1. Host Name Resolution

2. Overview Name resolution Name resolution Addressing a host Addressing a host Host names Host names Host name resolution Host name resolution HOSTS file HOSTS file DNS DNS DNS Name space DNS Name space Zones Zones Registering a domain Registering a domain Demo Demo

3. Name Resolution TCP/IP can only address another host via it’s IP address TCP/IP can only address another host via it’s IP address IP addresses are difficult for humans to remember 128.187.60.28 IP addresses are difficult for humans to remember 128.187.60.28 Names are easy to remember www.isysguy.com Names are easy to remember www.isysguy.com Need for a way to map Names to IP addresses Need for a way to map Names to IP addresses

4. Addressing a Host UNIX UNIX IP address IP address Host name Host name Domain Domain Windows Windows IP address IP address Host names Host names Domain Names Domain Names NetBIOS Names NetBIOS Names

5. Host Names A host name is a friendly name used to identify a computer so users don’t have to remember IP addresses A host name is a friendly name used to identify a computer so users don’t have to remember IP addresses There is not a 1 to 1 correlation between host names and IP addresses There is not a 1 to 1 correlation between host names and IP addresses Entries are stored in the Hosts File or DNS (more on DNS later) Entries are stored in the Hosts File or DNS (more on DNS later) Use hostname utility to view Use hostname utility to view

6. Host Name Resolution Process of mapping a host name to an IP address Process of mapping a host name to an IP address Standard methods Standard methods Local host name Local host name “hosts” file “hosts” file Domain Name System (DNS) Domain Name System (DNS) Microsoft Specific Microsoft Specific NetBIOS name server NetBIOS name server Local broadcast Local broadcast LMHOSTS file LMHOSTS file

7. Resolving names with the HOSTS File Host B pings host A Host B pings host A B> ping A B> ping A Host B checks its own host name to see if it is “A” Host B checks its own host name to see if it is “A” If not, the HOSTS file is parsed looking for an entry for “A” If not, the HOSTS file is parsed looking for an entry for “A” If no entry is found other methods are used such as Domain Name System (DNS) If no entry is found other methods are used such as Domain Name System (DNS) If the host name is not resolved an error is returned If the host name is not resolved an error is returned

8. Hosts File C:\windows\system32\drivers\etc\hosts C:\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

9. Microsoft Methods of Resolving Host Names 1. Local host name 2. HOSTS file 3. DNS 4. NetBIOS name server (WINS) 5. Broadcast 6. LMHOSTS file

10. Domain Name System (DNS) A DNS server is a host that maintains a database of Fully Qualified Domain Names(FQDN). The DNS server resolves FQDNs to IP addresses A DNS server is a host that maintains a database of Fully Qualified Domain Names(FQDN). The DNS server resolves FQDNs to IP addresses If configured, a DNS server is queried to resolve host names not resolved via the HOSTS file If configured, a DNS server is queried to resolve host names not resolved via the HOSTS file If the primary DNS server is not available the next DNS server in the list is tried If the primary DNS server is not available the next DNS server in the list is tried

11. Domain Name System In the beginning, all host names were stored in a master file hosts.txt at Stanford Research Institute’s Network Information Center In the beginning, all host names were stored in a master file hosts.txt at Stanford Research Institute’s Network Information Center All hosts on ARPANET would periodically copy the hosts.txt file to their local machine All hosts on ARPANET would periodically copy the hosts.txt file to their local machine After a while this became a bottleneck on the network After a while this became a bottleneck on the network Hosts file is a flat name space Hosts file is a flat name space Problem was solved by introduction of DNS (RFCs 1034, 1035 & 1591) Problem was solved by introduction of DNS (RFCs 1034, 1035 & 1591)

12. How DNS works The Domain Name System is implemented as a client-server hierarchical distributed database The Domain Name System is implemented as a client-server hierarchical distributed database DNS has two main components DNS has two main components The Resolver The Resolver The DNS Server The DNS Server

