Network Programming

1. Network Programming UNIX Internet Socket API

2. Everything in Unix is a File • When Unix programs do any sort of I/O, they do it by reading or writing to a file descriptor. • A file descriptor is simply an integer associated with an open file. • The file can be: • Network connection. • Pipes. • A real file on-the-disk. • Just about anything else.

3. Two Types of Network Sockets Connection Oriented Sockets Datagram Sockets

4. Connection Oriented Sockets • Stream sockets are reliable two-way connected communication streams, both FIFO and Error free. • The “Transmission Control Protocol", otherwise known as "TCP“. • TCP makes sure your data arrives sequentially and error-free. • Used by Applications/Protocols: • Telnet • HTTP • FTP

5. Datagram sockets • Connectionless? You don't have to maintain an open connection as you do with stream sockets. You just build a packet and send it out. • Whenever you send a datagram: • it may arrive. • It may arrive out of order or duplicate. • If it arrives, the data within the packet will be error-free. • The “User Datagram Protocol ", otherwise knownas “UDP“. • What is it good for?

6. Technical Stuff

7. struct sockaddr struct sockaddr { unsigned short sa_family; char sa_data[14]; }; • Address family in this presentation:AF_INET • Contains a destination address and port number for the socket. The port number is used by thekernel to match an incoming packet to acertain process's socket descriptor.

8. struct sockaddr_in struct sockaddr_in { short int sin_family; unsigned short int sin_port; struct in_addr sin_addr; unsigned char sin_zero[8]; }; This structure makes it easy to reference elements of the socket address. Note that sin_zero should be set to all zeroswith the function memset().

9. struct sockaddr_in • A pointer to a struct sockaddr_incan be cast to a pointer to a struct sockaddrand vice-versa. • Also, notice thatsin_family corresponds to sa_family in a struct sockaddr and should be set to "AF_INET". • Finally, thesin_port and sin_addr (unsigned long ) must be in Network Byte Order! • struct in_addr { uint32_t s_addr; };

10. structsand Data Handling • A socket descriptor is just a regular int. • There are two byte orderings: • Most significant byte first a.k.a. "Network Byte Order". • Least significant byte first. • In order to convert "Host Byte Order“ to Network Byte Order, you have to call a function.

11. Big\Little Endian

12. Convert! • There are two types that you can convert: short and. These functions work for the unsigned variations as well. • htons() - "Host to Network Short" • htonl() - "Host to Network Long" • ntohs() - "Network to Host Short" • ntohl() - "Network to Host Long“ • Be portable! Remember: put your bytes inNetwork Byte Order before you put them on the network.

13. IP Addresses #include <sys/socket.h> #include <netinet/in.h> #include <arpa/inet.h> struct sockaddr_in my_addr; my_addr.sin_family = AF_INET; my_addr.sin_port = htons(3490); inet_aton("10.12.110.57", &(my_addr.sin_addr)); memset(&(my_addr.sin_zero), '\0', 8);inet_aton(), unlike practically every other socket-related function, returns non-zero on success,and zero on failure.

14. Making the Connection

15. socketsystem call #include <sys/types.h> #include <sys/socket.h> int socket(int domain, int type, int protocol); • domain - should be set to PF_INET. • type - SOCK_STREAM or SOCK_DGRAM. • protocol -set to 0, Let the kernelchoose the correct protocol based on the type. • socket() simply returns to you a file (i.e. socket) descriptor that you can use in later system calls, or -1 on error and sets errno tothe error's value.

16. bindsystem call • Once you have a socket, you might have to associate that socket with a port on your local machine (address). • This is commonly done if you're going to listen() for incoming connections on a specific port. int bind(int sockfd, struct sockaddr *my_addr, int addrlen);

17. bindsystem call cont. • All ports below 1024 are reserved. • HTTP 80 • Telnet 23 • You can have any available port number above that, right up to 65535 • In order to use my IP address. my_addr.sin_addr.s_addr = htonl(INADDR_ANY); bind() also returns -1 on error and setserrno to the error's value.

