![Socket Structures
●
struct sockaddr: Holds socket address information
for many types of sockets
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struct sockaddr_in: A parallel structure that makes
it easy to reference elements of the socket address
●
sin_port and sin_addr must be in Network Byte
Order
struct sockaddr {
unsigned short sa_family; //address family AF_xxx
unsigned short sa_data[14]; //14 bytes of protocol addr
}
struct sockaddr_in {
short int sin_family; // set to AF_INET
unsigned short int sin_port; // Port number
struct in_addr sin_addr; // Internet address
unsigned char sin_zero[8]; //set to all zeros
}](https://image.slidesharecdn.com/sockets-4-20-231111143122-d030d969/85/sockets-7-320.jpg)
sockets
- 1. Byte Ordering ● Two types of “ Byte ordering” – Network Byte Order: High-order byte of the number is stored in memory at the lowest address – Host Byte Order: Low-order byte of the number is stored in memory at the lowest address – Network stack (TCP/IP) expects Network Byte Order ● Conversions: – htons() - Host to Network Short – htonl() - Host to Network Long – ntohs() - Network to Host Short – ntohl() - Network to Host Long
- 2. What is a socket? ● Socket: An interface between an application process and transport layer – The application process can send/receive messages to/from another application process (local or remote)via a socket ● In Unix jargon, a socket is a file descriptor – an integer associated with an open file ● Types of Sockets: Internet Sockets, unix sockets, X.25 sockets etc – Internet sockets characterized by IP Address (4 bytes), port number (2 bytes)
- 5. Types of Internet Sockets ● Stream Sockets (SOCK_STREAM) – Connection oriented – Rely on TCP to provide reliable two-way connected communication ● Datagram Sockets (SOCK_DGRAM) – Rely on UDP – Connection is unreliable
- 6. socket() -- Get the file descriptor ● int socket(int domain, int type, int protocol); – domain should be set to PF_INET – type can be SOCK_STREAM or SOCK_DGRAM – set protocol to 0 to have socket choose the correct protocol based on type – socket() returns a socket descriptor for use in later system calls or -1 on error int sockfd; sockfd = socket (PF_INET, SOCK_STREAM, 0);
- 7. Socket Structures ● struct sockaddr: Holds socket address information for many types of sockets ● struct sockaddr_in: A parallel structure that makes it easy to reference elements of the socket address ● sin_port and sin_addr must be in Network Byte Order struct sockaddr { unsigned short sa_family; //address family AF_xxx unsigned short sa_data[14]; //14 bytes of protocol addr } struct sockaddr_in { short int sin_family; // set to AF_INET unsigned short int sin_port; // Port number struct in_addr sin_addr; // Internet address unsigned char sin_zero[8]; //set to all zeros }
- 8. Dealing with IP Addresses ● ● int inet_aton(const char *cp, struct in_addr *inp); – inet_aton() gives non-zero on success; zero on failure ● To convert binary IP to string: inet_noa() printf(“ %s” ,inet_ntoa(my_addr.sin_addr)); struct sockaddr_in my_addr; my_addr.sin_family = AF_INET; my_addr.sin_port = htons(MYPORT); inet_aton(“10.0.0.5”,&(my_addr.sin_addr)); memset(&(my_addr.sin_zero),'0',8); struct in_addr { unsigned long s_addr; // that's a 32bit long, or 4 bytes };
- 9. bind() - what port am I on? ● Used to associate a socket with a port on the local machine – The port number is used by the kernel to match an incoming packet to a process ● int bind(int sockfd, struct sockaddr *my_addr, int addrlen) – sockfd is the socket descriptor returned by socket() – my_addr is pointer to struct sockaddr that contains information about your IP address and port – addrlen is set to sizeof(struct sockaddr) – returns -1 on error – my_addr.sin_port = 0; //choose an unused port at random – my_addr.sin_addr.s_addr = INADDR_ANY; //use my IP adr
