Network Programming

Sockets and Winsock

Please Be Responsible

• We all know that the Internet is full of security holes– most of them do not require any technical

expertise to exploit.

• Security = Inconvenience or Inaccessibility– the more systems that are hacked the harder it

will be to get system administrators to offer services.

• Federal Laws are in place and are being enforced

Host-dependent Data Representation

• Big Endian and Little Endian– How do we store the integer 0x12345678 ?

0x12

0x34

0x56

0x78

0x101

0x102

0x103

0x100 0x78

0x56

0x34

0x12

0x101

0x102

0x103

0x100

Big Endian Little Endian

Big and Little Endian Users*

• Big Endian– PowerPc

– Sun Sparc

– HP Workstation

• Little Endian– Dec Alpha

– Intel Pentium

Some systems (MIPS 2000 and Intel i860) can use either big endian or little endian. The Intel i860 can even change modes while a program is running!

*Source: Unix network programming, W. Richard Stevens

Network Protocols Must Adopt One of the Two Byte Orders

• Network Byte Order– The protocol byte order

• Host Byte Order– The native machine byte order

• Conversion Functions – htons : convert 16-bit value from host byte order to

network byte order. (ntohs provides the inverse)– htonl: convert 32-bit value from host byte order to

network byte order. (ntohl provides the inverse)

Sockets

• Machines can talk to each other regardless of type

• Provides a relatively simple API that will work with many different protocols

• Has become a standard for network programming across platforms

Making a Connection To A Server from a Client

• socket() provides the client with a socket– Specifies protocol and service type

• Protocol is typically Internet (AF_INET)

• Service is typically SOCK_STREAM (for reliable connection-oriented service or SOCK_DGRAM (for unreliable connectionless service)

• connect()– Specifies ip address and port number of server.

Starting a server

• socket() – same as client

• bind() – specify address family, IP address, and port number

• listen() – specify the number of clients that can be waiting for service (not the total number of clients)

• accept() – wait for someone to connect

WinSock

• Derived from Berkeley Sockets (Unix)– includes many enhancements for programming in

the windows environment

• Open interface for network programming under Microsoft Windows– API freely available– Multiple vendors supply winsock– Source and binary compatibility

• Collection of function calls that provide network services

gethostname()

• Get the name of the host the program is running on.– int gethostname(char *hostname, int

bufferLength)• Upon return hostname holds the name of the host

• bufferLength provides a limit on the number of bytes that gethostname can write to hostname.

Domain Name Library Routine(gethostbyname)

• gethostbyname(): Given a host name (such as acavax.lynchburg.edu) get host information.

– struct hostent* getbyhostname(char *hostname)• char* h_name; // official name of host

• char** h_aliases; // alias list

• short h_addrtype; // address family (e.g., AF_INET)

• short h_length; // length of address (4 for AF_INET)

• char** h_addr_list; // list of addresses (null pointer terminated)

• unsigned long inet_addr(char *address);– converts address in dotted form to a 32-bit

numeric value in network byte order• (e.g., “128.173.41.41”)

• char* inet_ntoa(struct in_addr address)– struct in_addr

• address.S_addr is the long int representation

Internet Address Library Routines(inet_addr() and inet_ntoa())

Internet Address Library Routines(gethostbyaddr)

• Get the name of the host the program is running on.– struct hostent* gethostbyaddr(char *address, int

addressLength, int type)• address is in network byte order

• addressLength is 4 if type is AF_INET

• type is the address family (e.g., AF_INET)

Differences Between Berkeley Sockets and WinSock

Berkeley WinSock

bzero() memset()

close() closesocket()

read() not required

write() not required

ioctl() ioctlsocket()

Additional Features of WinSock 1.1 • WinSock supports three different modes

– Blocking mode• socket functions don’t return until their jobs are done

• same as Berkeley sockets

– Nonblocking mode• Calls such as accept() don’t block, but simply return a

status

– Asynchronous mode• Uses Windows messages

– FD_ACCEPT - connection pending

– FD_CONNECT - connection established

– etc.

WinSock 2

• Supports protocol suites other than TCP/IP– DecNet– IPX/SPX– OSI

• Supports network-protocol independent applications

• Backward compatible with WinSock 1.1

WinSock 2 (Continued)

• Uses different files– winsock2.h– different DLL (WS2_-32.DLL)

• API changes– accept() becomes WSAAccept()– connect() becomes WSAConnect()– inet_addr() becomes WSAAddressToString()– etc.

