17 - All About DNS
- Talking about DNS
	- What is the point?
		- turns Domain names into IP addresses
		- Common analogy: phonebook
			- not exactly right
- DNS is heirarchical
	- Top Level Domains, TLD
		- com
		- org
		- gov
		- edu
		- biz
		- net
		- countries can have them too
			- ie
			- us
			- uk
			- tv
				- *Connor's Note* .tv belongs to Tuvalu
			- me
			- au
			- nz
			- de
	- under these are subdomains, which we think of as the domains
		- google.com
		- apple.com
		- typicalmacuser.com
		- impodcast.tv
		- iss-foundation.org
		- connorp.me
		- bartb.ie
	- and an infinite number of subdomains under these
- Who decides these and has authority over a section of DNS?
	- Root Domain name servers
		- most important servers on the internet
		- tell who has authority over the TLDs
		- the TLDs then give authority to the subdomains
		- ad infinitum
- DNS is a bunch of records
	- four important types
		- NS Record
			- Nameservers
			- who has authority over the DNS for a URL
		- A Record
			- maps a name to an IP address
				- www.typicalmacuser.com <http://www.typicalmacuser.com/> to an IP address
		- MX Record
			- Mail Exchange
				- which mailserver is responsible for email to @aurl.com
		- CNAME
			- alias
- Finding an IP address for a URL via DNS is a multistep process
	- first your machine goes to the root servers and asks for the TLD for the URL
	- then it asks the TLD for the NS Record of the URL
	- then it asks the Nameserver and asks for the IP address
- This would put a large amount of strain on the servers if this happened each time
	- to fix this, DNS servers, your machine, and even routers cache this information
		- how long this is kept is based upon a Time to Live, or TTL, set by the DNS record.
		- This is why when you change DNS it takes time for the change to propogate thru the entire world
- host command, while simple, is very powerful
	- host www.typicalmacuser.com <http://www.typicalmacuser.com/>
	- output:
	- www.typicalmacuser.com <http://www.typicalmacuser.com/> is an alias for typicalmacuser.com.
	typicalmacuser.com has address 68.178.254.63
	typicalmacuser.com mail is handled by 0 smtp.secureserver.net.
	typicalmacuser.com mail is handled by 10 mailstore1.secureserver.net.
	- 1st line is a CNAME
	- 2nd Line is an A record
	- 3rd and 4th lines are MX records
- Another simple command, but very detailed output
	- asks a specific DNS server for the answer
	- dig
		- dig adomainname.com
		- dig www.bartb.ie <http://www.bartb.ie/>
			- output:
			- ; <<>> DiG 9.4.3-P1 <<>> www.bartb.ie <http://www.bartb.ie/>
			;; global options:  printcmd
			;; Got answer:
			;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 53330
			;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0
			
			;; QUESTION SECTION:
			;www.bartb.ie <http://www.bartb.ie/>.			IN	A
			
			;; ANSWER SECTION:
			www.bartb.ie <http://www.bartb.ie/>.		14400	IN	A	70.38.18.241
			
			;; Query time: 266 msec
			;; SERVER: 10.0.1.1#53(10.0.1.1)
			;; WHEN: Sat Jun 27 21:25:54 2009
			;; MSG SIZE  rcvd: 46
			
				- first chunk: information on dig command
				- second chunk: the question you asked
				- third chunk: the answer
				- fourth chunk: some stats about the connection and request
		- Find the MX record
			- dig bartb.ie MX
		- Or the nameserver
			- dig bartb.ie NS
		- Now to ask the actual Nameserver
			- dig url.com@dnsserver
			- dig "ns1.panelboxmanager.com <http://www.bartb.ie@ns1.panelboxmanager.com/>"
- Bart Busschots
	- bartb.ie <http://bartb.ie/>
	- impodcast.tv <http://impodcast.tv/>
	- podfeet.com <http://podfeet.com/>