DNS

1. DNS

2. DNS services Hostname to IP address translation Host aliasing Canonical and alias names Mail server aliasing Load distribution Replicated Web servers: set of IP addresses for one canonical name DNS: Domain Name System

3. Root DNS Servers` org DNS servers edu DNS servers com DNS servers fsu.edu DNS servers umass.edu DNS servers pbs.org DNS servers yahoo.com DNS servers amazon.com DNS servers Distributed, Hierarchical Database

4. contacted by local name server that cannot resolve name root name server: contacts authoritative name server if name mapping not known gets mapping returns mapping to local name server http://root-servers.org/ a Verisign, Dulles, VA c Cogent, Herndon, VA (also Los Angeles) d U Maryland College Park, MD g US DoD Vienna, VA h ARL Aberdeen, MD j Verisign, ( 11 locations) k RIPE London (also Amsterdam, Frankfurt) i Autonomica, Stockholm (plus 3 other locations) m WIDE Tokyo e NASA Mt View, CA f Internet Software C. Palo Alto, CA (and 17 other locations) b USC-ISI Marina del Rey, CA l ICANN Los Angeles, CA DNS: Root name servers 13 root name servers worldwide

5. TLD and Authoritative Servers • Top-level domain (TLD) servers: responsible for com, org, net, edu, etc, and all top-level country domains cn, ca, fr, jp, uk etc. • Network solutions maintains servers for com TLD • Educause for edu TLD • Authoritative DNS servers: organization’s DNS servers, providing authoritative hostname to IP mappings for organization’s servers (e.g., Web and mail). • Can be maintained by organization or service provider

6. Local Name Server • Each ISP (residential ISP, company, university) has one. • Also called “default name server” • When a host makes a DNS query, query is sent to its local DNS server • Acts as a proxy, forwards query into hierarchy.

7. iterated query: contacted server replies with name of server to contact “I don’t know this name, but ask this server” local DNS server dns.poly.edu Iterative Queries root DNS server 2 3 TLD DNS server 4 5 6 7 1 8 authoritative DNS server dns.cs.umass.edu requesting host cis.poly.edu gaia.cs.umass.edu

8. root DNS server 2 3 6 7 TLD DNS server 4 local DNS server dns.poly.edu 5 1 8 authoritative DNS server dns.cs.umass.edu requesting host cis.poly.edu gaia.cs.umass.edu Recursive queries recursive query: puts burden of name resolution on contacted name server heavy load?

9. once (any) name server learns mapping, it caches mapping cache entries timeout (disappear) after some time TLD servers typically cached in local name servers Thus root name servers not often visited DNS: caching and updating records

10. DNS: distributed db storing resource records (RR) Type=NS name is domain (e.g. foo.com) value is IP address of authoritative name server for this domain RR format: (name, ttl, class, type, value DNS records • Type=A • name is hostname • value is IP address • Type=CNAME • name is alias name for some “canonical” (the real) name • www.ibm.com is really • servereast.backup2.ibm.com • value is canonical name • Type=MX • value is name of mail server associated with name

11. The dig command

12. The dig command

13. www.someschool.edu/someDept/pic.gif path name host name Web and HTTP First some jargon • Web page consists of objects • Object can be HTML file, JPEG image, Java applet, audio file,… • Web page consists of base HTML-file which includes several referenced objects • Each object is addressable by a URL • Example URL:

14. HTTP: hypertext transfer protocol Web’s application layer protocol client/server model client: browser that requests, receives, “displays” Web objects server: Web server sends objects in response to requests HTTP 1.0: RFC 1945 HTTP 1.1: RFC 2068 HTTP overview HTTP request PC running Explorer HTTP response HTTP request Server running Apache Web server HTTP response Mac running Navigator

15. Over TCP: client initiates TCP connection (creates socket) to server, port 80 server accepts TCP connection from client HTTP messages (application-layer protocol messages) exchanged between browser (HTTP client) and Web server (HTTP server) TCP connection closed HTTP overview (continued)

