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

2. 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

3. 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.

4. 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.

5. 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.

6. 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.

7. 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

8. 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.

9. 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

10. 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?

11. 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

12. 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

13. 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
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

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

15. 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)