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Inter-Host Communication. Berkeley Sockets.

Inter-Host Communication. Berkeley Sockets. Berkeley Sockets This is API (Application Program Interface) for different Communication Protocol Suites (TCP/IP, Unix Domain, XNS, etc) Socket API contains the set of system calls for establishing of network connection and transfer of data:

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Inter-Host Communication. Berkeley Sockets.

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  1. Inter-Host Communication.Berkeley Sockets. • Berkeley Sockets • This is API (Application Program Interface) for different Communication Protocol Suites • (TCP/IP, Unix Domain, XNS, etc) • Socket API contains the set of system calls for establishing of network connection and transfer of data: • socket() create endpoint • bind() bind address • listen() specify request queue • accept() wait for connection • connect() connect to peer • read(), write(), transfer data • recv(), send(), • recvfrom(), sendto(), • recvmsg(), sendmsg() • close(), shutdown() terminate connection Introduction to Network Programming in UNIX & LINUX

  2. 5-Tuple Association and Socket Address • Generic Model • Association = { Protocol, Port A, Address A, Port B , Address B } • Socket Address = { Protocol, Port, Address } • TCP/IP Model • TCP Association = {TCP, TCP Port A, IP Address A, TCP Port B, IP Address B } • UDP Association = {UDP, UDP Port A, IP Address A, UDP Port B, IP Address B } • Socket Address = {TCP/UDP, Port, IP Address } • Note: • In IPv4 and IPv6 the Socket Address has different format because of • different length of IP Address ( 32 bits for IPv4 and 128 bit for IPv6) • Unix Domain • The Unix Domain protocols are not an actual protocol suite, but a way of performing client / server • Communication on a single host using the same API that is used for clients and servers on different hosts. • The Unix Domain protocols are an alternative to the inter-process communication (IPC). There are two protocols: • - UNIXSTR Stream Protocol (analog of TCP) • - UNIXDG Datagram Protocol (analog of UDP) • The Unix Domain socket binding is provided to file path. • Unix Domain Association = { UNIXSTR / UNIXDG, File Path A, 0, File Path B, 0 } • Socket Address = { UNIXSTR / UNIXDG, File Path } # netstat -a Active Internet connections (servers and established) Proto Recv-Q Send-Q Local Address Foreign Address State tcp 0 0 *:bootpc *:* LISTEN tcp 0 0 *:x11 *:* LISTEN tcp 0 0 *:2055 *:* LISTEN tcp 0 0 Knoppix:2055 Knoppix:44992 ESTABLISHED tcp 0 0 Knoppix:44992 Knoppix:2055 ESTABLISHED udp 0 0 *:2055 *:* Active UNIX domain sockets (servers and established) Proto RefCnt Flags Type State I-Node Path unix 2 [ ACC ] STREAM LISTENING 10226 /var/run/dbus/system_bus_socket unix 2 [ ACC ] STREAM LISTENING 13345 /ramdisk/tmp/ksocket-knoppix/kdeinit__0 unix 2 [ ] DGRAM 2318 @/org/kernel/udev/udevd Introduction to Network Programming in UNIX & LINUX

  3. Note: Due to the standard, the fields of Socket Address structure would be filled with Network byte order. This is big-endian order, when upper byte has higher address in memory. Socket Address Representation Generic Socket Address Structure A socket address structure is always passed by reference when passed as an argument to any socket functions. All these functions are declared to accept the following generic socket address structure: #include <sys/socket.h> struct sockaddr { sa_family_t sa_family; /* address family: AF_LOCAL, AF_INET, AF_INET6, … */ char sa_data[14]; /* protocol-specific address */ }; IPv4 Socket Address Structure #include <netinet/in.h> struct in_addr { in_addr_t s_addr; /* 32-bit IPv4 address, network byte ordered */ }; struct sockaddr_in { sa_family_t sin_family; /* AF_INET */ in_port_t sin_port; /* 16-bit TCP or UDP port number, network byte ordered */ struct in_addr sin_addr; /* 32-bit IPv4 address, network byte ordered */ char sin_zero[8]; /* unused */ }; Unix Domain Address Structure #include <sys/un.h> struct sockaddr_un { sa_family_t sun_family; /* AF_LOCAL */ char sun_path[108]; /* null-terminated pathname */ }; Introduction to Network Programming in UNIX & LINUX

  4. Socket Address Encoding Functions Byte Ordering Functions #include <netinet/in.h> uint16_t htons(uint16_t host16bitvalue) ; /* Host TONetwork Short converter */ uint32_t htonl(uint32_t host32bitvalue) ; /* Host TONetwork Long converter */ uint16_t ntohs(uint16_t net16bitvalue) ; /* Network TOHost Short converter */ uint32_t ntohl(uint32_t net32bitvalue) ; /* Network TOHost Long converter */ Byte Manipulation Functions #include <strings.h> void bzero(void *dest, size_t nbytes); /* places n null bytes in the string dest */ void bcopy(const void *src, void *dest, size_t nbytes); /* copies n bytes from src to dest */ int bcmp(const void *ptr1, const void *ptr2, size_t nbytes); /* returns 0 if strings are identical, 1 otherwise */ Address Conversion Functions These functions convert IP Address from decimal dotted notation to integer and vice versa. #include <arpa/inet.h> int inet_aton(const char *strptr, struct in_addr *addrptr); /* Address TONetwork. Return:1-success / 0-error */ in_addr_t inet_addr(const char *strptr); /* Deprecated, returns INADDR_NONE on error */ int inet_pton(int af, const char *strptr, void *addrptr); /* Presentation TONetwork. Analog of inet_aton(), supporting different address families */ char *inet_ntoa(struct in_addr inaddr); /* Network TOAddress. Return: pointer to string */ const char *inet_ntop(int af, const void *addrptr, /* Network TOPresentation. Analog of inet_ntoa(), char *strptr, size_t strlen); supporting different address families */ Introduction to Network Programming in UNIX & LINUX

  5. Create The Socket #include <sys/types.h> #include <sys/socket.h> int socket (int family, int type, int protocol); • Creates the Socket - the endpoint for communication • Parameter family specifies Address (or Protocol) Family: • AF_INET IPv4 • AF_INET6 IPv6 • AF_LOCAL (or AF_UNIX) Unix Domain • Parameter type specifies the type of socket: • SOCK_DGRAM datagram socket • SOCK_STREAM stream socket • SOCK_RAW raw socket (exit to Network Layer) • Parameter protocol defines the protocol. • If 0, SOCK_DGRAM UDP, SOCK_STREAM  TCP. • Other values: • IPPROTO_UDP • IPPROTO_TCP • IPPROTO_ICMP (used only with SOCK_RAW type) • IPPROTO_RAW (used only with SOCK_RAW type) • Returns Socket Descriptor on success, -1 on error (errno specifies the error) Note: Each AF_... constant has corresponded PF_... constant of the same value, which could be used in the same way. Introduction to Network Programming in UNIX & LINUX

