1. Guide to TCP/IP Fourth Edition
Chapter 8:
Name Resolution on IP Networks

2. Objectives
Describe the characteristics of the various name resolution protocols, such as WINS, DNS, and LLMNR
Explain how name resolution works in IPv4 networks, including the DNS database structure, the DNS namespace, DNS database records, the delegation of DNS authority, and the different types of DNS servers, and explain how name servers work
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3. Objectives (cont'd.)
Describe how name resolution works on IPv6 networks, including the use of AAAA records, how forward and reverse mapping works, the use of source and destination address selection, how rules are organized by the source and destination address algorithms, and the end-to-end address selection process
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4. Objectives (cont'd.)
Explain how name resolution is supported in Windows operating systems, including how host files are used, the function of the DNS server service and DNS dynamic updates, how Windows manages source and destination address selection, LLMNR support, working with ipv6-literal.net names, and the use of the peer name resolution protocol
Describe the common sources for name resolution failure and use common name resolution troubleshooting tools such as NBTSTAT, NETSTAT, AND NSLOOKUP
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5. Understanding Name Resolution Fundamentals
Process by which a computer maps the human-readable names to the numeric addresses
Before a network device can send an IP packet
Name-to-address resolution must occur
Methods
Consult a name-to-address file or table on hard drive
Send a network broadcast requesting the destination computer’s IP address
Contact a server that maintains a database of name-to-address entries
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6. Network Name Resolution Protocols
Procedures that govern the rules and conventions used in manually and dynamically providing for name resolution systems in a networked environment
Provide the definitions and mechanisms involved in client and server applications that are used in name resolution
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7. NetBIOS over TCP/IP
Allow Windows 2000/XP computers to talk with devices running older Windows OSs
Maintains a list of unique names assigned to network resources
Two serious drawbacks
Does not have a network component to its namespace
Constantly sends short messages for a wide variety of purposes
Defined by RFC 1001
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8. WINS Windows Internet Name Service (WINS)
Service that resolves NetBIOS names to IP addresses in routed networks
Use of a WINS server on a network automates dynamic name resolution
WINS servers rely on direct communications (unicasts) between themselves and the clients
WINS-enabled clients can be configured to use more than one WINS server
Support a special name registration regime called burst mode
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9. DNS Domain Name System (DNS)
Described by RFCs 1034 and 1035
System used for naming computers and network services
Uses a hierarchical structure for organizing those objects into domains
RFC 3596 describes the DNS extensions for IPv6
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10. LLMNR Link-Local Multicast Name Resolution (LLMNR)
Defined by RFC 4795
Protocol based on the DNS packet format
Allows IPv4 and IPv6 network nodes to perform name resolution for other devices connected to the same local link
Usage limited to a single network segment
Ideal for smaller networks and other environments
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11. LLMNR (cont’d.)
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12. Name Resolution in IPv4 Networks
RFCs 882 and 883
Original RFCs for DNS
Created by Paul Mockapetris (also created JEEVES)
BIND (Berkeley Internet Name Domain)
Written by Kevin Dunlap in 1988
Database segments
Include only a portion of the overall namespace that DNS can access for its clients
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13. Name Resolution in IPv4 Networks (cont'd.)
DNS combines the following virtues
Allows local control over domain name database segments
Data from all database segments is available everywhere
Database information is robust and highly available
DNS
One of the most effective uses of distributed database technology in the world today
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14. DNS Database Structure
Mirrors structure of the domain namespace itself
Top-level domains in the U.S.
.com
.edu
.gov
.mil
.net
.org
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15. DNS Database Structure (cont’d.)
