udpread.c.bad

unix网络编程卷1：套接口API的全书源码

#include	"udpcksum.h"

struct udpiphdr *udp_check (char *, int);

/*
 * Read from the network until a UDP datagram is read that matches
 * the arguments.
 */

/* include udp_read */
struct udpiphdr *
udp_read (void)
{
  int len;
  char *ptr;
  struct ether_header *eptr;

  for (;;)
    {
      ptr = next_pcap (&len);

      switch (datalink)
	{
	case DLT_NULL:		/* loopback header = 4 bytes */
	  return (udp_check (ptr + 4, len - 4));

	case DLT_EN10MB:
	  eptr = (struct ether_header *) ptr;
	  if (ntohs (eptr->ether_type) != ETHERTYPE_IP)
	    err_quit ("Ethernet type %x not IP", ntohs (eptr->ether_type));
	  return (udp_check (ptr + 14, len - 14));

	case DLT_SLIP:		/* SLIP header = 24 bytes */
	  return (udp_check (ptr + 24, len - 24));

	case DLT_PPP:		/* PPP header = 24 bytes */
	  return (udp_check (ptr + 24, len - 24));

	default:
	  err_quit ("unsupported data link (%d)", datalink);
	}
    }
}
/* end udp_read */

/*
 * Check the received packet.
 * If UDP and OK, return pointer to packet.
 * If ICMP error, return NULL.
 * We assume the filter picks out desired UDP datagrams.
 */

/* include udp_check */
struct udpiphdr *
udp_check (char *ptr, int len)
{
  int hlen;
  struct ip *ip;
  struct udpiphdr *ui;

  if (len < sizeof (struct ip) + sizeof (struct udphdr))
      err_quit ("len = %d", len);

  /* minimal verification of IP header */
  ip = (struct ip *) ptr;
  if (ip->ip_v != IPVERSION)
    err_quit ("ip_v = %d", ip->ip_v);
  hlen = ip->ip_hl << 2;
  if (hlen < sizeof (struct ip))
      err_quit ("ip_hl = %d", ip->ip_hl);
  if (len < hlen + sizeof (struct udphdr))
      err_quit ("len = %d, hlen = %d", len, hlen);

  if ((ip->ip_sum = in_cksum ((u_short *) ip, hlen)) != 0)
    err_quit ("ip checksum error");

  if (ip->ip_p == IPPROTO_UDP)
    {
      ui = (struct udpiphdr *) ip;
      return (ui);
    }
  else
    err_quit ("not a UDP packet");
}
/* end udp_check */