16. HTTP request message • two types of HTTP messages: request, response • HTTP request message: • ASCII (human-readable format) request line (GET, POST, HEAD commands) GET /somedir/page.html HTTP/1.1 Host: www.someschool.edu User-agent: Mozilla/4.0 Connection: close Accept-language:fr (extra carriage return, line feed) header lines Carriage return, line feed indicates end of message

17. HTTP request message: general format

18. HTTP response message status line (protocol status code status phrase) HTTP/1.1 200 OK Connection close Date: Thu, 06 Aug 1998 12:00:15 GMT Server: Apache/1.3.0 (Unix) Last-Modified: Mon, 22 Jun 1998 …... Content-Length: 6821 Content-Type: text/html data data data data data ... header lines data, e.g., requested HTML file

19. 1. Telnet to your favorite Web server: Trying out HTTP (client side) for yourself telnet www.cs.fsu.edu 80 Opens TCP connection to port 80 (default HTTP server port) at www.cs.fsu.edu. Anything typed in sent to port 80 at www.cs.fsu.edu • 2. Type in a GET HTTP request: By typing this in (hit carriage return twice), you send this minimal (but complete) GET request to HTTP server GET index.html / HTTP/1.1 Host: www.cs.fsu.edu 3. Look at response message sent by HTTP server!

20. telnet www.cs.fsu.edu 80 Trying 192.168.23.10... Connected to www.cs.fsu.edu (192.168.23.10). Escape character is '^]'. GET /index.html /HTTP/1.1 Host: www.cs.fsu.edu HTTP/1.1 200 OK Date: Wed, 28 Nov 2007 18:34:29 GMT Server: Apache/2.0.52 (Scientific Linux) Last-Modified: Mon, 29 Aug 2005 18:02:35 GMT ETag: "1defce0-29c5-4cd2a4c0" Accept-Ranges: bytes Content-Length: 10693 Connection: close Content-Type: text/html; charset=ISO-8859-1 <html> <head> <title>Computer Science @ Florida State University</title> <base HREF="http://www.cs.fsu.edu/"> <meta NAME="resource-type" CONTENT="document"> <meta NAME="description" CONTENT="Website for the Computer Science Department at Florida State University"> <meta NAME="keywords" CONTENT="Florida State University, Computer Science, Internet2, CS"> <meta NAME="distribution" CONTENT="global"> <meta NAME="author" CONTENT="Kendal Van Dyke">

21. HTTP is stateless. two requests are treated independently. Why stateless? What is the problem with a stateless http? E-commence: People buy things by making many requests. Need the ability to bind the requests from the same customer together. Solution: cookies User-server state: cookies

22. client server usual http request msg usual http response + Set-cookie: 1678 Cookie file Cookie file Cookie file amazon: 1678 ebay: 8734 ebay: 8734 amazon: 1678 ebay: 8734 cookie- specific action usual http request msg cookie: 1678 usual http request msg cookie: 1678 usual http response msg usual http response msg cookie- spectific action Cookies: keeping “state” (cont.) server creates ID 1678 for user entry in backend database access access one week later:

23. What cookies can bring: authorization shopping carts recommendations user session state (Web e-mail) Cookies (continued) aside • Cookies and privacy: • cookies permit sites to learn a lot about you • you may supply name and e-mail to sites • search engines use redirection & cookies to learn yet more • advertising companies obtain info across sites

24. Some issues in HTTP: • Mainly due to its popularity • Cache support. • Insufficient in http/1.0, improved in http/1.1 • Intermediate nodes, encoding, etc • Dynamically generated date • Not reliable in http/1.0 • Performance • Persistent or non-persistent TCP connection • Download the whole file or part of a file • User preference • Security

25. Content-Delivery • Akamai. • With 18,000 servers all around the world. • When a requesting for a webpage, the original server will reply the text html file while asking Akamai servers to send the large files. • Exactly how this done – some uses DNS redirecting.