l9 DNS - Administrivia • Midterm exam Thursday- Open...

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Unformatted text preview: Administrivia • Midterm exam Thursday- Open book, Open notes, no electronic devices allowed- Feel free to print out and bring lecture slides • SCPD students:- Email [email protected] with your exam monitor information- Please ensure the email subject is “ exam monitor ” • Any other students with special exam needs- Please email cs144-staff to make arrangements Outline • DNS architecture • DNS protocol and resource records (RRs) • Record types: A, NS, glue, MX, SOA, CNAME • Reverse lookup • Load balancing • DNS security Parsing a URL http://cs144.scs.stanford.edu/labs/sc.html File Host Protocol Motivation Name server Mail program User TCP IP 2 cs.princeton.edu 192.12.69.5 3 user @ cs.princeton.edu 1 192.12.69.5 4 192.12.69.5 5 • Users can’t remember IP addresses- Need to map symbolic names ( www.stanford.edu ) → IP addr • Implemented by library functions & servers- getaddrinfo () talks to server over UDP (sometimes TCP) • Actually, more generally, need to map symbolic names to values hosts.txt system • Originally, hosts were listed in a file, hosts.txt- Email global network administrator when you add a host- Administrator mails out new hosts.txt file every few days • Would be completely impractical today- hosts.txt today would be huge (Gigabytes)- What if two people wanted to add same name?- Who is authorized to change address of a name?- People need to change name mappings more often than every few days (e.g., Dynamic IP addresses) Goals of DNS • Scalability- Must handle huge number of records- Potentially exponential in name size—because custom software may synthesize names on-the-fly • Distributed control- Let people control their own names • Fault-tolerance- Old software assumed hosts.txt always there- Bad potential failure modes when name lookups fail- Minimize lookup failures in the face of other network problems The good news • Properties that make DNS goals easier to achieve: 1. Read-only or read-mostly database- People typically look up hostnames much more often than they are updated 2. Loose consistency- When adding a machine, may be okay if info takes minutes or hours to propagate • These suggest approach w. aggressive caching- Once you have looked up hostname, remember result- Don’t need to look it up again in near future Domain Name System (DNS) edu com princeton … mit cs ee ux01 ux04 physics cisco … yahoo nasa … nsf arpa … navy acm … ieee gov mil org net uk fr • Break namespace into a bunch of zones- . (“root”), edu. , stanford.edu. , cs.stanford.edu. , ...- Zones separately administered = ⇒ delegation- Parent zones tell you how to find servers for dubdomains. • Each zone served from several replicated servers Root servers • Root (and TLD) servers must be widely replicated- For some, use various tricks like IP anycast DNS software architecture • Two types of query- Recursive- Non-Recursive • Apps make recursive queries to local DNS server (1) • Local server queries remote...
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This note was uploaded on 04/02/2012 for the course CS 144 at Stanford.

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l9 DNS - Administrivia • Midterm exam Thursday- Open...

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