  6. Bind: Assign the Local Address to Socket #include <sys/types.h> #include <sys/socket.h> int bind (int sockfd, const struct sockaddr * addr, socklen_t addrlen); • Assigns the local IP Address and Port, specified by addr parameter, to the socket. • Non-specific IP Address is specified by constant INADDR_ANY, non-specific port could be specified by value 0. • Server binds well-known IP Address and Port. • If INADDR_ANY and specific Port is specified, the Server will listen on this Port from any of host’s IP Addresses. • Clientbinds the specific or non-specific IP Address and Port. • Returns 0 on success, -1 on error Example: Server Socket Binding #define SERV_HOST_ADDR “145.9.112.75” #define SERV_TCP_PORT 5678 struct sockaddr_in servAddr; int sockfd; /* create socket */ sockfd = socket(AF_INET, SOCK_STREAM, 0); if (sockfd < 0) {perror….;} /* prepare Server binding */ bzero((char*)&servAddr, sizeof(servAddr)); servAddr.sin_family = AF_INET; If (inet_aton(SERV_HOST_ADDR, &servAddr.sin_addr) == 0) {perror…;} servAddr.sin_port = htons(SERV_TCP_PORT); /* bind the socket */ if(bind(sockfd, (struct sockaddr*)&servAddr, sizeof(servAddr))<0) {perror….;} Example: Client Socket Binding struct sockaddr_in cliAddr; int sockfd; /* create socket */ sockfd = socket(AF_INET, SOCK_STREAM, 0); if (sockfd < 0) {perror….;} /* prepare Client binding */ bzero((char*)&cliAddr, sizeof(cliAddr)); cliAddr.sin_family = AF_INET; cliAddr.sin_addr.s_addr =htonl(INADDR_ANY); cliAddr.sin_port = htons(0); /* bind the socket */ if(bind(sockfd, (struct sockaddr*)&cliAddr, sizeof(cliAddr))<0) {perror….;} Introduction to Network Programming in UNIX & LINUX

  7. Connect: Assign the Foreign Address to Socket #include <sys/types.h> #include <sys/socket.h> int connect(int sockfd, const struct sockaddr *addr, socklen_t addrlen); • Used to specify foreign Address. • For connection-oriented protocols (TCP, socket type = SOCK_STREAM) this call is used only by Client. • It establishes actual connection with Server, using address, specified by addr parameter. • (TCP sends SYN segment, waits for ACK segment and analyzes the cause of possible connection failure). • For connectionless protocols (UDP, socket type = SOCK_DGRAM) this call is optional. • It could be used by Server or by Client to store the already known foreign Address and to use it for following • datagram sending / receiving. In this case foreign Address would not be specified for each sending datagram • and would not be extracted from each receiving datagram. • For connectionless protocol the actual connection with Server is not established. • “Connection refused” error could be identified only after next system call, really sending the data. • Before calling connect() Client does not have to perform bind() call. In this case connect() call will assign also • the local address to the socket (as it done by bind(), called by Client with INADDR_ANY and port 0 ). • Connection-oriented sockets can successfully connect() only once. • Connectionless sockets can use connect() multiple times to change their association. • Connectionless sockets can dissolve the association, specifying AF_UNSPEC family in call to connect() • (or NULL pointer to address structure on some UNIX systems – see ‘man connect’). • Returns 0 on success, -1 in case of error. • If connect() fails, connection-oriented the socket is no longer usable and must be closed. Introduction to Network Programming in UNIX & LINUX

  8. Send Data Through Socket #include <sys/types.h> #include <sys/socket.h> ssize_t write ( int sockdescr, const void* buf, size_t buflen); ssize_t send ( int sockdescr, const void* buf, size_t buflen, int flags); ssize_t sendto ( int sockdescr, const void* buf, size_t buflen, int flags, const struct sockaddr *to, int tolen); struct msghdr { void * msg_name; /* optional address */ socklen_t msg_namelen; /* size of address */ struct iovec * msg_iov; /* scatter/gather array – handles the list of memory fragments to be read/written by single system call*/ size_t msg_iovlen; /* number of elements in msg_iov */ void * msg_control; /* ancillary data*/ socklen_t msg_controllen; /* ancillary data buffer len */ }; ssize_t sendmsg( int sockdescr, const struct msghdr *msg, int flags); • These calls are used to transmit a message buf of length buflen to another transport end-point. • Calls send() and write() may be used only when the socket is in a connected state • (Note! On some UNIX systems sending of empty datagram on connected UDP socket • is impossible with write() call. The call send() would be used instead. ) • Calls sendto() and sendmsg() may be used at any time. • The target address is specified by parameter to of length tolen . • If the message is too long to pass atomically through the underlying protocol, then the error EMSGSIZE is • returned, and the message is not transmitted. • If the socket does not have enough buffer space available to hold the message being sent, send() blocks, • unless the socket has been placed in non-blocking I/O mode (see fcntl). • The flags parameter is formed from the bitwise OR of zero or more of the following: • MSG_OOB • Send "out-of-band" data. Supported only by SOCK_STREAM sockets of AF_INET (AF_INET6) families. • MSG_DONTROUTE • The SO_DONTROUTE option is turned on for the duration of the operation. • It is used only by diagnostic or routing programs. • The call sendmsg() call uses a msghdr structure to minimize the number of directly supplied parameters. • These calls return the number of bytes sent, or -1 if an error occurred. Introduction to Network Programming in UNIX & LINUX

  9. Receive Data from Socket #include <sys/types.h> #include <sys/socket.h> #include <sys/uio.h> ssize_t read (int sockdescr, void *buf, size_t buflen); ssize_t recv ( int sockdescr, void *buf, size_t buflen, int flags); ssize_t recvfrom ( int sockdescr, void *buf, size_t buflen, int flags, struct sockaddr *from, int *fromlen); ssize_t recvmsg ( int sockdescr, struct msghdr *msg, int flags); • These calls are used to receive message from another socket to buffer buf of length buflen. • Calls read(), recv() may be used only on a connected socket • Calls recvfrom() and recvmsg() may be used to receive data on a socket whether it is in a connected state or not. • If parameter from is not a NULL pointer, the source address of the message is filled in. • Parameter fromlen is a value-result parameter, initialized to the size of the buffer from, and modified on return to • indicate the actual size of the address stored there. • If a message is too long to fit in the supplied buffer, excess bytes may be discarded depending on the type of • socket the message is received from. • If no messages are available at the socket, the receive call waits for a message to arrive, unless the socket is non- • blocking, in which case -1 is returned with errno set to EWOULDBLOCK. ( =EAGAIN. See fcntl() ) • The flags parameter is formed by ORing one or more of the following: • MSG_OOB • Read any "out-of-band" data present on the socket rather than the regular "in-band" data. • MSG_PEEK • "Peek" at the data present on the socket; the data is returned, but not consumed, • so that a subsequent receive operation will see the same data. • The call recvmsg() call uses a msghdr structure to minimize the number of directly supplied parameters. • These calls return the number of bytes received, or -1 if an error occurred. Introduction to Network Programming in UNIX & LINUX

  10. Get Local and Foreign Address of the Socket Close the Socket #include <sys/socket.h> int getsockname(int sockfd, struct sockaddr *localaddr, socklen_t *addrlen); int getpeername(int sockfd, struct sockaddr *peeraddr, socklen_t *addrlen); • These functions used to extract the local and foreign IP Address and Port of the Socket • Return 0 on success, -1 on error. int close (int sockdescr); int shutdown (int sockdescr, int how); • These calls close the socket connection. • Call shutdown() call shuts down all or part of a full-duplex connection. • If how is 0, then further receives will be disallowed. • If how is 1, then further sends will be disallowed. • If how is 2, then further sends and receives will be disallowed. • The system returns from these calls immediately, but in case of TCP protocol the kernel still tries to send • already queued data (if SO_LINGER socket option is not specified). • These calls return 0 on success , -1 in case of failure. Introduction to Network Programming in UNIX & LINUX

  11. UDP Client UDP Server socket( ) socket( ) well-known port bind( ) data (request) blocks until datagram received from client sendto( ) recvfrom( ) …process request… data (reply) sendto( ) recvfrom( ) no …process reply… end session ? yes no no end session ? stop server ? yes yes close( ) close( ) exit( ) exit( ) UDP Socket Example 1. Iterative Server Introduction to Network Programming in UNIX & LINUX

