietf.org The domain name system (DNS) is an Internet-wide distributed database that translates between domain names and IP addresses How important is DNS? Imagine what happens when the local DNS server is down.

Before there was DNS . . there was the HOSTS.TXT file Before DNS (until 1985), the name-to-IP address was done by downloading a single file (hosts.txt) from a central server with FTP. – Names in hosts.txt are not structured. – The hosts.txt file still works on most operating systems. It can be used to define local names.

Resolver and name server Reverse lookups are also possible, i.e., find the hostname given an IP address HTTP IP address (128.143.71.21) Resolver Name server Hostname (neon.tcpip-lab.edu) Hostname (neon.tcpip-lab.edu) IP address (128.143.71.21) 1. An application program on a host accesses the domain system through a DNS client, called the resolver 2. Resolver contacts DNS server, called name server 3. DNS server returns IP address to resolver which passes the IP address to application

Design principle of DNS The naming system on which DNS is based is a hierarchical and logical tree structure called the domain namespace. An organization obtains authority for parts of the name space, and can add additional layers of the hierarchy Names of hosts can be assigned without regard of location on a link layer network, IP network or autonomous system In practice, allocation of the domain names generally follows the allocation of IP address, e.g., – All hosts with network prefix 130.207/16 have domain name suffix gatech.edu – All hosts on network 130.207.3/24 are in the College of Computing of Georgia Tech

DNS Name hierarchy DNS hierarchy can be represented by a tree Root and top-level domains are administered by an Internet central name registration authority (ICANN) Below top-level domain, administration of name space is delegated to organizations Each organization can delegate further . (root) org gov edu uci.edu Top-level Domains com toronto.edu Managed by UofT math.toronto.edu ece.toronto.edu Managed by ECE Dept. neon.ece.toronto.edu

Domain name system Each node in the DNS tree represents a DNS name Each branch below a node is a DNS domain. – DNS domain can contain hosts or other domains (subdomains) Example: DNS domains are . edu virginia.edu www.virginia.edu cs.virginia.edu ., edu, virginia.edu, cs.virginia.edu neon.cs.virginia.edu

Domain names Hosts and DNS domains are named based on their position in the domain tree Every node in the DNS domain tree can be identified by a unique Fully Qualified Domain Name (FQDN). The FQDN gives the position in the DNS tree. cs.virginia.edu or cs.virginia.edu. A FQDN consists of labels (“cs”,“virginia”,”edu”) separated by a period (“.”) There can be a period (“.”) at the end. Each label can be up to 63 characters long FQDN contains characters, numerals, and dash character (“-”) FQDNs are not case-sensitive

Top-level domains Three types of top-level domains: – Organizational: 3-character code indicates the function of the organization Used primarily within the US Examples: gov, mil, edu, org, com, net – Geographical: 2-character country or region code Examples: us, va, jp, de – Reverse domains: A special domain (in-addr.arpa) used for IP address-to-name mapping There are more than 200 top-level domains.

Organizational top-level domains com Commercial organizations edu Educational institutions gov Government institutions int International organizations mil U.S. military institutions net Networking organizations org Non-profit organizations

Hierarchy of name servers The resolution of the hierarchical name space is done by a hierarchy of name servers Each server is responsible (authoritative) for a contiguous portion of the DNS namespace, called a zone. Zone is a part of the subtree DNS server answers queries about hosts in its zone 130.207.3.207: authoritative name server for cc.gatech.edu 130.207.244.244: authoritative name server for gatech.edu root server org server uci.edu server edu server gov server .virginia.edu server cs.virginia.edu server com server

Authority and delegation Authority for the root domain is with the Internet Corporation for Assigned Numbers and Names (ICANN) ICANN delegates to accredited registrars (for gTLDs) and countries for country code top level domains (ccTLDs) Authority can be delegated further Chain of delegation can be obtained by reading domain name from right to left. Unit of delegation is a “zone”.

DNS domain and zones Each zone is anchored at a specific domain node, but zones are not domains. . (root) Zone A DNS domain is a branch of the namespace A zone is a portion of the DNS namespace generally stored in a file (It could consist of multiple nodes) A server can divide part of its zone and delegate it to other servers .edu .uci.edu math.virginia.edu Zone and domain .virginia.edu cs.virginia.edu Domain

Primary and secondary name servers For each zone, there must be a primary name server and a secondary name server – The primary server (master server) maintains a zone file which has information about the zone. Updates are made to the primary server – The secondary server copies data stored at the primary server. Adding a host: When a new host is added (“gold.cs.virginia.edu”) to a zone, the administrator adds the IP information on the host (IP address and name) to a configuration file on the primary server

Root name servers The root name servers know how to find the authoritative name servers for all top-level zones. There are only 13 root name servers Root servers are critical for the proper functioning of name resolution

