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Page 1: Host Name Resolution. Overview Name resolution Name resolution Addressing a host Addressing a host Host names Host names Host name resolution Host name

Host Name ResolutionHost Name Resolution

Page 2: Host Name Resolution. Overview Name resolution Name resolution Addressing a host Addressing a host Host names Host names Host name resolution Host name

OverviewOverview

Name resolutionName resolution Addressing a hostAddressing a host

Host namesHost names Host name resolutionHost name resolution HOSTS fileHOSTS file DNSDNS DNS Name spaceDNS Name space ZonesZones

Registering a domainRegistering a domain DemoDemo

Page 3: Host Name Resolution. Overview Name resolution Name resolution Addressing a host Addressing a host Host names Host names Host name resolution Host name

Name ResolutionName Resolution

TCP/IP can only address another host via TCP/IP can only address another host via it’s IP addressit’s IP address

IP addresses are difficult for humans to IP addresses are difficult for humans to remember remember 128.187.60.28128.187.60.28

Names are easy to remember Names are easy to remember www.isysguy.comwww.isysguy.com

Need for a way to map Names to IP Need for a way to map Names to IP addressesaddresses

Page 4: Host Name Resolution. Overview Name resolution Name resolution Addressing a host Addressing a host Host names Host names Host name resolution Host name

Addressing a HostAddressing a Host

UNIXUNIX IP addressIP addressHost nameHost nameDomainDomain

WindowsWindows IP addressIP addressHost namesHost namesDomain NamesDomain NamesNetBIOS NamesNetBIOS Names

Page 5: Host Name Resolution. Overview Name resolution Name resolution Addressing a host Addressing a host Host names Host names Host name resolution Host name

Host NamesHost Names

A host name is a friendly name used to A host name is a friendly name used to identify a computer so users don’t have to identify a computer so users don’t have to remember IP addressesremember IP addresses

There is not a 1 to 1 correlation between There is not a 1 to 1 correlation between host names and IP addresseshost names and IP addresses

Entries are stored in the Hosts File or DNS Entries are stored in the Hosts File or DNS (more on DNS later)(more on DNS later)

Use Use hostnamehostname utility to view utility to view

Page 6: Host Name Resolution. Overview Name resolution Name resolution Addressing a host Addressing a host Host names Host names Host name resolution Host name

Host Name ResolutionHost Name Resolution

Process of mapping a host name to an IP Process of mapping a host name to an IP addressaddress

Standard methodsStandard methods Local host nameLocal host name ““hosts” filehosts” file Domain Name System (DNS)Domain Name System (DNS)

Microsoft SpecificMicrosoft Specific NetBIOS name serverNetBIOS name server Local broadcastLocal broadcast LMHOSTS fileLMHOSTS file

Page 7: Host Name Resolution. Overview Name resolution Name resolution Addressing a host Addressing a host Host names Host names Host name resolution Host name

Resolving names with the HOSTS FileResolving names with the HOSTS File

Host B pings host AHost B pings host A B> ping AB> ping A

Host B checks its own host name to see if it is Host B checks its own host name to see if it is “A”“A”

If not, the HOSTS file is parsed looking for an If not, the HOSTS file is parsed looking for an entry for “A”entry for “A”

If no entry is found other methods are used such If no entry is found other methods are used such as Domain Name System (DNS)as Domain Name System (DNS)

If the host name is not resolved an error is If the host name is not resolved an error is returnedreturned

Page 8: Host Name Resolution. Overview Name resolution Name resolution Addressing a host Addressing a host Host names Host names Host name resolution Host name

Hosts FileHosts FileC:\windows\system32\drivers\etc\hostsC:\windows\system32\drivers\etc\hosts

# Copyright (c) 1993-1999 Microsoft Corp.## This is a sample HOSTS file used by Microsoft TCP/IP for Windows.## This file contains the mappings of IP addresses to host names. Each# entry should be kept on an individual line. The IP address should# be placed in the first column followed by the corresponding host name.# The IP address and the host name should be separated by at least one# space.## Additionally, comments (such as these) may be inserted on individual# lines or following the machine name denoted by a '#' symbol.## For example:## 102.54.94.97 rhino.acme.com # source server# 38.25.63.10 x.acme.com # x client host

127.0.0.1 localhost

Page 9: Host Name Resolution. Overview Name resolution Name resolution Addressing a host Addressing a host Host names Host names Host name resolution Host name

Microsoft Methods of Resolving Microsoft Methods of Resolving Host NamesHost Names

1.1. Local host nameLocal host name

2.2. HOSTS fileHOSTS file

3.3. DNSDNS

4.4. NetBIOS name server (WINS)NetBIOS name server (WINS)

5.5. BroadcastBroadcast

6.6. LMHOSTS fileLMHOSTS file

Page 10: Host Name Resolution. Overview Name resolution Name resolution Addressing a host Addressing a host Host names Host names Host name resolution Host name