  12. UDP Client UDP Server socket( ) socket( ) well-known port connect( ) bind( ) data (request) send( ) recvfrom( ) blocks until datagram received from client connect( ) …process request… data (reply) recv( ) send( ) no end session ? …process reply… yes connect(..AF_UNSPEC or NULL.. ) no end session ? yes no stop server ? yes close( ) close( ) exit( ) exit( ) UDP Socket Example 2. Using connect(). Introduction to Network Programming in UNIX & LINUX

  13. UDP Server UDP Client socket( ) socket( ) well-known port bind( ) data (request) recvfrom( ) sendto( ) blocks until datagram received from client fork( ) subserver no is parent ? yes no close parent socket stop server ? close( ) yes close( ) recvfrom(..MSG_PEEK.. ) socket( ) data (1st reply) exit( ) connect( ) connect( ) …process request… data (reply) recv( ) send( ) no end session ? yes …process reply… close( ) no end session ? yes exit( ) close( ) exit( ) UDP Socket Example 3. Concurrent Server. Introduction to Network Programming in UNIX & LINUX

  14. UDP Examples Synchronization Problems • Problem 1 End of response notification • Client would know, when the last portion of data arrived from Server. • Possible Solution • Last datagram sent by Server would be empty. • Problem 2 Connection timeout • Client not always could receive “connection refused” error message even if it uses connect() system call. • Possible Solution • The connection timeout would be handled by Client. • Problem 3 Lost and disordered datagrams • Some of datagrams could be lost because of: • Socket buffer overflow, when Server sends more quickly than Client is able to receive and process • Because of network problems • Because of unexpected termination of peer process • The datagrams could be received by Client in another order, than they were sent by the Server, because • each datagram could have its own route. • Possible Solution • Implementing of sequence control and flow by means of Acknowledgement with Retransmission algorithms Introduction to Network Programming in UNIX & LINUX

  15. read request (RRQ) 1 file name 0 transfer mode 0 2 bytes write request (WRQ) 2 file name 0 transfer mode 0 3 block # data up to 512 bytes data 2 bytes acknowledgement (WRQ) 4 block # 5 errcode error message 0 error 2 bytes TFTP – Trivial File Transfer Protocol(Example of UDP-based standard application protocol) TFTP is standard UDP-based application protocol, providing simple method of transferring files between two hosts. Developed and standardized in 1981. It is much smaller and easier than FTP (File Transfer Protocol). Unlike FTP, the TFTP provides only file transfer and does not provide user authentication, directory listing, etc. Because of simplicity, TFTP could be used for bootstrap of LAN workstations. TFTP Message Formats TFTP supports 2 transfer modes: “octet” and “netascii” Error codes: 1- File not found 2- Access violation 3- Disk full 4- Illegal FTP operation 5- Unknown port 6- File already exists 7- No such user All 2-bytes fields: opcode, block#, errcode are stored in Network Byte Order Introduction to Network Programming in UNIX & LINUX

  16. File Client (receiver) Server (sender) RRQ data, block #1 ACK, block #1 data, block #2 ACK, block #2 data, block #3 . . . . . . . . . File Client (sender) Server (receiver) WRQ ACK, block #0 data, block #1 ACK, block #1 data, block #2 ACK, block #2 . . . . . . . . . TFTP Server is standard UNIX daemon. TFTP Client is realized as standard UNIX utility tftp TFTP Transfer Scenarios • TFTP Server is concurrent. • TFTP Server well-known port is 69/udp • TFTP Client initiates connection, sending • RRQ (read from Server) or WRQ (write to Server) • first datagram. • Server main process spawns child Sub-Server • process to handle Client request. Main process • then returns to listening on well-known port. • Sub-Server creates new socket and binds it with • unique local port. All following responses to the • Client then sent from this port. • Client reads Sub-Server address from 1st response • and then uses it for all following session. • The data block shorter than 512 bytes is • recognized as the last data portion. Introduction to Network Programming in UNIX & LINUX

  17. send DATA(n) (time out) retransmit DATA(n) receive ACK(n) send DATA(n+1) receive ACK(n) (duplicate) send DATA(n+1) (duplicate) receive ACK(n+1) send DATA(n+2) receive ACK(n+1) (duplicate) send DATA(n+2) (duplicate) . . . receive DATA(n) send ACK(n) receive DATA(n) (duplicate) send ACK(n) (duplicate) receive DATA(n+1) send ACK(n+1) receive DATA(n+1) (duplicate) send ACK(n+1) (duplicate) receive DATA(n+2) send ACK(n+2) . . . send DATA(n) (time out) retransmit DATA(n) receive ACK(n) send DATA(n+1) receive ACK(n) (duplicate) (don’t send anything) receive ACK(n+1) send DATA(n+2) . . . receive DATA(n) send ACK(n) receive DATA(n) (duplicate) send ACK(n) (duplicate) receive DATA(n+1) send ACK(n+1) receive DATA(n+2) send ACK(n+2) . . . “Sorcerer’s Apprentice Syndrome” Sorcerer's Apprentice Syndrome (SAS) is a particularly bad network protocol flaw in the original versions of TFTP. Occurred in case of packet delay, which was not taken into account when the protocol was designed. It was named after the “Sorcerer's Apprentice” segment of Walt Disney motion picture Fantasia. Packet delay led to growing number of duplicated packets with following “chain reaction”, congestive collapse of network and transfer failure. • Fixed Design: • Receiver does not have retransmission timer • Sender ignores duplicated acknowledgement • Original Bad Design: • Both Server and Receiver use timeout with retransmission • Receiving duplicated acknowledgement, Server retransmits the data Introduction to Network Programming in UNIX & LINUX

  18. server sum of both queues cannot exceed backlog accept TCP connection queues three-way handshake complete arriving SYN 3-Way handshake and TCP connection queues client server connect called SYN n create entry on incomplete queue RTT (round-trip time) SYN k, ACK n+1 RTT (round-trip time) connect returns ACK k+1 entry moved to completed queue accept returns Listen, Accept - TCP Server-Specific System Calls #include <sys/socket.h> int listen (int sockfd, int backlog); • Assigns the length of the queue • for TCP connection requests. • Parameter backlog specifies the number of • pending requests queued by system. • If queue full, Client performs connection request • retransmission. • Returns 0 on success, -1 on error completed connection queue (ESTABLISHED state) incomplete connection queue (SYN_RCVD state) #include <sys/socket.h> int accept (int sockfd, struct sockaddr *cliaddr, socklen_t *addrlen); • Returns the next completed connection from • the front of the completed connection queue. • If the completed connection queue is empty, • the process is put to sleep • The cliaddr and addrlen parameters are used • to return the address of connected Client. • Returns new socket descriptor automatically • created by the kernel. Returns -1 in case of error. • Usually a Server creates only one listening socket, • which then exists for the all lifetime of the server. • The kernel creates one connected socket for each accept()-ed client connection. • When a Server (or concurrent Sub-Server) is finished serving a given client, • the connected socket is closed. Introduction to Network Programming in UNIX & LINUX

  19. TCP Client TCP Server listen (listenSock,…) acceptSock = accept (listenSock,…) fork( ) bind (listenSock,…) listenSock = socket(...) close (listenSock) recv (acceptSock,…) …process request… socket( ) close (acceptSock) close (acceptSock) exit( ) connect( ) send( ) recv( ) …process reply… exit( ) close( ) send (acceptSock,…) connection establishment blocks until connection request from client concurrent server only yes is parent ? no data (request) blocks until data received from client data (reply) no stop server ? yes no end session ? close(listenSock) yes no end session ? exit( ) yes iterative server only concurrent sub-server only TCP Socket Examples. Iterative & Concurrent Server. Introduction to Network Programming in UNIX & LINUX

