src/dump.c

/* dnsmasq is Copyright (c) 2000-2021 Simon Kelley

   This program is free software; you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation; version 2 dated June, 1991, or
   (at your option) version 3 dated 29 June, 2007.
 
   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.
     
   You should have received a copy of the GNU General Public License
   along with this program.  If not, see <http://www.gnu.org/licenses/>.
*/

#include "dnsmasq.h"

#ifdef HAVE_DUMPFILE

static u32 packet_count;

/* https://wiki.wireshark.org/Development/LibpcapFileFormat */
struct pcap_hdr_s {
        u32 magic_number;   /* magic number */
        u16 version_major;  /* major version number */
        u16 version_minor;  /* minor version number */
        u32 thiszone;       /* GMT to local correction */
        u32 sigfigs;        /* accuracy of timestamps */
        u32 snaplen;        /* max length of captured packets, in octets */
        u32 network;        /* data link type */
};

struct pcaprec_hdr_s {
        u32 ts_sec;         /* timestamp seconds */
        u32 ts_usec;        /* timestamp microseconds */
        u32 incl_len;       /* number of octets of packet saved in file */
        u32 orig_len;       /* actual length of packet */
};


void dump_init(void)
{
  struct stat buf;
  struct pcap_hdr_s header;
  struct pcaprec_hdr_s pcap_header;

  packet_count = 0;
  
  if (stat(daemon->dump_file, &buf) == -1)
    {
      /* doesn't exist, create and add header */
      header.magic_number = 0xa1b2c3d4;
      header.version_major = 2;
      header.version_minor = 4;
      header.thiszone = 0;
      header.sigfigs = 0;
      header.snaplen = daemon->edns_pktsz + 200; /* slop for IP/UDP headers */
      header.network = 101; /* DLT_RAW http://www.tcpdump.org/linktypes.html */

      if (errno != ENOENT ||
	  (daemon->dumpfd = creat(daemon->dump_file, S_IRUSR | S_IWUSR)) == -1 ||
	  !read_write(daemon->dumpfd, (void *)&header, sizeof(header), 0))
	die(_("cannot create %s: %s"), daemon->dump_file, EC_FILE);
    }
  else if ((daemon->dumpfd = open(daemon->dump_file, O_APPEND | O_RDWR)) == -1 ||
	   !read_write(daemon->dumpfd, (void *)&header, sizeof(header), 1))
    die(_("cannot access %s: %s"), daemon->dump_file, EC_FILE);
  else if (header.magic_number != 0xa1b2c3d4)
    die(_("bad header in %s"), daemon->dump_file, EC_FILE);
  else
    {
      /* count existing records */
      while (read_write(daemon->dumpfd, (void *)&pcap_header, sizeof(pcap_header), 1))
	{
	  lseek(daemon->dumpfd, pcap_header.incl_len, SEEK_CUR);
	  packet_count++;
	}
    }
}

void dump_packet(int mask, void *packet, size_t len, union mysockaddr *src, union mysockaddr *dst)
{
  struct ip ip;
  struct ip6_hdr ip6;
  int family;
  struct udphdr {
    u16 uh_sport;               /* source port */
    u16 uh_dport;               /* destination port */
    u16 uh_ulen;                /* udp length */
    u16 uh_sum;                 /* udp checksum */
  } udp;
  struct pcaprec_hdr_s pcap_header;
  struct timeval time;
  u32 i, sum;
  void *iphdr;
  size_t ipsz;
  int rc;
  
  if (daemon->dumpfd == -1 || !(mask & daemon->dump_mask))
    return;
  
