Network ProgrammingNetwork Programming

TopicsTopics Programmer’s view of the Internet Sockets interface Writing clients and servers

CS 105“Tour of the Black Holes of Computing!”

– 2 – CS 105

Client-Server TransactionsClient-Server Transactions

Clientprocess

Serverprocess

1. Client sends request

2. Server handlesrequest

3. Server sends response4. Client handles

response

Resource

Every network application is based on client-server Every network application is based on client-server model:model: 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)

– 3 – CS 105

Programmer’s View of InternetProgrammer’s View of Internet

1. Hosts are mapped to set of 32-bit or 128-bit 1. Hosts are mapped to set of 32-bit or 128-bit IP IP addressesaddresses 134.173.42.2 2001:1878:301:902:214:22ff:fe7c:883f

2. IP addresses are mapped to identifiers called Internet 2. IP addresses are mapped to identifiers called Internet domain namesdomain names 134.173.42.2 maps to www.cs.hmc.edu In general, mapping is many-to-many

3. Process on one Internet host can communicate with 3. Process on one Internet host can communicate with process on another over a process on another over a connectionconnection

– 4 – CS 105

1. IP Addresses1. IP Addresses

Computers are identified by Computers are identified by IP addressesIP addresses

Two flavors: IPv4 (old) and IPv6 (new)Two flavors: IPv4 (old) and IPv6 (new)

Both are stored in an Both are stored in an IP address struct IP address struct of appropriate of appropriate typetype in_addr for IPv4 in6_addr for IPv6

Details matter; new struct and library functions are very Details matter; new struct and library functions are very different and very powerful.different and very powerful.

– 5 – CS 105

3. Internet Connections3. Internet Connections

Connection socket pair(128.2.194.242:51213, 134.173.42.2:80)

Server(port 80)

Client

Client socket address128.2.194.242:51213

Server socket address134.173.42.2:80

Client host address128.2.194.242

Server host address134.173.42.2

Clients and servers communicate by sending streams Clients and servers communicate by sending streams of bytes over of bytes over connectionsconnections

Connections are point-to-point, full-duplex (2-way Connections are point-to-point, full-duplex (2-way communication), and can be reliablecommunication), and can be reliable

Note: 51213 is anephemeral port allocated

by the kernel

Note: 80 is a well-known portassociated with Web servers

– 6 – CS 105

ClientsClientsExamples of client programsExamples of client programs

Web browsers, ftp, telnet, ssh

How does a client find the server?How does a client find the server? IP address in server socket address identifies host (more

precisely, an adapter on the host) (Well-known) port in server socket address identifies

service, and thus implicitly identifies server process that provides it

Examples of well-known ports: /etc/servicesPort 7: Echo serverPort 22: Ssh serverPort 25: Mail serverPort 80: Web server

– 7 – CS 105

Using Ports to Identify ServicesUsing Ports to Identify Services

Web server(port 80)

Client host

Server host 134.173.42.2

Echo server(port 7)

Service request for134.173.42.2:80

(i.e., Web server)

Web server(port 80)

Echo server(port 7)

Service request for134.173.42.2:7

(i.e., echo server)

Kernel

Kernel

Client

Client

– 8 – CS 105

ServersServersServers are long-running processes (daemons).Servers are long-running processes (daemons).

Created at boot time (typically) by init process (process 1) Run continuously until machine is turned off Or spawned by inetd in response to connection to port

Each server waits for requests to arrive on well-known Each server waits for requests to arrive on well-known port associated with that particular serviceport associated with that particular service Port 7: echo server Port 22: ssh server Port 25: mail server Port 80: HTTP server

Machine that runs a server process is also often referred Machine that runs a server process is also often referred to as a to as a ““serverserver””

– 9 – CS 105

Server ExamplesServer Examples

Web server (port 80)Web server (port 80) Resource: files/compute cycles (CGI programs) Service: retrieves files and runs CGI programs on behalf of

the client

FTP server (20, 21)FTP server (20, 21) Resource: files Service: stores and retrieve files

Ssh server (22)Ssh server (22) Resource: terminal Service: proxies a terminal on the server machine

Mail server (25)Mail server (25) Resource: email “spool” file Service: stores mail messages in spool file

See /etc/services for a comprehensive list of the services (potentially) available on a Linux machine.