  20. UDP and TCP Server Comparison Introduction to Network Programming in UNIX & LINUX

  21. #include <sys/socket.h> int getsockopt(int sockfd, int level, int optname, void *optval, socklen_t *optlen); int setsockopt(int sockfd, int level, int optname, const void *optval, socklen_t optlen); Socket Options and Control Operations • There are various ways to get and set the options that affect a socket: • System call fcntl() - provides control functions on files and sockets • System call ioctl() - provides control functions on files, sockets, terminal and devices • System call setsockopt() • These system calls get / set the value of Socket Option for socket, specified by descriptor sockfd • Parameter level specifies subsystem responsible for the option, for example: • SOL_SOCKET – general socket code • IPPROTO_IP, IPPROTO_TCP – protocol-specific code • Parameter optname specifies ID of the specific option. • Parameter optval used to specify/extract the specific value of the option. • Parameter optlen specifies length of optval. • The option could be binary flag (0 or 1) or more complex value. • The following socket options are inherited by a connected TCP socket from the listening socket: SO_DEBUG, SO_DONTROUTE, SO_KEEPALIVE, SO_LINGER, SO_OOBINLINE, SO_RCVBUF, SO_RCVLOWAT, SO_SNDBUF, SO_SNDLOWAT, TCP_MAXSEG, and TCP_NODELAY. To ensure that one of these socket options is set for the connected “accept” socket when the three-way handshake completes, we must set that option for the “listen” socket. Example:don't wait for TIME_WAIT state delay expiration before TCP Server restart. int on=1; …. if(setsockopt(sockListen,SOL_SOCKET,SO_REUSEADDR,(char *)&on,sizeof(on)) < 0) { perror(…); } Introduction to Network Programming in UNIX & LINUX

  22. Socket Options Level Name Description Settable Data Type IPPROTO_IP IP_OPTIONS options in IP header yes char[] IPPROTO_UDP UDP_NOCHECKSUM enables sending if datagrams with checksum=0 yes int (bool) IPPROTO_TCP TCP_MAXSEG get TCP maximum segment size no int TCP_NODELAY enables or disables the Nagle optimization algorithm for TCP sockets yes int (bool) SOL_SOCKET SO_DEBUG enables recording of debugging information (for processes with EUID=0 only) yes int (bool) SO_REUSEADDR enables local address reuse yes int (bool) SO_KEEPALIVE enables sending keep connections alive messages yes int (bool) SO_DONTROUTE enables routing bypass for outgoing messages same as flag MSG_DONTROUT) yes int (bool) SO_LINGER linger if data present - block shutdown(), close() calls until queued data sent or timeout expired yes struct linger SO_BROADCAST permits to transmit broadcast messages yes int (bool) SO_OOBINLINE enables reception of out-of-band data in band (else, OOB data is received with MSG_OOB flag only) yes int SO_SNDBUF buffer size for output yes int SO_RCVBUF buffer size for input yes int SO_SNDLOWAT minimum byte count for output no in Linux int SO_RCVLOWAT minimum byte count for input no in Linux int SO_SNDTIMEO timeout value for output no in Linux int SO_RCVTIMEO timeout value for input no in Linux int SO_TYPE get the type of the socket no int SO_ERROR get and clear error on the socket no int Introduction to Network Programming in UNIX & LINUX

  23. Category request Description Datatype Socket SIOCATMARK At out-of-band mark ? int SIOCSPGRP Set process ID / group ID of socket int SIOCGPGRP Get process ID / group ID of socket int #include <unistd.h> int ioctl(int fd, int request, … /* void *arg */); FIONBIO Set/clear nonblocking flag int File FIOASYNC Set/clear asynchronous I/O flag int FIONREAD Get # of bytes in receive buffer int FIOSETOWN Set process ID / group ID of file int FIOGETOWN Get process ID / group ID of file int SIOCGUFCONF Get list of all interfaces struct ifconf Interface SIOCSIFADDR Set interface address struct ifreq SIOCGIFADDR Get interface address struct ifreq SIOCSIFFLAGS Set interface flags struct ifreq SIOCGIFFLAGS Get interface flags struct ifreq SIOCSIFDSTADDR Set point-to-point address struct ifreq SIOCGIFDSTADDR Get point-to-point address struct ifreq SIOCSIFBRDADDR Set broadcast address struct ifreq SIOCGIFBRDADDR Get broadcast address struct ifreq SIOCSIFNETMASK Set subnet mask struct ifreq SIOCGIFNETMASK Get subnet mask struct ifreq SIOCSIFMETRIC Set interface metric struct ifreq SIOCGIFMETRIC Get interface metric struct ifreq #include <unistd.h> #include <fcntl.h> int fcntl(int fildes, int cmd, /*arg*/ ...); SIOCGIFMTU Get interface MTU struct ifreq SIOCxxx (many more; implementation-dependent) SIOCSARP Create/modify ARP entry struc arpreq ARP SIOCGARP Get ARP entry struc arpreq SIOCDARP Delete ARP entry struc arpreq SIOCADDRT Add route struct rtentry Routing SIOCDELRT Delete route struct rtentry Socket Control Operations • A common use of ioctl() • by network programs • (typically servers) is • to obtain information • on all the host's interfaces, • the interface addresses, • whether the interface • supports broadcasting, • multicasting, etc. Introduction to Network Programming in UNIX & LINUX

  24. fd sd1 sd2 sd2 fd sd1 fd I/O Multiplexing Solution 1. Fork child process per channel I/OMultiplexing • Input/Output Multiplexing • is simultaneous handling of 2 or more different I/O channels. • Examples • 1) Printer connected to network waits simultaneously for requests: • - from local host processes - on UNIX Domain STREAM socket • from remote processes - on IPv4 TCP socket • 2) Superserver inetd – waits for requests for multiple different services on • different ports and invokes corresponding service as separate sub-server • process, when specific request is accepted. Process sock1 sock2 kernel fork Process Child Process 1 Child Process 2 fork read read read write write pipe sock1 sock2 kernel Introduction to Network Programming in UNIX & LINUX

  25. Note: The following two error codes are equal: EWOULDBLOCK = EAGAIN I/O Multiplexing Solution 2. Polling • Set both sockets to non-blocking mode • fcntl (sd1, F_SETFL, fcntl(sd1, F_GET_FL,0) | O_NONBLOCK); • fcntl (sd2, F_SETFL, fcntl(sd2, F_GET_FL,0) | O_NONBLOCK); • Read from both sockets and, if nothing available, wait timeout • while(…) • { • /* try to read from 1st socket */ • len = read (sd1, buff); • if (len >=0) break; /*go to data processing */ • if (errno != EWOULDBLOCK) { perror(…);} • /* try to read from 2nd socket */ • len = read (sd2, buff); • if (len >=0) break; /*go to data processing */ • if (errno != EWOULDBLOCK) { perror(…);} • /* provide polling timeout sleep */ • sleep(TIMEOUT); • } • /* begin accepted data processing */ • … Introduction to Network Programming in UNIX & LINUX

