Applications DNS HTTP and the WWW EECS 122 Lecture 6 Department of Electrical Engineering and Computer Sciences University of California Berkeley What we ve covered so far Basic Background General Overview of different kinds of networks General Design Principles Architecture Performance How to write a network application Now let s get into how things really work Lecture 6 Spring 2003 A Parekh EE122 Revised and upd ated F2002 Lecture by Ion Stoica 2 Applications Application Must separate the application processing from the application protocols Example WWW Browser Server HTTP Also applications can be run on the end hosts or inside the network cloud WWW on end hosts DNS in the network cloud February 5 2003 TCP UDP IP Network HTTP DNS TCP UDP IP Ethernet Abhay Parekh EE122 S2003 Updat ed from Stoica EE122 F2002 FDDI Token Etc 3 Today DNS WWW HTTP Both are client server applications have decentralized management enable access to vast amounts of distributed information are based on open protocols are distributed databases February 5 2003 Abhay Parekh EE122 S2003 Updat ed from Stoica EE122 F2002 4 Domain Name Service Resolves a host name names into an IP address Why host names To organize machines Why IP addresss Eg robotics eecs berkeley edu This conveys more information to humans than 128 32 48 234 The network needs an address to route Host Names yield information to people and IP addresses yield information to routers February 5 2003 Abhay Parekh EE122 S2003 Updat ed from Stoica EE122 F2002 5 DNS History Initially all host addess mappings were in a file called hosts txt in etc hosts As the internet grew this system broke down because Changes were submitted to SRI by email New versions of hosts txt were ftp d periodically from SRI An administrator could pick names at their discretion SRI couldn t handled the load The system was unreliable since there was a single point of contact Names were not unique Many hosts had inaccurate copies of hosts txt Internet growth was threatened February 5 2003 Abhay Parekh EE122 S2003 Updat ed from Stoica EE122 F2002 6 DNS Features Hierarchical Namespace Distributed architecture for storing names Administration divided along the same hierarchy Nameservers assigned zones of the hierarchical namespace Backup servers available for redundancy DNS client is simple Resolver Client server interaction on UDP Port 53 but can use TCP if desired February 5 2003 Abhay Parekh EE122 S2003 Updat ed from Stoica EE122 F2002 7 Host names are organized hierarchically root edu gov com berkeley mit eecs sims argus February 5 2003 org net uk fr The first level names are called Top Level Domains Depth of tree is arbitrary limit 128 Domains are subtrees mil E g berkeley edu and eecs berkeley edu Name collision avoided E g berkeley edu and berkeley com Abhay Parekh EE122 S2003 Updat ed from Stoica EE122 F2002 8 Host names are administered hierarchically root edu berkeley eecs sims com gov mil org net uk fr mit A zone corresponds to an administrative authority that is responsible for that portion of the hierarchy argus February 5 2003 Abhay Parekh EE122 S2003 Vers ion draws from Stoica EE122 F20 9 Server Hierarchy Servers are organized in hierarchies Each server has authority over a portion of the hierarchy A server maintains only a subset of all names Each server contains all the records for the hosts in its zone Each server needs to know other servers that are responsible for the other portions of the hierarchy Every server knows the root Root server knows about all top level domains February 5 2003 Abhay Parekh EE122 S2003 Updat ed from Stoica EE122 F2002 10 DNS Example Recursive Query root name server Host whistler cs cmu edu wants IP address of www berkeley edu 1 Contacts its local DNS server mango srv cs cmu edu 2 mango srv cs cmu edu contacts root name server if necessary 3 Root name server contacts authoritative name server ns1 berkeley edu if 2 5 3 4 local name server authorititive name server mango srv cs cmu edu 1 ns1 berkeley edu 6 necessary requesting host www berkeley edu whistler cs cmu edu February 5 2003 Abhay Parekh EE122 S2003 Updat ed from Stoica EE122 F2002 11 DNS Example Recursive Query root name server Root name server May not know authoritative name server May know intermediate name server who to contact to find authoritative name server Recursive query 2 6 7 local name server mango srv cs cmu edu Puts burden of name resolution on contacted name 1 8 server Heavy load requesting host 3 intermediate name server edu server 4 5 authoritative name server ns1 berkeley edu whistler cs cmu edu www berkeley edu February 5 2003 Abhay Parekh EE122 S2003 Updat ed from Stoica EE122 F2002 12 DNS Iterated Queries Iterated query root name server Contacted server 2 replies with name 3 of server to contact I don t know this name but ask this local name server server mango srv cs cmu edu 1 iterated query 4 5 intermediate name server edu server 6 8 requesting host 7 authoritative name server ns1 berkeley edu whistler cs cmu edu February 5 2003 Abhay Parekh EE122 S2003 Updat ed from Stoica EE122 F2002 www berkeley edu 13 Robustness and Security For non root severs multiple servers are common as well Caching provides another form of redundancy and quicker response time DOS attack in October 2002 Secure DNS A M Root Servers Net February 5 2003 Abhay Parekh EE122 S2003 Updat ed from Stoica EE122 F2002 14 Examples February 5 2003 Abhay Parekh EE122 S2003 Updat ed from Stoica EE122 F2002 15 DNS and Mail Mail Exchange Point A host that either processes or forwards mail Why should the DNS just resolve IP addresses MX records map a name to the name of the mail exchange point for that name Example www tecknowbasic com IN 10 formidible cnchost com www tecknowbasic com IN 20 zealous cnchost com www tecknowbasic com IN 30 inflexible cnchost com Lower numbers imply higher preference February 5 2003 Abhay Parekh EE122 S2003 Updat ed from Stoica EE122 F2002 16 DNS and Virtual IP addresses DNS records don t have to store the real IP address of the host All hosts in the acme com may have the same IP address A firewall at this IP address decides whether to admit a transport level connection firewall to the host x acme com A load balancer decides to forward the connection to one of several identical servers In both cases the gateway must use a local lookup to decide which end host to direct the connection Redirection to be to anywhere Even another country Allows for distributed caching
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