  /* So wireshark can Id the packet. */
  udp.uh_sport = udp.uh_dport = htons(NAMESERVER_PORT);

  if (src)
    family = src->sa.sa_family;
  else
    family = dst->sa.sa_family;

  if (family == AF_INET6)
    {
      iphdr = &ip6;
      ipsz = sizeof(ip6);
      memset(&ip6, 0, sizeof(ip6));
      
      ip6.ip6_vfc = 6 << 4;
      ip6.ip6_plen = htons(sizeof(struct udphdr) + len);
      ip6.ip6_nxt = IPPROTO_UDP;
      ip6.ip6_hops = 64;

      if (src)
	{
	  memcpy(&ip6.ip6_src, &src->in6.sin6_addr, IN6ADDRSZ);
	  udp.uh_sport = src->in6.sin6_port;
	}
      
      if (dst)
	{
	  memcpy(&ip6.ip6_dst, &dst->in6.sin6_addr, IN6ADDRSZ);
	  udp.uh_dport = dst->in6.sin6_port;
	}
            
      /* start UDP checksum */
      for (sum = 0, i = 0; i < IN6ADDRSZ; i+=2)
	{
	  sum += ip6.ip6_src.s6_addr[i] + (ip6.ip6_src.s6_addr[i+1] << 8) ;
	  sum += ip6.ip6_dst.s6_addr[i] + (ip6.ip6_dst.s6_addr[i+1] << 8) ;
	  
	}
    }
  else
    {
      iphdr = &ip;
      ipsz = sizeof(ip);
      memset(&ip, 0, sizeof(ip));
      
      ip.ip_v = IPVERSION;
      ip.ip_hl = sizeof(struct ip) / 4;
      ip.ip_len = htons(sizeof(struct ip) + sizeof(struct udphdr) + len); 
      ip.ip_ttl = IPDEFTTL;
      ip.ip_p = IPPROTO_UDP;
      
      if (src)
	{
	  ip.ip_src = src->in.sin_addr;
	  udp.uh_sport = src->in.sin_port;
	}

      if (dst)
	{
	  ip.ip_dst = dst->in.sin_addr;
	  udp.uh_dport = dst->in.sin_port;
	}
      
      ip.ip_sum = 0;
      for (sum = 0, i = 0; i < sizeof(struct ip) / 2; i++)
	sum += ((u16 *)&ip)[i];
      while (sum >> 16)
	sum = (sum & 0xffff) + (sum >> 16);  
      ip.ip_sum = (sum == 0xffff) ? sum : ~sum;
      
      /* start UDP checksum */
      sum = ip.ip_src.s_addr & 0xffff;
      sum += (ip.ip_src.s_addr >> 16) & 0xffff;
      sum += ip.ip_dst.s_addr & 0xffff;
      sum += (ip.ip_dst.s_addr >> 16) & 0xffff;
    }
  
  if (len & 1)
    ((unsigned char *)packet)[len] = 0; /* for checksum, in case length is odd. */

  udp.uh_sum = 0;
  udp.uh_ulen = htons(sizeof(struct udphdr) + len);
  sum += htons(IPPROTO_UDP);
  sum += htons(sizeof(struct udphdr) + len);
  for (i = 0; i < sizeof(struct udphdr)/2; i++)
    sum += ((u16 *)&udp)[i];
  for (i = 0; i < (len + 1) / 2; i++)
    sum += ((u16 *)packet)[i];
  while (sum >> 16)
    sum = (sum & 0xffff) + (sum >> 16);
  udp.uh_sum = (sum == 0xffff) ? sum : ~sum;

  rc = gettimeofday(&time, NULL);
  pcap_header.ts_sec = time.tv_sec;
  pcap_header.ts_usec = time.tv_usec;
  pcap_header.incl_len = pcap_header.orig_len = ipsz + sizeof(udp) + len;
  
  if (rc == -1 ||
      !read_write(daemon->dumpfd, (void *)&pcap_header, sizeof(pcap_header), 0) ||
      !read_write(daemon->dumpfd, iphdr, ipsz, 0) ||
      !read_write(daemon->dumpfd, (void *)&udp, sizeof(udp), 0) ||
      !read_write(daemon->dumpfd, (void *)packet, len, 0))
    my_syslog(LOG_ERR, _("failed to write packet dump"));
  else
    my_syslog(LOG_INFO, _("dumping UDP packet %u mask 0x%04x"), ++packet_count, mask);

}

#endif