  26. I/O Multiplexing Solution 3. Signal-Driven I/O • Establish handler for SIGIO signal. • void iohandler (int sig) • {…} • … • signal (SIGIO, iohandler); /* or using sigaction() */ • Declare process as sockets owner • fcntl (sd1, F_SETOWN, getpid( ) ); • fcntl (sd2, F_SETOWN, getpid( ) ); • Enable signal-driven I/O • fcntl (sd1, F_SETFL, fcntl(sd1, F_GET_FL,0) | O_ASYNC); • fcntl (sd2, F_SETFL, fcntl(sd2, F_GET_FL,0) | O_ASYNC); • The Problems • Signal does not contain information, on which descriptor the event occurred. • This model works good with UDP – signal is sent by kernel when: • - new datagram arrives • - asynchronous error occurs • In case of TCP the signal is sent on any socket status change: • - connect request accepted, connection established, disconnect request accepted, disconnected, • data arrived, data sent, error occurred, etc. • As result, it is difficult to recognize the proper event to handle the data. Introduction to Network Programming in UNIX & LINUX

  27. I/O Multiplexing: System Call select FD Set Type fd_set is bitmask. Each bit flag in the mask corresponds to file descriptor which equal to the index of this bit flag. FD Set Modification Macros: FD_ZERO (fd_set *set); /*empty the set*/ FD_SET (int fd, fd_set *set); /* set one bit ON */ FD_CLR (int fd, fd_set *set); /* set one bit OFF */ FD_ISSET (int fd, fd_set *set); /* test one bit */ #include <sys/select.h> /* According to POSIX */ #include <sys/time.h> /* According to earlier standards */ #include <sys/types.h> #include <unistd.h> struct timeval { long tv_sec; /* seconds */ long tv_usec; /* microseconds */ }; int select (int maxFDplus1, fd_set *readFDs, fd_set *writeFDs, fd_set *exceptFDs, struct timeval *timeout); • This system call provides the process with possibility • to wait for a number of file descriptors to change status: • - Descriptors in set readFDs are watched to became ready • for non-blocking read • - Descriptors in set writeFDs are watched to became ready • for non-blocking write • - Descriptors in set exceptFDs are watched for exceptions • Specific events are not watched, if FD Set parameter is NULL. • Parameter maxFDplus1 specifies the maximal width of FD Sets • to avoid check of all the bit flags in each FD Set. • Parameter timeout specifies the wait period as follows: • - NULL – wait until I/O event occurs. • - pointer to structure with positive values • – wait no more than specified time. • - pointer to structure with tv_sec=tv_usec=0 • – don’t wait (non-blocking check, used by polling) • Return value is number of descriptors where event occurred. • The 3 FD Sets are modified to point on affected descriptors. • Return value is 0 if timeout expired, -1 if error occurred. Example. Wait for input [fd(0)] during 5 seconds #include <sys/time.h> #include <sys/types.h> #include <unistd.h> … fd_set rfds; struct timeval tv; int retval; FD_ZERO(&rfds); FD_SET(0, &rfds); tv.tv_sec = 5; tv.tv_usec = 0; retval = select(1, &rfds, NULL, NULL, &tv); if (retval == -1) { perror("select()"); } else if (retval) { printf("Data is available now.\n"); /* FD_ISSET(0, &rfds) will be true. */ }else{ printf("No data within five seconds.\n"); } Introduction to Network Programming in UNIX & LINUX

  28. I/O Multiplexing: System Call pselect #include <sys/select.h> /* According to POSIX */ #include <sys/time.h> /* According to earlier standards */ #include <sys/types.h> #include <unistd.h> struct timespec { long tv_sec; /* seconds */ long tv_nsec; /* nanoseconds */ }; int pselect (int maxFDplus1, fd_set *readFDs, fd_set *writeFDs, fd_set *exceptFDs, const struct timespec *timeout, const sigset_t *sigmask); 2 IN 1 select() sigsuspend() = pselect() • This system call provides the process with possibility to wait simultaneously: • - for I/O events (for a number of file descriptors to change status) • - for signals • It differs from select() call only by format of timeout parameter and by additional parameter sigmask, • which specifies the signal disposition for accepting of desirable signals. I/O Multiplexing with simultaneous signal handling. Example 2. Reliable solution. Example 1. Race condition. if the signal occurs in critical region, it will be lost if select blocks forever. … sigset_t newmask, oldmask, zeromask; sigemptyset(&zeromask); sigemptyset(&newmask); sigaddset(&newmask, SIGINT); sigprocmask (SIG_BLOCK, &newmask, &oldmask); /* block SIGINT */ if (intr_flag) handle_intr(); /* handle the signal */ if ( (nready = pselect ( ... , &zeromask) ) < 0) { if (errno == EINTR) { if (intr_flag) handle_intr (); } ... } void sighandler(int sig){ intr_flag=1; } … if (intr_flag) handle_intr(); /* handling before select */ if ( (nready = select( ... ) ) < 0) { if (errno == EINTR) { if (intr_flag) handle_intr(); /* handling after select */ } ... } critical region Introduction to Network Programming in UNIX & LINUX

  29. I/O Multiplexing: System Call poll Event Mask Bits: #define POLLIN 0x0001 /* There is data to read */ #define POLLPRI 0x0002 /* There is urgent data to read */ #define POLLOUT 0x0004 /* Writing now will not block */ #define POLLERR 0x0008 /* Error condition */ #define POLLHUP 0x0010 /* Hung up */ #define POLLNVAL 0x0020 /* Invalid request: fd not open */ #ifdef _XOPEN_SOURCE #define POLLRDNORM 0x0040 /* Normal data may be read */ #define POLLRDBAND 0x0080 /* Priority data may be read */ #define POLLWRNORM 0x0100 /* Writing now will not block */ #define POLLWRBAND 0x0200 /* Priority data may be written */ #endif #ifdef _GNU_SOURCE /* Linux only */ #define POLLMSG 0x0400 #endif #include <sys/poll.h> struct pollfd { int fd; /* file descriptor */ short events; /* requested events */ short revents; /* returned events */ }; int poll(struct pollfd *fdArray, unsigned int fdArrayLen, int timeout); • The system call poll() is variation of system call select() • The parameter fdArray is array of structures of length fdArrayLen. • Each structure in fdArray corresponds to single file descriptor to be watched for events. • Requested and returned events are specified by separate event masks (events, revents), constructed • from Event Mask Bits. • The timeout value specified as number of milliseconds and has following meaning: • - timeout = -1 – wait until I/O event occurs. • - timeout > 0 – no more than specified time. • - timeout = 0 – don’t wait (non-blocking check, used by polling) • Return value is number of descriptors where event occurred. • The occurred event type is described by revents field of fdArray element, corresponded to specific descriptor. • Return value is 0 if timeout expired, -1 if error occurred. Introduction to Network Programming in UNIX & LINUX

  30. I/O Models: Blocking, Non-Blocking application kernel I/OModels system call no datagram ready recvfrom wait for data 1. Blocking I/O Model process blocks in call to recvfrom datagram ready copy datagram copy data from kernel to user return OK copy complete process datagram application kernel system call no datagram ready recvfrom EWOULDBLOCK system call no datagram ready recvfrom EWOULDBLOCK wait for data system call no datagram ready recvfrom 2. Non- Blocking I/O Model EWOULDBLOCK process repeatedly calls recvfrom, waiting for an OK return (polling) system call recvfrom datagram ready copy datagram copy data from kernel to user return OK copy complete process datagram Introduction to Network Programming in UNIX & LINUX

  31. I/O Models: Multiplexing, Signal-Driven application kernel I/OModels system call no datagram ready select process blocks in call to select, waiting for one of sockets to become readable wait for data 3. I/O Multiplexing Model return readable datagram ready copy datagram system call recvfrom process blocks while data copied into application buffer copy data from kernel to user return OK copy complete process datagram application kernel system call no datagram ready sigaction (set SIGIO handler) return process continues executing wait for data 4. Signal-Driven I/O Model deliver SIGIO datagram ready copy datagram SIGIO handler system call recvfrom process blocks while data copied into application buffer copy data from kernel to user return OK copy complete process datagram Introduction to Network Programming in UNIX & LINUX

