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CMU CS 15213 - Lecture

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Network Programming November 21, 2007Packet SniffingA Client-Server TransactionA Programmer’s View of the Internet1. IP Addresses2. Domain Naming System (DNS)3. Internet ConnectionsClientsUsing Ports to Identify ServicesServersServer ExamplesSockets InterfaceSocketsOverview of the Sockets InterfaceSocket Address StructuresSocket Address StructuresExample: Echo Client and ServerEcho Client Main RoutineEcho Client: open_clientfdEcho Client: open_clientfd (socket)Echo Client: open_clientfd (gethostbyname)A Careful Look at bcopy ArgumentsEcho Client: open_clientfd (connect)Echo Server: Main RoutineEcho Server: open_listenfdEcho Server: open_listenfd (cont)Echo Server: open_listenfd (socket)Echo Server: open_listenfd (setsockopt)Echo Server: open_listenfd (initialize socket address)Echo Server: open_listenfd (bind)Echo Server: open_listenfd (listen)Echo Server: Main LoopEcho Server: acceptEcho Server: accept IllustratedConnected vs. Listening DescriptorsEcho Server: Identifying the ClientEcho Server: echoTesting Servers Using telnetTesting the Echo Server With telnetFor More InformationNetwork ProgrammingNovember 21, 2007Network ProgrammingNovember 21, 2007TopicsTopics Peeking at Internet traffic Programmer’s view of the Internet (review) Sockets interface Writing clients and servers Understanding protocol15-213class23.ppt–2–15-213, F’07Packet SniffingPacket SniffingProgram That Records Network Traffic Visible at NodeProgram That Records Network Traffic Visible at Node Promiscuous Modez Record traffic that does not have this host as source or destination–3–15-213, F’07A Client-Server TransactionA Client-Server TransactionClientprocessServerprocess1. Client sends request2. Server handlesrequest3. Server sends response4. Client handlesresponseResourceMost network applications are based on the clientMost network applications are based on the client--server model:server model: A server process and one or more client processes Server manages some resource. Server provides service by manipulating resource for clients.Note: clients and servers are processes running on hosts (can be the same or different hosts).–4–15-213, F’07A Programmer’s View of the InternetA Programmer’s View of the Internet1. Hosts are mapped to a set of 321. Hosts are mapped to a set of 32--bit bit IP addressesIP addresses.. 128.2.203.1792. The set of IP addresses is mapped to a set of 2. The set of IP addresses is mapped to a set of identifiers called Internet identifiers called Internet domain namesdomain names.. 128.2.203.179 is mapped to www.cs.cmu.edu3. A process on one Internet host can communicate 3. A process on one Internet host can communicate with a process on another Internet host over a with a process on another Internet host over a connectionconnection..–5–15-213, F’071. IP Addresses1. IP Addresses3232--bit IP addresses are stored in an bit IP addresses are stored in an IP address IP address structstruct IP addresses are always stored in memory in network byte order (big-endian byte order) True in general for any integer transferred in a packet header from one machine to another.z E.g., the port number used to identify an Internet connection./* Internet address structure */struct in_addr {unsigned int s_addr; /* network byte order (big-endian) */};Handy network byte-order conversion functions:htonl: convert long int from host to network byte order.htons: convert short int from host to network byte order.ntohl: convert long int from network to host byte order.ntohs: convert short int from network to host byte order.–6–15-213, F’072. Domain Naming System (DNS)2. Domain Naming System (DNS)The Internet maintains a mapping between IP addresses The Internet maintains a mapping between IP addresses and domain names in a huge worldwide distributed and domain names in a huge worldwide distributed database called database called DNSDNS.. Conceptually, programmers can view the DNS database as a collection of millions of host entry structures:Functions for retrieving host entries from DNS:Functions for retrieving host entries from DNS: gethostbyname: query key is a DNS domain name. gethostbyaddr: query key is an IP address./* DNS host entry structure */ struct hostent { char *h_name; /* official domain name of host */ char **h_aliases; /* null-terminated array of domain names */ int h_addrtype; /* host address type (AF_INET) */ int h_length; /* length of an address, in bytes */ char **h_addr_list; /* null-terminated array of in_addr structs */ };–7–15-213, F’073. Internet Connections3. Internet ConnectionsConnection socket pair(128.2.194.242:51213, 208.216.181.15:80)Server(port 80)ClientClient socket address128.2.194.242:51213Server socket address208.216.181.15:80Client host address128.2.194.242Server host address208.216.181.15Clients and servers communicate by sending streams Clients and servers communicate by sending streams of bytes over of bytes over connectionsconnections..Connections are pointConnections are point--toto--point, fullpoint, full--duplex (2duplex (2--way way communication), and reliable.communication), and reliable.Note: 51213 is anephemeral port allocatedby the kernel Note: 80 is a well-known portassociated with Web servers–8–15-213, F’07ClientsClientsExamples of client programsExamples of client programs Web browsers, ftp, telnet, sshHow does a client find the server?How does a client find the server? The IP address in the server socket address identifies the host (more precisely, an adapter on the host) The (well-known) port in the server socket address identifies the service, and thus implicitly identifies the server process that performs that service. Examples of well know portsz Port 7: Echo serverz Port 23: Telnet serverz Port 25: Mail serverz Port 80: Web server–9–15-213, F’07Using Ports to Identify ServicesUsing Ports to Identify ServicesWeb server(port 80)Client hostServer host 128.2.194.242Echo server(port 7)Service request for128.2.194.242:80(i.e., the Web server)Web server(port 80)Echo server(port 7)Service request for128.2.194.242:7(i.e., the echo server)KernelKernelClientClient–10–15-213, F’07ServersServersServers are longServers are long--running processes (daemons).running processes (daemons). Created at boot-time (typically) by the init process (process 1) Run continuously until the machine is turned off.Each server waits for


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