Connectionless Servers

Some services use datagram sockets. The rwho(1) service provides status information on hosts connected to a local area network. (Avoid running in.rwhod(1M) because it causes heavy network traffic.) This service requires the ability to broadcast information to all hosts connected to a particular network. It is an example of datagram socket use.

A user on a host running the rwho(1) server can get the current status of another host with ruptime(1). Typical output is illustrated in Example 2-10.

Example 2-10 Output of ruptime(1) Program

itchy up 9:45, 5 users, load 1.15, 1.39, 1.31
scratchy up 2+12:04, 8 users, load 4.67, 5.13, 4.59
click up 10:10, 0 users, load 0.27, 0.15, 0.14
clack up 2+06:28, 9 users, load 1.04, 1.20, 1.65
ezekiel up 25+09:48, 0 users, load 1.49, 1.43, 1.41
dandy 5+00:05, 0 users, load 1.51, 1.54, 1.56
peninsula down 0:24
wood down 17:04
carpediem down 16:09
chances up 2+15:57, 3 users, load 1.52, 1.81, 1.86

Status information is periodically broadcast by the rwho(1) server processes on each host. The server process also receives the status information and updates a database. This database is interpreted for the status of each host. Servers operate autonomously, coupled only by the local network and its broadcast capabilities.

Use of broadcast is fairly inefficient because a lot of net traffic is generated. Unless the service is used widely and frequently, the expense of periodic broadcasts outweighs the simplicity.

Example 2-11 shows a simplified version of the rwho(1) server. It performs two tasks: receives status information broadcast by other hosts on the network and supplies the status of its host. The first task is done in the main loop of the program: Packets received at the rwho(1) port are checked to be sure they were sent by another rwho(1) server process, and are stamped with the arrival time. They then update a file with the status of the host. When a host has not been heard from for an extended time, the database routines assume the host is down and logs it. This application is prone to error, as a server might be down while a host is up.

Example 2-11 rwho(1) Server

main()
{
   ...
   sp = getservbyname("who", "udp");
   net = getnetbyname("localnet");
   sin.sin6_addr = inet_makeaddr(net->n_net, in6addr_any);
   sin.sin6_port = sp->s_port;
   ...
   s = socket(AF_INET6, SOCK_DGRAM, 0);
   ...
   on = 1;
   if (setsockopt(s, SOL_SOCKET, SO_BROADCAST, &on, sizeof on)
         == -1) {
      syslog(LOG_ERR, "setsockopt SO_BROADCAST: %m");
      exit(1);
   }
   bind(s, (struct sockaddr *) &sin, sizeof sin);
   ...
   signal(SIGALRM, onalrm);
   onalrm();
   while(1) {
      struct whod wd;
	      int cc, whod, len = sizeof from;
      cc = recvfrom(s, (char *) &wd, sizeof(struct whod), 0,
         (struct sockaddr *) &from, &len);
      if (cc <= 0) {
      if (cc == -1 && errno != EINTR)
         syslog(LOG_ERR, "rwhod: recv: %m");
      continue;
      }
      if (from.sin6_port != sp->s_port) {
         syslog(LOG_ERR, "rwhod: %d: bad from port",
            ntohs(from.sin6_port));
         continue;
      }
      ...
      if (!verify( wd.wd_hostname)) {
         syslog(LOG_ERR, "rwhod: bad host name from %x",
            ntohl(from.sin6_addr.s6_addr));
         continue;
      }
      (void) sprintf(path, "%s/whod.%s", RWHODIR, wd.wd_hostname);
      whod = open(path, O_WRONLY|O_CREAT|O_TRUNC, 0666);
      ...
      (void) time(&wd.wd_recvtime);
      (void) write(whod, (char *) &wd, cc);
      (void) close(whod);
   }
   exit(0);
}

The second server task is to supply the status of its host. This requires periodically acquiring system status information, packaging it in a message, and broadcasting it on the local network for other rwho(1) servers to hear. This task is run by a timer and triggered with a signal. Locating the system status information is involved but uninteresting.

Status information is broadcast on the local network. For networks that do not support broadcast, use another scheme.