  32. I/O Models: Asynchronous application kernel I/O Models system call no datagram ready aio_read (set signal handler) return • 5.Asynchronous I/O Model • The POSIX defines set of system calls, • providing the API for implementation • of asynchronous I/O on set of descriptors: • aio_read(), aio_write() • These functions allow the calling process to initiate single read (write) asynchronous I/O request. • aio_error() • This function returns the error status associated with the single asynchronous I/O request. • It is equivalent to errno value that would be set by the corresponding read() or write() system call. • If the operation has not yet completed, then the error status will be equal to EINPROGRESS. • aio_return() • This function returns the result associated with the single asynchronous I/O request after its completion. • lio_listio() • This function allows the calling process to initiate a list of I/O requests within a single function call. • Depending on passed argument values, the function could wait until all I/O is complete or • to return immediately after request scheduling, with following signal notification. wait for data process continues executing datagram ready copy datagram copy data from kernel to user deliver signal copy complete signal handler process datagram Introduction to Network Programming in UNIX & LINUX

  33. Blocking I/O Non-Blocking I/O I/O Multiplexing Signal-Driven I/O Asynchronous I/O check check check check check check check check initiate initiate blocked wait for data blocked ready notification initiate initiate blocked copy data from kernel to user blocked blocked complete complete complete notification complete handles both phases 1st phase handled differently 2nd phase handled the same (blocked in call to recvfrom) I/O Models Comparison • Each input operation normally has two distinct phases: • Waiting for the data to be ready • Copying the data from the kernel to the process • POSIX gives the following definitions of Synchronous and Asynchronous Input / Output: • A Synchronous I/O operation causes the requesting process to be blocked until that I/O operation completes. • An Asynchronous I/O operation does not cause the requesting process to be blocked. • Using these definitions, the first four I/O models—Blocking, Non-Blocking, I/O Multiplexing, and Signal-Driven I/O—are all Synchronous because the actual I/O operation blocks the process. • And only the Asynchronous I/O model matches the definition of Asynchronous I/O. Introduction to Network Programming in UNIX & LINUX

  34. Daemon Process • Daemon is a process that runs in the background and is not associated with a controlling terminal. • Many standard UNIX network services: printer, remote login, file transfer, tasks scheduling (cron) • are provided by servers which running as daemons. • Who starts the daemons ? • Daemons started during system startup • These daemons are started by scripts /etc/rc…and have superuser privileges. • In such a way traditionally the following daemons are started: • Server cron (initializes the specified tasks in scheduled time) • Superserver inetd(listens on multiple sockets and spawns sub-servers per service request) • Web Server • Mail Server • Server syslogd (provides logging services for all running daemons) • Daemons started by superserver inetd • These are FTP Server, TFTP Server, Telnet Server, Rlogin Server, etc. • These servers are spawned by inetd for handling of specific request and run as daemons. • Daemons started by cron server • These are the programs configured (in system file /usr/lib/cronab ) to be started in specific scheduled time. • A program also could be scheduled by means of crontab and at UNIX commands. • All programs started in specific moment of time by cron server, are executed as daemons. • Programs started from user terminal • How could a process to become a daemon ? • Pass to background • Disassociate from process group • Disassociate from control terminal • Close stdin, stdout, stderr and all inherited unnecessary file descriptors • Reset working directory and file creation mask. Introduction to Network Programming in UNIX & LINUX

  35. #include <syslog.h> void syslog(int priority, const char *message, ... ); void openlog(const char *ident, int options, int facility); void closelog( ); Daemon syslogd • Since daemons do not have a controlling terminal, they can not use standard output and standard error streams. • To provide information output from daemons, the standard syslogddaemon is used. • The syslogd daemon life cycle: • starts during UNIX startup; • reads its configuration (file /etc/syslog.conf ), specifying where the accepted messages would be logged; • opens Unix Domain socket and binds it to well-known name (/var/run/log or /dev/log); • listens for messages from all other daemons and handles them according to specified configuration. • To communicate with syslogd daemon, other daemon processes could use the following functions: • Function syslog() establishes the connection with daemon syslogd and logs the message. • Parameter message is format string (as in printf() ) extended with %m pattern, • which is replaced with the error message corresponding to the current value of errno. • Parameter priority is combination of 2 values: • - level (severity) (0=LOG_EMERG – highest severity, …, 7=LOG_DEBUG – lowest severity) • - facility (functional area) (LOG_CRON, LOG_FTP, LOG_MAIL, LOG_USER, etc.) • The level and facility values are used in configuration file /etc/syslog.conf for specification, • to where the syslogd will forward specific messages. (See full level and facility list in man description) • Function openlog( ) could be called in the beginning of a process to specify common prefix ident, default value • of facility and additional options (output to console, print PID, etc.) for all the upcoming messages. • Function closelog( ) could be called when the application is finished sending log messages. Introduction to Network Programming in UNIX & LINUX

  36. #include <syslog.h> int closeAll {…} /* To find all actually open descriptors, navigate through /proc/self/fd/ or use system-dependent system calls like fcloseall(), closefrom() etc. */ int daemon_init(const char *prog_name, int facility) { int i; pid_t pid; if ( (pid = fork()) < 0){ return (-1); }else if (pid > 0){ exit(0); /* parent terminates */ } /*--- child 1 continues in background... ----*/ if (setsid() < 0) { /* become session leader */ return (-1); } signal(SIGHUP, SIG_IGN); /* disassociate from control terminal */ if ( (pid = fork()) < 0) { return (-1); }else if (pid > 0) { exit(0); /* child 1 terminates */ } /*--- child 2 continues, it is not session leader ... ----*/ chdir("/"); /* change working directory */ closeAll(); /* close all file descriptors */ open("/dev/null",O_RDONLY); /* redirect stdin, stdout, and stderr */ open("/dev/null",O_RDWR); open("/dev/null",O_RDWR); openlog(pname, LOG_PID, /* pre-configure syslog output */ facility); return (0); /* initialization success */ } Daemon Initialization • This example shows the function, • which “demonizes” the process. • Some of UNIX / LINUX systems • provide daemon () function with • the same functionality. • The First fork passes the 1st child • process to background • To disassociate from control • terminal, process became • session (and group) leader • The fork guarantees, • that the 2nd child is no longer • a session leader, so it cannot • acquire a control terminal. • The standard input, output and • error are redirected to /dev/null • to avoid the errors, when these • descriptors unexpectedly assigned • to files, sockets, and then printf or • perror is called. • Working directory and file mask • could be reset to specific values, • depending on process functionality. Introduction to Network Programming in UNIX & LINUX

