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transmission/libtransmission/tr-lpd.cc

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// Except where noted, this file Copyright © 2010-2023 Johannes Lieder.
// It may be used under the MIT (SPDX: MIT) license.
// License text can be found in the licenses/ folder.
#include <algorithm>
#include <array>
#include <chrono>
#include <cstdint>
#include <functional>
#include <memory>
#include <optional>
#include <string>
#ifdef _WIN32
#include <ws2tcpip.h>
#else
#include <ctime>
#include <sys/socket.h> /* socket(), bind() */
#include <netinet/in.h> /* sockaddr_in */
#endif
#include <event2/event.h>
#include <fmt/core.h>
#include "libtransmission/transmission.h"
#include "libtransmission/crypto-utils.h" // for tr_rand_obj()
#include "libtransmission/log.h"
#include "libtransmission/net.h"
#include "libtransmission/timer.h"
#include "libtransmission/tr-assert.h"
#include "libtransmission/tr-lpd.h"
#include "libtransmission/utils.h" // for tr_net_init()
#include "libtransmission/utils-ev.h" // for tr_net_init()
using namespace std::literals;
// Code in this namespace Copyright © 2022 Mnemosyne LLC.
// It may be used under GPLv2 (SPDX: GPL-2.0-only), GPLv3 (SPDX: GPL-3.0-only), MIT (SPDX: MIT),
// or any future license endorsed by Mnemosyne LLC.
// License text can be found in the licenses/ folder.
namespace
{
// opaque value, allowing the sending client to filter out its
// own announces if it receives them via multicast loopback
auto makeCookie()
{
static auto constexpr Pool = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"sv;
auto buf = tr_rand_obj<std::array<char, 12>>();
for (auto& ch : buf)
{
ch = Pool[static_cast<unsigned char>(ch) % std::size(Pool)];
}
return std::string{ std::data(buf), std::size(buf) };
}
constexpr char const* const McastGroup = "239.192.152.143"; /**<LPD multicast group */
auto constexpr McastPort = tr_port::fromHost(6771); /**<LPD source and destination UPD port */
/*
* A LSD announce is formatted as follows:
*
* ```
* BT-SEARCH * HTTP/1.1\r\n
* Host: <host>\r\n
* Port: <port>\r\n
* Infohash: <ihash>\r\n
* cookie: <cookie (optional)>\r\n
* \r\n
* \r\n
* ```
*
* An announce may contain multiple, consecutive Infohash headers
* to announce the participation in more than one torrent. This
* may not be supported by older implementations. When sending
* multiple infohashes the packet length should not exceed 1400
* bytes to avoid MTU/fragmentation problems.
*/
auto makeAnnounceMsg(std::string_view cookie, tr_port port, std::vector<std::string_view> const& info_hash_strings)
{
auto ret = fmt::format(
"BT-SEARCH * HTTP/1.1\r\n"
"Host: {:s}:{:d}\r\n"
"Port: {:d}\r\n",
McastGroup,
McastPort.host(),
port.host());
for (auto const& info_hash : info_hash_strings)
{
ret += fmt::format("Infohash: {:s}\r\n", tr_strupper(info_hash));
}
if (!std::empty(cookie))
{
ret += fmt::format("cookie: {:s}\r\n", cookie);
}
return ret + "\r\n\r\n";
}
struct ParsedAnnounce
{
int major;
int minor;
tr_port port;
std::vector<std::string_view> info_hash_strings;
std::string_view cookie;
};
std::optional<ParsedAnnounce> parseAnnounceMsg(std::string_view announce)
{
static auto constexpr CrLf = "\r\n"sv;
auto ret = ParsedAnnounce{};
// get major, minor
auto key = "BT-SEARCH * HTTP/"sv;
if (auto const pos = announce.find(key); pos != std::string_view::npos)
{
// parse `${major}.${minor}`
auto walk = announce.substr(pos + std::size(key));
if (auto const major = tr_num_parse<int>(walk, &walk); major && tr_strv_starts_with(walk, '.'))
