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33 * This is a simple program which demonstrates how to query the kernel
34 * routing mechanism using only a UDP socket. Pass it a hostname on
35 * the command line (sorry, it doesn't parse dotted decimal) and it will
36 * print out an IP address which names the interface over which UDP
37 * packets intended for that destination would be sent.
38 * A more sophisticated program might use the list obtained from SIOCGIFCONF
39 * to match the address with an interface name, but applications programmers
40 * much more often need to know the address of the interface rather than
43 #include <sys/types.h>
44 #include <sys/socket.h>
46 #include <netinet/in.h>
47 #include <arpa/inet.h>
57 main(int argc, char **argv)
59 struct sockaddr_in local, remote;
66 errx(EX_USAGE, "must supply a hostname");
69 hp = gethostbyname(*argv);
71 errx(EX_NOHOST, "cannot resolve hostname: %s", *argv);
74 memcpy(&remote.sin_addr, hp->h_addr_list[0], sizeof remote.sin_addr);
75 remote.sin_port = htons(60000);
76 remote.sin_family = AF_INET;
77 remote.sin_len = sizeof remote;
79 local.sin_addr.s_addr = htonl(INADDR_ANY);
80 local.sin_port = htons(60000);
81 local.sin_family = AF_INET;
82 local.sin_len = sizeof local;
84 s = socket(PF_INET, SOCK_DGRAM, 0);
86 err(EX_OSERR, "socket");
89 rv = bind(s, (struct sockaddr *)&local, sizeof local);
90 local.sin_port = htons(ntohs(local.sin_port) + 1);
91 } while(rv < 0 && errno == EADDRINUSE);
94 err(EX_OSERR, "bind");
97 rv = connect(s, (struct sockaddr *)&remote, sizeof remote);
98 remote.sin_port = htons(ntohs(remote.sin_port) + 1);
99 } while(rv < 0 && errno == EADDRINUSE);
102 err(EX_OSERR, "connect");
104 namelen = sizeof local;
105 rv = getsockname(s, (struct sockaddr *)&local, &namelen);
107 err(EX_OSERR, "getsockname");
109 printf("Route to %s is out %s\n", *argv, inet_ntoa(local.sin_addr));