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16. The DNS Namespace DNS Domains (such as ibm.com) Any valid domain name
Arbitrarily partitions tree and creates subtrees for database information
Domains (such as ibm.com)
Can be broken into subdomains (such as clearlake.ibm.com)
Any valid domain name
Ultimately resides within some specific DNS database
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17. DNS Database Records Resource records
Stores data associated with domain names, address records, and other specific data
Most commonly used types
Address (A) record
Canonical name (CNAME) record
Host information (HINFO) record
Mail exchange (MX) record
Name server (NS) record
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18. Delegating DNS Authority
Permits database record for primary DNS server to delegate authority to DNS servers lower in domain namespace
Once authority is delegated
Database for name server includes NS records that point to name servers
Organization of global DNS database
Designed to make it quick and easy for name servers to point to other name servers
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19. Types of DNS Servers Primary master name DNS server Primary master
Where the primary DNS database files for the domain(s) or subdomain(s) reside
Primary master
Distinguished from other name servers for a domain
For any DNS zone
There can be only one primary master name server
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20. Secondary DNS Server
Gets its data for the zone from the master server for that zone
Zone data on a secondary server
Always originates from a primary server
Zone transfer
Secondary DNS server gets data for the zone from the master server for that zone
Secondary, or slave, DNS servers
Provide a back-up copy of the domain database for a specific zone
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21. Caching Servers Store recently accessed DNS records from other domains
Caching-only server
Speeds access to specific domain names by storing a copy of the lookup data locally
Size and Internet access volume
Factors that determine if an organization implements separate caching-only servers
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22. How Domain Name Servers Work
A TCP/IP client
Usually some application or service that encounters a domain name for which it needs an IP address
Servers
Queried in the order in which they appear in TCP/IP configuration files (from top down)
DNS servers
Handle real name resolution
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23. Recursive Query Used by DNS resolvers to:
Delegate the first DNS server that they contact to go out and find the necessary address translation
In the grand DNS server hierarchy
Any DNS server can issue iterative queries
Only a DNS client or a root server can issue recursive queries
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24. Iterative or Non-Recursive Queries
Issued when one DNS server receives a recursive request
Do not cause other queries to be issued
Reason some recursive name queries involve a root server
Root server always knows how to find whatever DNS server is authoritative for the domain
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25. Importance of DNS Caching
All data in a DNS cache
Has an expiration value
DNS servers
Cache name and address pairs for addresses they resolved
Keep information about name requests that result in error messages
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26. DNS Configuration Files and Resource Record Formats
domain.dns
The files that map host names to addresses
addr.in-addr.arpa.dns
Files that map addresses to domain names for reverse lookups
Every DNS zone file must contain:
SOA and NS records
Records about host names or addresses in that zone
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27. Start of Authority Record
Identifies the current name server as the best source of information for data in its zone
Both secondary and primary name servers:
Can designate themselves as authoritative in their own SOA records
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28. Address and Canonical Name Records
DNS, by default
Accesses only the first IP address for a host when multiple entries for a single domain name are defined
DNS round robin load balancing
Permits a DNS server to keep track of which IP addresses it has provided for a specific translation
Rotates the IP addresses within the list of addresses available
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29. Mapping Addresses to Names
Records in the db.addr file
Provided to support reverse DNS lookups
Reverse address lookups
Used primarily to determine if IP address that user presents matches originating domain name
Classful
File structure of reverse DNS lookups
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30. Name Resolution in IPv6 Networks
DNS continues to operate in IPv6 environments
Basic mechanisms of DNS continue unaltered
Task of name resolution is made more complex
IPv6 offers backup service that can stand in for DNS
LLMNR protocol uses the same message format that conventional DNS uses
But runs on different ports
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31. DNS in IPv6 RFC 1886 (obsoleted by RFC 3596) AAAA record
Defined the DNS extensions supporting IPv6
AAAA record
Developed to accommodate larger IPv6 addresses
ip6.int (substituted with ip6.arpa)
Created to support IPv6 reverse-mapping domain
Forward mapping
Involves sending a request to a remote host with its domain name and requesting its IP address
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32. DNS in IPv6 (cont’d.)
