DNS
===

- Replaced hosts.txt.  What's wrong with hosts.txt?

- What are requirements of Domain Name system?
   - Huge number of records
   - Distributed control of database contents
   - Fault-tolerance

- What properties can the implementation can take advantage of?
   - Loose consistency
   - Read-only (or rare updates)

- Architecture of DNS
   - Break namespace into a bunch of zones
     root ("."), "edu.", "nyu.edu.", "cs.nyu.edu.", etc.
   - Each zone served from several replicated servers
   - Zones separately administered--delegation
   - Parent zones tell you how to find servers for dubdomains.

- DNS protocol
   - TCP/UDP port 53.
   - Most traffic uses UDP.
   - Lightweight protocol has 512 byte UDP message limit.
     (retry w. TCP if UDP fails)
   - Clients typically query a local DNS server for all lookups
   - server "recurses" and asks other servers

- Queries and answers are in terms of "resource records"
   - A record binds name to address (www.nyu.edu. A 128.122.108.9)
   - MX record says where to send mail (with priorities)
   - NS record gives name server for a zone
        nyu.edu. NS nyu.edu.
        nyu.edu. NS EGRESS.nyu.edu.
        nyu.edu. NS NYUNSB.nyu.edu.
   - CNAME is host alias (spunky.nyu.edu. CNAME dept.cs.nyu.edu.)
   - PTR record binds IP addresses to names (in-addr.arpa. / ip6.arpa.)
   - SOA - information about zone
   - HINFO, TXT (mostly useless)

- How do you get name servers for root domain?  Prime cache with them

- How do you get name server for subdomain, given NS record
   - responses have "additional records"
   - response format:  answers, authority records, additional records
	;; QUESTIONS:
	;;      www.nyu.edu, type = A, class = IN

	;; ANSWERS:
	www.nyu.edu.    60      CNAME   WWWSERVER.nyu.edu.
	WWWSERVER.nyu.edu.      345600  A       128.122.108.9

	;; AUTHORITY RECORDS:
	nyu.edu.        345600  NS      nyu.edu.
	nyu.edu.        345600  NS      EGRESS.nyu.edu.
	nyu.edu.        345600  NS      NYUNSB.nyu.edu.

	;; ADDITIONAL RECORDS:
	nyu.edu.        60      A       128.122.253.92
	nyu.edu.        60      A       192.76.177.18
	EGRESS.nyu.edu. 60      A       128.122.128.24
	NYUNSB.nyu.edu. 345600  A       128.122.253.37
   - Name servers cache all records they see for good performance (security?)

- How does consitency work?  (TTL)
- What about replicas (zone transfer, SOA):
	nyu.edu.        241165  SOA     nyu.edu. HOSTMASTER.nyu.edu. (
				200104101       ; serial
				86400   ; refresh (1 day)
				7200    ; retry (2 hours)
				2592000 ; expire (30 days)
				345600 )        ; minimum (4 days)
- Can have multiple resource records for a domain name
   - Name servers return records in round-robin order