Connectionless Servers

Some services use datagram sockets. The rwho service provides status information on hosts connected to a local area network. (Avoid running in.rwhod 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 server can get the current status of another host with ruptime. Typical output is illustrated in Example 2-11.

Example 2-11 Output of ruptime 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 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 2-12 shows a simplified version of the rwho 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 port are checked to be sure they were sent by another rwho 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 2-12 rwho Server

main()
{
   ...
   sp = getservbyname("who", "udp");
   net = getnetbyname("localnet");
   sin.sin_addr = inet_makeaddr(net->n_net, INADDR_ANY);
   sin.sin_port = sp->s_port;
   ...
   s = socket(AF_INET, 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.sin_port != sp->s_port) {
         syslog(LOG_ERR, "rwhod: %d: bad from port",
            ntohs(from.sin_port));
         continue;
      }
      ...
      if (!verify( wd.wd_hostname)) {
         syslog(LOG_ERR, "rwhod: bad host name from %x",
            ntohl(from.sin_addr.s_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 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.

It is important that software operating in a distributed environment not have any site-dependent information compiled into it. This would require a separate copy of the server at each host and make maintenance a severe problem. The system isolates host-specific data from applications by providing function calls that return the required data. (For example, uname() returns the host's official name.) The SIOCGIFCONF ioctl() call lets you find the networks to which a host is directly connected. A local network broadcasting mechanism has been implemented at the socket level. Combining these two features lets a process broadcast on any directly connected local network that supports broadcasting in a site-independent manner. This solves the problem of deciding how to propagate status with rwho, or more generally in broadcasting. Such status is broadcast to connected networks at the socket level, where the connected networks have been obtained through the appropriate ioctl() calls. "Broadcasting and Determining Network Configuration" details the specifics of broadcasting.