{
ret.major = *major;
}
else
{
return {};
}
walk.remove_prefix(1); // the '.' between major and minor
if (auto const minor = tr_num_parse<int>(walk, &walk); minor && tr_strv_starts_with(walk, CrLf))
{
ret.minor = *minor;
}
else
{
return {};
}
}
key = "Port: "sv;
if (auto const pos = announce.find(key); pos != std::string_view::npos)
{
auto walk = announce.substr(pos + std::size(key));
if (auto const port = tr_num_parse<uint16_t>(walk, &walk); port && tr_strv_starts_with(walk, CrLf))
{
ret.port = tr_port::fromHost(*port);
}
else
{
return {};
}
}
key = "cookie: "sv;
if (auto const pos = announce.find(key); pos != std::string_view::npos)
{
auto walk = announce.substr(pos + std::size(key));
if (auto const end = walk.find(CrLf); end != std::string_view::npos)
{
ret.cookie = walk.substr(0, end);
}
else
{
return {};
}
}
key = "Infohash: "sv;
for (;;)
{
if (auto const pos = announce.find(key); pos != std::string_view::npos)
{
announce.remove_prefix(pos + std::size(key));
}
else
{
break;
}
if (auto const end = announce.find(CrLf); end != std::string_view::npos)
{
ret.info_hash_strings.push_back(announce.substr(0, end));
announce.remove_prefix(end + std::size(CrLf));
}
else
{
return {};
}
}
return ret;
}
} // namespace
class tr_lpd_impl final : public tr_lpd
{
public:
tr_lpd_impl(Mediator& mediator, struct event_base* event_base)
: mediator_{ mediator }
, announce_timer_{ mediator.timerMaker().create([this]() { announceUpkeep(); }) }
, dos_timer_{ mediator.timerMaker().create([this]() { dosUpkeep(); }) }
{
if (!init(event_base))
{
return;
}
announce_timer_->start_repeating(AnnounceInterval);
announceUpkeep();
dos_timer_->start_repeating(DosInterval);
dosUpkeep();
}
tr_lpd_impl(tr_lpd_impl&&) = delete;
tr_lpd_impl(tr_lpd_impl const&) = delete;
tr_lpd_impl& operator=(tr_lpd_impl&&) = delete;
tr_lpd_impl& operator=(tr_lpd_impl const&) = delete;
~tr_lpd_impl() override
{
event_.reset();
if (mcast_rcv_socket_ != TR_BAD_SOCKET)
{
evutil_closesocket(mcast_rcv_socket_);
}
if (mcast_snd_socket_ != TR_BAD_SOCKET)
{
evutil_closesocket(mcast_snd_socket_);
}
tr_logAddTrace("Done uninitialising Local Peer Discovery");
}
private:
bool init(struct event_base* event_base)
{
if (initImpl(event_base))
{
return true;
}
auto const err = sockerrno;
evutil_closesocket(mcast_rcv_socket_);
evutil_closesocket(mcast_snd_socket_);
mcast_rcv_socket_ = TR_BAD_SOCKET;
mcast_snd_socket_ = TR_BAD_SOCKET;
tr_logAddWarn(fmt::format(
_("Couldn't initialize LPD: {error} ({error_code})"),
fmt::arg("error", tr_strerror(err)),
fmt::arg("error_code", err)));
return false;
}
/**
* @brief Initializes Local Peer Discovery for this node
*
* For the most part, this means setting up an appropriately configured multicast socket
* and event-based message handling.
*
* @remark Since the LPD service does not use another protocol family yet, this code is
* IPv4 only for the time being.
*/
bool initImpl(struct event_base* event_base)
{
int const opt_on = 1;
static_assert(AnnounceScope > 0);
tr_logAddDebug("Initialising Local Peer Discovery");
/* setup datagram socket (receive) */
{
mcast_rcv_socket_ = socket(PF_INET, SOCK_DGRAM, 0);
if (mcast_rcv_socket_ == TR_BAD_SOCKET)
{
return false;
}
if (evutil_make_socket_nonblocking(mcast_rcv_socket_) == -1)
{
return false;
}
if (setsockopt(
mcast_rcv_socket_,
SOL_SOCKET,
SO_REUSEADDR,
reinterpret_cast<char const*>(&opt_on),
