A simple stun client in C++ (posix only)

stun.cpp

// How to compile:
// clang++ -std=c++11 stun.cpp

#include <arpa/inet.h>
#include <netdb.h>
#include <stdio.h>
#include <sys/socket.h>
#include <unistd.h>
#include <array>
#include <cstring>
#include <string>

constexpr uint16_t kBindingRequest = 0x0001;
constexpr uint16_t kBindingResponse = 0x0101;
constexpr uint16_t kXorMappedAddress = 0x0020;
constexpr uint32_t kMagicCookie = 0x2112A442;

// https://datatracker.ietf.org/doc/html/rfc5389#section-6
struct __attribute__((packed)) StunRequest {
  StunRequest() {
    for (int i = 0; i < transaction_id_.size(); i++) {
      transaction_id_[i] = rand() % 256;
    }
  }
  const int16_t stun_message_type_ = htons(kBindingRequest);
  const int16_t message_length_ = htons(0x0000);
  const int32_t magic_cookie_ = htonl(kMagicCookie);
  std::array<uint8_t, 12> transaction_id_;
};

// https://datatracker.ietf.org/doc/html/rfc5389#section-7
struct __attribute__((packed)) StunResponse {
  int16_t stun_message_type_;
  int16_t message_length_;
  int32_t magic_cookie_;
  std::array<uint8_t, 12> transaction_id_;
  std::array<uint8_t, 1000> attributes_;
};

struct IPAndPort {
  std::string ip_;
  uint16_t port_ = 0;
};

// Converts a host name into an IP address.
std::string HostnameToIP(std::string const& hostname) {
  char ip[100];
  struct addrinfo hints, *servinfo, *p;
  struct sockaddr_in* h;
  int rv;
  memset(&hints, 0, sizeof(hints));
  hints.ai_family = AF_INET;
  hints.ai_socktype = SOCK_STREAM;

  if ((rv = getaddrinfo(hostname.c_str(), "http", &hints, &servinfo)) != 0) {
    return "";
  }
  for (p = servinfo; p != NULL; p = p->ai_next) {
    h = (struct sockaddr_in*)p->ai_addr;
    strcpy(ip, inet_ntoa(h->sin_addr));
  }
  freeaddrinfo(servinfo);
  return std::string(ip);
}

// Returns true on success
bool PerformStunRequest(int socket_fd,
                        std::string const& stun_server_ip,
                        short stun_server_port,
                        short local_port,
                        IPAndPort& ip_and_port) {
  struct sockaddr_in servaddr;
  struct sockaddr_in localaddr;

  StunRequest stun_request;
  StunResponse stun_response;

  bzero(&servaddr, sizeof(servaddr));
  servaddr.sin_family = AF_INET;
  inet_pton(AF_INET, stun_server_ip.c_str(), &servaddr.sin_addr);
  servaddr.sin_port = htons(stun_server_port);

  bzero(&localaddr, sizeof(localaddr));
  localaddr.sin_family = AF_INET;
  localaddr.sin_port = htons(local_port);

  int err = bind(socket_fd, (struct sockaddr*)&localaddr, sizeof(localaddr));
  if (err < 0) {
    printf("bind error\n");
    return false;
  }

  printf("Sending STUN request to %s:%d\n", stun_server_ip.c_str(),
         stun_server_port);
  err = sendto(socket_fd, &stun_request, sizeof(stun_request), 0,
               (struct sockaddr*)&servaddr, sizeof(servaddr));
  if (err < 0) {
    printf("sendto error\n");
    return false;
  }

  err = recvfrom(socket_fd, &stun_response, sizeof(stun_response), 0, NULL, 0);
  if (err < 0) {
    printf("recvfrom error\n");
    return false;
  }

