A modified version of the UDP echo server from CMU. Modified to echo back the number of bytes sent, not just the strlen of the data sent. Fixes issue with the SmartRG CPES where they send 24 NULL bytes.

udpserver.c

/* 
 * https://www.cs.cmu.edu/afs/cs/academic/class/15213-f99/www/class26/udpserver.c
 * udpserver.c - A simple UDP echo server 
 * usage: udpserver <port>
 */

#include <stdio.h>
#include <stdbool.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <netdb.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>

#define BUFSIZE 1024

/*
 * error - wrapper for perror
 */
void error(char *msg) {
    perror(msg);
    exit(1);
}

int main(int argc, char **argv) {
    int sockfd;                    /* socket */
    int portno;                    /* port to listen on */
    socklen_t clientlen;           /* byte size of client's address */
    struct sockaddr_in serveraddr; /* server's addr */
    struct sockaddr_in clientaddr; /* client addr */
    char buf[BUFSIZE];             /* message buf */
    char *hostaddrp;               /* dotted decimal host addr string */
    int optval;                    /* flag value for setsockopt */
    int n;                         /* message byte size */

    /* 
     * check command line arguments 
     */
    if (argc != 2) {
        fprintf(stderr, "usage: %s <port>\n", argv[0]);
        exit(1);
    }
    portno = atoi(argv[1]);

    // socket: create the parent socket 
    sockfd = socket(AF_INET, SOCK_DGRAM, 0);
    if (sockfd < 0) {
        error("ERROR opening socket");
    }

    /* setsockopt: Handy debugging trick that lets 
     * us rerun the server immediately after we kill it; 
     * otherwise we have to wait about 20 secs. 
     * Eliminates "ERROR on binding: Address already in use" error. 
     */
    optval = 1;
    setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, (const void *)&optval, sizeof(int));

    //Init the buffer to null
    memset(buf, 0, BUFSIZE);

    // build the server's Internet address
    bzero((char *)&serveraddr, sizeof(serveraddr));
    serveraddr.sin_family = AF_INET;
    serveraddr.sin_addr.s_addr = htonl(INADDR_ANY);
    serveraddr.sin_port = htons((unsigned short)portno);

    // bind: associate the parent socket with a port 
    if (bind(sockfd, (struct sockaddr *)&serveraddr, sizeof(serveraddr)) < 0) {
        error("ERROR on binding");
    }

    // main loop: wait for a datagram, then echo it
    clientlen = sizeof(clientaddr);
    while (true) {
        // recvfrom: receive a UDP datagram from a client
        bzero(buf, BUFSIZE);
        n = recvfrom(sockfd, buf, BUFSIZE, 0, (struct sockaddr *)&clientaddr, &clientlen);

        hostaddrp = inet_ntoa(clientaddr.sin_addr);

        if (n < 0) {
            error("ERROR in recvfrom");
        } else if (n > BUFSIZE) {
            snprintf(buf, BUFSIZE, "ERROR amount of data received from %s is greater than buffer size of %d", hostaddrp, BUFSIZE);
            error(buf);
        }

        printf("Received %d/%d bytes from %s\n", (int)strlen(buf), n, hostaddrp);

        // sendto: echo the input back to the client 
        n = sendto(sockfd, buf, n, 0, (struct sockaddr *)&clientaddr, clientlen);
        if (n < 0) {
            error("ERROR in sendto");
        }
    }
}