– 10 – CS 105

Sockets InterfaceSockets Interface

Created in the early 80Created in the early 80’’s as part of original Berkeley s as part of original Berkeley distribution of Unix that contained an early version distribution of Unix that contained an early version of the Internet protocolsof the Internet protocols

Designed for Designed for ““anyany”” network protocol network protocol

Provides a user-level (OS, kernel vs user) interface to Provides a user-level (OS, kernel vs user) interface to the networkthe network

Underlying basis for all Internet applicationsUnderlying basis for all Internet applications

Based on client/server programming modelBased on client/server programming model

– 11 – CS 105

Overview of Sockets InterfaceOverview of Sockets InterfaceClient Server

socket socket

bind

listen

accept

read

read

write

close

read

connect

write

close

Connectionrequest

EOF

Await connectionrequest fromnext client

open_listenfdbook code lib

open_clientfdbook code lib

– 12 – CS 105

SocketsSockets

What is a socket?What is a socket? To kernel, socket is endpoint of communication To application, socket is file descriptor that lets application

read from or write to networkRemember: All Unix I/O devices, including networks, are

modeled as files

Clients and servers communicate with each other by Clients and servers communicate with each other by reading from and writing to socket descriptorsreading from and writing to socket descriptors

Main distinction between regular file I/O and socket I/O Main distinction between regular file I/O and socket I/O is how application is how application ““opensopens”” socket descriptors socket descriptors

– 13 – CS 105

Socket Address StructuresSocket Address StructuresGeneric Generic (old) (old) socket address structure (socket address structure (struct struct

sockaddrsockaddr):): For address arguments to connect, bind, and accept Necessary because other networks (non-TCP/IP) have

different address formats But doesn’t work for IPv6!

Internet-specific socket address structures:Internet-specific socket address structures: IPv4 uses sockaddr_in IPv6 uses sockaddr_in6 Must use union to be sure of having enough space (cool!!)

– 14 – CS 105

Ex: Echo Client Main RoutineEx: Echo Client Main Routine/* #include lots of stuff */

/* usage: ./echoclient host port */int main(int argc, char **argv){ int clientfd; size_t n; char *host, *port, buf[MAXLINE]; host = argv[1]; port = argv[2];

// open_clientfd does socket setup, details in next slides if ((clientfd = open_clientfd(host, port)) == -1) exit(1);// read/write from/to the socket while (fgets(buf, sizeof buf - 1, stdin) != NULL) { write(clientfd, buf, strlen(buf)); n = read(clientfd, buf, sizeof buf - 1); if (n >= 0) { buf[n] = '\0'; fputs(buf, stdout); } } close(clientfd); // you opened it, you close it exit(0);}

– 15 – CS 105

Echo Client: open_clientfdEcho Client: open_clientfdint open_clientfd(char *hostname, char *port){ int clientfd, error; struct addrinfo hints, *hostaddresses = NULL;

/* Find out the server's IP address and port */ memset(&hints, 0, sizeof hints); // 0 struct hints.ai_flags = AI_ADDRCONFIG | AI_V4MAPPED; hints.ai_family = AF_INET6; hints.ai_socktype = SOCK_STREAM; if (getaddrinfo(hostname, port, &hints, &hostaddresses) != 0) return -2; }

/* We take advantage of the fact that AF_* and PF_* are identical */ clientfd = socket(hostaddresses->ai_family, hostaddresses->ai_socktype, hostaddresses->ai_protocol); if (clientfd == -1) return -1; /* check errno for cause of error */ /* Establish a connection with the server */ if (connect(clientfd, hostaddresses->ai_addr, hostaddresses->ai_addrlen) == -1) return -1; freeaddrinfo(hostaddresses); return clientfd;}

This function opens connection from client to server at hostname:portDetails follow….

