1 domain name system (dns). 2 dns: domain name system internet hosts: – ip address (32 bit) - used...
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Domain Name System (DNS)
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DNS: Domain Name System
Internet hosts:– IP address (32 bit) - used for addressing
datagrams– “name”, e.g., www.yahoo.com - used by humans
DNS: provides translation between host name and IP address– distributed database implemented in hierarchy of
many name servers– Distributed for scalability & reliability
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DNS Services
Hostname to IP address translation Host aliasing
– Canonical and alias names
Mail server aliasing Load distribution
– Replicated Web servers: set of IP addresses for one canonical name
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Names are valuable!
A New York Times article from August 22, 2000
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Growth of DNS registrations
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Names Hierarchy in DNS
hostname = “dot” separated concatenation of domain names along path toward the root
– unc.edu– cs.unc.edu– classroom.cs.unc.edu
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requesting hostcis.poly.edu
gaia.cs.umass.edu
root DNS server
local DNS serverdns.poly.edu
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authoritative DNS serverdns.cs.umass.edu
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TLD DNS server
DNS Infrastructure
Host at cis.poly.edu wants IP address for gaia.cs.umass.edu
Infrastructure:– Client resolver– Local DNS server– Authoritative DNS
Server– Root DNS Server– Top-Level Domain
DNS Server
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Distributed, Hierarchical Database
Root servers and TLD servers typically do not contain hostname to IP mappings; they contain mappings for locating authoritative servers.
Root DNS Servers
com DNS servers ca DNS servers edu DNS servers
poly.eduDNS servers
umass.eduDNS servers
yahoo.comDNS servers
amazon.comDNS servers
ucalgary.caDNS servers
TLDServers
usask.caDNS servers
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DNS: Root name servers
contacted by local name server that can not resolve name
root name server:– contacts authoritative name server if name mapping
not known– gets mapping– returns mapping to local name server
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DNS: Root name servers
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TLD and Authoritative Servers
Top-level domain (TLD) servers: responsible for com, org, net, edu, etc, and all top-level country domains uk, fr, ca, jp.– Network solutions maintains servers for com TLD– Educause for edu 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
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TLD servers
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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.– Reduces lookup latency for commonly searched
hostnames
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requesting hostcis.poly.edu
gaia.cs.umass.edu
root DNS server
local DNS serverdns.poly.edu
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2
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authoritative DNS serverdns.cs.umass.edu
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TLD DNS server
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Recursive queries
recursive query: puts burden of name
resolution on contacted name server
heavy load?
iterated query: contacted server
replies with name of server to contact
“I don’t know this name, but ask this server”
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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
update/notify mechanisms under design by IETF– RFC 2136– http://www.ietf.org/html.charters/dnsind-charter.html
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DNS records
DNS: distributed db storing resource records (RR)
Type=NS– name is domain (e.g.
foo.com)– value is IP address of
authoritative name server for this domain
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
Type=MX– value is name of mailserver
associated with name
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DNS protocol, messages
DNS protocol : query and reply messages, both with same message format
msg header identification: 16 bit # for
query, reply to query uses same #
flags:– query or reply– recursion desired – recursion available– reply is authoritative
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DNS protocol, messages
Name, type fields for a query
RRs in reponseto query
records forauthoritative servers
additional “helpful”info that may be used
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Inserting records into DNS
Example: just created startup “Network Utopia” Register name networkuptopia.com at a registrar (e.g.,
Network Solutions)– Need to provide registrar with names and IP addresses of
your authoritative name server (primary and secondary)– Registrar inserts two RRs into the com TLD server:
(networkutopia.com, dns1.networkutopia.com, NS)(dns1.networkutopia.com, 212.212.212.1, A)
Put in authoritative server Type A record for www.networkuptopia.com and Type MX record for networkutopia.com
How do people get the IP address of your Web site?
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A Real DNS Answer
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A Real DNS Answer (cont.)
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A Real DNS Answer (cont.)
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Example (cont.)
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Another Example
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Summary
F gets 270,000,000+ hits per day
– Other servers have comparable load
The Verisign TLD servers answer 5,000,000,000 queries per day
Clearly the DNS would collapse without:
– Hierarchy– Distributed processing– Caching
If DNS fails, Internet services stop working!