13. Resolvers The DNS client is called a resolver The DNS client is called a resolver Responsible to formulate queries for the name servers to resolve a FQDN to an IP address Responsible to formulate queries for the name servers to resolve a FQDN to an IP address Often implemented as a standard library on system Often implemented as a standard library on system Unix gethostbyname() Unix gethostbyname() Can be called by any application e.g. your web browser Can be called by any application e.g. your web browser

14. Windows XP DNS Resolver Windows XP allows multiple DNS servers to be specified. The first sever is the preferred sever. Windows XP allows multiple DNS servers to be specified. The first sever is the preferred sever. 1. Check local cache. 2. IF not in cache, first server Queried and waits 1 second 3. If no response The first listed DNS server on each adapter is queried. The resolver waits 2 seconds for a response. 4. If no response the resolver sends the query to all DNS servers on all adapters and waits another 2 seconds. 5. If no response the query is sent on all adapters and waits 4 seconds 6. If no response the query is sent on all adapters and waits 8 seconds 7. If the resolver receives a positive response it adds the response to the local cache and terminates all remaining queries. 8. If no response, the resolver responds to the requesting application with a time-out. If a server is not heard from within 30 seconds that server is removed from the list of active servers. If a server is not heard from within 30 seconds that server is removed from the list of active servers.

15. DNS Resolver Cache Service Window has a DNS resolver cache service that caches DNS queries to speed further DNS queries Window has a DNS resolver cache service that caches DNS queries to speed further DNS queries Caches positive and negative responses Caches positive and negative responses Timeouts are configurable Timeouts are configurable Default Negative cache timeout is 300 seconds Default Negative cache timeout is 300 seconds Default Network Failure Cache timeout is 30 seconds Default Network Failure Cache timeout is 30 seconds

16. Windows XP DNS commands ipconfig /flushdns ipconfig /flushdns Clears local DNS cache Clears local DNS cache ipconfig /displaydns ipconfig /displaydns Shows local DNS resolutions in cache Shows local DNS resolutions in cache ipconfig /registerdns ipconfig /registerdns Forces adapters to re-register their hostname and IP address(es) with their name server. Only useful if DNS supports dynamic updates Forces adapters to re-register their hostname and IP address(es) with their name server. Only useful if DNS supports dynamic updates

17. Domain Name System Server Name servers Name servers Responsible for maintaining lists of host names and IP addresses for resolution Responsible for maintaining lists of host names and IP addresses for resolution Each server only maintains a small portion of the DNS name space Each server only maintains a small portion of the DNS name space Name servers may either resolve the query or forward the query to another server Name servers may either resolve the query or forward the query to another server Implementations Implementations Berkley Internet Name Daemon (BIND) Berkley Internet Name Daemon (BIND) Microsoft DNS server Microsoft DNS server others others

18. Name Space.eduorg byu ebay isysguy uscom Root level domain Top Level Domains lindstrom FQDN www.isysguy.com. www

19. Zones A DNS zone is a portion of the DNS database. A DNS zone is a portion of the DNS database. A zone is typically everything at the second level and below A zone is typically everything at the second level and below Zone of authority. The area of the DNS database that an organization is responsible for maintaining. Zone of authority. The area of the DNS database that an organization is responsible for maintaining.

20. Zone of Authority. com Root level domain students craig isysguy www mail Zone of Authority

21. Types of name servers Primary Primary Keeps the authoritive copy of the DNS database for its zone of authority Keeps the authoritive copy of the DNS database for its zone of authority Secondary Secondary Keeps a redundant copy of a zone Keeps a redundant copy of a zone Master Master Source of zone information for Secondaries Source of zone information for Secondaries Caching Caching Perform queries only. Do not contain zone information other than cached resolutions Perform queries only. Do not contain zone information other than cached resolutions

22. Name Resolution Your ISPs Name Server. (root) Name Server Edu. Name Server byu.edu. Name Sever DNS Client Recursive Query Iterative Queries 1 2 3 4 5 6 7 8 www.byu.edu.

