Dave’s DNS Show

Read at your own risk

Domain Name Service

• Maps IP addresses to more human readable domain names

• Every domain name ultimately resolves to an IP address that the IP network can then route correctly for delivery.

Domain Names

www.openwave.com• Top Level domain name (also called first level) consists

of several hundred – com, edu, gov, etc.

• Second-level domain name is a huge list – yahoo, google, openwave, etc.

• There can be up to 127 levels, but in practice only 4 are ever used. – In the example ads.bbc.co.uk, “bbc” is a third-level domain

name

• Left most term is the hostname

whois Database

• A central database is required to ensure the uniqueness of domain names. The whois database is the master repository for all domain names.

• The whois database is maintained by Network Solutions• Registrars (like your ISP) work with Network Solutions to

add/modify/delete entries from the whois database.• The contents of the whois database are read twice a day

by the DNS database system.

DNS is a distributed DB

• While the whois database is a central repository for all of the domain names, the information about each domain is distributed.

• Every domain has an associated domain name server (DNS).

DNS Resolution example1. Client makes a request to the primary DNS server to resolve www.openwave.com.

The primary DNS server IP address is configured into the client (the Windows IP stack, for example).

2. If at any hop, a DNS server has cached an IP value for the queried hostname, it returns it. Let’s assume in this cases there are no cached values.

3. The primary DNS server makes a request to the Root Server for www.openwave.com. Root servers are distributed and know the IP addresses of all the name servers that handle top level (.com, .gov, etc.) domains. The list of all the Root Servers (and their IP addresses) are configured into every DNS server.

4. The Root Server returns the IP address of a DNS server for the particular Top Level domain. In this case it returns the IP address of a DNS server handling the .com Top Level domain.

5. The primary DNS server now queries the .com DNS server for www.openwave.com. The .com DNS server knows the DNS servers for each of the second level domains, and returns the one for openwave.com.

6. The primary DNS server then makes a request to the DNS server for openwave.com for www.openwave.com, and gets back the desired IP address.

7. The primary DNS server returns the IP address for www.openwave.com to the client.

Client Primary Root

Top Level .com

2nd Level openwave.com

www.openwave.com

www.openwave.com

.com server

www.openwave.com

openwave.com server

www.openwave.com

12.25.201.26

12.25.201.26

Key Attributes of DNS System

• Redundancy– At any level there are multiple DNS servers

• Caching– Each DNS server caches the results of a

query

• Time to Live (TTL)– Each DNS result includes a TTL directive to

determine how long it should be cached.

How are Domains Created?

• A domain is created via a Registrar and placed into the whois database. The DNS servers for the new domain are also registered.

• Top Level DNS servers are refreshed 2 times a day, so they pick up the DNS servers for all the second-level domains (yahoo, google, openwave, etc.)

DNS Record Types – Address (A) Records

Address (A) recordsA records resolve a hostname to an IP address. The same hostname can resolve to multiple IP addresses (use case: round robin DNS).

EXAMPLE:

www.openwave.com. IN A 12.25.201.26

www.openwave.com. IN A 12.25.201.27

DNS Record Types – CNAME Records

Canonical Name (CNAME) recordsCNAME records allow a machine to be known by more than one hostname. Also knows as an alias.

EXAMPLE:inside.openwave.com. IN CNAME 12.25.201.26

DNS Record Types – MX Records

Mail Exchange (MX) recordsMX records specify a host that all mail bound for a domain should be routed to. Email clients use these records to determine where the mail should be sent. There can be multiple records, and each can specify a priority (lower is higher).

EXAMPLE

openwave.com. IN MX 10 mail.openwave.com.

DNS Record Types – PTR Records

Pointer (PTR) recordsPTR records enable reverse lookups (provide a hostname provided an IP address).

EXAMPLE12.25.201.26.in-addr.arpa. IN PTR www.openwave.com.

DNS Record Types – NS Records

Name Server (NS) RecordsState the authoritative name servers for the domain.

EXAMPLEopenwave.com. IN NS NS4.OPENWAVE.COM.openwave.com. IN NS NS5.OPENWAVE.COM.

DNS Record Types – SOA Records

• TBD

Dynamic DNS

• There is an extension to DNS that allows a client to update the IP address of a hostname. This is built into routers and there are also software clients.

• Windows LANs support a flavor of this, when you obtain an IP address via DHCP an entry is added to the DNS server.

• Good to allow those on DHCP to host servers.• Dynamic DNS Service Providers

www.dynip.comwww.zoneedit.comwww.dnsexit.comwww.no-ip.com

Referenceshttp://computer.howstuffworks.com/dns1.htmhttp://support.algx.net/cst/dns/dns2.htmlhttp://www.technopagan.org/dynamic/http://www.sentman.com/dyndns/http://www.webopedia.com/TERM/D/dynamic_DNS.htmlhttp://technet2.microsoft.com/WindowsServer/en/Library/e760737e-

9e55-458d-b5ed-a1ae9e04819e1033.mspxhttp://kb.iu.edu/data/akir.htmlhttp://www.simpledns.com/help/index.html?df_dyndns.htmhttp://powder.physics.sunysb.edu/xitami/index13.htmhttp://www.die.net/doc/linux/man/man8/nsupdate.8.htmlhttp://linux.yyz.us/nsupdate/