  if (stun_response.magic_cookie_ != htonl(kMagicCookie)) {
    printf("magic cookie of response does not match request\n");
    return false;
  }
  if (stun_response.transaction_id_ != stun_request.transaction_id_) {
    printf("incorrect transaction id\n");
    return false;
  }
  if (stun_response.stun_message_type_ != htons(kBindingResponse)) {
    printf("incorrect message type\n");
    return false;
  }
  auto const& attributes = stun_response.attributes_;
  int16_t attributes_length =
      std::min<int16_t>(htons(stun_response.message_length_),
                        stun_response.attributes_.size());
  int i = 0;
  while (i < attributes_length) {
    auto attribute_type = htons(*(int16_t*)(&attributes[i]));
    auto attribute_length = htons(*(int16_t*)(&attributes[i + 2]));
    if (attribute_type == kXorMappedAddress) {
      uint16_t port = ntohs(*(uint16_t*)(&attributes[i + 6]));
      port ^= (kMagicCookie >> 16);
      std::string ip = std::to_string(attributes[i + 8] ^ ((kMagicCookie & 0xff000000) >> 24)) + "." +
                        std::to_string(attributes[i + 9] ^ ((kMagicCookie & 0x00ff0000) >> 16)) + "." +
                        std::to_string(attributes[i + 10] ^ ((kMagicCookie & 0x0000ff00) >> 8)) + "." +
                        std::to_string(attributes[i + 11] ^ ((kMagicCookie & 0x000000ff) >> 0)) + ".";
      ip_and_port.ip_ = ip;
      ip_and_port.port_ = port;
      return true;
    }
    i += (4 + attribute_length);
  }
  return false;
}

int main(int argc, char* argv[]) {
  const char* stun_server_host_name = "stun2.l.google.com";
  std::string ip = HostnameToIP(stun_server_host_name);
  printf("%s resolved to %s\n", stun_server_host_name, ip.c_str());
  
  IPAndPort ip_and_port;
  int socket_fd = socket(AF_INET, SOCK_DGRAM, 0);
  bool stun_request_succesfull =
      PerformStunRequest(socket_fd, ip, 19305, 9910, ip_and_port);
  close(socket_fd);
  if (stun_request_succesfull) {
    printf("local ip:port = %s:%d\n", ip_and_port.ip_.c_str(),
           ip_and_port.port_);
  } else {
    printf("Failed to perform stun request");
  }
}

This is nice but it has bugs. The conversion of the port number will not work for a port number that will not fit into a signed 16 bit integer. And it would be nice to use the actual message_cookie for the xor calculations. Anyways converting a port number greater than 32767 will give you a negative port number with this algorithm.

Fixed the bug and addressed your clean up suggestion. Thank you!

I have a question, where do you get the attribute types from?
I read the site about it, but I can't find where the signature of the types are.
For example, the constexpr uint16_t kXorMappedAddress = 0x0020;
But very nice work!

See section 18.2:

Comprehension-required range (0x0000-0x7FFF):
0x0000: (Reserved)
0x0001: MAPPED-ADDRESS
0x0002: (Reserved; was RESPONSE-ADDRESS)
0x0003: (Reserved; was CHANGE-ADDRESS)
0x0004: (Reserved; was SOURCE-ADDRESS)
0x0005: (Reserved; was CHANGED-ADDRESS)
0x0006: USERNAME
0x0007: (Reserved; was PASSWORD)
0x0008: MESSAGE-INTEGRITY
0x0009: ERROR-CODE
0x000A: UNKNOWN-ATTRIBUTES
0x000B: (Reserved; was REFLECTED-FROM)
0x0014: REALM
0x0015: NONCE
0x0020: XOR-MAPPED-ADDRESS

See section 18.2:

Comprehension-required range (0x0000-0x7FFF): 0x0000: (Reserved) 0x0001: MAPPED-ADDRESS 0x0002: (Reserved; was RESPONSE-ADDRESS) 0x0003: (Reserved; was CHANGE-ADDRESS) 0x0004: (Reserved; was SOURCE-ADDRESS) 0x0005: (Reserved; was CHANGED-ADDRESS) 0x0006: USERNAME 0x0007: (Reserved; was PASSWORD) 0x0008: MESSAGE-INTEGRITY 0x0009: ERROR-CODE 0x000A: UNKNOWN-ATTRIBUTES 0x000B: (Reserved; was REFLECTED-FROM) 0x0014: REALM 0x0015: NONCE 0x0020: XOR-MAPPED-ADDRESS

Oh i didnt saw that thank you for your fast reply. I thought it was under STUN Attributes.