UDP broadcast discovery

budp.c

/* minimalistic UDP-based broadcast discovery tool

(C)2018 Simon Urbanek <simon.urbanek@R-project.org>
License: MIT

*/

#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <errno.h>

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

static char buf[512];

static int active = 1;

const char *nomatch = "";

int main(int ac, char **av) {
    int verb = 0;
    int port = -1;
    int prefix = 0;
    int maxat = 8;
    int UDPs = socket(AF_INET,SOCK_DGRAM,0);

    int reuse=1;
    if (setsockopt(UDPs, SOL_SOCKET, SO_REUSEADDR, &reuse, sizeof(reuse))) /* allow reuse */
	perror("WARNING: Cannot activate socket addres re-usage for UDP");
    if (setsockopt(UDPs, SOL_SOCKET, SO_BROADCAST, &reuse, sizeof(reuse)))
	perror("WARNING: Cannot activate broadcast for UDP");

    const char *req = 0;

    int i = 0;
    while (++i < ac)
	if (av[i][0] == '-') switch(av[i][1]) {
	    case 'v':
		verb++;
		break;
	    case 'h':
		printf("\n Usage: %s [-p=<port>] [-s] [-c=<query-content>] [-m=<max-attempts>] [<key>=<value> [...]]\n\n Default server port is 9552, default client port is 9551.\n If no query is specified server mode is assumed.\n\n",
		       av[0]);
		return 0;
	    case 'p':
		if (av[i][2] == '=')
		    port = atoi(av[i] + 3);
		else {
		    fprintf(stderr, "ERROR: missing port in -p=<port>\n");
		    return 1;
		}		
		break;
	    case 's':
		prefix = 1;
		break;
	    case 'm':
		if (av[i][2] == '=')
		    maxat = atoi(av[i] + 3);
		else {
		    fprintf(stderr, "ERROR: missing value in -m=<max.attempts>\n");
		    return 1;
		}
		break;
	    case 'c':
		if (av[i][2] == '=')
		    req = av[i] + 3;
		else {
		    fprintf(stderr, "ERROR: missing content in -c=<content>\n");
		    return 1;
		}
		break;
	    default:
		fprintf(stderr, "WARNING: unsupported option %s\n", av[i]);
	    }

    if (port < 1)
	port = req ? 9551 : 9552;

    struct sockaddr_in UDPsrv, peer;
    memset(&UDPsrv,0,sizeof(UDPsrv));
    UDPsrv.sin_family = AF_INET;
    UDPsrv.sin_port   = htons(port);
    UDPsrv.sin_addr.s_addr = htonl(INADDR_ANY);
    if (bind(UDPs, (struct sockaddr*)&UDPsrv, sizeof(UDPsrv))) {
	perror("Can't bind UDP server");
	return 1;
    }

    if (req) {
	socklen_t len = sizeof(peer);
	memset(&peer, 0, sizeof(UDPsrv));
	peer.sin_family = AF_INET;
	peer.sin_port = htons(9552);
	peer.sin_addr.s_addr = htonl(INADDR_ANY); // inet_addr("127.0.0.1");
	int sn = (int) sendto(UDPs, req, strlen(req), 0, (const struct sockaddr*) &peer, len);
	if (verb)
	    printf("INFO: sendto returned %d\n", sn);
	if (sn < 0)
	    perror("ERROR: send failed");
	struct timeval tv;
	tv.tv_sec = 0;
	tv.tv_usec = 250000;
	if (setsockopt(UDPs, SOL_SOCKET, SO_RCVTIMEO, &tv, sizeof(tv)) < 0) {
	    perror("Cannot set UDP receive timeout");
	    return 1;
	}
    }

    while (active) {
	socklen_t len = sizeof(peer);
	int n = recvfrom(UDPs, buf, sizeof(buf) - 1, 0, (struct sockaddr*) &peer, &len);
	if (verb)
	    printf("%d\n", n);
	if (n >= 0) {
	    buf[n] = 0;
	    if (verb)
		printf("%x:%d \"%s\"\n", peer.sin_addr.s_addr, ntohs(peer.sin_port), buf);
	    if (req) {
		if (prefix)
		    printf("%s:", inet_ntoa(peer.sin_addr));
		puts(buf);
		break;
	    }
	    
	    int answ = 0;
	    i = 0;
	    while (++i < ac) {
		if (strncmp(av[i], buf, n) == 0 &&
		    av[i][n] == '=') {
		    int sn = (int) sendto(UDPs, av[i] + n + 1, strlen(av[i] + n + 1),
					  0, (const struct sockaddr*) &peer, len);
		    if (sn < 0)
			perror("Failed to send (valid) response");
		    answ = 1;
		}
	    }
	} else if (req) {
	    maxat--;
	    if (maxat < 1) {
		fprintf(stderr, "ERROR: maximum number of attempts exhausted, giving up\n");
		return 1;
	    }
	    int sn = (int) sendto(UDPs, req, strlen(req), 0, (const struct sockaddr*) &peer, len);
	    if (verb)
		printf("INFO: sendto returned %d\n", sn);
	    if (sn < 0)
		perror("ERROR: send failed");
	}
	fflush(stdout);
    }
    close(UDPs);
    return 0;
}