– 16 – CS 105

Echo Client: open_clientfd (getaddrinfo)Echo Client: open_clientfd (getaddrinfo)

hints.ai_flags = AI_ADDRCONFIG | AI_V4MAPPED; hints.ai_family = AF_INET6; hints.ai_socktype = SOCK_STREAM; if (getaddrinfo(hostname, port, &hints, &hostaddresses) != 0)

... (more)

getaddrinfo getaddrinfo finds out about an Internet hostfinds out about an Internet host AI_ADDRCONFIG: only gives IPv6 address if our machine can

talk IPv6; likewise for IPv4 AI_V4MAPPED: translate IPv6 to IPv4 when needed AF_INET6: prefer IPv6 to IPv4 SOCK_STREAM: selects a reliable byte-stream (TCP) connection

– 17 – CS 105

Echo Client: open_clientfd (socket)Echo Client: open_clientfd (socket)

int clientfd; /* socket descriptor */

clientfd = socket(hostaddresses->ai_family, hostaddresses->ai_socktype, hostaddresses->ai_protocol);

... (more)

socketsocket creates socket descriptor on client creates socket descriptor on client All details provided by getaddrinfo Possibility of multiple addresses (must loop & try all)

– 18 – CS 105

Echo Client: open_clientfd (connect)Echo Client: open_clientfd (connect)Finally, client creates connection with serverFinally, client creates connection with server

Client process suspends (blocks) until connection is created After resuming, client is ready to begin exchanging messages with server

via Unix I/O calls on descriptor sockfd hostaddresses is linked list, must be freed

Including on error returns (not shown for brevity)

int clientfd; /* socket descriptor */ ... /* Establish a connection with the server */ if (connect(clientfd, hostaddresses->ai_addr, hostaddresses->ai_addrlen) == -1) return -1; freeaddrinfo(hostaddresses);

– 19 – CS 105

Echo Server: Main RoutineEcho Server: Main Routineint main(int argc, char **argv) { int listenfd, connfd, clientlen, error; char * port; union {struct sockaddr_in client4; struct sockaddr_in6 client6; } clientaddr; char hostname[NI_MAXHOST], hostaddr[NI_MAXHOST];

listenfd = open_listenfd(argv[1]); if (listenfd < 0) exit(1); while (1) { clientlen = sizeof clientaddr; connfd = accept(listenfd, (struct sockaddr *)&clientaddr, &clientlen); if (connfd == -1) continue; error = getnameinfo((struct sockaddr*)&clientaddr, clientlen, hostname, sizeof hostname, NULL, 0, 0); if (error != 0) continue; getnameinfo((struct sockaddr*)&clientaddr, clientlen, hostaddr, sizeof hostaddr, NULL, 0, NI_NUMERICHOST); printf("server connected to %s (%s)\n", hostname, hostaddr); echo(connfd); close(connfd); }}

This program repeatedly waits for connections, then calls echo(). Details will follow after we look at open_listenfd()…

– 20 – CS 105

int open_listenfd(char *port){ int listenfd, optval=1, error; struct addrinfo hints; struct addrinfo *hostaddresses = NULL;

/* Find out the server's IP address and port */ memset(&hints, 0, sizeof hints); hints.ai_flags = AI_ADDRCONFIG | AI_V4MAPPED | AI_PASSIVE; hints.ai_family = AF_INET6; hints.ai_socktype = SOCK_STREAM; error = getaddrinfo(NULL, port, &hints, &hostaddresses); if (error != 0) return -2; if ((listenfd = socket(hostaddresses->ai_family, hostaddresses->ai_socktype, hostaddresses->ai_protocol)) == -1) return -1; /* Eliminates "Address already in use" error from bind. */ if (setsockopt(listenfd, SOL_SOCKET, SO_REUSEADDR, (const void *)&optval , sizeof optval) == -1) return -1; /* Listenfd will be an endpoint for all requests to port */ if (bind(listenfd, hostaddresses->ai_addr, hostaddresses->ai_addrlen) == -1) return -1; /* Make it a listening socket ready to accept connection requests */ if (listen(listenfd, LISTEN_MAX) == -1) return -1; return listenfd;}

Echo Server: open_listenfdEcho Server: open_listenfd

This function opens a file descriptor on which server can listen for incoming connections. Details follow…

– 21 – CS 105

Here, Here, getaddrinfogetaddrinfo sets up to create generic sets up to create generic ““portport”” Most options same as for open_clientfd AI_PASSIVE: allow any host to connect to us (because we’re a

server) First argument to getaddrinfo is NULL because we won’t be

connecting to a specific host

Echo Server: open_listenfd(getaddrinfo)Echo Server: open_listenfd(getaddrinfo)

hints.ai_flags = AI_ADDRCONFIG | AI_V4MAPPED | AI_PASSIVE; hints.ai_family = AF_INET6; hints.ai_socktype = SOCK_STREAM; error = getaddrinfo(NULL, port, &hints, &hostaddresses);

