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

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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
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Outline DNS architecture DNS protocol and resource records (RRs) Record types: A, NS, glue, MX, SOA, CNAME Reverse lookup Load balancing DNS security
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Parsing a URL http://cs144.scs.stanford.edu/labs/sc.html File Host Protocol
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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
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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)
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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
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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
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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
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Root servers Root (and TLD) servers must be widely replicated - For some, use various tricks like IP anycast
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DNS software architecture Two types of query - Recursive - Non-Recursive Apps make recursive queries to local DNS server (1) Local server queries remote servers non-recursively (2, 4, 6) - Aggressively caches result - E.g., only contact root on first query ending .umass.edu
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