apt-pkg/contrib/srvrec.cc

   1 // -*- mode: cpp; mode: fold -*-
   2 // Description                                                          /*{{{*/
   3 /* ######################################################################
   4
   5    SRV record support
   6
   7    ##################################################################### */
   8                                                                         /*}}}*/
   9 #include <config.h>
  10
  11 #include <netdb.h>
  12
  13 #include <netinet/in.h>
  14 #include <arpa/nameser.h>
  15 #include <resolv.h>
  16 #include <time.h>
  17
  18 #include <algorithm>
  19
  20 #include <apt-pkg/configuration.h>
  21 #include <apt-pkg/error.h>
  22 #include <apt-pkg/strutl.h>
  23
  24
  25 #include "srvrec.h"
  26
  27
  28 bool GetSrvRecords(std::string host, int port, std::vector<SrvRec> &Result)
  29 {
  30    std::string target;
  31    int res;
  32    struct servent s_ent_buf;
  33    struct servent *s_ent = nullptr;
  34    std::vector<char> buf(1024);
  35
  36    res = getservbyport_r(htons(port), "tcp", &s_ent_buf, buf.data(), buf.size(), &s_ent);
  37    if (res != 0 || s_ent == nullptr)
  38       return false;
  39
  40    strprintf(target, "_%s._tcp.%s", s_ent->s_name, host.c_str());
  41    return GetSrvRecords(target, Result);
  42 }
  43
  44 bool GetSrvRecords(std::string name, std::vector<SrvRec> &Result)
  45 {
  46    unsigned char answer[PACKETSZ];
  47    int answer_len, compressed_name_len;
  48    int answer_count;
  49
  50    if (res_init() != 0)
  51       return _error->Errno("res_init", "Failed to init resolver");
  52
  53    answer_len = res_query(name.c_str(), C_IN, T_SRV, answer, sizeof(answer));
  54    if (answer_len == -1)
  55       return false;
  56    if (answer_len < (int)sizeof(HEADER))
  57       return _error->Warning("Not enough data from res_query (%i)", answer_len);
  58
  59    // check the header
  60    HEADER *header = (HEADER*)answer;
  61    if (header->rcode != NOERROR)
  62       return _error->Warning("res_query returned rcode %i", header->rcode);
  63    answer_count = ntohs(header->ancount);
  64    if (answer_count <= 0)
  65       return _error->Warning("res_query returned no answers (%i) ", answer_count);
  66
  67    // skip the header
  68    compressed_name_len = dn_skipname(answer+sizeof(HEADER), answer+answer_len);
  69    if(compressed_name_len < 0)
  70       return _error->Warning("dn_skipname failed %i", compressed_name_len);
  71
  72    // pt points to the first answer record, go over all of them now
  73    unsigned char *pt = answer+sizeof(HEADER)+compressed_name_len+QFIXEDSZ;
  74    while ((int)Result.size() < answer_count && pt < answer+answer_len)
  75    {
  76       u_int16_t type, klass, priority, weight, port, dlen;
  77       char buf[MAXDNAME];
  78
  79       compressed_name_len = dn_skipname(pt, answer+answer_len);
  80       if (compressed_name_len < 0)
  81          return _error->Warning("dn_skipname failed (2): %i",
  82                                 compressed_name_len);
  83       pt += compressed_name_len;
  84       if (((answer+answer_len) - pt) < 16)
  85          return _error->Warning("packet too short");
  86
  87       // extract the data out of the result buffer
  88       #define extract_u16(target, p) target = *p++ << 8; target |= *p++;
  89
  90       extract_u16(type, pt);
  91       if(type != T_SRV)
  92          return _error->Warning("Unexpected type excepted %x != %x",
  93                                 T_SRV, type);
  94       extract_u16(klass, pt);
  95       if(klass != C_IN)
  96          return _error->Warning("Unexpected class excepted %x != %x",
  97                                 C_IN, klass);
  98       pt += 4;  // ttl
  99       extract_u16(dlen, pt);
 100       extract_u16(priority, pt);
 101       extract_u16(weight, pt);
 102       extract_u16(port, pt);
 103
 104       #undef extract_u16
 105
 106       compressed_name_len = dn_expand(answer, answer+answer_len, pt, buf, sizeof(buf));
 107       if(compressed_name_len < 0)
 108          return _error->Warning("dn_expand failed %i", compressed_name_len);
 109       pt += compressed_name_len;
 110
 111       // add it to our class
 112       Result.emplace_back(buf, priority, weight, port);
 113    }
 114
 115    // implement load balancing as specified in RFC-2782
 116
 117    // sort them by priority
 118    std::stable_sort(Result.begin(), Result.end());
 119
 120    for(std::vector<SrvRec>::iterator I = Result.begin();
 121       I != Result.end(); ++I)
 122    {
 123       if (_config->FindB("Debug::Acquire::SrvRecs", false) == true)
 124       {
 125          std::cerr << "SrvRecs: got " << I->target
 126                    << " prio: " << I->priority
 127                    << " weight: " << I->weight
 128                    << std::endl;
 129       }
 130    }
 131
 132    return true;
 133 }
 134
 135 SrvRec PopFromSrvRecs(std::vector<SrvRec> &Recs)
 136 {
 137    // FIXME: instead of the simplistic shuffle below use the algorithm
 138    //        described in rfc2782 (with weights)
 139    //        and figure out how the weights need to be adjusted if
 140    //        a host refuses connections
 141
 142 #if 0  // all code below is only needed for the weight adjusted selection
 143    // assign random number ranges
 144    int prev_weight = 0;
 145    int prev_priority = 0;
 146    for(std::vector<SrvRec>::iterator I = Result.begin();
 147       I != Result.end(); ++I)
 148    {
 149       if(prev_priority != I->priority)
 150          prev_weight = 0;
 151       I->random_number_range_start = prev_weight;
 152       I->random_number_range_end = prev_weight + I->weight;
 153       prev_weight = I->random_number_range_end;
 154       prev_priority = I->priority;
 155
 156       if (_config->FindB("Debug::Acquire::SrvRecs", false) == true)
 157          std::cerr << "SrvRecs: got " << I->target
 158                    << " prio: " << I->priority
 159                    << " weight: " << I->weight
 160                    << std::endl;
 161    }
 162
 163    // go over the code in reverse order and note the max random range
 164    int max = 0;
 165    prev_priority = 0;
 166    for(std::vector<SrvRec>::iterator I = Result.end();
 167       I != Result.begin(); --I)
 168    {
 169       if(prev_priority != I->priority)
 170          max = I->random_number_range_end;
 171       I->random_number_range_max = max;
 172    }
 173 #endif
 174
 175    // shuffle in a very simplistic way for now (equal weights)
 176    std::vector<SrvRec>::iterator I = Recs.begin();
 177    std::vector<SrvRec>::iterator const J = std::find_if(Recs.begin(), Recs.end(),
 178          [&I](SrvRec const &J) { return I->priority != J.priority; });
 179
 180    // clock seems random enough.
 181    I += clock() % std::distance(I, J);
 182    SrvRec const selected = std::move(*I);
 183    Recs.erase(I);
 184
 185    if (_config->FindB("Debug::Acquire::SrvRecs", false) == true)
 186       std::cerr << "PopFromSrvRecs: selecting " << selected.target << std::endl;
 187
 188    return selected;
 189 }