UNIT IV - SOCKET OPTIONS, ELEMENTARY UDP SOCKETS

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1 UNIT IV - SOCKET OPTIONS, ELEMENTARY UDP SOCKETS Socket options getsocket and setsocket functions generic socket options IP socket options ICMP socket options TCP socket options Elementary UDP sockets UDP echo Server UDP echo Client Multiplexing TCP and UDP sockets Domain name system gethostbyname function Ipv6 support in DNS gethostbyadr function getservbyname and getservbyport functions. Socket Options There are various ways to get and set the options that affect the socket. 1) Getsockopt and setsockopt functions 2) fcnt1 function 3) ioct1 function #include <sys/socket.h> intgetsockopt(intsockfd,, int level, intoptname, void *optval, socklent_t *optlen); intsetsockopt(intsockfd, int level, intoptname, const void *optval, socklent_toptlen); sockfd =>open socket descriptor level =>code in the system to interprete the option(generic, IPv4, IPv6, TCP) optval =>pointer to a variable from which the new value of option is fetched bysetsockopt, or into which the current value of the option is stored bysetsockopt. optlen => the size of the option variable. Generic Socket Option socket state We must set that option for the listening socket => because connected socket is not returned to a server by accept until the three way handshake is completed by the TCP layer. 1) SO_BROCAST =>enable or disable the ability of the process to send broadcast message.(only datagram socket : Ethernet, token ring..) 2) SO_DEBUG =>kernel keep track of detailed information about all packets sent or received by TCP(only supported by TCP) 3) SO_DONTROUTE=>outgoing packets are to bypass the normal routing mechanisms of the underlying protocol. 4) SO_ERROR=>when error occurs on a socket, the protocol module in a Berkeley-derived kernel sets a variable named so_error for that socket. Process can obtain the value of so_error by fetching the SO_ERROR socket option 5) SO_KEEPALIVE=>wait 2hours, and then TCP automatically sends a keepalive probe to the peer. a. Peer response 1 CCET PREPARED BY S.PON SANGEETHA

2 i. ACK(everything OK) ii. RST(peer crashed and rebooted):econnreset iii. no response:etimeout =>socket closed b. example: Rlogin, Telnet 6) SO_LINGER SO_LINGER =>specify how the close function operatesfor a connection-oriented protocol(default:close returns immediately) struct linger{ intl_onoff; /* 0 = off, nonzero = on */ intl_linger; /*linger time : second*/ }; l_onoff = 0 : turn off, l_linger is ignored l_onoff = nonzero and l_linger is 0:TCP abort the connection, discard any remaining data in send buffer. l_onoff = nonzero and l_linger is nonzero : process wait until remained data sending, or until linger time expired. If socket has been set nonblocking it will not wait for the close to complete, even if linger time is nonzero. 7) SO_RCVBUF, SO_SNDBUF let us change the default send-buffer, receive-buffer size. Default TCP send and receive buffer size : 4096bytes bytes Default UDP buffer size : 9000bytes, bytes SO_RCVBUF option must be setting before connection established. TCP socket buffer size should be at least three times the MSSs. 2 CCET PREPARED BY S.PON SANGEETHA

3 8) SO_RCVLOWAT, SO_SNDLOWAT Every socket has a receive low-water mark and send low-water mark.(used by select function) Receive low-water mark: the amount of data that must be in the socket receive buffer for select to return readable. Default receive low-water mark : 1 for TCP and UDP Send low-water mark: the amount of available space that must exist in the socket send buffer for select to return writable Default send low-water mark : 2048 for TCP UDP send buffer never change because dose not keep a copy of send datagram. 9) SO_RCVTIMEO, SO_SNDTIMEO allow us to place a timeout on socket receives and sends. Default disabled. 10) SO_REUSEADDR, SO_REUSEPORT Allow a listening server to start and bind itswell known port even if previously established connection exist that use this port as their local port. Allow multiple instance of the same server to be started on the same port, as long as each instance binds a different local IP address. Allow a single process to bind the same port to multiple sockets, as long as each bind specifies a different local IP address. Allow completely duplicate bindings : multicasting. 11) SO_TYPE Return the socket type. Returned value is such as SOCK_STREAM, SOCK_DGRAM... 12) SO_USELOOPBACK This option applies only to sockets in the routing domain(af_route). The socket receives a copy of everything sent on the socket. IP Socket Options 1) Level => IPPROTO_IP 2) IP_HDRINCL =>If this option is set for a raw IP socket, we must build our IP header for all the datagrams that we send on the raw socket. 3) IP_OPTIONS=>allows us to set IP option in IPv4 header. 4) IP_RECVDSTADDR=>This socket option causes the destination IP address of a received UDP datagram to be returned as ancillary data by recvmsg. 3 CCET PREPARED BY S.PON SANGEETHA