Domain Name System (DNS)Domain Name System (DNS)

A DNS server is a host that maintains a A DNS server is a host that maintains a database of Fully Qualified Domain database of Fully Qualified Domain Names(FQDN). The DNS server resolves Names(FQDN). The DNS server resolves FQDNs to IP addressesFQDNs to IP addresses

If configured, a DNS server is queried to If configured, a DNS server is queried to resolve host names not resolved via the resolve host names not resolved via the HOSTS fileHOSTS file

If the primary DNS server is not available If the primary DNS server is not available the next DNS server in the list is triedthe next DNS server in the list is tried

Page 11: Host Name Resolution. Overview Name resolution Name resolution Addressing a host Addressing a host Host names Host names Host name resolution Host name

Domain Name SystemDomain Name System

In the beginning, all host names were stored in a In the beginning, all host names were stored in a master file master file hosts.txthosts.txt at Stanford Research at Stanford Research Institute’s Network Information CenterInstitute’s Network Information Center

All hosts on ARPANET would periodically copy All hosts on ARPANET would periodically copy the hosts.txt file to their local machinethe hosts.txt file to their local machine

After a while this became a bottleneck on the After a while this became a bottleneck on the networknetwork

Hosts file is a flat name spaceHosts file is a flat name space Problem was solved by introduction of DNS Problem was solved by introduction of DNS

(RFCs 1034, 1035 & 1591)(RFCs 1034, 1035 & 1591)

Page 12: Host Name Resolution. Overview Name resolution Name resolution Addressing a host Addressing a host Host names Host names Host name resolution Host name

How DNS worksHow DNS worksThe Domain Name System is The Domain Name System is

implemented as a implemented as a client-server client-server hierarchical distributed databasehierarchical distributed database

DNS has two main componentsDNS has two main componentsThe ResolverThe ResolverThe DNS ServerThe DNS Server

Page 13: Host Name Resolution. Overview Name resolution Name resolution Addressing a host Addressing a host Host names Host names Host name resolution Host name

ResolversResolvers

The DNS client is called a resolverThe DNS client is called a resolverResponsible to formulate queries for the Responsible to formulate queries for the

name servers to resolve a FQDN to an IP name servers to resolve a FQDN to an IP addressaddress

Often implemented as a standard library Often implemented as a standard library on systemon systemUnix gethostbyname()Unix gethostbyname()

Can be called by any application e.g. your Can be called by any application e.g. your web browserweb browser

Page 14: Host Name Resolution. Overview Name resolution Name resolution Addressing a host Addressing a host Host names Host names Host name resolution Host name

Windows XP DNS ResolverWindows XP DNS Resolver Windows XP allows multiple DNS servers to be specified. The first Windows XP allows multiple DNS servers to be specified. The first

sever is the preferred sever.sever is the preferred sever.1.1. Check local cache.Check local cache.2.2. IF not in cache, first server Queried and waits 1 second IF not in cache, first server Queried and waits 1 second 3.3. If no response The first listed DNS server on each adapter is queried. If no response The first listed DNS server on each adapter is queried.

The resolver waits The resolver waits 22 seconds for a response. seconds for a response. 4.4. If no response the resolver sends the query to all DNS servers on all If no response the resolver sends the query to all DNS servers on all

adapters and waits another adapters and waits another 22 seconds. seconds. 5.5. If no response the query is sent on all adapters and waits If no response the query is sent on all adapters and waits 44 seconds seconds6.6. If no response the query is sent on all adapters and waits If no response the query is sent on all adapters and waits 88 seconds seconds7.7. If the resolver receives a positive response it adds the response to the If the resolver receives a positive response it adds the response to the

local cache and terminates all remaining queries.local cache and terminates all remaining queries.8.8. If no response, the resolver responds to the requesting application with If no response, the resolver responds to the requesting application with

a time-out.a time-out. If a server is not heard from within If a server is not heard from within 3030 seconds that server is seconds that server is

removed from the list of active servers.removed from the list of active servers.

