Chabot College ELEC 99.08 Name Resolution

Name Resolution Topics Concepts name to IP address mapping “fully qualified” domain names Methods host tables domain name services (DNS) Commands ip host ip domain-lookup ip name-server ip domain-name show hosts

Name Resolution Purposes 1. Convenience of humans. Which is easier to remember? 198.133.219.25 www.cisco.com

Name Resolution Purposes 2. Ability to move machines with no disruption of access. What happens when we move a server to a different network or subnet? How do users find it? 198.133.219.25 becomes 209.42.167.99 but www.cisco.com remains www.cisco.com

Fully Qualified Domain Name Unique in the world - no duplicates. Example: library.stanford.edu Heirarchical structure: Top level domain (e.g. edu) Domain (e.g. stanford) Host (e.g. library)

Fully Qualified Domain Name chabotde.clpccd.cc.ca.us Host name Domain name Top-Level Domain Sub-Domains

Fully Qualified Domain Name For our labs, we’ll assume this domain name has been assigned: lab.edu Our router’s fully qualified domain names will therefore be: fremont.lab.edu hayward.lab.edu oakland.lab.edu

Default Domain Name TCP/IP configurations include a default domain to be appended to any unqualified hostname (e.g. hayward). For our labs, we’ll use this default domain: lab.edu Then when you ping fremont, TCP/IP will actually ping fremont.lab.edu Saves a lot of typing!

Resolving Names to Addresses Host Tables Domain Name Services

Host Table Created manually on each router or computer Maps names to IP addresses fremont.lab.edu 192.168.1.1 hayward.lab.edu 192.168.2.1 Provides name resolution only on the host on which the table was created. Fast, low overhead, cached in RAM.

Host Table Commands ip host [name] [address] May include up to 8 addresses per name Global config mode oak(config)#ip host fremont 192.168.1.1 10.1.0.1 oak(config)#ip host hayward 192.168.2.1 10.2.0.1 oak(config)#ip host oakland 192.168.3.1 10.2.0.2

Domain Name Services (DNS) A network service Runs on one or two servers that are used by all hosts on the network Maps names to IP addresses fremont.lab.edu 192.168.1.1 hayward.lab.edu 192.168.2.1 Used throughout the global Internet

DNS - Special Considerations Billions of IP addresses in use, many mapped to a name. (We could never fit them in a hosts file.) IP addresses and domain names change daily, and new ones are assigned daily. (We could never keep hosts files up to date.) DNS is like a global phone book that must be kept up-to-date very fast in its response

How DNS Works Local host sends DNS query to the local name server defined in the host’s TCP/IP config. (UDP port 53) If the local name server knows the address of the target name, it sends back a reply. Otherwise, it forwards the query to one of the 13 root name servers on the Internet. The root server then returns the IP address of the primary name server responsible for the domain being requested. The local name server then begins a recursive process, contacting the primary name server and then the authoritative name server for the target domain. The local name server then fills the local host’s request for the target ip address. The host then uses the supplied IP address to contact its target.

How DNS Works - Example To find the IP address of technet.micrsoft.com... your PC asks the Chabot name server, which then performs the following steps: Asks one of the 13 root name servers on the Internet for the addresses of the com name servers Asks one of these com servers for the address of the microsoft.com name server Ask the microsoft.com name server for the address of the host: technet.microsoft.com. The chabot name server then replies to your PC with the target IP address

How DNS Works

DNS Commands no ip domain-lookup Turns off the use of DNS to look up the ip address mapped to a hostname Used in our practice lab to prevent delays due to mistyped commands Global config mode oak(config)#no ip domain-lookup oak(config)#

DNS Commands ip domain-lookup Turns on the use of DNS to look up the ip address mapped to a hostname This is the default for all routers, so this command is only needed if lookup was previously turned off with no ip domain-lookup Global config mode oak(config)#ip domain-lookup oak(config)#