- 10. Example int sockfd; struct sockaddr_in my_addr; sockfd = socket(PF_INET, SOCK_STREAM, 0); my_addr.sin_family = AF_INET; // host byte order my_addr.sin_port = htons(MYPORT); // short, network byte order my_addr.sin_addr.s_addr = inet_addr("172.28.44.57"); memset(&(my_addr.sin_zero), '0', 8); // zero the rest of the struct bind(sockfd, (struct sockaddr *)&my_addr, sizeof(struct sockaddr)); /****** Code needs error checking. Don't forget to do that ****** /
- 11. connect() - Hello! ● Connects to a remote host ● int connect(int sockfd, struct sockaddr *serv_addr, int addrlen) – sockfd is the socket descriptor returned by socket() – serv_addr is pointer to struct sockaddr that contains information on destination IP address and port – addrlen is set to sizeof(struct sockaddr) – returns -1 on error ● No need to bind(), kernel will choose a port
- 12. Example #define DEST_IP "172.28.44.57" #define DEST_PORT 5000 main(){ int sockfd; struct sockaddr_in dest_addr; // will hold the destination addr sockfd = socket(PF_INET, SOCK_STREAM, 0); dest_addr.sin_family = AF_INET; // host byte order dest_addr.sin_port = htons(DEST_PORT); // network byte order dest_addr.sin_addr.s_addr = inet_addr(DEST_IP); memset(&(dest_addr.sin_zero), '0', 8); // zero the rest of the struct connect(sockfd, (struct sockaddr *)&dest_addr, sizeof(struct sockaddr)); /******* Don't forget error checking ********/
- 13. listen() - Call me please! ● Waits for incoming connections ● int listen(int sockfd, int backlog); – sockfd is the socket file descriptor returned by socket() – backlog is the number of connections allowed on the incoming queue – listen() returns -1 on error – Need to call bind() before you can listen() ● socket() ● bind() ● listen() ● accept()
- 14. accept() - Thank you for calling ! ● accept() gets the pending connection on the port you are listen()ing on ● int accept(int sockfd, void *addr, int *addrlen); – sockfd is the listening socket descriptor – information about incoming connection is stored in addr which is a pointer to a local struct sockaddr_in – addrlen is set to sizeof(struct sockaddr_in) – accept returns a new socket file descriptor to use for this accepted connection and -1 on error
- 15. Example #include <string.h> #include <sys/types.h> #include <sys/socket.h> #include <netinet/in.h> #define MYPORT 3490 // the port users will be connecting to #define BACKLOG 10 // pending connections queue will hold main(){ int sockfd, new_fd; // listen on sock_fd, new connection on new_fd struct sockaddr_in my_addr; // my address information struct sockaddr_in their_addr; // connector's address information int sin_size; sockfd = socket(PF_INET, SOCK_STREAM, 0);
- 16. Cont... my_addr.sin_family = AF_INET; // host byte order my_addr.sin_port = htons(MYPORT); // short, network byte order my_addr.sin_addr.s_addr = INADDR_ANY; // auto-fill with my IP memset(&(my_addr.sin_zero), '0', 8); // zero the rest of the struct // don't forget your error checking for these calls: bind(sockfd, (struct sockaddr *)&my_addr, sizeof(struct sockaddr)); listen(sockfd, BACKLOG); sin_size = sizeof(struct sockaddr_in); new_fd = accept(sockfd, (struct sockaddr *)&their_addr, &sin_size);
- 17. send() and recv() - Let's talk! ● The two functions are for communicating over stream sockets or connected datagram sockets. ● int send(int sockfd, const void *msg, int len, int flags); – sockfd is the socket descriptor you want to send data to (returned by socket() or got from accept()) – msg is a pointer to the data you want to send – len is the length of that data in bytes – set flags to 0 for now – sent() returns the number of bytes actually sent (may be less than the number you told it to send) or -1 on error