Page 15: Host Name Resolution. Overview Name resolution Name resolution Addressing a host Addressing a host Host names Host names Host name resolution Host name

DNS Resolver Cache ServiceDNS Resolver Cache ServiceWindow has a Window has a DNS resolver cache service DNS resolver cache service

that caches DNS queries to speed further that caches DNS queries to speed further DNS queriesDNS queries

Caches positive and negative responsesCaches positive and negative responsesTimeouts are configurableTimeouts are configurableDefault Negative cache timeout is 300 Default Negative cache timeout is 300

secondssecondsDefault Network Failure Cache timeout is Default Network Failure Cache timeout is

30 seconds30 seconds

Page 16: Host Name Resolution. Overview Name resolution Name resolution Addressing a host Addressing a host Host names Host names Host name resolution Host name

Windows XP DNS commandsWindows XP DNS commands

ipconfig /flushdnsipconfig /flushdnsClears local DNS cacheClears local DNS cache

ipconfig /displaydnsipconfig /displaydnsShows local DNS resolutions in cacheShows local DNS resolutions in cache

ipconfig /registerdnsipconfig /registerdnsForces adapters to re-register their hostname Forces adapters to re-register their hostname

and IP address(es) with their name server. and IP address(es) with their name server. Only useful if DNS supports dynamic updatesOnly useful if DNS supports dynamic updates

Page 17: Host Name Resolution. Overview Name resolution Name resolution Addressing a host Addressing a host Host names Host names Host name resolution Host name

Domain Name System ServerDomain Name System Server

Name serversName servers Responsible for maintaining lists of host names and Responsible for maintaining lists of host names and

IP addresses for resolutionIP addresses for resolution Each server only maintains a small portion of the DNS Each server only maintains a small portion of the DNS

name spacename space Name servers may either resolve the query or forward Name servers may either resolve the query or forward

the query to another serverthe query to another server ImplementationsImplementations

Berkley Internet Name Daemon (BIND)Berkley Internet Name Daemon (BIND) Microsoft DNS serverMicrosoft DNS server othersothers

Page 18: Host Name Resolution. Overview Name resolution Name resolution Addressing a host Addressing a host Host names Host names Host name resolution Host name

Name SpaceName Space

.

eduorg

byuebayisysguy

us com

Root level domain

Top Level Domains

lindstrom

FQDN www.isysguy.com.

www

Page 19: Host Name Resolution. Overview Name resolution Name resolution Addressing a host Addressing a host Host names Host names Host name resolution Host name

ZonesZones

A DNS zone is a portion of the DNS A DNS zone is a portion of the DNS database. database.

A zone is typically everything at the A zone is typically everything at the second level and belowsecond level and below

Zone of authority. The area of the DNS Zone of authority. The area of the DNS database that an organization is database that an organization is responsible for maintaining.responsible for maintaining.

Page 20: Host Name Resolution. Overview Name resolution Name resolution Addressing a host Addressing a host Host names Host names Host name resolution Host name

Zone of Zone of AuthorityAuthority

.

com

Root level domain

students

craig

isysguy

www

mail

Zone of Authority

Page 21: Host Name Resolution. Overview Name resolution Name resolution Addressing a host Addressing a host Host names Host names Host name resolution Host name

Types of name serversTypes of name servers

PrimaryPrimary Keeps the authoritive copy of the DNS database for Keeps the authoritive copy of the DNS database for

its zone of authorityits zone of authority SecondarySecondary

Keeps a redundant copy of a zoneKeeps a redundant copy of a zone MasterMaster

Source of zone information for SecondariesSource of zone information for Secondaries CachingCaching

Perform queries only. Do not contain zone Perform queries only. Do not contain zone information other than cached resolutionsinformation other than cached resolutions

Page 22: Host Name Resolution. Overview Name resolution Name resolution Addressing a host Addressing a host Host names Host names Host name resolution Host name

Name ResolutionName Resolution

Your ISPsName Server . (root)

Name Server

Edu.Name Server

byu.edu.Name Sever

DNS Client

RecursiveQuery

IterativeQueries

11

2233

44

5566

7788

www.byu.edu.

Page 23: Host Name Resolution. Overview Name resolution Name resolution Addressing a host Addressing a host Host names Host names Host name resolution Host name

Record TypesRecord Types

Full list of record types Full list of record types http://www.iana.org/assignments/dns-parametershttp://www.iana.org/assignments/dns-parameters

Type Meaning Contents

A Host Address 32-bit IP address

CNAME Canonical Name Canonical Domain Name for an alias

HINFO CPU & OS Name of CPU and Operating System

MINFO Mailbox info Information about a mailbox or mail list

MX Mail Exchanger 16-bit preference and name of host that acts as mail exchanger for the domain

NS Name Server Name of authoritative server for domain

PTR Pointer Pointer from IP address to domain name

SOA Start of Authority Multiple fields that specify which parts of the naming hierarchy a server implements

TXT Arbitrary Text Uninterrupted string of ASCII text

Page 24: Host Name Resolution. Overview Name resolution Name resolution Addressing a host Addressing a host Host names Host names Host name resolution Host name

Zone FilesZone Files; Database file isysguy.com.dns for isysguy.com zone.; Zone version: 222@ IN SOA ns2.drycreek.us.

