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Sigo intentando descubrir porque no captura algo que deberia hacerlo con facilidad, ya que no estoy enviando nada a otra ip, sino a la del mismo pc. Quizas por la longitud de los paquetes?
He comprobado el estado del paquete con wireshark y no me dice nada fuera de lo comun.

Es extraño.

Hay otra forma de hacerlo??, que es lo que bloquea la captura??.

[Por cierto, estos codigos los he encontrado en Internet.]

Estoy desarrollando una pequeña aplicación en el que un dispositivo me envía un paquete UDP y me gustaría saber porque si los captura con winpcap y no con winsock de windows.

Mi intención es desarrollar la aplicación sin añadirle la librería winpcap. Os dejo el código de las dos aplicaciones.

Código [Seleccionar] Expandir

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

void error(const char *msg)
{
   perror(msg);
   exit(1);
}

int main(int argc, char *argv[])
{
  WSADATA wsda;
  SOCKET s;
  SOCKADDR_IN addr, remote_addr;
  u_short iPort = 12134;
  int i, ret, iRemoteAddrLen;
  char hex[16] = "123456789ABCDEF";
  char szMessage[512], szMessageString[512*3];

  WSAStartup(MAKEWORD(2,2), &wsda);
   s = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
  //s = socket(AF_INET, SOCK_RAW, IPPROTO_UDP);
 
  if(s == SOCKET_ERROR)
  {
     printf("Error\nCall to socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP); failed with:\n%d\n", WSAGetLastError());
     exit(1);
  }

  addr.sin_family = AF_INET;
  addr.sin_port = htons(iPort);
  addr.sin_addr.s_addr = INADDR_ANY;

  if(bind(s, (struct sockaddr *) &addr, sizeof(addr)) == SOCKET_ERROR)
  {
     printf("Error\nCall to bind(s, (struct sockaddr *) &addr, sizeof(addr)); failed with:\n%d\n", WSAGetLastError());
     exit(1);
  }

  printf("Waiting for packets (Press Ctrl-C to exit)...\n");
  while(1)
  {
  iRemoteAddrLen = sizeof(remote_addr);
  ret = recvfrom(s, szMessage, sizeof(szMessage), 0, (struct sockaddr *) &remote_addr, &iRemoteAddrLen);

  if(ret == SOCKET_ERROR)
  {
 printf("Error\nCall to recvfrom(s, szMessage, sizeof(szMessage), 0, (struct sockaddr *) &remote_addr, &iRemoteAddrLen); failed with:\n%d\n", WSAGetLastError());
 exit(1);
  }

  for(i=0; i<ret; i++)
  {
  szMessageString[i*3+0] = hex[szMessage[i]>>4];
  szMessageString[i*3+1] = hex[szMessage[i]&0xF];
  szMessageString[i*3+2] = ' ';
  }
  szMessageString[i*3-1] = 0;

 printf("Received %d bytes from %s\n", ret, inet_ntoa(remote_addr.sin_addr));
  }

  closesocket(s);
  WSACleanup();

  return 0;

 }

Código [Seleccionar] Expandir

#define HAVE_REMOTE
#include "pcap.h"
#include "remote-ext.h"

/* 4 bytes IP address */
typedef struct ip_address{
u_char byte1;
u_char byte2;
u_char byte3;
u_char byte4;
}ip_address;

/* IPv4 header */
typedef struct ip_header{
u_char ver_ihl; // Version (4 bits) + Internet header length (4 bits)
u_char tos; // Type of service
u_short tlen; // Total length
u_short identification; // Identification
u_short flags_fo; // Flags (3 bits) + Fragment offset (13 bits)
u_char ttl; // Time to live
u_char proto; // Protocol
u_short crc; // Header checksum
ip_address saddr; // Source address
ip_address daddr; // Destination address
u_int op_pad; // Option + Padding
}ip_header;

/* UDP header*/
typedef struct udp_header{
u_short sport; // Source port
u_short dport; // Destination port
u_short len; // Datagram length
u_short crc; // Checksum
}udp_header;

/* prototype of the packet handler */
void packet_handler(u_char *param, const struct pcap_pkthdr *header, const u_char *pkt_data);


int main()
{
pcap_if_t *alldevs;
pcap_if_t *d;
int inum;
int i=0;
pcap_t *adhandle;
char errbuf[PCAP_ERRBUF_SIZE];
char source[10] = "rpcap://";
u_int netmask;
char packet_filter[] = "ip and udp";
struct bpf_program fcode;

/* Retrieve the device list */
if (pcap_findalldevs_ex(source, NULL, &alldevs, errbuf) == -1)
{
fprintf(stderr,"Error in pcap_findalldevs: %s\n", errbuf);
exit(1);
}