Addresses of root servers A.ROOT-SERVERS.NET. B.ROOT-SERVERS.NET. C.ROOT-SERVERS.NET. D.ROOT-SERVERS.NET. E.ROOT-SERVERS.NET. F.ROOT-SERVERS.NET. G.ROOT-SERVERS.NET. H.ROOT-SERVERS.NET. I.ROOT-SERVERS.NET. J.ROOT-SERVERS.NET. K.ROOT-SERVERS.NET. L.ROOT-SERVERS.NET. M.ROOT-SERVERS.NET. (formerly NS1.ISI.EDU) (formerly C.PSI.NET) (TERP.UMD.EDU) (NS.NASA.GOV) (NS.ISC.ORG) (NS.NIC.DDN.MIL) (AOS.ARL.ARMY.MIL) (NIC.NORDU.NET) (at NSI (InterNIC)) (operated by RIPE NCC) (at ISI (IANA)) (operated by WIDE, Japan) 202.12.27.33 198.41.0.4 128.9.0.107 192.33.4.12 128.8.10.90 192.203.23 192.5.5.241 192.112.36.4 128.63.2.53 192.36.148.17 198.41.0.10 193.0.14.129 198.32.64

Domain name resolution Hostname (neon.tcpip-lab.edu) HTTP IP address (128.143.71.21) Resolver Name server Hostname (neon.tcpip-lab.edu) IP address (128.143.71.21) 1. User program issues a request for the IP address of a hostname 2. Local resolver formulates a DNS query to the name server of the host 3. Name server checks if it is authorized to answer the query. a) If yes, it responds. b) Otherwise, it will query other name servers, starting at the root tree 4. When the name server has the answer it sends it to the resolver.

Recursive and Iterative Queries There are two types of queries: – Recursive queries – Iterative (non-recursive) queries The type of query is determined by a bit in the DNS query Recursive query: When the name server of a host cannot resolve a query, the server issues a query to resolve the query Iterative queries: When the name server of a host cannot resolve a query, it sends a referral to another server to the resolver

Recursive queries Referral to edu name server If the server cannot supply the answer, it will send the query to the “closest known” authoritative name server (here: In the worst case, the closest known server is the root server) 2nd query: neon.cs.virginia.edu Referral to virginia.edu name server Name server response The root sever sends a referral to the “edu” server. Querying this server yields a referral to the server of “virginia.edu” and so on root server 1st query: neon.cs.virginia.edu query In a recursive query, the resolver expects the response from the name server rd 3 query: neon.cs.virginia.edu Referral to cs.virginia.edu name server edu server virginia.edu server 4th query: neon.cs.virginia.edu Resolver IP address of neon.cs.virginia.edu cs.virginia.edu server

Iterative queries root server In an iterative query, the name server sends a closest known authoritative name server a referral to the root server. query referral to root server This involves more work for the resolver Name server Resolver u ed . ia r gin ve r r i .v se s e c . am on n e n du y: e r ue u l to st q a .ed r r a 1 i edu server fe gin e r i e R .v m .cs na n o u e d :n a.e y i r n i e u g n d qu .ed vir rver a i n o 2 t se gi .vir ral r s c e . f n Re neo virginia.edu : y r edu . rd que a i server 3 rgin server i v . s to c name l a r er Ref 4th query: neon.cs.virginia.edu IP address of neon.cs.virginia.edu cs.virginia.edu server

Caching To reduce DNS traffic, name servers caches information on domain name/IP address mappings When an entry for a query is in the cache, the server does not contact other servers Note: If an entry is sent from a cache, the reply from the server is marked as “unauthoritative”

Resource Records The database records of the distributed data base are called resource records (RR) Resource records are stored in configuration files (zone files) at name servers. Left Resource records for a zone: db.mylab.com TTL 86400 mylab.com. IN SOA PC4.mylab.com. hostmaster.mylab.com. ( 1 ; serial 28800 ; refresh 7200 ; retry 604800 ; expire 86400 ; ttl ) ; mylab.com. IN ; localhost PC4.mylab.com. PC3.mylab.com. PC2.mylab.com. PC1.mylab.com. NS PC4.mylab.com. A A A A A 127.0.0.1 10.0.1.41 10.0.1.31 10.0.1.21 10.0.1.11

Resource Records db.mylab.com TTL 86400 mylab.com. IN SOA PC4.mylab.com. hostmaster.mylab.com. ( 1 ; serial 28800 ; refresh 7200 ; retry 604800 ; expire 86400 ; ttl ) ; mylab.com. IN ; localhost PC4.mylab.com. PC3.mylab.com. PC2.mylab.com. PC1.mylab.com. NS PC4.mylab.com. A A A A A 127.0.0.1 10.0.1.41 10.0.1.31 10.0.1.21 10.0.1.11 Max. age of cached data in seconds * Start of authority (SOA) record. Means: “This name server is authoritative for the zone Mylab.com” * PC4.mylab.com is the name server * [email protected] is the email address of the person in charge Name server (NS) record. One entry for each authoritative name server Address (A) records. One entry for each hostaddress

DNS timers Refresh interval. Used to determine how often other DNS servers that load and host the zone must attempt to renew the zone. Retry interval. Used to determine how often other DNS servers that load and host the zone are to retry a request for update of the zone each time that the refresh interval occurs. Expire interval. Used by other DNS servers that are configured to load and host the zone to determine when zone data expires if it is not renewed.