  37. FIELD DESCRIPTION • service-name must be in /etc/services • socket-type stream or dgram • protocol tcp or udp (must be in /etc/protocols) • wait-flag wait (iterative) or nowait (concurrent) • user-name from /etc/passwd, typically root • server-program full pathname to be used for exec • server-program-arguments arguments for exec ftp stream tcp nowait root /usr/bin/ftpd ftpd -1 telnet stream tcp nowait root /usr/bin/telnetd telnetd login stream tcp nowait root /usr/bin/rlogind rlogind -s tftp dgram udp wait nobody /usr/bin/tftpd tftpd -s /tftpboot … Superserver inetd • This server simultaneously waits for requests for multiple different services on different ports and invokes corresponding service as separate sub-server process, when specific request is accepted. • The Superserver inetd has the following advantages: • 1. It allows a single process inetd to be waiting for incoming client requests for multiple services, • instead of one process for each service. This reduces the total number of processes in the system. • 2. It simplifies writing daemon processes since most of the startup details are handled by inetd. • The “price” for these advantages is execution of fork() and exec() for every handling request. • The Superserver inetd life cycle: • Starts during UNIX system startup • Reads its configuration from file /etc/inetd.conf • Opens all sockets specified by configuration and performs simultaneous wait for request • Accepting request, spawns ( fork() + exec() ) specified sub-server to handle specific request • and passes actual server name as first argument (argv[0]) to spawned process. The structure of configuration file /etc/inetd.conf : The example of configuration file /etc/inetd.conf : Introduction to Network Programming in UNIX & LINUX

  38. For each service listed in file /etc/inetd.conf socket( ) bind( ) listen( ) (TCP only) ..add to FD Set.. select( ) (Read events) accept( ) (TCP only) fork( ) no is parent ? yes close accept-ed socket (TCP only) close all FDs other than socket no dup socket FD to FDs: 0,1,2 is “wait” service ? yes temporary remove FD from FD Set while child is running close socket FD setgid() setuid() (if user not root) exec( ) server Superserver inetdwork schema • On startup, inetd reads the /etc/inetd.conf file and creates a socket of the appropriate type (stream or datagram) for all the specified services. It binds the sockets and for TCP sockets also performs listen. • Simultaneous waiting for request on all open sockets is performed with system call select. • For each arrived request sub-server process is forked. It duplicates stdin, stdout and stderr to be a socket descriptor, sets GID and UID and performs exec call to actual server. • Parent process in the same time continues to wait for other requests. Also it periodically accepts and handles SIGCHILD when sub-servers terminate. • For concurrent services (nowait) corresponded bits in fd_set bitmask are always reset to ON before next select call. • For iterative services (wait) the corresponded bit flags restored only during handling of SIGCHILD signal after termination of previous sub-server of the same type. • Note: Configuring of UDP services as nowait • can cause a race condition, where: • the inetd program selects on the socket • and the server program reads from the socket. Introduction to Network Programming in UNIX & LINUX

  39. application application code function call function return UDP request local name server other name server resolver code UDP reply resolver configuration files Name and Address Conversions • There are four types of network information that an application might want to look up: • Hosts • Networks • Protocols • Services • Protocol and Service information is always obtained from static files (/etc/protocols, /etc/services). • Host and Network information could be obtained from • different sources: • Domain Name System (DNS) • Static files (/etc/hosts, /etc/networks) • Network Information System (NIS) • Lightweight Directory Access Protocol (LDAP). • The specific type of name service used by specific • host is depends on configuration, provided by administrator. • The user application, independently on specific name service • configuration, obtains this information using Resolver – • standard functionality, providing interface to name service. Resolver Functionality • The standard API provides the following conversion methods: • Hosts information gethostbyaddr(), gethostbyname() • Networks information getnetbyaddr(), getnetbyname() • Protocols information getprotobyname(), getprotobynumber() • Services information getservbyname(), getservbyport() Introduction to Network Programming in UNIX & LINUX

  40. hostent { } h_name official hostname \0 h_aliases #include <netdb.h> struct hostent { char *h_name; /* official (canonical) name of host */ char **h_aliases; /* pointer to array of pointers to alias names */ int h_addrtype; /* host address type: AF_INET */ int h_length; /* length of address: 4 */ char **h_addr_list; /* ptr to array of ptrs with IPv4 addrs */ }; struct hostent *gethostbyname (const char *hostname); struct hostent *gethostbyaddr (const char *addr, socklen_t len, int family); h_addrtype alias #1 \0 AF_INET 4 h_length alias #2 \0 h_addr_list NULL in_addr{ } IP addr #1 in_addr{ } IP addr #2 in_addr{ } NULL IP addr #3 h_length=4 Host Information Utilities • Both these methods provide host information by Domain Name or IP Address • Return value is pointer to hostent structure on success, NULL on failure (h_errno specifies the error) Example. Extract IP Address by Host Name … char [ ] hostName=“www.google.com”; sockaddr_in servAddr; struct hostent* pEntry; … pEntry=gethostbyname(hostName); if (pEntry != null) { servAddr.sin_family = pEntry -> h_addrtype; bcopy(pEntry -> h_addr_list, (char*) &servAddr.sin_addr, pEntry ->h_length); } … Introduction to Network Programming in UNIX & LINUX

  41. Example of gethostbyname() usage. #include <stdio.h> #include <netdb.h> #include <sys/types.h> #include <netinet/in.h> #include <sys/socket.h> #include <arpa/inet.h> int main(int argc, char **argv) { char *ptr, **pptr; char str [INET_ADDRSTRLEN]; struct hostent *hptr; while (--argc > 0) { ptr = *++argv; if ( (hptr = gethostbyname (ptr) ) == NULL) { fprintf (stderr, "gethostbyname error for host: %s, h_errno= %d\n", ptr, h_errno ); continue; } printf ("official hostname: %s\n", hptr->h_name); for (pptr = hptr->h_aliases; *pptr != NULL; pptr++) printf ("\talias: %s\n", *pptr); switch (hptr->h_addrtype) { case AF_INET: pptr = hptr->h_addr_list; for ( ; *pptr != NULL; pptr++) printf ("\taddress: %s\n", inet_ntoa (*pptr)); break; default: perror ("unknown address type"); break; } } exit(0); } Introduction to Network Programming in UNIX & LINUX

  42. #include <netdb.h> struct servent { char *s_name; /* official service name */ char **s_aliases; /* alias list */ int s_port; /* port number, network-byte order */ char *s_proto; /* protocol to use */ }; struct servent *getservbyname (const char *servname, const char *protoname); struct servent *getservbyport (int port, const char *protoname); Example. Extract Port Number by Service Name … struct servent* pEntry; int tftpPort; pEntry=getservbyname(“tftp”, “udp”); if (pEntry != null) { tftpPort = pEntry -> s_port; } … Service Information Utilities • Both these methods provide service information by service name or port number • Some Internet services are provided using either TCP or UDP. • In this case parameter protoname could specify specific protocol of interest (“tcp”, “udp”) • Return value is pointer to servent structure, NULL on failure. Introduction to Network Programming in UNIX & LINUX

  43. Example. Create and Bind Unix Domain Stream Socket. #include <socket.h> #include <sys/un.h> … int sockFd; struct sockaddr_un servAddr; char [ ] filePath=“/tmp/anyname”; /* create Unix Domain stream socket */ sockFd = socket (AF_LOCAL, SOCK_STREAM, 0); /* file to be used as address, would not exist before binding */ unlink(filePath); /* bind the socket */ bzero((char*)&servAddr, sizeof(servAddr)); servAddr.sun_family = AF_LOCAL; strncpy(servAddr.sun_path, filePath, strlen(filePath)); bind(sockFd, &servAddr, sizeof(servAddr); … Unix Domain Sockets The Unix Domain protocols are not an actual protocol suite, but a way of performing Inter-Process Communication on a single host using socket API. The Unix Domain socket binding is provided to file path. To connect() to Unix Domain socket, the process would have the same permissions as required to open() the file. Unix Domain Address Structure #include <sys/un.h> struct sockaddr_un { sa_family_t sun_family; /* AF_LOCAL (AF_UNIX) */ char sun_path[108]; /* pathname\0*/ }; Introduction to Network Programming in UNIX & LINUX