/* Print the list */
for(d=alldevs; d; d=d->next)
{
printf("%d. %s", ++i, d->name);
if (d->description)
printf(" (%s)\n", d->description);
else
printf(" (No description available)\n");
}

if(i==0)
{
printf("\nNo interfaces found! Make sure WinPcap is installed.\n");
return -1;
}

printf("Enter the interface number (1-%d):",i);
scanf("%d", &inum);

if(inum < 1 || inum > i)
{
printf("\nInterface number out of range.\n");
/* Free the device list */
pcap_freealldevs(alldevs);
return -1;
}

/* Jump to the selected adapter */
for(d=alldevs, i=0; i< inum-1 ;d=d->next, i++);

/* Open the adapter */
if ( (adhandle= pcap_open(d->name, // name of the device
65536, // portion of the packet to capture.
// 65536 grants that the whole packet will be captured on all the MACs.
1, // promiscuous mode
1000, // read timeout
NULL, // remote authentication
errbuf // error buffer
) ) == NULL)
{
fprintf(stderr,"\nUnable to open the adapter. %s is not supported by WinPcap\n");
/* Free the device list */
pcap_freealldevs(alldevs);
return -1;
}

/* Check the link layer. We support only Ethernet for simplicity. */
if(pcap_datalink(adhandle) != DLT_EN10MB)
{
fprintf(stderr,"\nThis program works only on Ethernet networks.\n");
/* Free the device list */
pcap_freealldevs(alldevs);
return -1;
}

if(d->addresses != NULL)
/* Retrieve the mask of the first address of the interface */
netmask=((struct sockaddr_in *)(d->addresses->netmask))->sin_addr.S_un.S_addr;
else
/* If the interface is without addresses we suppose to be in a C class network */
netmask=0xffffff;


//compile the filter
if (pcap_compile(adhandle, &fcode, packet_filter, 1, netmask) <0 )
{
fprintf(stderr,"\nUnable to compile the packet filter. Check the syntax.\n");
/* Free the device list */
pcap_freealldevs(alldevs);
return -1;
}

//set the filter
if (pcap_setfilter(adhandle, &fcode)<0)
{
fprintf(stderr,"\nError setting the filter.\n");
/* Free the device list */
pcap_freealldevs(alldevs);
return -1;
}

printf("\nlistening on %s...\n", d->description);

/* At this point, we don't need any more the device list. Free it */
pcap_freealldevs(alldevs);

/* start the capture */
pcap_loop(adhandle, 0, packet_handler, NULL);

return 0;
}

/* Callback function invoked by libpcap for every incoming packet */
void packet_handler(u_char *param, const struct pcap_pkthdr *header, const u_char *pkt_data)
{
struct tm ltime;
char timestr[16];
ip_header *ih;
udp_header *uh;
u_int ip_len;
u_short sport,dport;
time_t local_tv_sec;

/*
* Unused variable
*/
(VOID)(param);

/* convert the timestamp to readable format */
local_tv_sec = header->ts.tv_sec;
//localtime(&ltime, &local_tv_sec);
strftime( timestr, sizeof timestr, "%H:%M:%S", &ltime);

/* print timestamp and length of the packet */
printf("%s.%.6d len:%d ", timestr, header->ts.tv_usec, header->len);

/* retireve the position of the ip header */
ih = (ip_header *) (pkt_data +
14); //length of ethernet header

/* retireve the position of the udp header */
ip_len = (ih->ver_ihl & 0xf) * 4;
uh = (udp_header *) ((u_char*)ih + ip_len);

/* convert from network byte order to host byte order */
sport = ntohs( uh->sport );
dport = ntohs( uh->dport );

/* print ip addresses and udp ports */
printf("%d.%d.%d.%d.%d -> %d.%d.%d.%d.%d\n",
ih->saddr.byte1,
ih->saddr.byte2,
ih->saddr.byte3,
ih->saddr.byte4,
sport,
ih->daddr.byte1,
ih->daddr.byte2,
ih->daddr.byte3,
ih->daddr.byte4,
dport);
}

Mientras que la aplicación con winsock no me captura nada, la aplicación con winpcap me muestra:

.175848 len:47 192.168.0.176.12134 -> 192.168.0.150.12134
.177304 len:47 192.168.0.176.12134 -> 192.168.0.150.12134
.180558 len:47 192.168.0.176.12134 -> 192.168.0.150.12134
.182848 len:47 192.168.0.176.12134 -> 192.168.0.150.12134

He utilizado el compilador Mingw y la herramienta de desarrollo Netbeans 7.1.1

Por cierto, estos codigos los he encontrado en Internet.