Chapter 9 Domain Name Servers Networking CS 3470

DNS: Domain Name System Major goals Map hostname to IP address Map IP address

Distributed, Hierarchical Database DNS is implemented as a distributed hierarchical database Root DNS Servers

Distributed, Hierarchical Database Root DNS servers know information for each top-level domain The Internet

Distributed, Hierarchical Database Below the top-level domain, there are more specific domain names for

DNS: Root Name Servers Contacted by local name server that cannot resolve name Root

TLD and Authoritative Servers Top-level domain (TLD) servers: responsible for com, org, net, edu,

Local Name Server Each ISP (residential ISP, company, university) has one. Also called “default

Iterative Queries root DNS server 2 4 iterated query: contacted server replies with name

Recursive Queriesroot DNS server 2 3 7 recursive query: puts burden of name resolution

Combination of Query Types root DNS server 2 In reality, we can see a

DNS: Caching and Updating Records Once (any) name server learns mapping, it caches mapping

DNS Records We can think of dns as a distributed db storing resource records

Other Information DNS uses UDP to exchange information Query is initiated from system calls

Host Configurations Two options Configure DNS manually – manually configure the name/IP address pairs

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Chapter 9: Domain Name Servers Networking CS 3470, Section 1

DNS: Domain Name System Major goals Map hostname to IP address Map IP address to hostname Also provides other services Host aliasing – multiple names for one server MX records (Mail e. Xchanger) – indicate mail servers for a domain Load balancing – many IP addresses associated with a hostname 2

Distributed, Hierarchical Database DNS is implemented as a distributed hierarchical database Root DNS Servers com DNS servers yahoo. com amazon. com DNS servers org DNS servers pbs. org DNS servers edu DNS servers uni. edu DNS servers fsu. edu DNS servers 3

Distributed, Hierarchical Database Root DNS servers know information for each top-level domain The Internet domain is partitioned into multiple top-level domains Root DNS Servers com DNS servers yahoo. com amazon. com DNS servers org DNS servers pbs. org DNS servers edu DNS servers uni. edu DNS servers fsu. edu DNS servers 4

Distributed, Hierarchical Database Below the top-level domain, there are more specific domain names for different organizations Root DNS Servers com DNS servers yahoo. com amazon. com DNS servers org DNS servers pbs. org DNS servers edu DNS servers uni. edu DNS servers fsu. edu DNS servers 5

DNS: Root Name Servers Contacted by local name server that cannot resolve name Root name server: Contacts authoritative name server if name mapping not known Gets mapping Returns mapping to local name server 6

TLD and Authoritative Servers Top-level domain (TLD) servers: responsible for com, org, net, edu, etc, and all top-level country domains cn, ca, fr, jp, uk etc. Ex: Network solutions maintains servers for com TLD Authoritative DNS servers: organization’s DNS servers, providing authoritative hostname to IP mappings for organization’s servers (e. g. , Web and mail). Can be maintained by organization or service provider 7

Local Name Server Each ISP (residential ISP, company, university) has one. Also called “default name server” When a host makes a DNS query, query is sent to its local DNS server Acts as a proxy, forwards query into hierarchy. 8

Iterative Queries root DNS server 2 4 iterated query: contacted server replies with name of server to contact “I don’t know this name, but ask this server” 3 TLD DNS server 5 local DNS server dns. poly. edu 1 8 requesting host 7 6 authoritative DNS server dns. cs. umass. edu cis. poly. edu gaia. cs. umass. edu 9

Recursive Queriesroot DNS server 2 3 7 recursive query: puts burden of name resolution on contacted name server heavy load? local DNS server dns. poly. edu 1 6 TLD DNS server 5 4 8 requesting host authoritative DNS server dns. cs. umass. edu cis. poly. edu gaia. cs. umass. edu 10

Combination of Query Types root DNS server 2 In reality, we can see a combination of iterative and recursive queries 3 TLD DNS Server 4 7 local DNS server dns. poly. edu 1 6 5 8 requesting host authoritative DNS server dns. cs. umass. edu cis. poly. edu gaia. cs. umass. edu 11

DNS: Caching and Updating Records Once (any) name server learns mapping, it caches mapping cache entries timeout (disappear) after some time TLD servers typically cached in local name servers Thus root name servers not often visited 12

DNS Records We can think of dns as a distributed db storing resource records (RR) RR format: (name, value, type, class, ttl) • Type=A name is hostname value is IP address • Type=NS name is domain (e. g. foo. com) value is IP address of authoritative name server for this domain • Type=CNAME name is alias name for some “canonical” (the real) name www. ibm. com is really servereast. backup 2. ibm. com value is canonical name • Type=MX value is name of mail server associated with name 13

Other Information DNS uses UDP to exchange information Query is initiated from system calls like gethostbyname and gethostbyaddr. 14

Host Configurations Two options Configure DNS manually – manually configure the name/IP address pairs for each host (usually done in a configuration file like “/etc/hosts”) DHCP – Get your local name server address from a dhcp server (along with your IP address and other routing information) 15