src/resolv.c

   1 /*
   2         libloc - A library to determine the location of someone on the Internet
   3
   4         Copyright (C) 2019 IPFire Development Team <info@ipfire.org>
   5
   6         This library is free software; you can redistribute it and/or
   7         modify it under the terms of the GNU Lesser General Public
   8         License as published by the Free Software Foundation; either
   9         version 2.1 of the License, or (at your option) any later version.
  10
  11         This library is distributed in the hope that it will be useful,
  12         but WITHOUT ANY WARRANTY; without even the implied warranty of
  13         MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  14         Lesser General Public License for more details.
  15 */
  16
  17 #include <arpa/nameser.h>
  18 #include <arpa/nameser_compat.h>
  19 #include <resolv.h>
  20 #include <string.h>
  21 #include <time.h>
  22
  23 #include <loc/format.h>
  24 #include <loc/private.h>
  25 #include <loc/resolv.h>
  26
  27 static int parse_timestamp(const unsigned char* txt, time_t* t) {
  28     struct tm ts;
  29
  30     // Parse timestamp
  31     char* p = strptime((const char*)txt, "%a, %d %b %Y %H:%M:%S GMT", &ts);
  32
  33     // If the whole string has been parsed, we convert the parse value to time_t
  34     if (p && !*p) {
  35         *t = timegm(&ts);
  36
  37     // Otherwise we reset t
  38     } else {
  39         *t = 0;
  40         return -1;
  41     }
  42
  43     return 0;
  44 }
  45
  46 LOC_EXPORT int loc_discover_latest_version(struct loc_ctx* ctx,
  47         unsigned int version, time_t* t) {
  48     // Initialise the resolver
  49     int r = res_init();
  50     if (r) {
  51         ERROR(ctx, "res_init() failed\n");
  52         return r;
  53     }
  54
  55     // Make domain
  56     char domain[64];
  57     snprintf(domain, 63, LOC_DATABASE_DOMAIN, version);
  58
  59     unsigned char answer[PACKETSZ];
  60     int len;
  61
  62     DEBUG(ctx, "Querying %s\n", domain);
  63
  64     // Send a query
  65     if ((len = res_query(domain, C_IN, T_TXT, answer, sizeof(answer))) < 0 || len > PACKETSZ) {
  66         ERROR(ctx, "Could not query %s: \n", domain);
  67
  68         return -1;
  69     }
  70
  71     unsigned char* end = answer + len;
  72     unsigned char* payload = answer + sizeof(HEADER);
  73
  74     // Expand domain name
  75     char host[128];
  76     if ((len = dn_expand(answer, end, payload, host, sizeof(host))) < 0) {
  77         ERROR(ctx, "dn_expand() failed\n");
  78         return -1;
  79     }
  80
  81     // Payload starts after hostname
  82     payload += len;
  83
  84     if (payload > end - 4) {
  85         ERROR(ctx, "DNS reply too short\n");
  86         return -1;
  87     }
  88
  89     int type;
  90     GETSHORT(type, payload);
  91     if (type != T_TXT) {
  92         ERROR(ctx, "DNS reply of unexpected type: %d\n", type);
  93         return -1;
  94     }
  95
  96     // Skip class
  97     payload += INT16SZ;
  98
  99     // Walk through CNAMEs
 100     unsigned int size = 0;
 101     int ttl __attribute__ ((unused));
 102     do {
 103         payload += size;
 104
 105         if ((len = dn_expand(answer, end, payload, host, sizeof(host))) < 0) {
 106             ERROR(ctx, "dn_expand() failed\n");
 107             return -1;
 108         }
 109
 110         payload += len;
 111
 112         if (payload > end - 10) {
 113             ERROR(ctx, "DNS reply too short\n");
 114             return -1;
 115         }
 116
 117         // Skip type, class, ttl
 118         GETSHORT(type, payload);
 119         payload += INT16SZ;
 120         GETLONG(ttl, payload);
 121
 122         // Read size
 123         GETSHORT(size, payload);
 124         if (payload + size < answer || payload + size > end) {
 125             ERROR(ctx, "DNS RR overflow\n");
 126             return -1;
 127         }
 128     } while (type == T_CNAME);
 129
 130     if (type != T_TXT) {
 131         ERROR(ctx, "Not a TXT record\n");
 132         return -1;
 133     }
 134
 135     if (!size || (len = *payload) >= size || !len) {
 136         ERROR(ctx, "Broken TXT record (len = %d, size = %d)\n", len, size);
 137         return -1;
 138     }
 139
 140     // Get start of the string
 141     unsigned char* txt = payload + 1;
 142     txt[len] = '\0';
 143
 144     DEBUG(ctx, "Resolved to: %s\n", txt);
 145
 146     // Parse timestamp
 147     r = parse_timestamp(txt, t);
 148
 149     return r;
 150 }