tcpdump 抓不到包
小弟初学网络编程，想用tcpdump抓包看一下，但是死活抓不到，求大神指点
服务器代码
[code=C/C++][/code]
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/wait.h>
#include <netinet/in.h>

#define SERVER_PORT 8003
#define MSG_BUF_SIZE 128

main()
{
  int sockfd;
  struct sockaddr_in my_addr;
  struct sockaddr_in their_addr;
  int addr_len,numbytes;
  char buffer[MSG_BUF_SIZE];

  if((sockfd = socket(AF_INET,SOCK_DGRAM,0)) == -1)
  {
  fprintf(stderr,"socket error");
  exit(1);
  }
  my_addr.sin_family = AF_INET;
  my_addr.sin_port = htons(SERVER_PORT);
  my_addr.sin_addr.s_addr = INADDR_ANY;
  bzero(&(my_addr.sin_zero),8);

  if(bind(sockfd,(struct sockaddr *)&my_addr,sizeof(struct sockaddr)) == -1)
  {
  fprintf(stderr,"bind error\n");
  exit(1);
  }
   
  addr_len = sizeof(struct sockaddr);

  if((numbytes = recvfrom(sockfd,buffer,MSG_BUF_SIZE,0,(struct sockaddr *)&their_addr,&addr_len)) == -1)
  {
  fprintf(stderr,"recvfrom error");
  exit(1);
  }

  printf("got packet from %s\n",inet_ntoa(their_addr.sin_addr));
  printf("packet is %d bytes long\n",numbytes);
  buffer[numbytes]='\0';
  printf("packet contains \"%s\"\n",buffer);
  close(sockfd);
}

客户端代码：
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <netdb.h>
#include <sys/socket.h>
#include <sys/wait.h>

#define SERVER_PORT 8003
int main(int argc,char *argv[])
{
  int sockfd;
  struct sockaddr_in their_addr;
  struct hostent *hostname;
  int num_bytes;

  if(argc != 3)
  {
  fprintf(stderr,"usage:talker hostname message\n");
  exit(1);
  }
  if((hostname = gethostbyname(argv[1])) == NULL)
  {
  fprintf(stderr,"gethostbyname");
  exit(1);
  }
  if((sockfd = socket(AF_INET,SOCK_DGRAM,0)) == -1)
  {
  fprintf(stderr,"socket");
  exit(1);
  }

  their_addr.sin_family = AF_INET;
  their_addr.sin_port = htons(SERVER_PORT);
  their_addr.sin_addr = *((struct in_addr *)hostname->h_addr);
  bzero(&(their_addr.sin_zero),8);

  if((num_bytes = sendto(sockfd,argv[2],strlen(argv[2]),0,(struct sockaddr *)&their_addr,sizeof(struct sockaddr))) == -1)
  {
  fprintf(stderr,"sendto");
  exit(1);
  }
  printf("sent %d bytes to %s\n",num_bytes,inet_ntoa(their_addr.sin_addr));
  close(sockfd);
  return 0;
}
所用的命令是 tcpdump port 8003
程序正常运行，但是抓不到包


------解决方案--------------------
本地环回不跑以太网卡，tcpdump -i lo 抓本地环回的。