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33. Source and Destination Address Selection
Specified by RFC 3484
IPv6 addressing
Allows multiple unicast addresses to be assigned to a computer’s network interface
Addresses can have different reachability scopes
For an IPv6 node with multiple addresses
Multiple IPv6 addresses are returned in the DNS Name Query Response message
Source and destination address must be matched to each other for both address scope and purpose
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34. Source and Destination Address Selection (cont’d.)
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35. Source Address Selection Algorithm
Eight rules processed sequentially
Prefer the source address that equals the destination address
Prefer the source address that has the appropriate scope for D-Addr
Prefer addresses that are not depreciated
Prefer a home address (for IPv6 mobile)
For routers, prefer the source address that is assigned to the next-hop interface pointing at D-Addr
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36. Source Address Selection Algorithm (cont’d.)
Eight rules processed sequentially (cont’d.)
Prefer the source address that has the same label in the prefix policy table as D-Addr
Prefer the source address that uses a public address over the source address that uses a temporary address
Prefer the source address that has the longest matching prefix with D-Addr
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37. Destination Address Selection Algorithm
Ten rules processed sequentially
Prefer a destination that is reachable over one that is not
Prefer the destination that matches the scope of the source address
Prefer a destination address with a source address that is not deprecated
Prefer a destination with a source address that is a home address (for IPv6 mobile)
Prefer a destination address that has the same label from the prefix policy table as its source address
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38. Destination Address Selection Algorithm (cont’d.)
Ten rules processed sequentially (cont’d.)
Prefer a destination address that has the highest precedence in the prefix policy table
Prefer a native IPv6 destination over an IPv6 transition technology destination
Prefer a destination address with the smallest scope
Prefer a destination address possessing the longest matching prefix length with its source address
Otherwise, leave the order unchanged
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39. Using Address Selection
Address selection from end to end
Operator on Node1 queries remote host for its configured addresses
Remote host replies with multiple addresses
Node1 uses source address selection algorithm
Node1 uses destination address selection algorithm
Application on Node1 is provided with the ordered destination addresses and related source addresses
Application attempts to use the source/destination address pairs until successfully establishing communication
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40. Using Address Selection (cont’d.)
Changing the destination address scope preference
Destination address selection algorithm rule 8
Gives preference to destination addresses with the smallest scope
You may want to change the policy table to reverse the default preference
Use Windows command netsh interface ipv6 set prefixpolicy
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41. Name Resolution Support in Windows Operating Systems
NetBIOS and WINS
Historical and native name resolution methods for Windows
Have been made obsolete by the ubiquitous presence of DNS
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42. Hosts File Stored locally on the Windows computer
Must be updated manually
On Windows 7
You can locate the hosts file at C:\Windows\System32\drivers\etc
Can map both IPv4 and IPv6 addresses to computer host names
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43. Hosts File (cont’d.)
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44. DNS Resolver Responsible for initiating and sequencing DNS queries
Two types of queries:
Nonrecursive query to a DNS server
Recursive query to a DNS server
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45. Client Side of DNS Resolvers An address request
Issue requests for service, called name queries or address requests, to domain name servers
An address request
Seeks to resolve a domain name to a corresponding numeric IP address
Name query (inverse DNS query)
Seeks to resolve an address to a domain name
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46. DNS Server Service
Older Windows servers such as Windows NT relied on NetBIOS and WINS
Windows Server 2003 and Windows Server 2008 naturally support DNS
Windows servers also support stub zones
Copies of a zone that contains only the resource records
DNS server service on Windows supports incremental zone transfers
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47. DNS Dynamic Update Dynamic DNS (DDNS) Steps
Allow automatic machine registration and record updating on DNS servers
Steps
Client sends DNS query to locate an authoritative DNS server
Local name server responds
Client attempts to dynamically update the authoritative DNS server
Authoritative DNS server replies with a success or failure message
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48. Source and Destination Address Selection
When a Windows computer has more than one IP address configured for a network interface
TCP/IP stack will choose one unicast address to use as the computer’s source IP address
In compliance with the standards set in RFC 3484
Windows Vista and Windows 7 computers support IPv6 destination address selection
As defined by RFC 3484
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49. LLMNR Support
Supported and enabled by default on Windows Vista/7/Server 2008
On client computers, it will attempt to search for a domain controller (DC) on the domain
Can be disabled either using:
Group Policy in AD domains
Registry for individual computers
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50. Working with ipv6-literal.net Names
Supported by Windows Vista/7/Server 2008
Can be used by applications and services that are unable to recognize the syntax of IPv6 addresses
Specified by RFC 2732
Obsoleted by RFC 3986
Provides generic syntax for Uniform Resource Identifiers (URIs) and addresses
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51. Peer Name Resolution Protocol
Microsoft Windows IPv6 proprietary peer-to-peer name resolution system
First developed for Windows XP SP2
Updated for Windows Vista
Peer name groups
Global cloud
Link-local cloud
Site-specific cloud
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52. Peer Name Resolution Protocol (cont’d.)
