support/support_format_dns_packet.c

   1 /* Convert a DNS packet to a human-readable representation.
   2    Copyright (C) 2016-2020 Free Software Foundation, Inc.
   3    This file is part of the GNU C Library.
   4
   5    The GNU C Library is free software; you can redistribute it and/or
   6    modify it under the terms of the GNU Lesser General Public
   7    License as published by the Free Software Foundation; either
   8    version 2.1 of the License, or (at your option) any later version.
   9
  10    The GNU C Library is distributed in the hope that it will be useful,
  11    but WITHOUT ANY WARRANTY; without even the implied warranty of
  12    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  13    Lesser General Public License for more details.
  14
  15    You should have received a copy of the GNU Lesser General Public
  16    License along with the GNU C Library; if not, see
  17    <https://www.gnu.org/licenses/>.  */
  18
  19 #include <support/format_nss.h>
  20
  21 #include <arpa/inet.h>
  22 #include <resolv.h>
  23 #include <stdbool.h>
  24 #include <support/check.h>
  25 #include <support/support.h>
  26 #include <support/xmemstream.h>
  27
  28 struct in_buffer
  29 {
  30   const unsigned char *data;
  31   size_t size;
  32 };
  33
  34 static inline bool
  35 extract_8 (struct in_buffer *in, unsigned char *value)
  36 {
  37   if (in->size == 0)
  38     return false;
  39   *value = in->data[0];
  40   ++in->data;
  41   --in->size;
  42   return true;
  43 }
  44
  45 static inline bool
  46 extract_16 (struct in_buffer *in, unsigned short *value)
  47 {
  48   if (in->size < 2)
  49     return false;
  50   *value = (in->data[0] << 8) | in->data[1];
  51   in->data += 2;
  52   in->size -= 2;
  53   return true;
  54 }
  55
  56 static inline bool
  57 extract_32 (struct in_buffer *in, unsigned *value)
  58 {
  59   if (in->size < 4)
  60     return false;
  61   unsigned a = in->data[0];
  62   unsigned b = in->data[1];
  63   unsigned c = in->data[2];
  64   unsigned d = in->data[3];
  65   *value = (a << 24) | (b << 16) | (c << 8) | d;
  66   in->data += 4;
  67   in->size -= 4;
  68   return true;
  69 }
  70
  71 static inline bool
  72 extract_bytes (struct in_buffer *in, size_t length, struct in_buffer *value)
  73 {
  74   if (in->size < length)
  75     return false;
  76   *value = (struct in_buffer) {in->data, length};
  77   in->data += length;
  78   in->size -= length;
  79   return true;
  80 }
  81
  82 struct dname
  83 {
  84   char name[MAXDNAME + 1];
  85 };
  86
  87 static bool
  88 extract_name (struct in_buffer full, struct in_buffer *in, struct dname *value)
  89 {
  90   const unsigned char *full_end = full.data + full.size;
  91   /* Sanity checks; these indicate buffer misuse.  */
  92   TEST_VERIFY_EXIT
  93     (!(in->data < full.data || in->data > full_end
  94        || in->size > (size_t) (full_end - in->data)));
  95   int ret = dn_expand (full.data, full_end, in->data,
  96                        value->name, sizeof (value->name));
  97   if (ret < 0)
  98     return false;
  99   in->data += ret;
 100   in->size -= ret;
 101   return true;
 102 }
 103
 104 char *
 105 support_format_dns_packet (const unsigned char *buffer, size_t length)
 106 {
 107   struct in_buffer full = { buffer, length };
 108   struct in_buffer in = full;
 109   struct xmemstream mem;
 110   xopen_memstream (&mem);
 111
 112   unsigned short txnid;
 113   unsigned short flags;
 114   unsigned short qdcount;
 115   unsigned short ancount;
 116   unsigned short nscount;
 117   unsigned short adcount;
 118   if (!(extract_16 (&in, &txnid)
 119         && extract_16 (&in, &flags)
 120         && extract_16 (&in, &qdcount)
 121         && extract_16 (&in, &ancount)
 122         && extract_16 (&in, &nscount)
 123         && extract_16 (&in, &adcount)))
 124     {
 125       fprintf (mem.out, "error: could not parse DNS header\n");
 126       goto out;
 127     }
 128   if (qdcount != 1)
 129     {
 130       fprintf (mem.out, "error: question count is %d, not 1\n", qdcount);
 131       goto out;
 132     }
 133   struct dname qname;
 134   if (!extract_name (full, &in, &qname))
 135     {
 136       fprintf (mem.out, "error: malformed QNAME\n");
 137       goto out;
 138     }
 139   unsigned short qtype;
 140   unsigned short qclass;
 141   if (!(extract_16 (&in, &qtype)
 142         && extract_16 (&in, &qclass)))
 143     {
 144       fprintf (mem.out, "error: malformed question\n");
 145       goto out;
 146     }
 147   if (qtype != T_A && qtype != T_AAAA && qtype != T_PTR)
 148     {
 149       fprintf (mem.out, "error: unsupported QTYPE %d\n", qtype);
 150       goto out;
 151     }
 152
 153   fprintf (mem.out, "name: %s\n", qname.name);
 154
 155   for (int i = 0; i < ancount; ++i)
 156     {
 157       struct dname rname;
 158       if (!extract_name (full, &in, &rname))
 159         {
 160           fprintf (mem.out, "error: malformed record name\n");
 161           goto out;
 162         }
 163       unsigned short rtype;
 164       unsigned short rclass;
 165       unsigned ttl;
 166       unsigned short rdlen;
 167       struct in_buffer rdata;
 168       if (!(extract_16 (&in, &rtype)
 169             && extract_16 (&in, &rclass)
 170             && extract_32 (&in, &ttl)
 171             && extract_16 (&in, &rdlen)
 172             && extract_bytes (&in, rdlen, &rdata)))
 173         {
 174           fprintf (mem.out, "error: malformed record header\n");
 175           goto out;
 176         }
 177       /* Skip non-matching record types.  */
 178       if ((rtype != qtype && rtype != T_CNAME) || rclass != qclass)
 179         continue;
 180       switch (rtype)
 181         {
 182         case T_A:
 183           if (rdlen == 4)
 184               fprintf (mem.out, "address: %d.%d.%d.%d\n",
 185                        rdata.data[0],
 186                        rdata.data[1],
 187                        rdata.data[2],
 188                        rdata.data[3]);
 189           else
 190             fprintf (mem.out, "error: A record of size %d: %s\n",
 191                      rdlen, rname.name);
 192           break;
 193         case T_AAAA:
 194           {
 195             if (rdlen == 16)
 196               {
 197                 char buf[100];
 198                 if (inet_ntop (AF_INET6, rdata.data, buf, sizeof (buf)) == NULL)
 199                   fprintf (mem.out, "error: AAAA record decoding failed: %m\n");
 200                 else
 201                   fprintf (mem.out, "address: %s\n", buf);
 202               }
 203             else
 204               fprintf (mem.out, "error: AAAA record of size %d: %s\n",
 205                        rdlen, rname.name);
 206           }
 207           break;
 208         case T_CNAME:
 209         case T_PTR:
 210           {
 211             struct dname name;
 212             if (extract_name (full, &rdata, &name))
 213               fprintf (mem.out, "name: %s\n", name.name);
 214             else
 215               fprintf (mem.out, "error: malformed CNAME/PTR record\n");
 216           }
 217         }
 218     }
 219
 220  out:
 221   xfclose_memstream (&mem);
 222   return mem.buffer;
 223 }