18. bindsystem call cont. • When we get - “Address already in use.” • We can wait, or we can add the following code: int yes=1; If (setsockopt (listener, SOL_SOCKET, SO_REUSEADDR, &yes,sizeof(int)) == -1) { perror("setsockopt"); exit(1); }

19. listensystem call int listen(int sockfd, int backlog); • Wait for incoming connections and handle them in some way. The process is two step: first you listen(), then you accept(). • sockfdis the usual socket file descriptor from the socket() system call. • backlog is the number of connections allowed on the incoming queue. • As usual, listen() returns -1 and setserrnoon error.

20. Stream Style

21. acceptsystem call Scenario: • A client will try to connect() to your machine on a port that you are listen()ing on. • Their connection will be queued up waiting to be accept()ed. • You call accept() and you tell it to get the pending connection. • It’ll return to you a brand new socket file descriptor to use for this single connection!

22. acceptsystem call cont. int accept(int sockfd, void *addr, int *addrlen); • sockfd is the listen()ing socket descriptor. • addr will usually be a pointer to a local structsockaddr_in. This is where the information about the incoming connection will go. • addrlenis a local integer variable that should be set to sizeof(struct sockaddr_in) before its address is passed to accept(). • As usual, accept() returns -1 and setserrno on error.

23. connectsystem call int connect(int sockfd, struct sockaddr *serv_addr, int addrlen); • sockfd is socket file descriptor. • serv_addris a struct sockaddr containing the destination port and IP address. • addrlen can be set to sizeof(struct sockaddr). • Be sure to check the return value fromconnect()-it'll return -1 on error and set the variable errno.

24. Summary • if you're going to be listening for incoming connections, the sequence of system calls you'll make is: • socket(); • bind(); • listen(); • accept();

25. send()system call int send(intsockfd, const void *msg, intlen, int flags); sockfd is the socket descriptor you want to send data to. msg is a pointer to the data you want to send. len is the length of that data in bytes. flags set to 0. send()returns the number of bytes actually sent out. -1 is returned on error, and errno is set to theerror number.

26. recv()system call int recv(int sockfd, void *buf, int len, unsigned int flags); • sockfd is the socket descriptor to read from • buf is the buffer to read the information into. • lenis the maximum length of the buffer, • flags can again be set to 0. • recv() returns the number of bytes actually read into the buffer, or -1 on error with errno set, accordingly. • recv() can return 0. This means the remoteside has closed the connection.

27. Datagram Style

28. sendto()system call int sendto(int sockfd, const void *msg, int len, unsigned int flags, const struct sockaddr *to, int tolen); • This call is basically the same as the call to send() with the addition of two other pieces of information. • to is a pointer to a struct sockaddr. • tolen can simply be set to sizeof(struct sockaddr). • Just like with send(),sendto() returnsthe number of bytes actually sent, or -1 onerror.

29. recvfrom()system call int recvfrom(int sockfd, void *buf, int len, unsigned int flags, struct sockaddr *from, int *fromlen); • This is just like recv() with the addition of a couple fields. • from is a pointer to a local struct sockaddr that will be filled with the IP address and port of the originating machine. • fromlen is a pointer to a local int that should be initialized to sizeof(struct sockaddr). When the function returns, fromlen will contain the length ofthe address actually stored in from. • recvfrom() returns the number of bytes received,or -1 on error with errno set accordingly.

30. closesystem call close(sockfd); This will prevent any more reads and writes to the socket. Anyone attempting to read or write the socket on the remote end will receive an error.

31. Summary Stream Socket • Server Side • socket(); • bind(); • listen(); • accept(); • send()/recv() • Client Side • socket(); • connect(); • send()/recv()

32. Summary Datagram Socket • Listener side: • socket(); • bind(); • recvfrom(); • Talker side: • socket(); • connect();//op • sendto(); • By using connect(), talker can send \ receive to \ from a specific address. For this purpose, you don't have to use sendto() and recvfrom()you can simply use send() and recv().

33. Blocking Vs. Non-Blocking

34. Blocking Synchronous When you first create the socket descriptor with socket(), the kernel sets it to blocking. If you don't want a socket to be blocking, you have to make a call to fcntl(): #include <unistd.h> #include <fcntl.h> #include <sys/socket.h> sockfd = socket(AF_INET, SOCK_STREAM, 0); fcntl(sockfd, F_SETFL, O_NONBLOCK); If you try to read from a non-blocking socket and there’sno data there, it’s not allowed to block - it will return -1and errno will be set to EWOULDBLOCK

35. Blocking Vs. Non Blocking If you put your program in a busy-wait looking for data on the socket, you’ll suck up CPU time. A more elegant solution for checking to see if there’s data waiting to be read comes in synchronous mechanism of select().

