const char *bin 就进坑了，const unsigned char *bin 可以出坑

std::string bin_to_hex(const char *bin, const size_t len) {
  std::string s;
  for (size_t k = 0; k < len; k++) {
    char buffer[3];
    std::snprintf(buffer, sizeof(buffer), "%02x", bin[k]);
    s += buffer;
  }

  return s;
}
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FROM 114.241.228.*