DNS Commands ip name-server xxx.xxx.xxx.xxx Sets the address for the DNS server to be used All DNS queries will go to this server Up to six server addresses may be listed, separated by spaces Global config mode oak(config)#ip name-server 172.28.118.12 oak(config)#

DNS Commands ip domain-name [name] Sets the default domain to be appended to “unqualified” names Saves repetitive typing of local domain Global config mode oak(config)#ip domain-name lab.edu oak(config)#

DNS Commands Typical configuration Global config mode oak(config)#ip domain-lookup oak(config)#ip name-server 172.28.118.12 oak(config)#ip domain-name lab.edu oak(config)# On a Windows PC, the same settings are made in the Network Control Panel, TCP/IP properties.

Show Hosts Commands show hosts Lists the host-address mappings cached in RAM privileged EXEC mode router#show hosts Default domain is CISCO.COM Name/address lookup uses domain service Name servers are 192.31.7.10 131.108.4.12 Host Flag Age Type Address(es) SLAG.CISCO.COM (perm, OK) 1 IP 131.108.4.10 CHAR.CISCO.COM (temp, OK) 8 IP 192.31.7.50 CHAOS.CISCO.COM (temp, OK) 8 IP 131.108.1.115 DIRT.CISCO.COM (temp, EX) 8 IP 131.108.1.111 DUSTBIN.CISCO.COM (temp, EX) 0 IP 131.108.1.27 DREGS.CISCO.COM (temp, EX) 24 IP 131.108.1.30

Show Hosts Commands Flags: perm - address resolved by host table temp - address resolved by DNS router#show hosts Default domain is CISCO.COM Name/address lookup uses domain service Name servers are 192.31.7.10 131.108.4.12 Host Flag Age Type Address(es) SLAG.CISCO.COM (perm, OK) 1 IP 131.108.4.10 CHAR.CISCO.COM (temp, OK) 8 IP 192.31.7.50 CHAOS.CISCO.COM (temp, OK) 8 IP 131.108.1.115 DIRT.CISCO.COM (temp, OK) 8 IP 131.108.1.111 DUSTBIN.CISCO.COM (temp, EX) 0 IP 131.108.1.27 DREGS.CISCO.COM (temp, EX) 24 IP 131.108.1.30

DNS Server Records A few DNS record types A (host address) CNAME (alias - cannonical name) PTR (domain name pointer - for reverse lookup) MX (mail exchange - mail server for domain) CNAME records mean that a host at one IP address can have many names.

Let’s try it! Point your browser at: http://www.clpccd.cc.ca.us http://porter.clpccd.cc.ca.us For the host at 209.129.156.3, Chabot’s DNS servers list porter as the host name (A-record) www as a name alias (CNAME-record)

Let’s try it! Point your browser at: http://porter On your PC, the default domain is set to clpccd.cc.ca.us Therefore, when you browse to porter, you reach the ip address of porter.clpccd.cc.ca.us

Let’s try it! At the DOS prompt: ping davis On your PC, the default domain is set to clpccd.cc.ca.us Therefore, when you ping davis, you reach the ip address of davis.clpccd.cc.ca.us

Lookup Sequence 1. Check cache in RAM. If entry not found, 2. Check hosts table. If entry not found, 3. Query local DNS server. If entry not found, refer to root server and then to authoritative DNS server for the target domain. Therefore, host table entries will be used first to satisfy a query.

Site Blocking - a simple approach... What does this address mean? 127.0.0.1 “loopback” address - your own NIC

Site Blocking - a simple approach... Suppose I wanted to prevent my browser from diplaying ads from ad1.doubleclick.net which DNS resolves to 206.65.183.110. Where would my browser go if my PC’s host table contained this entry? 127.0.0.1 ad1.doubleclick.net Take a look at this hosts file, which is designed to support ad blocking. For more info, point your browswer at http://www.accs-net.com/hosts/index.html