23. Record Types Full list of record types http://www.iana.org/assignments/dns-parameters Full list of record types http://www.iana.org/assignments/dns-parameters http://www.iana.org/assignments/dns-parameters TypeMeaningContents AHost Address32-bit IP address CNAMECanonical NameCanonical Domain Name for an alias HINFOCPU & OSName of CPU and Operating System MINFOMailbox infoInformation about a mailbox or mail list MXMail Exchanger16-bit preference and name of host that acts as mail exchanger for the domain NSName ServerName of authoritative server for domain PTRPointerPointer from IP address to domain name SOAStart of AuthorityMultiple fields that specify which parts of the naming hierarchy a server implements TXTArbitrary TextUninterrupted string of ASCII text

24. Zone 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 @ NSns2.drycreek.us. ns2.drycreek.us. 3600A128.187.60.28 @ NSns1.drycreek.us. ; Zone records @ 3600A128.187.60.28 @ MX10mail.isysguy.com. bigo A128.187.60.18 craig 3600A128.187.60.25 ftp CNAMEwww.isysguy.com. ilh 3600A128.187.60.25 lab CNAMEwww.isyslab.com. mail 3600A128.187.60.28 msdnaa A128.187.60.110 portal A128.187.60.24 www 3600A128.187.60.28

25. Root Hints The ICANN (Internet Corporation for Assigned Names and Numbers) maintains and organizes the assignment of domain names and certifies domain registrars The ICANN (Internet Corporation for Assigned Names and Numbers) maintains and organizes the assignment of domain names and certifies domain registrars ICANN is a private not for profit organization that maintains the internet names and number systems. ICANN is a private not for profit organization that maintains the internet names and number systems. The registrars are responsible for maintaining the DNS root name servers The registrars are responsible for maintaining the DNS root name servers Every RFC compliant implementation of DNS must include a root-hints cache file Every RFC compliant implementation of DNS must include a root-hints cache file Contains a list of root name servers Contains a list of root name servers Can be found at ftp://rs.internic.net/domain/named.cache 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 root servers and querying the DNS for a resolution. The root servers will refer to the appropriate name servers. Each DNS server can find any FQDN by starting at the root servers and querying the DNS for a resolution. The root servers will refer to the appropriate name servers.

26. Testing with NSLookup NSLookup is a utility that queries a name server for name resolutions NSLookup is a utility that queries a name server for name resolutions nslookup FQDN nslookup FQDN Looks up the IP address for FQDN Looks up the IP address for FQDN nslookup nslookup Interactive mode for multiple queries Interactive mode for multiple queries exit to quit exit to quit ? For commands ? For commands

27. Troubleshooting For DNS to work there must be a path from the root servers to the lowest level domain. For DNS to work there must be a path from the root servers to the lowest level domain. A good resource to troubleshoot DNS problems is http://www.squish.net/dnscheck/ A good resource to troubleshoot DNS problems is http://www.squish.net/dnscheck/ http://www.squish.net/dnscheck/ Performs full traversal of DNS servers for target name Performs full traversal of DNS servers for target name

28. Registering a Domain All domains are controlled by a domain registration authority. All domains are controlled by a domain registration authority. Initially all.com,.edu, and.org domains were administered by Network Solutions Initially all.com,.edu, and.org domains were administered by Network Solutions Now there are many other authorized domain registrars such as godaddy.com Now there are many other authorized domain registrars such as godaddy.com When you register a domain you become authoritive for that zone When you register a domain you become authoritive for that zone Two DNS severs are required to register a domain Two DNS severs are required to register a domain

29. Dynamic DNS Most DNS records are statically mapped Most DNS records are statically mapped Dynamic IP addresses cause inconsistencies in DNS databases Dynamic IP addresses cause inconsistencies in DNS databases Dynamic DNS allows hosts to update their own host record in the DNS Dynamic DNS allows hosts to update their own host record in the DNS MS DNS allows dynamic updates for MS clients MS DNS allows dynamic updates for MS clients http://www.dyndns.org/ DNS that allows dynamic updates http://www.dyndns.org/ DNS that allows dynamic updates http://www.dyndns.org/

30. Installing and Administering a Microsoft DNS Install from Add-Remove Software Install from Add-Remove Software Manage with management console Manage with management console Demo Demo