4 5) IP_RECVIF- Cause the index of the interface on which a UDP datagram is received to be returned as ancillary data by recvmsg. 6) IP_TOS-lets us set the type-of-service(tos) field in IP header for a TCP or UDP socket.if we call getsockopt for this option, the current value that would be placed into the TOS(type of service) field in the IP header is returned. 7) IP_TTLWe can set and fetch the default TTL(time to live field). ICMP SOCKET OPTIONS This socket option is processed by ICMPv6 and has a level of IPPROTO_ICMPV6. ICMP6_FILTER =>lets us fetch and set an icmp6_filter structure that specifies which of the 256possible ICMPv6 message types are passed to the process on a raw socket. TCP SOCKET OPTIONS There are five socket option for TCP, but three are new with Posix.1g and not widely supported. Specify the level as IPPROTO_TCP. 1) TCP_KEEPALIVE This is new with Posix.1g It specifies the idle time in second for the connection before TCP starts sending keepalive probe. Default 2hours this option is effective only when the SO_KEEPALIVE socket option enabled. 2) TCP_MAXRT This is new with Posix.1g. It specifies the amount of time in seconds before a connection is broken once TCP starts retransmitting data. 0 : use default -1:retransmit forever positive value:rounded up to next transmission time 3) TCP_MAXSEG This allows us to fetch or set the maximum segment size(mss) for TCP connection. 4) TCP_NODELAY This option disables TCP s Nagle algorithm. (default this algorithm enabled) purpose of the Nagle algorithm. ==>prevent a connection from having multiple small packets outstanding at any time. Small packet => any packet smaller than MSS. 4 CCET PREPARED BY S.PON SANGEETHA

5 Nagle algorithm: Default enabled. Reduce the number of small packet on the WAN. If given connection has outstanding data, then no small packet data will be sent on connection until the existing data is acknowledged. Nagle algorithm disabled Nagle algorithm enabled TCP Echo Server We now redo our TCP echo serve using poll instead of select. In the previous version using select, we had to allocate a client array along with a descriptor set named rset. With poll, we must allocate an array of pollfd structures to maintain the client information instead of allocating another array. We handle the fd member of this array the same way we handled the client array in Figure 6.15: a value of 1 means the entry is not in use; otherwise, it is the descriptor value. Recall from the previous section that any entry in the array of pollfd structures passed to poll with a negative value for the fd member is just ignored. Figure 6.25 shows the first half of our server. Figure 6.25 First half of TCP server using poll. tcpcliserv/tcpservpoll01.c 1 #include "unp.h" 5 CCET PREPARED BY S.PON SANGEETHA

6 2 #include <limits.h> /* for OPEN_MAX */ 3 int 4 main(int argc, char **argv) 5 { 6 int i, maxi, listenfd, connfd, sockfd; 7 int nready; 8 ssize_t n; 9 char buf[maxline]; 10 socklen_t clilen; 11 struct pollfd client[open_max]; 12 struct sockaddr_in cliaddr, servaddr; 13 listenfd = Socket(AF_INET, SOCK_STREAM, 0); 14 bzero(&servaddr, sizeof(servaddr)); 15 servaddr.sin_family = AF_INET; 16 servaddr.sin_addr.s_addr = htonl(inaddr_any); 17 servaddr.sin_port = htons(serv_port); 18 Bind(listenfd, (SA *) &servaddr, sizeof(servaddr)); 19 Listen(listenfd, LISTENQ); 20 client[0].fd = listenfd; 21 client[0].events = POLLRDNORM; 22 for (i = 1; i < OPEN_MAX; i++) 23 client[i].fd = -1; /* -1 indicates available entry */ 24 maxi = 0; /* max index into client[] array */ Allocate array of pollfd structures 11 We declare OPEN_MAX elements in our array of pollfd structures. Determining the maximum number of descriptors that a process can have open at any one time is difficult. One way is to call the POSIX sysconf function with an argument of _SC_OPEN_MAX and then dynamically allocate an array of the appropriate size. But one of the possible returns from sysconf is "indeterminate," meaning we still have to guess a value. Here, we just use the POSIX OPEN_MAX constant. Initialize We use the first entry in the client array for the listening socket and set the descriptor for the remaining entries to 1. We also set the POLLRDNORM event for this descriptor, to be notified by poll when a new connection is ready to be accepted. The variable maxi contains the largest index of 6 CCET PREPARED BY S.PON SANGEETHA