sizeof(opt_on)) == -1)
{
return false;
}
#if HAVE_SO_REUSEPORT
if (setsockopt(
mcast_rcv_socket_,
SOL_SOCKET,
SO_REUSEPORT,
reinterpret_cast<char const*>(&opt_on),
sizeof(opt_on)) == -1)
{
return false;
}
#endif
mcast_addr_ = {};
mcast_addr_.sin_family = AF_INET;
mcast_addr_.sin_port = McastPort.network();
mcast_addr_.sin_addr.s_addr = INADDR_ANY;
if (bind(mcast_rcv_socket_, reinterpret_cast<sockaddr*>(&mcast_addr_), sizeof(mcast_addr_)) == -1)
{
return false;
}
if (evutil_inet_pton(mcast_addr_.sin_family, McastGroup, &mcast_addr_.sin_addr) == -1)
{
return false;
}
/* we want to join that LPD multicast group */
struct ip_mreq mcast_req = {};
mcast_req.imr_multiaddr = mcast_addr_.sin_addr;
mcast_req.imr_interface.s_addr = INADDR_ANY;
if (setsockopt(
mcast_rcv_socket_,
IPPROTO_IP,
IP_ADD_MEMBERSHIP,
reinterpret_cast<char const*>(&mcast_req),
sizeof(struct ip_mreq)) == -1)
{
return false;
}
}
/* setup datagram socket (send) */
{
unsigned char const scope = AnnounceScope;
mcast_snd_socket_ = socket(PF_INET, SOCK_DGRAM, 0);
if (mcast_snd_socket_ == TR_BAD_SOCKET)
{
return false;
}
if (evutil_make_socket_nonblocking(mcast_snd_socket_) == -1)
{
return false;
}
if (setsockopt(
mcast_snd_socket_,
SOL_SOCKET,
SO_REUSEADDR,
reinterpret_cast<char const*>(&opt_on),
sizeof(opt_on)) == -1)
{
return false;
}
if (auto [ss, sslen] = mediator_.bind_address(TR_AF_INET).to_sockaddr({});
bind(mcast_snd_socket_, reinterpret_cast<sockaddr*>(&ss), sslen) == -1)
{
return false;
}
/* configure outbound multicast TTL */
if (setsockopt(
mcast_snd_socket_,
IPPROTO_IP,
IP_MULTICAST_TTL,
reinterpret_cast<char const*>(&scope),
sizeof(scope)) == -1)
{
return false;
}
}
/* Note: lpd_unsolicitedMsgCounter remains 0 until the first timeout event, thus
* any announcement received during the initial interval will be discarded. */
event_.reset(event_new(event_base, mcast_rcv_socket_, EV_READ | EV_PERSIST, event_callback, this));
event_add(event_.get(), nullptr);
tr_logAddDebug("Local Peer Discovery initialised");
return true;
}
/**
* @brief Processing of timeout notifications and incoming data on the socket
* @note maximum rate of read events is limited according to @a lpd_maxAnnounceCap
* @see DoS */
static void event_callback(evutil_socket_t /*s*/, short type, void* vself)
{
if ((type & EV_READ) != 0)
{
static_cast<tr_lpd_impl*>(vself)->onCanRead();
}
}
void onCanRead()
{
if (!mediator_.allowsLPD())
{
return;
}
// process announcement from foreign peer
struct sockaddr_in foreign_addr = {};
auto addr_len = socklen_t{ sizeof(foreign_addr) };
auto foreign_msg = std::array<char, MaxDatagramLength>{};
auto const res = recvfrom(
mcast_rcv_socket_,
std::data(foreign_msg),
MaxDatagramLength,
0,
reinterpret_cast<sockaddr*>(&foreign_addr),
&addr_len);
// If we couldn't read it or it was too big, discard it
if (res < 1 || static_cast<size_t>(res) > MaxDatagramLength)
{
return;
}
// If it doesn't look like a BEP14 message, discard it
auto const msg = std::string_view{ std::data(foreign_msg), static_cast<size_t>(res) };
if (static auto constexpr SearchKey = "BT-SEARCH * HTTP/"sv; msg.find(SearchKey) == std::string_view::npos)
{
return;
}
// If we're receiving too many, discard it
if (++messages_received_since_upkeep_ > MaxIncomingPerUpkeep)
{
return;
}
// If it's an invalid message or the wrong protocol version, discard it.
