c - 标记数据包以通过原始套接字发送

Question

我有以下通过原始套接字发送数据包的函数。

#include <unistd.h>
#include <stdio.h>
#include <sys/socket.h>
#include <netinet/ip.h>
#include <netinet/udp.h>

#include "pkt-types.h"
#include "pkt-log.h"
#include "pkt-utils.h"

int
send_packet_raw (void *data, int size)
{
  log_message (LOG_DEBUG, " inside send_packet_raw");
  int sd;
  struct iphdr *iph = (struct iphdr *) data;
  struct udphdr *udph = (struct udphdr *) (data + sizeof (struct ip));
  struct sockaddr_in sin;
  // needed for notify kernel to not to build header for this
  int one = 1;
  const int *val = &one;
  // creating a socket
  if ((sd = socket (PF_INET, SOCK_RAW, IPPROTO_UDP)) < 0)
    {
      log_message (LOG_ERROR, " problem creating a socket");
      return EXITCODE_SOCK_CREATION_FAILED;
    }
  // setting address family
  sin.sin_family = AF_INET;
  // setting port
  sin.sin_port = udph->dest;
  // setting ip
  sin.sin_addr.s_addr = iph->daddr;
  // notifying kernel do not fill up the packet structure.
  if (setsockopt (sd, IPPROTO_IP, IP_HDRINCL, val, sizeof (one)) < 0)
    {
      log_message (LOG_ERROR, "error notifying kernel about raw socket");
      return EXITCODE_SOCK_KERN_NOTIF_FAILED;
    }
  /* setting socket option to use MARK value */
  if (setsockopt (sd, SOL_SOCKET, SO_MARK, val, sizeof (one)) < 0)
  {
    log_message (LOG_ERROR, "error notifying kernel about MARK");
    return EXITCODE_SOCK_MARK_FAILED;
  }
  #ifdef CHECKSUM
  /* compute checksum */
  udph->check = udp_checksum (data + IP_OFFSET, size - IP_OFFSET, iph->saddr, iph->daddr);
  /* testing purposed */
  #else
  udph->check = 0x00;
  #endif
  /* dscp 101000 means express forwarding */
  if (sendto (sd,               /* our socket */
              data,             /* data to send */
              size,     /* total length of our ip packet */
              0,                /* routing flag, normally always zero */
              (struct sockaddr *) &sin, /* socket addr */
              sizeof (sin)) < 0)
    {
      log_message (LOG_ERROR, "sending over raw socket failed");
      return EXITCODE_SOCK_SEND_FAILED;
    }
  else
  {
    /* shutdown the socket */
    if(shutdown (sd, 2)) /* shutdown ok */
      return EXITCODE_OK;
  }
}

现在我正在从 libnetfilter_queue 的 nfq_set_verdict2() 设置标记:http://www.netfilter.org/projects/libnetfilter_queue/doxygen/group__Queue.html

int nfq_set_verdict2    (   struct nfq_q_handle *   qh,
u_int32_t   id,
u_int32_t   verdict,
u_int32_t   mark,
u_int32_t   data_len,
const unsigned char *   buf  
)           
nfq_set_verdict2 - like nfq_set_verdict, but you can set the mark.

Parameters:
qh  Netfilter queue handle obtained by call to nfq_create_queue().
id  ID assigned to packet by netfilter.
verdict     verdict to return to netfilter (NF_ACCEPT, NF_DROP)
mark    mark to put on packet
data_len    number of bytes of data pointed to by buf
buf     the buffer that contains the packet data

当我收到来自 netfilter_queue 的数据包时，我会执行以下操作:

nfq_set_verdict(..,NF_DROP,MARK,...);
process_packet();

这个 process_packet() 调用 send_packet_raw()。

关联的 iptable 规则:

$iptables -t mangle -A PREROUTING -m mark --mark 0xa -j ACCEPT
$iptables -t mangle -A PREROUTING -p udp --dport $PORT -j NFQUEUE
$iptables -t mangle -A OUTPUT -m mark --mark 0xa -j ACCEPT
$iptables -t mangle -A OUTPUT -p udp --sport $PORT -j NFQUEUE

我还设置了一些 -j 日志规则来查看数据包是否实际匹配。但由于没有显示日志条目，因此数据包似乎既没有发出也没有进入。无法理解如何在此处找到问题。

Answer 1

不太确定问题是什么，但是

nfq_set_verdict(..,NF_DROP,MARK,...);
process_packet();

看起来很糟糕。我不会在处理数据包之前调用 NF_DROP。我写了几个隧道程序，我首先处理数据包，把它放在我的缓冲区中，发出一个 NF_DROP。在此之后，我可以使用原始套接字从缓冲区重新发出数据包。所以:

process_packet();    
nfq_set_verdict(..,NF_DROP,MARK,...);

会更好。至少在发布判决之前复制数据包数据。