7 the client array currently in use. The second half of our function is shown in Figure Figure 6.26 Second half of TCP server using poll. tcpcliserv/tcpservpoll01.c 25 for ( ; ; ) { 26 nready = Poll(client, maxi + 1, INFTIM); 27 if (client[0].revents & POLLRDNORM) { /* new client connection */ 28 clilen = sizeof(cliaddr); 29 connfd = Accept(listenfd, (SA *) &cliaddr, &clilen); 30 for (i = 1; i < OPEN_MAX; i++) 31 if (client[i].fd < 0) { 32 client[i].fd = connfd; /* save descriptor */ 33 break; 34 } 35 if (i == OPEN_MAX) 36 err_quit("too many clients"); 37 client[i].events = POLLRDNORM; 38 if (i > maxi) 39 maxi = i; /* max index in client[] array */ 40 if (--nready <= 0) 41 continue; /* no more readable descriptors */ 42 } 43 for (i = 1; i <= maxi; i++) { /* check all clients for data */ 44 if ( (sockfd = client[i].fd) < 0) 45 continue; 46 if (client[i].revents & (POLLRDNORM POLLERR)) { 47 if ( (n = read(sockfd, buf, MAXLINE)) < 0) { 48 if (errno == ECONNRESET) { 49 /* connection reset by client */ 50 Close(sockfd); 51 client[i].fd = -1; 52 } else 53 err_sys("read error"); 54 } else if (n == 0) { 7 CCET PREPARED BY S.PON SANGEETHA

8 55 /* connection closed by client */ 56 Close(sockfd); 57 client[i].fd = -1; 58 } else 59 Writen(sockfd, buf, n); 60 if (--nready <= 0) 61 break; /* no more readable descriptors */ 62 } 63 } 64 } 65 } Call poll, check for new connection We call poll to wait for either a new connection or data on existing connection. When a new connection is accepted, we find the first available entry in the client array by looking for the first one with a negative descriptor. Notice that we start the search with the index of 1, since client[0] is used for the listening socket. When an available entry is found, we save the descriptor and set the POLLRDNORM event. Check for data on an existing connection The two return events that we check for are POLLRDNORM and POLLERR. The second of these we did not set in the events member because it is always returned when the condition is true. The reason we check for POLLERR is because some implementations return this event when an RST is received for a connection, while others just return POLLRDNORM. In either case, we call read and if an error has occurred, it will return an error. When an existing connection is terminated by the client, we just set the fd member to 1. Elementary UDP Sockets 8 CCET PREPARED BY S.PON SANGEETHA

9 #include<sys/socket.h> ssize_t recvfrom(int sockfd, void *buff, size_t nbyte, int flag, struct sockaddr *from, socklen_t *addrlen); ssize_t sendto(int sockfd, const void *buff, size_t nbyte, int flag, const struct sockaddr *to, socklen_t addrlen); Both return: number of bytes read or written if OK,-1 on error Main Function UDP Echo Server #include unp.h int main(int argc, char **argv) { int sockfd; struck sockaddr_in servaddr,cliaddr; sockfd=socket(af_inet,sock_dgram,0); bzero(&servaddr,sizeof(servaddr)); servaddr.sin_fammily=af_inet; servaddr.sin_addr.s_addr=htonl(inaddr_any); servaddr.sin_port=htons(serv_port); bind(sockfd, (SA *) &servaddr,sizeof(servaddr)); dg_echp(sockfd, (SA *) &cliaddr,sizeof(cliaddr)); } #include unp.h void dg_echo(int sockfd, SA *pcliaddr, socklen_t clilen) { int n; socklen_t len; char mesg[maxline]; for( ; ; ) { len=clilen; n=recvfrom(sockfd, mseg, MAXLINE, 0, pcliaddr, &len); sendto(sockfd, mesg, n, 0, pcliaddr, len); } } 9 CCET PREPARED BY S.PON SANGEETHA

10 UDP Echo Client Main Function: #include unp.h int main(int argc, char **argv) { int sockfd; struct sockaddr_in servaddr; if (argc!= 2) err_quit( usage : udpcli <Ipaddress> ); 10 CCET PREPARED BY S.PON SANGEETHA

