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 eXchanger) – indicate mail servers for a domain Load balancing – many IP addresses associated with a hostname

Distributed, Hierarchical Database DNS is implemented as a distributed hierarchical database Root DNS Servers com DNS servers org DNS servers edu DNS servers uni.edu DNS servers fsu.edu yahoo.com amazon.com pbs.org DNS is implemented as a distributed hierarchical database. This figure shows the structure of DNS service deployment. At the top we have a number of root DNS servers, which knows top level domain’s DNS server information. The Internet domain is partitioned into multiple top-level domains, for example, .edu is for educational institute, .com is for companies/business, and .org is normally for non-profit organizations. There are other top-level domains such as .gov for government, .mil for military, and .us for US, .cn for China. Below the top-level domain, there are more specific domain names for different organizations, for example yahoo.com for Yahoo!, fsu.edu for FSU.

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 org DNS servers edu DNS servers uni.edu DNS servers fsu.edu yahoo.com amazon.com pbs.org DNS is implemented as a distributed hierarchical database. This figure shows the structure of DNS service deployment. At the top we have a number of root DNS servers, which knows top level domain’s DNS server information. The Internet domain is partitioned into multiple top-level domains, for example, .edu is for educational institute, .com is for companies/business, and .org is normally for non-profit organizations. There are other top-level domains such as .gov for government, .mil for military, and .us for US, .cn for China. Below the top-level domain, there are more specific domain names for different organizations, for example yahoo.com for Yahoo!, fsu.edu for FSU.

Distributed, Hierarchical Database Below the top-level domain, there are more specific domain names for different organizations Root DNS Servers com DNS servers org DNS servers edu DNS servers uni.edu DNS servers fsu.edu yahoo.com amazon.com pbs.org DNS is implemented as a distributed hierarchical database. This figure shows the structure of DNS service deployment. At the top we have a number of root DNS servers, which knows top level domain’s DNS server information. The Internet domain is partitioned into multiple top-level domains, for example, .edu is for educational institute, .com is for companies/business, and .org is normally for non-profit organizations. There are other top-level domains such as .gov for government, .mil for military, and .us for US, .cn for China. Below the top-level domain, there are more specific domain names for different organizations, for example yahoo.com for Yahoo!, fsu.edu for FSU.

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 In a sense, you can consider Root DNS servers know everything (not accurate). If a local DNS server cannot resolve a domain host name, it will sent a request to a root DNS server. If the root DNS server also does not know the exact mapping. It will contact the corresponding authoritative name server, that is, the official DNS server that is responsible for the domain.

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

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.

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

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

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

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

RR format: (name, value, type, class, ttl) 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=CNAME name is alias name for some “canonical” (the real) name www.ibm.com is really servereast.backup2.ibm.com value is canonical name The only widely-used class is the one used by the Internet (denoted IN). Type=NS name is domain (e.g. foo.com) value is IP address of authoritative name server for this domain Type=MX value is name of mail server associated with name

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

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)