– 22 – CS 105

socketsocket creates socket descriptor on the server creates socket descriptor on the server All important parameters provided by getaddrinfo Saves us from worrying about IPv4 vs. IPv6

Echo Server: open_listenfd(socket)Echo Server: open_listenfd(socket)

int listenfd; /* listening socket descriptor */ /* Create a socket descriptor */ if ((listenfd = socket(hostaddresses->ai_family, hostaddresses->ai_socktype, hostaddresses->ai_protocol)) == -1) return -1;

– 23 – CS 105

Echo Server: open_listenfd(setsockopt)Echo Server: open_listenfd(setsockopt)

The socket can be given some attributesThe socket can be given some attributes

Handy trick that allows us to rerun the server Handy trick that allows us to rerun the server immediately after we kill itimmediately after we kill it Otherwise we would have to wait about 15 seconds Eliminates “Address already in use” error from bind()

Strongly suggest you do this for all your servers to Strongly suggest you do this for all your servers to simplify debuggingsimplify debugging

.../* Eliminates "Address already in use" error from bind(). */ if (setsockopt(listenfd, SOL_SOCKET, SO_REUSEADDR, (const void *)&optval , sizeof optval) == -1) return -1;

– 24 – CS 105

Echo Server: open_listenfd (bind)Echo Server: open_listenfd (bind)

bind bind associates socket with socket address we just associates socket with socket address we just createdcreated

Again, important parameters come from Again, important parameters come from getaddrinfogetaddrinfo

int listenfd; /* listening socket */

... /* listenfd will be an endpoint for all requests to port on any IP address for this host */ if (bind(listenfd, hostaddresses->ai_addr, hostaddresses->ai_addrlen) == -1) return -1;

– 25 – CS 105

Echo Server: open_listenfd (listen)Echo Server: open_listenfd (listen)

listenlisten indicates that this socket will accept indicates that this socket will accept connection (connection (connectconnect) requests from clients) requests from clients

WeWe’’re finally ready to enter main server loop that re finally ready to enter main server loop that accepts and processes client connection requestsaccepts and processes client connection requests

int listenfd; /* listening socket */

... /* Make it a listening socket ready to accept connection requests */ if (listen(listenfd, LISTEN_MAX) == -1) return -1; return listenfd; }

– 26 – CS 105

Echo Server: Main LoopEcho Server: Main Loop

Server loops endlessly, waiting for connection Server loops endlessly, waiting for connection requests, then reading input from client and echoing requests, then reading input from client and echoing it back to clientit back to client

main() {

/* create and configure the listening socket */

while(1) { /* accept(): wait for a connection request */ /* echo(): read and echo input lines from client til EOF */ /* close(): close the connection */ }}

– 27 – CS 105

accept()accept() blocks waiting for connection request blocks waiting for connection request

acceptaccept returns returns connected descriptor connected descriptor ((connfdconnfd) with ) with same properties as same properties as listening descriptorlistening descriptor ( (listenfd)listenfd) Returns when connection between client and server is

created and ready for I/O transfers All I/O with client will be done via connected socket

accept accept also fills in clientalso fills in client’’s IP addresss IP address

Echo Server: acceptEcho Server: accept

int listenfd; /* listening descriptor */ int connfd; /* connected descriptor */ union { struct sockaddr_in client4; struct sockaddr_in6 client6; } clientaddr; int clientlen; clientlen = sizeof(clientaddr); connfd = accept(listenfd, (struct sockaddr *)&clientaddr, &clientlen);

– 28 – CS 105

Echo Server: accept IllustratedEcho Server: accept Illustrated

listenfd(3)

Client

1. Server blocks in accept, waiting for connection request on listening descriptor listenfdclientfd

Server

listenfd(3)

Client

clientfd

Server2. Client makes connection request by calling and blocking in connect

Connectionrequest

listenfd(3)

Client

clientfd

Server

3. Server returns connfd from accept. Client returns from connect. Connection is now established between clientfd and connfd

connfd(4)