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53. Troubleshooting Name Resolution Problems and Failures
DNS shortcomings
Database updates usually require a qualified administrator or special purpose tools
Propagation delay
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54. Common Sources of Failure
Two common sources of name resolution failure
Negative response to a query
Positive response to a query with an incorrect name
Common causes of a negative result
Incorrect domain suffix appended to a queried name
Incorrect IP configuration on a client or server
Querying a name server that is not authoritative
Inability to connect to the correct name server
Causes for positive but incorrect name server
Incorrect data stored in name server’s resolver cache
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55. Tools for Troubleshooting NetBIOS and WINS Problems
Tools that are useful for diagnosing and troubleshooting TCP/IP networks in general
Also useful in maintaining NetBIOS and WINS services
Ping
Excellent way to test connectivity
Traceroute and Netstat
Useful diagnostic tools
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56. Tools for Troubleshooting DNS Problems
Process of troubleshooting DNS for IPv4 and IPv6 is essentially the same
Differences
Knowing how to specify an IPv6 name server
Knowing how to format forward and reverse mappings for each IP version
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57. Nbtstat Command-line program that returns statistics on NetBIOS
A fast way to:
Check the status of a particular NetBIOS host
Get a quick snapshot of NetBIOS name resolution activity on the local network segment
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58. Netstat
Shows active TCP connections, listening ports, Ethernet statistics, IPv4 statistics, and IPv6 statistics
Available on Windows, UNIX, and UNIX-like computers
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59. Nslookup Supported by Windows and UNIX
Provides access to all kinds of DNS information
Essential tool for testing, when configuring or troubleshooting a DNS server
Syntax
nslookup domain-name [name-server]
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60. Nslookup (cont’d.)
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62. Using Nslookup set OPTION command ls –a or ls –d
Used to examine specific types of resource records
ls –a or ls –d
Used to extract information from certain well-known name servers
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63. Nslookup (cont’d.)
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64. Nslookup (cont’d.)
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65. Nslookup and IPv6
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66. Summary The Domain Name System Impetus for DNS DNS name servers
Provides key address resolution service that makes today’s Internet possible
Impetus for DNS
Arose from difficulty of maintaining static HOSTS files for computers on the ARPANET
DNS name servers
Come in multiple varieties
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67. Summary (cont'd.) DNS DNS databases DNS clients
Maintains its data on a large collection of name servers around the Internet
DNS databases
Consist of a collection of resource records (RRs)
DNS clients
Rely on resolver to interact with available DNS server for name resolution services
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68. Summary (cont'd.) DNS packet structures
Incorporate type information that identifies the kind of RR being carried
IPv6 networks use DNS extensions but must be able to work in hybrid IPv4–IPv6 environments
IPv6 source and destination address selection is managed by algorithms that use a set of rules
Windows operating system supports a variety of name resolution technologies
There are a number of common causes of name resolution problems and failures
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