Cabecalho UDP 
        +-----------------------+-----------------------+ 
        |     Porta de origem   |   Porta de destino    |  \
        |       (16 bits)       |       (16 bits)       |   \
        +-----------------------+-----------------------+    -- 8 bytes
        |        Tamanho        |       Checksum        |   /
        |       (16 bits)       |       (16 bits)       |  /
        +-----------------------+-----------------------+ 
        |                                               |  \
       <                      Dados                      >  -- Max. 65507 bytes
        |                     (...)                     |  /    (teoricamente)
        +-----------------------------------------------+

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

#define LOCAL_SERVER_PORT 6666
#define MAX_MSG 100

int main(int argc, char *argv[]) {
  
  int sd, rc, n, cliLen;
  struct sockaddr_in cliAddr, servAddr;
  char msg[MAX_MSG];

  // criamos o socket usando SOCK_DGRAM para UDP
  sd=socket(AF_INET, SOCK_DGRAM, 0);
  if(sd<0) {
    printf("%s: cannot open socket \n",argv[0]);
    exit(1);
  }

  // bind no local
  servAddr.sin_family = AF_INET;
  servAddr.sin_addr.s_addr = htonl(INADDR_ANY);
  servAddr.sin_port = htons(LOCAL_SERVER_PORT);
  rc = bind (sd, (struct sockaddr *) &servAddr,sizeof(servAddr));
  if(rc<0) {
    printf("%s: cannot bind port number %d \n",
      argv[0], LOCAL_SERVER_PORT);
    exit(1);
  }

  printf("%s: waiting for data on port UDP %u\n",
      argv[0],LOCAL_SERVER_PORT);

  // loop infinito pra escuta
  while(1) {
    
    // buffer
    memset(msg,0x0,MAX_MSG);


    // separando dados
    cliLen = sizeof(cliAddr);
    n = recvfrom(sd, msg, MAX_MSG, 0,
       (struct sockaddr *) &cliAddr, &cliLen);

    if(n<0) {
      printf("%s: cannot receive data \n",argv[0]);
      continue;
    }
      
    //mostrando dados
    printf("%s: from %s:UDP%u : %s \n",
      argv[0],inet_ntoa(cliAddr.sin_addr),
      ntohs(cliAddr.sin_port),msg);
    
  }

return 0;

}

#include "sys/types.h"
#include "sys/socket.h"
#include "netinet/in.h"
#include "arpa/inet.h"
#include "netdb.h"
#include "stdlib.h"
#include "stdio.h"
#include "unistd.h"
#include "string.h"
#include "sys/time.h"

#define REMOTE_SERVER_PORT 6666
#define MAX_MSG 100


int main(int argc, char *argv[]) {
  
  int sd, rc, i;
  struct sockaddr_in cliAddr, remoteServAddr;
  struct hostent *h;

  // checamos o argumento
  if(argc<3) {
    printf("usage : %s <server> <data1> ... <dataN> \n", argv[0]);
    exit(1);
  }

  // verificamos o Host
  h = gethostbyname(argv[1]);
  if(h==NULL) {
    printf("%s: unknown host '%s' \n", argv[0], argv[1]);
    exit(1);
  }

  printf("%s: sending data to '%s' (IP : %s) \n", argv[0], h->h_name,
    inet_ntoa(*(struct in_addr *)h->h_addr_list[0]));

  remoteServAddr.sin_family = h->h_addrtype;
  memcpy((char *) &remoteServAddr.sin_addr.s_addr,
    h->h_addr_list[0], h->h_length);
  remoteServAddr.sin_port = htons(REMOTE_SERVER_PORT);

  // criamos nossa socket repare só o SOCK_DGRAM isso por que vamos usar UDP
  sd = socket(AF_INET,SOCK_DGRAM,0);
  if(sd<0) {
    printf("%s: cannot open socket \n",argv[0]);
    exit(1);
  }
  
  // bind na porta
  cliAddr.sin_family = AF_INET;
  cliAddr.sin_addr.s_addr = htonl(INADDR_ANY);
  cliAddr.sin_port = htons(0);
  
  rc = bind(sd, (struct sockaddr *) &cliAddr, sizeof(cliAddr));
  if(rc<0) {
    printf("%s: cannot bind port\n", argv[0]);
    exit(1);
  }


  // mandamos os dados
  for(i=2;i<argc;i++) {
    rc = sendto(sd, argv[i], strlen(argv[i])+1, 0,
      (struct sockaddr *) &remoteServAddr,
      sizeof(remoteServAddr));

    if(rc<0) {
      printf("%s: cannot send data %d \n",argv[0],i-1);
      close(sd);
      exit(1);
    }

  }
  
  return 1;

}