36. select()Synchronous I/O Multiplexing select() gives you the power to monitor several sockets at the same time. It will tell you which ones are ready for reading, which are ready for writing, and which sockets have raised exceptions. #include <sys/time.h> #include <sys/types.h> #include <unistd.h> int select(intnumfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds, structtimeval *timeout);

37. select() • The parameter numfds should be set to the values of the highest file descriptor plus one. • In order to manipulate fd_setuse the following macros: • FD_ZERO(fd_set *set) //clears the set • FD_SET(int fd, fd_set *set) //adds fd to the set • FD_CLR(int fd, fd_set *set) //removes fd from the set • FD_ISSET(int fd, fd_set *set) //tests to see if fd is in the set.

38. select() • If the time specified in structtimeval is exceeded and select() still hasn't found any ready file descriptors, it'll return so you can continue processing. structtimeval { inttv_sec; //seconds inttv_usec; //microseconds }; • If you set the fields in your structtimeval to 0, select() will timeout immediately, effectivelypolling all the file descriptors in your sets. • If you set the parameter timeout to NULL, it willnever timeout, and will wait until the first file descriptor is ready.

39. select() if you don't care about waiting for a certain set, you can just set it to NULL in the call to select(). if you have a socket that you are listen()'ing to, you can check for a new connection by putting that socket’s file descriptor in the readfds set. On success, select() returns the number ready descriptors contained in the descriptor sets, which may be zero if the timeout expires before anything interesting happens. On error, -1 is returned, and errno is set appropriately;

40. Example int main() { structtimevaltv; fd_setreadfds; tv.tv_sec = 2; tv.tv_usec = 500000; FD_ZERO(&readfds); FD_SET(STDIN, &readfds); select(STDIN+1, &readfds, NULL, NULL, &tv); if (FD_ISSET(STDIN, &readfds)) printf("A key was pressed!\n"); else printf("Timed out.\n"); return 0; }

41. Address Related System Calls

42. getpeername The function getpeername() will tell you who is at the other end of a connected stream socket. The synopsis: • intgetpeername(intsockfd, structsockaddr *addr, int *addrlen); • sockfd is the descriptor of the connected stream socket. • addr is a pointer to a structsockaddr that will hold the information about the other side of the connection, • addrlen is a pointer to an int, that should beinitialized to sizeof(structsockaddr). • The function returns -1 on error and setserrnoaccordingly.

43. Domain Name Service -gethostname • DNS is an acronym for Domain Name Service. This service maps human-readable address (a.k.a. host names) to IP addresses. This allows computers to be accessed remotely by name instead of number. • The function gethostname() returns the name of the computer that your program is running on. The name can then be used by gethostbyname() to determine the IP address of your local machine. #include <unistd.h> int gethostname(char *hostname, size_t size);

44. Domain Name Service - gethostbyname #include <netdb.h> struct hostent * gethostbyname(const char *name); returns a pointer to the filled struct hostent, or NULL on error.

45. struct hostent struct hostent { char *h_name;//Official name of the host. char **h_aliases;//Alternate names. int h_addrtype;//usually AF_INET. int h_length;//length of the address. char **h_addr_list; //network addresses for the host in N.B.O. }; #define h_addr h_addr_list[0]

46. Example int main(intargc, char *argv[]) { structhostent *h; if (argc != 2) { fprintf(stderr, "usage: getip address\n"); exit(1); } if ((h=gethostbyname(argv[1])) == NULL) { herror("gethostbyname"); exit(1); } printf("Host name : %s\n", h->h_name); printf("IP Address : %s\n", inet_ntoa(*((structin_addr *)h->h_addr))); return 0; }

47. IPv6 - The next-generation IPv4 is finished (all addresses given out) Bigger address space (128 bit) More security \ New features Supported by all the new operating system butstill not so widespread….