register.isysguy.com. ( 222 ; serial number 300 ; refresh 120 ; retry 600 ; expire 300 ) ; default TTL; Zone NS records@ NS ns2.drycreek.us.ns2.drycreek.us. 3600 A 128.187.60.28@ NS ns1.drycreek.us.; Zone records@ 3600 A 128.187.60.28@ MX 10 mail.isysguy.com.bigo A 128.187.60.18craig 3600 A 128.187.60.25ftp CNAME www.isysguy.com.ilh 3600 A 128.187.60.25lab CNAME www.isyslab.com.mail 3600 A 128.187.60.28msdnaa A 128.187.60.110portal A 128.187.60.24www 3600 A 128.187.60.28

Page 25: Host Name Resolution. Overview Name resolution Name resolution Addressing a host Addressing a host Host names Host names Host name resolution Host name

Root HintsRoot Hints The ICANN (Internet Corporation for Assigned Names The ICANN (Internet Corporation for Assigned Names

and Numbers) maintains and organizes the assignment and Numbers) maintains and organizes the assignment of domain names and certifies domain registrarsof domain names and certifies domain registrars ICANN is a private not for profit organization that maintains the ICANN is a private not for profit organization that maintains the

internet names and number systems.internet names and number systems. The registrars are responsible for maintaining the DNS The registrars are responsible for maintaining the DNS

root name serversroot name servers Every RFC compliant implementation of DNS must Every RFC compliant implementation of DNS must

include a root-hints cache fileinclude a root-hints cache file Contains a list of root name serversContains a list of root name servers Can be found at Can be found at ftp://rs.internic.net/domain/named.cacheftp://rs.internic.net/domain/named.cache

Each DNS server can find any FQDN by starting at the Each DNS server can find any FQDN by starting at the root servers and querying the DNS for a resolution. The root servers and querying the DNS for a resolution. The root servers will refer to the appropriate name servers.root servers will refer to the appropriate name servers.

Page 26: Host Name Resolution. Overview Name resolution Name resolution Addressing a host Addressing a host Host names Host names Host name resolution Host name

Testing with NSLookupTesting with NSLookup

NSLookup is a utility that queries a name NSLookup is a utility that queries a name server for name resolutionsserver for name resolutions

nslookup FQDNnslookup FQDNLooks up the IP address for FQDNLooks up the IP address for FQDN

nslookupnslookup Interactive mode for multiple queriesInteractive mode for multiple queriesexitexit to quit to quit?? For commands For commands

Page 27: Host Name Resolution. Overview Name resolution Name resolution Addressing a host Addressing a host Host names Host names Host name resolution Host name

TroubleshootingTroubleshooting

For DNS to work there must be a path For DNS to work there must be a path from the root servers to the lowest level from the root servers to the lowest level domain.domain.

A good resource to troubleshoot DNS A good resource to troubleshoot DNS problems is problems is http://www.squish.net/dnscheck/http://www.squish.net/dnscheck/ Performs full traversal of DNS servers for Performs full traversal of DNS servers for

target nametarget name

Page 28: Host Name Resolution. Overview Name resolution Name resolution Addressing a host Addressing a host Host names Host names Host name resolution Host name

Registering a DomainRegistering a Domain

All domains are controlled by a domain All domains are controlled by a domain registration authority.registration authority.

Initially all .com, .edu, and .org domains were Initially all .com, .edu, and .org domains were administered by Network Solutionsadministered by Network Solutions

Now there are many other authorized domain Now there are many other authorized domain registrars such as godaddy.comregistrars such as godaddy.com

When you register a domain you become When you register a domain you become authoritive for that zoneauthoritive for that zone

Two DNS severs are required to register a Two DNS severs are required to register a domaindomain

Page 29: Host Name Resolution. Overview Name resolution Name resolution Addressing a host Addressing a host Host names Host names Host name resolution Host name

Dynamic DNSDynamic DNS

Most DNS records are statically mappedMost DNS records are statically mappedDynamic IP addresses cause Dynamic IP addresses cause

inconsistencies in DNS databasesinconsistencies in DNS databasesDynamic DNS allows hosts to update their Dynamic DNS allows hosts to update their

own host record in the DNSown host record in the DNSMS DNS allows dynamic updates for MS MS DNS allows dynamic updates for MS

clientsclientshttp://www.dyndns.org/http://www.dyndns.org/ DNS that allows DNS that allows

dynamic updatesdynamic updates

Page 30: Host Name Resolution. Overview Name resolution Name resolution Addressing a host Addressing a host Host names Host names Host name resolution Host name

Installing and Administering a Installing and Administering a Microsoft DNSMicrosoft DNS

Install from Add-Remove SoftwareInstall from Add-Remove SoftwareManage with management consoleManage with management consoleDemoDemo