11 bzero(&servaddr, sizeof(servaddr); servaddr.sin_family = AF_INET; servaddr.sin_port = htons(serv_port); Inet_pton(AF_INET, argv[1], &servaddr.sin_addr); sockfd = Socket(AF_INET, SOCK_DGRAM, 0); dg_cli(stdin, sockfd, (SA *) &servaddr, sizeof(servaddr); exit(0); } #include unp.h void dg_cli(file *fp, int sockfd, const SA *pservaddr, soklen_t servlen) { int n; char sendline[maxline], recvline[maxline+1]; while(fgets(sendline, MAXLINE, fp)!= NULL) { sendto(sockfd, sendline, strlen(sendline), 0, pservaddr, servlen); n = Recvfrom(sockfd, recvline, MAXLINE, 0, NULL, NULL); recvline[n] = 0; /* null terminate */ Fputs(recvline,stdout); } } I/O Multiplexing We often need to be able to monitor multiple descriptors: a generic TCP client (like telnet) a server that handles both TCP and UDP Client that can make multiple concurrent requests browser The Domain Name System The domain name system is usually used to translate a host name into an IP address. Domain names comprise a hierarchy so that names are unique, yet easy to remember. DNS Hierarchy 11 CCET PREPARED BY S.PON SANGEETHA

12 Host name structure Each host name is made up of a sequence of labels separated by periods. Each label can be up to 63 characters The total name can be at most 255 characters. Examples: whitehouse.gov barney.the.purple.dinosaur.com monica.cs.rpi.edu Domain Name The domain name for a host is the sequence of labels that lead from the host (leaf node in the naming tree) to the top of the worldwide naming tree. A domain is a subtree of the worldwide naming tree. Top level domains edu, gov, com, net, org, mil, Countries each have a top level domain (2 letter domain name). New top level domains include:.aero.biz.coop.info.name.pro DNS Organization Distributed Database The organization that owns a domain name is responsible for running a DNS server that can provide the mapping between hostnames within the domain to IP addresses. So - some machine run by RPI is responsible for everything within the rpi.edu domain. DNS Distributed Database There is one primary server for a domain, and typically a number of secondary servers containing replicated databases. 12 CCET PREPARED BY S.PON SANGEETHA

13 DNS Clients A DNS client is called a resolver. A call to getbyname(host)is handled by a resolver (typically part of the client). Most Unix workstations have the file /etc/resolv.conf that contains the local domain and the addresses of DNS servers for that domain. DNS Servers Servers handle requests for their domain directly. Servers handle requests for other domains by contacting remote DNS server(s). Servers cache external mappings. DNS Data DNS databases contain more than just hostname-to-address records: Name server records NS Hostname aliases CNAME Mail Exchangers MX Host Information HINFO Resolvers and Name Servers #include <netdb.h> Gethostbyname Function 13 CCET PREPARED BY S.PON SANGEETHA

14 struct hostent *gethostbyname (const char *hostname); struct hostent { char *h_name; /* official (canonical) name of host */ char **h_aliases; /* pointer to arrayof opinters to alises names */ int h_addrtype; /* host address type : AF_INET or AF_INET6 */ int h_length; /* length of address : 4 or 6 */ char **h_addr_list; /* ptr to array of ptrs with IPv4 or IPv6 addrs */ }; RES_USE_INET6 Resolver Option 1. #include <resolv.h> res_init(); _res.option = RES_USE_INET6; 2. export RES_OPTION=inet6; (shell once) (in.profile One User Only) 3. options inet6 (in /etc/resolv.conf) if all program support IPv6 Gethostbyname2 Function #include <netdb.h> struct hostent *gethostbyname2 (const char *hostname, int family); 14 CCET PREPARED BY S.PON SANGEETHA

15 Gethostbyaddr Function #include <netdb.h> struct hostent *gethostbyaddr(const chr *addr, size_t len,int family); The field of interest in this structure is normally h_name, the canonical hostname uname Function #include <netdb.h> int uname(struct utsname *name); #define _UTS_NAMESIZE 16 #define _UTS_NODESIZE 256 struct ntsname { char sysname[_uts_namesize]; /* name of this operationg system */ char sysname[_uts_namesize]; /* name of this node */ char sysname[_uts_namesize]; /* O. S. release level */ char sysname[_uts_namesize]; /* O. S. version level */ char sysname[_uts_namesize]; /* hardware type */ }; gethostname Function #include <netdb.h> int gethostname(char *name, size_t namelen); getservbyname Function #include <netdb.h> 15 CCET PREPARED BY S.PON SANGEETHA

16 struct servent *getservbyname(const char *servname, const char *protoname); struct servent { char *s_name; /* official service name */ char **s_ aliases; /* aliases list*/ int s_port; /* port number, network by order */ char *s_proto; /* protocol to use */ }; getservbyport Function #include <netdb.h> struct servent *getservbyport(int port, const char *protoname); 16 CCET PREPARED BY S.PON SANGEETHA

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