Basic Socket Calls(socket)

// Berkeley form

int socket(int family

int type,

int protocol);

// WinSock form

SOCKET socket (int family,

int type,

int protocol);

socket (continued)

SOCKET socket (int family,

int type,

int protocol);

– family is address family

• AF_INET // internet protocols

• AF_UNIX // unix internal protocols

• AF_NS // Xerox NS protocols

• AF_IMPLINK // Interface Message Processor

– type is

• SOCK_STREAM // stream socket

• SOCK_DGRAM // datagram socket

• SOCK_RAW // raw socket

– protocol is usually zero in applications programming

main(int argc, char** argv){ const int expectedArguments = 1; if (argc != expectedArguments+1) { cerr << “usage: << argv[0] << “<serverName>” << endl; } else { cerr << “server name is “ << argv[1] << endl; // The rest of the client code goes here }}

Basic Socket Calls(bind)

// Berkeley form

int bind(int sockfd

struct sockaddr* addr,

int addrLen);

// WinSock form

int bind(SOCKET sockfd

struct sockaddr* addr,

int addrLen);

bind (continued)

int bind (SOCKET sockfd,

struct sockaddr* addr,

int addrLen);– sockfd was returned from the socket() call– addr is pointer to a sockaddr_in structure that

contains the server IP address and port number• struct sockaddr_in

– short sin_family // address family

– u_short sin_port // port number

– struct in_addr sin_addr //IP address (32-bits)

– addrLen - sizeof (struct sockaddr_in)

Basic Socket Calls(listen)

// Berkeley form

int listen(int s

int backlog);

// WinSock form

int listen (SOCKET s,

int backlog);

listen (continued)

int listen (SOCKET s,

int backlog);– s was returned from the socket() call– 1<=backlog<=5

• Backlog specifies the number of connection requests that may be simultaneously waiting for service.

• This is NOT the number of clients that can be receiving service simultaneously.

Basic Socket Calls(accept)

// Berkeley form

int accept(int s

struct sockaddr* addr,

int* pointerToAddrLen);

// WinSock form

SOCKET accept (SOCKET s,

struct sockaddr* addr,

int* pointerToAddrLen);

accept (continued)

SOCKET accept (SOCKET s,

struct sockaddr* addr,

int* pointerToAddrLen);– s was returned from the socket() call– addr is pointer to a sockaddr structure that will

contain the client information• struct sockaddr

– unsigned short sa_family // Address Family

» AF_INET, AF_UNIX, AF_NS, AF_IMPLINK

– char sa_data[14] // up to 14 bytes of protocol-specific address

• PointerToAddrLen can be set by accept• The returned socket provides communication with the

client.

Basic Socket Calls(connect)

// Berkeley form

int connect(int s

struct sockaddr* addr,

int sizeOfAddr);

// WinSock form

int connect (SOCKET s,

struct sockaddr* addr,

int sizeOfAddr);

connect (continued)

int connect (SOCKET s,

struct sockaddr* addr,

int sizeOfAddr);– s was returned from the socket() call– addr is pointer to a sockaddr structure that will

contain the server information• struct sockaddr_in

– short sin_family // address family

– u_short sin_port // port number

– struct in_addr sin_addr //IP address (32-bits)

• sizeOfAddr is sizeOf(struct sockaddr)

Basic Socket Calls(send)

// Berkeley form

int send(int s

const char* bytesToSend,

int nBytes,

int flags);

// WinSock form

int send (SOCKET s,

const char* bytesToSend,

int nBytes,

int flags);

send (continued)

int send (SOCKET s,

const char* bytesToSend,

int nBytes,

int flags);

– s was returned from the socket() call

– bytesToSend is a pointer to the data to send

– nBytes is the number of bytes to send

– flags

• e.g., MSG_OOB

• Note: There is no guarantee that send will send all the data requested. The number of bytes that send actually transmits is returned as a the result of the function. It may be necessary to call send repeatedly in a loop to achieve the desired result.

send (continued)

// Sample code that shows repeated calls to send

int mustSend (SOCKET s,

const char* bytesToSend,

int nBytes,

int flags)

{

int bytesSent = 0;

while(bytesSent < nBytes)

{

bytesSent += send(s, bytesToSend+bytesSent, nBytes-bytesSent, flags);

}

return(bytesSent);

}

Basic Socket Calls(recv)

// Berkeley form

int recv(int s

char* bytesToReceive,

int nBytes,

int flags);

// WinSock form

int recv(SOCKET s,

char* bytesToReceive,

int nBytes,

int flags);

recv (continued)

int recv (SOCKET s,

char* bytesToReceive,

int nBytes,

int flags);

– s was returned from the socket() call

– bytesToReceive is a pointer to the data buffer

– nBytes is the maximum number of bytes to receive

– flags

• e.g., MSG_OOB

• Note: There is no guarantee that recv will recv all the data requested. The number of bytes that recv actually received is returned by the function. It may be necessary to call recv repeatedly in a loop to achieve the desired result.

while( (*_readPtr != _delimiter) && (fieldCursor < maxFieldLength-1) ) { if (_charsInReadBuffer > 0) // Is there data to read? { field[fieldCursor] = *_readPtr; // Copy read data ++fieldCursor; ++_readPtr; --_charsInReadBuffer; }

// Do we need to read more data? while (_charsInReadBuffer == 0) { _charsInReadBuffer = recv(_socketfd, _readBuffer, maxFieldLength, 0); _readPtr = _readBuffer; }

}

Basic Socket Calls(write)

// Berkeley form

int write(int s

const char* bytesToSend,

int nBytes);

// WinSock form

// Not necessarily supported

// Uses Berkeley form when supported

write (continued)

int write (int s,

char* bytesToSend,

int nBytes);– s was returned from the socket() call

– bytesToSend is a pointer to the data to send

– nBytes is the number of bytes to send

• Note: There is no guarantee that write will send all the data requested. The number of bytes that write actually transmits is returned as a the result of the function. It may be necessary to call write repeatedly in a loop to achieve the desired result.