– 29 – CS 105

Connected vs. Listening DescriptorsConnected vs. Listening Descriptors

Listening descriptorListening descriptor End point for client connection requests Created once and exists for lifetime of server

Connected descriptorConnected descriptor End point of connection between client and server New descriptor is created each time server accepts

connection request from a client Exists only as long as it takes to service client

Why the distinction?Why the distinction? Allows for concurrent servers that can communicate over

many client connections simultaneouslyE.g., each time we receive a new request, we fork a child or

spawn a thread to handle it

Can be closed to break connection to particular client

– 30 – CS 105

Echo Server: Identifying ClientEcho Server: Identifying Client

Server can determine domain name and IP address of Server can determine domain name and IP address of clientclient

char hostname[NI_MAXHOST], hostaddr[NI_MAXHOST]; … error = getnameinfo((struct sockaddr*)&clientaddr, clientlen, hostname, sizeof hostname, NULL, 0, 0); if (error != 0) continue; getnameinfo((struct sockaddr*)&clientaddr, clientlen, hostaddr, sizeof hostaddr, NULL, 0, NI_NUMERICHOST); printf("server connected to %s (%s)\n", hostname, hostaddr);

– 31 – CS 105

Echo Server: echoEcho Server: echo

void echo(int connfd){ size_t n; char buf[MAXLINE];

while((n = read(connfd, buf, sizeof buf)) > 0) { printf("server received %d bytes\n", n); write(connfd, buf, n); }}

Server uses Unix I/O to read and echo text lines until Server uses Unix I/O to read and echo text lines until EOF (end-of-file) is encounteredEOF (end-of-file) is encountered EOF notification caused by client calling close(clientfd) IMPORTANT: EOF is a condition, not a particular data byte

– 32 – CS 105

Testing Servers Using telnetTesting Servers Using telnet

The The telnet telnet program is invaluable for testing servers program is invaluable for testing servers that transmit ASCII strings over Internet connectionsthat transmit ASCII strings over Internet connections Our simple echo server Web servers Mail servers

Usage: Usage: unix> telnet <host> <portnumber> Creates connection with server running on <host> and

listening on port <portnumber>

– 33 – CS 105

Testing Echo Server With telnetTesting Echo Server With telnetmallet> ./echoserver 5000server connected to bow-vpn.cs.hmc.edu (::ffff:192.168.6.5)server received 5 bytesserver connected to bow-vpn.cs.hmc.edu (::ffff:192.168.6.5)server received 8 bytes

bow> telnet mallet-vpn 5000Trying 192.168.6.1...Connected to mallet-vpn.Escape character is '^]'.123123Connection closed by foreign host.bow> telnet mallet-vpn 5000Trying 192.168.6.1...Connected to mallet-vpn.Escape character is '^]'.456789456789Connection closed by foreign host.bow>

– 34 – CS 105

Running Echo Client and ServerRunning Echo Client and Servermallet> echoserver 5000server connected to bow-vpn.cs.hmc.edu (::ffff:192.168.6.5)server received 4 bytesserver connected to bow-vpn.cs.hmc.edu (::ffff:192.168.6.5)server received 7 bytes...

bow> echoclient mallet-vpn 5000123123

bow> echoclient mallet-vpn 5000456789456789bow>

– 35 – CS 105

One More Important FunctionOne More Important Function

Real servers often want to handle multiple clientsReal servers often want to handle multiple clients

Problem: you have 3 clients. Only B wants service. Problem: you have 3 clients. Only B wants service. You can’t really write You can’t really write serve(A); serve(B); serve(A); serve(B); serve(C); serve(C); because B must wait for A to ask for because B must wait for A to ask for serviceservice

Solution: Solution: selectselect system call system call Accepts set of file descriptors you’re interested in Tells you which ones have input waiting or are ready for

output Then you can read from/write to only the active ones For more info, see man 2 select and Chapter 13

– 36 – CS 105

For More InformationFor More Information

W. Richard Stevens, W. Richard Stevens, ““Unix Network Programming: Unix Network Programming: Networking APIs: Sockets and XTINetworking APIs: Sockets and XTI””, Volume 1, , Volume 1, Second Edition, Prentice Hall, 1998Second Edition, Prentice Hall, 1998 THE network programming bible

Versions of the echo client and server are in the text, Versions of the echo client and server are in the text, but use RIObut use RIO