  44. #include <sys/socket.h> int socketpair( int family, /* AF_LOCAL */ int type, /*SOCK_STREAM / SOCK_DGRAM */ int protocol, /* 0 */ int sockfd[2] ); /* (output) array of 2 descriptors */ struct msghdr { void * msg_name; /* address */ socklen_t msg_namelen; /* size of address */ struct iovec * msg_iov; /* scatter/gather array */ size_t msg_iovlen; /* msg_iov array length */ void * msg_control; /* ancillary data*/ socklen_t msg_controllen; /* ancillary data length */ }; Unix Domain Socket Features Socket-Based Pipe • Creates the pair of already connected unnamed sockets. • If SOCK_STREAM protocol is used, the full-duplex stream pipe is created. • On success returns 0 and fills sockfd[0] and sockfd[1] . • On error returns -1, errno specifies the error. Unix Domain & Ancillary Data • Ancillary Data is control information passed by means • of sendmsg(), recvmsg() system calls. • The fields msg_control and msg_controllen of the • structure msghdr are used for Ancillary Data. • The following types of Ancillary Data are used with • Unix Domain sockets: • Passing Descriptors: • Sender opens resource (file) and “sends” the descriptor, allocated in its process. • Receiver “receives” newly allocated descriptor, pointing to the same resource. • Passing Credentials: • Sender “sends” standard structure • Receiver “receives” it filled with sender credentials: PID, UID, EUID, GID, etc. • To see the specific data structures for handling of Ancillary Data on specific Linux/Unix system, • see man pages for system calls recvmsg and sendmsg. Introduction to Network Programming in UNIX & LINUX

  45. Host 1 Host 2 Host Non-Distributed Application Distributed Application Client Distributed Application Server Distributed Application Distributed Application is an application made up of distinct components that are physically located on different computer systems, connected by a network. The components of Distributed Application are distributed across multiple computers on a network, but seem to be running on the same user's computer. Distribution Network Request Stub of Component A Component A Component A Stub of Component B Component B Component B Distributed Application Service is call to remote component with passing of input and output parameters Distributed Application Server is responsible to accept Client requests and to provide the call Service Distributed Application Client sends requests to the Server and accepts remote call results. • Distributed Application Design Tasks: • Discover the desired Server Host • Discover and connect to the desired Server Process • Serialization / Deserialization of input/output • parameters passed over network. • Examples of Distributed Application Technologies: • Sun RPC (Remote Procedure Call) • CORBA (Common Object Request Broker Architecture) • Java RMI (Remote Method Invocation) • Microsoft DCOM (Distributed Common Object Model) Introduction to Network Programming in UNIX & LINUX

  46. Sun RPC Sun RPC (Remote Procedure Call) is a powerful technique for constructing distributed, client-server based applications. It allows the execution of individual routines on remote computers across a network. RPC isolates the application from the physical and logical elements of the data communications, and hides the call to subroutine on remote server under the “traditional” local function call interface. Non-Distributed Application RPC Client RPC Server Server Stub of Procedure B Procedure A Procedure A Server Communication Client Stub of Procedure B RPC Conversion Client Interface Server Interface Procedure B Client Communication Procedure B • To develop an RPC application the following steps are needed: • Specify the protocol for client server communication • Develop the client program • Develop the server program Introduction to Network Programming in UNIX & LINUX

  47. RPC Application Development Before Distribution After Distribution program main ( ) calls local procedures client main( ) calls client stub procedures client program link P_main.c main.c client calls P_clnt.c client stub RPC specification XDR file non-distributed program RPC run-time library rpcgen link prog P.x P.h common include file calls server stub calls P_svc.c link server proc.c P_proc.c server program server procedures called by server stub local procedures called from program main() • The remote procedure in RPC is identified by triplet: • Program ID – unique hexadecimal id of Server Program, • Version – version id of Server Program • Procedure ID – serial number of procedure under the specific Server Program • The client-server communication interface is defined using XDR (eXternal Data Representation) protocol • The XDR defines the set of serializable data types and syntax for Program ID, Version, Procedure ID definition. • Standard RPC compiler rpcgen compiles XDR interface definitions and builds C code for interface parts of • Server Stub and Client Stub, and also H file with common interface constants. • RPC run-time library is linked during build of Server and Client application and provides Client and Server • communication functionalities. Introduction to Network Programming in UNIX & LINUX

  48. P_main.c – after distribution #include <rpc/rpc.h> #include “P.h” void main() { char [ ] msg = “test”; int * pResult; CLIENT* pClnt ; /* Connection Handle */ pClnt = clnt_create(“MyHost”, MESSAGEPROG, MESAGEVERS, “udp”); pResult = printmessage_1( &msg, pClnt); clnt_destroy(pClnt); } main.c – before distribution int printmessage(char* msg); void main() { char [ ] msg = “test”; int result; result = printmessage (msg); } P.x - the XDR definition of interface P.h – generated by rpcgen program MESSAGEPROG { version MESSAGEVERS { int PRINTMESSAGE(string) = 1; } = 1; } = 0x20000099; #define MESSAGEPROG 0x20000099 #define MESSAGEVERS 1 #define PRINTMESSAGE 1 int * printmessage_1(char**, CLIENT*); int * printmessage_1_svc(char **, struct svc_req *); P_proc.c – after distribution proc.c – before distribution #include <rpc/rpc.h> #include “P.h” int * printmessage_1_svc(char ** args, struct svc_req *); { static int result; /*must be static to return by pointer */ char * msg = *args; /*extract the argument passed by pointer */ /* print msg */ return &result; /* return result by pointer */ } int printmessage(char* msg) { /* print msg */ return 0; } RPC Simple Example rpcgen Introduction to Network Programming in UNIX & LINUX

  49. Client Host Server Host Registration Map Program ID, Version RPC Server Port Server Stub Procedure Connection to RPC Server Port Mapper (Port 111) Program ID, Version RPC Server Port Number RPC Client Procedure ID, parameters Client Stub Register on startup RPC Server (Ephemeral Port) Result Data • Port Mapper is standard daemon, listening on port 111 UDP (TCP) • and handling map (Program ID, Version) -> (Port Number) • Each RPC Server starts on ephemeral port and registers with Port Mapper • Each RPC Client calls Port Mapper on specific host to accept the port of target RPC Server • Than RPC Client calls the RPC Server with request, containing Procedure ID and parameters • Note: • RPC does not provide automatic discovery of Server Host. • To accept the RPC service, the RPC Client is responsible to know the name of the target Server Host., • where the proper RPC Server is running. Introduction to Network Programming in UNIX & LINUX

  50. / /dir123 NFS: Network File System NFS provides transparent file access for clients to files and filesystems on a remote server. NFS accesses only the portions of a file that a process references, and a goal of NFS is to make this access transparent(user process accesses local and remote filesystems and files information in the same way). When user process accesses remote filesystem or file, the local NFS Client sends a request to remote NFS Server, which performs the requested operation and provides the requested information in its reply. Before the local NFS Client can access files from remote NFS Server’s filesystem, this remote filesystem must be mounted to the local mount point at the NFS Client’s host via NFS Mount Protocol. / NFS mount /var /mydir123 file0 file1 file1 To reference particular filesystem or file on the remote NFS Server, the NFS Client obtains a File Handle,an opaque object generated by NFS Server. To perform any following operation on remote file or filesystem, the Client sends back to NFS Server the corresponded File Handle. NFS Client calls are performed by the client kernel, on behalf of client user processes. NFS Servers, for efficiency, are implemented within the server kernel. NFS implementation is based on RPC. NFS was originally written to use RPC over UDP. Newer implementations, however, also support RPC over TCP. Introduction to Network Programming in UNIX & LINUX

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