DNS DNS Domain Name System DNS services Hostname

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DNS

DNS

DNS: Domain Name System DNS services • Hostname to IP address translation • Host

DNS: Domain Name System 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 2

Distributed, Hierarchical Database Root DNS Servers` com DNS servers yahoo. com amazon. com DNS

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

DNS: Root name servers • • contacted by local name server that cannot resolve

DNS: Root name servers • • 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 13 root name servers – http: //root-servers. org/ a Verisign, Dulles, VA c Cogent, Herndon, VA (also Los Angeles) d U Maryland College Park, MD k RIPE London (also Amsterdam, g US Do. D Vienna, VA Frankfurt) Stockholm (plus 3 i Autonomica, h ARL Aberdeen, MD other locations) j Verisign, ( 11 locations) worldwide 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 4

TLD and Authoritative Servers • Top-level domain (TLD) servers: responsible for com, org, net,

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 5

Local Name Server • Each ISP (residential ISP, company, university) has one. – Also

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. 6

root DNS server Iterative Queries 2 4 iterated query: • • contacted server replies

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

Recursive queries root DNS server 2 7 recursive query: puts burden of name resolution

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

DNS: caching and updating records • once (any) name server learns mapping, it caches

DNS: caching and updating records • 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 9

DNS records DNS: distributed db storing resource records (RR) RR format: (name, ttl, class,

DNS records DNS: distributed db storing resource records (RR) RR format: (name, ttl, class, type, value) • 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. backup 2. ibm. com • Type=NS – name is domain (e. g. foo. com) – value is IP address of authoritative name server for this domain value is canonical name • Type=MX value is name of mail server associated with name 10

The dig command

The dig command

The dig command

The dig command

Web and HTTP First some jargon • Web page consists of objects • Object

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: www. someschool. edu/some. Dept/pic. gif host name path name

HTTP overview HTTP: hypertext transfer protocol • Web’s application layer protocol • client/server model

HTTP overview 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 HT TP req ues PC running HT t TP res Explorer pon se st ue q e r P nse Server T o p running HT es r P T Apache Web HT server Mac running Navigator

HTTP overview (continued) Over TCP: • client initiates TCP connection (creates socket) to server,

HTTP overview (continued) 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 request message • two types of HTTP messages: request, response • HTTP request

HTTP request message • two types of HTTP messages: request, response • HTTP request message: – ASCII (human-readable format) request line (GET, POST, HEAD commands) header lines Carriage return, line feed indicates end of message 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)

HTTP request message: general format

HTTP request message: general format

HTTP response message status line (protocol status code status phrase) header lines data, e.

HTTP response message status line (protocol status code status phrase) header lines data, e. g. , requested HTML file 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. . .

Trying out HTTP (client side) for yourself 1. Telnet to your favorite Web server:

Trying out HTTP (client side) for yourself 1. Telnet to your favorite Web server: 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: GET index. html / HTTP/1. 1 Host: www. cs. fsu. edu By typing this in (hit carriage return twice), you send this minimal (but complete) GET request to HTTP server 3. Look at response message sent by HTTP server!

telnet www. cs. fsu. edu 80 Trying 192. 168. 23. 10. . . Connected

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: "1 defce 0 -29 c 5 -4 cd 2 a 4 c 0" 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, Internet 2, CS"> <meta NAME="distribution" CONTENT="global"> <meta NAME="author" CONTENT="Kendal Van Dyke"> 20

User-server state: cookies • HTTP is stateless. – two requests are treated independently. –

User-server state: cookies • 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

Cookies: keeping “state” (cont. ) client ebay: 8734 Cookie file amazon: 1678 ebay: 8734

Cookies: keeping “state” (cont. ) client ebay: 8734 Cookie file amazon: 1678 ebay: 8734 usual http request msg usual http response + Set-cookie: 1678 usual http request msg cookie: 1678 usual http response msg Cookie file amazon: 1678 ebay: 8734 cookiespecific action ss acce ac ce one week later: e n server da try i tab n b creates ID as ac ke e nd 1678 for user ss Cookie file server usual http request msg cookie: 1678 usual http response msg cookiespectific action

Cookies (continued) What cookies can bring: • authorization • shopping carts • recommendations •

Cookies (continued) What cookies can bring: • authorization • shopping carts • recommendations • user session state (Web e-mail) 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

 • Some issues in HTTP: • Mainly due to its popularity – Cache

• 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

Content-Delivery • Akamai. • With 18, 000 servers all around the world (in 2010).

Content-Delivery • Akamai. • With 18, 000 servers all around the world (in 2010). • Most likely what happens when a request for a webpage, the original server will reply the text html file while asking Akamai servers to send the large files. • Good for the clients, and good for the ISPs because it reduces the upstream traffic.

DNS Redirecting • First time, the client needs to look up www. cdn. com

DNS Redirecting • First time, the client needs to look up www. cdn. com so it will get the DNS reply from the CDN DNS server. • The CDN DNS server finds which server best serves this client and returns the ip address of this server, considering the network capacity, the network delay, and the network load. • Akamai also has to consider where to put the servers.