// Note this comes *after* incrementing the count since there is some
// small CPU overhead in parsing, so don't do it for *every* message
auto const parsed = parseAnnounceMsg(msg);
if (!parsed || parsed->major != 1 || parsed->minor < 1 || parsed->cookie == cookie_)
{
tr_logAddTrace("Discarded invalid multicast message");
return;
}
auto peer_addr = tr_address{};
peer_addr.addr.addr4 = foreign_addr.sin_addr;
for (auto const& hash_string : parsed->info_hash_strings)
{
if (!mediator_.onPeerFound(hash_string, peer_addr, parsed->port))
{
tr_logAddDebug(fmt::format(FMT_STRING("Cannot serve torrent #{:s}"), hash_string));
}
}
}
void announceUpkeep()
{
if (!mediator_.allowsLPD())
{
return;
}
auto torrents = mediator_.torrents();
// remove torrents that don't need to be announced
auto const now = tr_time();
auto const needs_announce = [&now](auto& info)
{
return info.allows_lpd && (info.activity == TR_STATUS_DOWNLOAD || info.activity == TR_STATUS_SEED) &&
(info.announce_after < now);
};
torrents.erase(
std::remove_if(std::begin(torrents), std::end(torrents), std::not_fn(needs_announce)),
std::end(torrents));
if (std::empty(torrents))
{
return;
}
// prioritize the remaining torrents
std::sort(
std::begin(torrents),
std::end(torrents),
[](auto const& a, auto const& b)
{
if (a.activity != b.activity)
{
return a.activity < b.activity;
}
if (a.announce_after != b.announce_after)
{
return a.announce_after < b.announce_after;
}
return false;
});
// cram in as many as will fit in a message
auto const baseline_size = std::size(makeAnnounceMsg(cookie_, mediator_.port(), {}));
auto const size_with_one = std::size(makeAnnounceMsg(cookie_, mediator_.port(), { torrents.front().info_hash_str }));
auto const size_per_hash = size_with_one - baseline_size;
auto const max_torrents_per_announce = (MaxDatagramLength - baseline_size) / size_per_hash;
auto info_hash_strings = std::vector<std::string_view>{};
info_hash_strings.resize(std::min(std::size(torrents), max_torrents_per_announce));
std::transform(
std::begin(torrents),
std::begin(torrents) + std::size(info_hash_strings),
std::begin(info_hash_strings),
[](auto const& tor) { return tor.info_hash_str; });
if (!sendAnnounce(info_hash_strings))
{
return;
}
auto const next_announce_after = now + TorrentAnnounceIntervalSec;
for (auto const& info_hash_string : info_hash_strings)
{
mediator_.setNextAnnounceTime(info_hash_string, next_announce_after);
}
}
void dosUpkeep()
{
if (messages_received_since_upkeep_ > MaxIncomingPerUpkeep)
{
tr_logAddTrace(fmt::format(
"Dropped {} announces in the last interval (max. {} allowed)",
messages_received_since_upkeep_ - MaxIncomingPerUpkeep,
MaxIncomingPerUpkeep));
}
messages_received_since_upkeep_ = 0;
}
/**
* @brief Announce the given torrent on the local network
*
* @return Returns a success flag
*
* Send a query for torrent t out to the LPD multicast group (or the LAN, for that
* matter). A listening client on the same network might react by adding us to his
* peer pool for torrent t.
*/
bool sendAnnounce(std::vector<std::string_view> const& info_hash_strings)
{
auto const announce = makeAnnounceMsg(cookie_, mediator_.port(), info_hash_strings);
TR_ASSERT(std::size(announce) <= MaxDatagramLength);
auto const res = sendto(
mcast_snd_socket_,
std::data(announce),
std::size(announce),
0,
reinterpret_cast<sockaddr const*>(&mcast_addr_),
sizeof(mcast_addr_));
auto const sent = res == static_cast<int>(std::size(announce));
return sent;
}
std::string const cookie_ = makeCookie();
Mediator& mediator_;
tr_socket_t mcast_rcv_socket_ = TR_BAD_SOCKET; /**<separate multicast receive socket */
tr_socket_t mcast_snd_socket_ = TR_BAD_SOCKET; /**<and multicast send socket */
libtransmission::evhelpers::event_unique_ptr event_;
static auto constexpr MaxDatagramLength = size_t{ 1400 };
sockaddr_in mcast_addr_ = {}; /**<initialized from the above constants in init() */
// BEP14: "To avoid causing multicast storms on large networks a
// client should send no more than 1 announce per minute."
static auto constexpr AnnounceInterval = 1min;
std::unique_ptr<libtransmission::Timer> announce_timer_;
// Flood Protection:
// To protect against message flooding, stop processing search messages
// after processing N per upkeep. If we hit that limit, we're either
// in a *very* crowded multicast group or a hostile host is sending us
// bogus data. Better to drop a few packets than get DoS'ed.
static auto constexpr DosInterval = 5s;
std::unique_ptr<libtransmission::Timer> dos_timer_;
static auto constexpr MaxIncomingPerSecond = int{ 10 };
static auto constexpr MaxIncomingPerUpkeep = std::chrono::duration_cast<std::chrono::seconds>(DosInterval).count() *
MaxIncomingPerSecond;
// @brief throw away messages after this number exceeds MaxIncomingPerUpkeep
size_t messages_received_since_upkeep_ = 0U;
static auto constexpr TorrentAnnounceIntervalSec = time_t{ 240U }; // how frequently to reannounce the same torrent
static auto constexpr TtlSameSubnet = int{ 1 };
static auto constexpr AnnounceScope = int{ TtlSameSubnet }; /**<the maximum scope for LPD datagrams */
};
std::unique_ptr<tr_lpd> tr_lpd::create(Mediator& mediator, struct event_base* event_base)
{
return std::make_unique<tr_lpd_impl>(mediator, event_base);
}
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