Basic Socket Calls(read)

// Berkeley form

int read(int s

char* bytesToReceive,

int nBytes);

// WinSock form

// Not necessarily supported

// Uses Berkeley form when supported

read (continued)

int read(SOCKET s,

char* bytesToReceive,

int nBytes);

– s was returned from the socket() call

– bytesToReceive is a pointer to the data buffer

– nBytes is the maximum number of bytes to receive

• Note: There is no guarantee that read will read all the data requested. The number of bytes that read actually received is returned by the function. It may be necessary to call read repeatedly in a loop to achieve the desired result.

Basic Socket Calls(shutdown)

// Berkeley form

int shutdown(int s, int howto);

// WinSock form

int shutdown(Socket s, int howto);

shutdown (continued)

int shutdown(SOCKET s, int howto);

– s was returned from the socket() call

– howto• SHUT_RD - shut down just the read direction. No more reads or

recvs can be issued on this socket. However, writes and sends can still be used.

• SHUT_WR - shutdown just the write direction. No more writes and sends can be issued on this socket. However, reads and recvs can still be used.

• SHUT_RDWR - this shuts down both directions. No more data can be transmitted using the socket. The socket still must be closed.

– Note: shutdown will terminate data transmission regardless of the socket reference count (i.e., even if other threads haven’t yet closed the socket.)

Basic Socket Calls(close/closesocket)

// Berkeley form

int close(int s);

// WinSock form

int closesocket(Socket s);

closesocket (continued)

int closesocket(SOCKET s);

– s was returned from the socket() call

• Note: There is no guarantee that read will read all the data requested. The number of bytes that read actually received is returned by the function. It may be necessary to call read repeatedly in a loop to achieve the desired result.

Basic Socket Calls(sendto)

// Berkeley form

int sendto(int s

const char* bytesToSend,

int nBytes,

int flags,

struct sockaddr *to,

int sizeOfSockaddr);

// WinSock form

int sendto(SOCKET s,

const char* bytesToSend,

int nBytes,

int flags,

struct sockaddr *to,

int sizeOfSockaddr);

sendto (continued)

int sendto (SOCKET s,

const char* bytesToSend,

int nBytes,

int flags

struct sockaddr *to,

int sizeOfSockaddr);

– to is a struct sockaddr_in• struct sockaddr_in

– short sin_family // address family– u_short sin_port // port number– struct in_addr sin_addr //IP address (32-bits)

• sizeOfSockaddr is sizeof(struct sockaddr_in)• Note: Although this function can be used with TCP connected sockets it is really

designed for use with UDP connectionless sockets. There is no guarantee that data sent with a successful send will arrive at its destination.

Basic Socket Calls(recvfrom)

// Berkeley form

int recvfrom(int s

char* receivedData,

int nBytes,

int flags,

struct sockaddr *from,

int sizeOfSockaddr);

// WinSock form

int recvfrom(SOCKET s,

char* receivedBytes,

int nBytes,

int flags,

struct sockaddr *from,

int sizeOfSockaddr);

recvfrom (continued)

int recvfrom (SOCKET s,

char* receivedData,

int nBytes,

int flags

struct sockaddr *from,

int sizeOfSockaddr);

– to is a struct sockaddr_in• struct sockaddr_in

– short sin_family // address family– u_short sin_port // port number– struct in_addr sin_addr //IP address (32-bits)

• sizeOfSockaddr is sizeof(struct sockaddr_in)• Note: Although this function can be used with TCP connected sockets it is really

designed for use with UDP connectionless sockets. There is no guarantee that data sent with a successful send will arrive at its destination.

Basic System Calls(fork)

// Berkeley form

int fork();

// WinSock form

// not necessarily available under windows

fork (continued)

int fork();

• Fork creates a new process– Typically the new process handles a client request while

the original process continues waiting for new clients.

for( ; ; ) { /* Wait for a connection. */ clilen = sizeof(cli_addr); newsockfd = accept(sockfd, (struct sockaddr *) &cli_addr, &clilen);

if (newsockfd < 0) { cerr << “accept failed, errno = “ << errno << endl; } else { // Create a new process to process the request int pid = 0; pid = fork(); // The child process handles the request if (pid != 0) processServerRequest(newsockfd); } }