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transmission/libtransmission/tr-lpd.cc

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// This file Copyright © 2010 Johannes Lieder.
// It may be used under the MIT (SPDX: MIT) license.
// License text can be found in the licenses/ folder.
#include <algorithm>
#include <cerrno>
#include <csignal> /* sig_atomic_t */
#include <cstring> /* strlen(), strncpy(), strstr(), memset() */
#include <string_view>
#include <type_traits>
#ifdef _WIN32
#include <ws2tcpip.h>
using in_port_t = uint16_t; /* all missing */
#else
#include <ctime>
#include <unistd.h> /* close() */
#include <sys/types.h>
#include <sys/socket.h> /* socket(), bind() */
#include <netinet/in.h> /* sockaddr_in */
#endif
#include <event2/event.h>
#include <event2/util.h>
#include <fmt/core.h>
#include "transmission.h"
#include "log.h"
#include "net.h"
#include "peer-mgr.h" /* tr_peerMgrAddPex() */
#include "session.h"
#include "torrent.h"
#include "tr-assert.h"
#include "tr-lpd.h"
#include "utils.h"
using namespace std::literals;
static auto constexpr SIZEOF_HASH_STRING = TR_SHA1_DIGEST_STRLEN;
/**
* @brief Local Peer Discovery
* @file tr-lpd.c
*
* This module implements the Local Peer Discovery (LPD) protocol as supported by the
* uTorrent client application. A typical LPD datagram is 119 bytes long.
*
*/
static void event_callback(evutil_socket_t, short /*type*/, void* /*unused*/);
static auto constexpr UpkeepIntervalSecs = int{ 5 };
static struct event* upkeep_timer = nullptr;
static tr_socket_t lpd_socket; /**<separate multicast receive socket */
static tr_socket_t lpd_socket2; /**<and multicast send socket */
static event* lpd_event = nullptr;
static tr_port lpd_port;
static tr_session* session;
static auto constexpr lpd_maxDatagramLength = int{ 200 }; /**<the size an LPD datagram must not exceed */
static char constexpr lpd_mcastGroup[] = "239.192.152.143"; /**<LPD multicast group */
static auto constexpr lpd_mcastPort = int{ 6771 }; /**<LPD source and destination UPD port */
static auto lpd_mcastAddr = sockaddr_in{}; /**<initialized from the above constants in tr_lpdInit */
/**
* @brief Protocol-related information carried by a Local Peer Discovery packet */
struct lpd_protocolVersion
{
int major;
int minor;
};
static auto constexpr lpd_ttlSameSubnet = int{ 1 };
// static auto constexpr lpd_ttlSameSite = int{ 32 };
// static auto constexpr lpd_ttlSameRegion = int{ 64 };
// static auto constexpr lpd_ttlSameContinent = int{ 128 };
// static auto constexpr lpd_ttlUnrestricted = int{ 255 };
static auto constexpr lpd_announceInterval = int{ 4 * 60 }; /**<4 min announce interval per torrent */
static auto constexpr lpd_announceScope = int{ lpd_ttlSameSubnet }; /**<the maximum scope for LPD datagrams */
/**
* @defgroup DoS Message Flood Protection
* @{
* We want to have a means to protect the libtransmission backend against message
* flooding: the strategy is to cap event processing once more than ten messages
* per second (that is, taking the average over one of our housekeeping intervals)
* got into our processing handler.
* If we'd really hit the limit and start discarding events, we either joined an
* extremely crowded multicast group or a malevolent host is sending bogus data to
* our socket. In this situation, we rather miss some announcements than blocking
* the actual task.
* @}
*/
/**
* @ingroup DoS
* @brief allow at most ten messages per second (interval average)
* @note this constraint is only enforced once per housekeeping interval */
static auto constexpr lpd_announceCapFactor = int{ 10 };
/**
* @ingroup DoS
* @brief number of unsolicited messages during the last HK interval
* @remark counts downwards */
static int lpd_unsolicitedMsgCounter;
/**
* @def CRLF
* @brief a line-feed, as understood by the LPD protocol */
#define CRLF "\r\n"
/**
* @defgroup HttpReqProc HTTP-style request handling
* @{
*/
/**
* @brief Checks for BT-SEARCH method and separates the parameter section
* @param[in] s The request string
* @param[out] ver If non-nullptr, gets filled with protocol info from the request
* @return Returns a relative pointer to the beginning of the parameter section.
* If result is nullptr, s was invalid and no information will be returned
* @remark Note that the returned pointer is only usable as long as the given
* pointer s is valid; that is, return storage is temporary.
*
* Determines whether the given string checks out to be a valid BT-SEARCH message.
* If so, the return value points to the beginning of the parameter section (note:
* in this case the function returns a character sequence beginning with CRLF).
* If parameter is not nullptr, the declared protocol version is returned as part
* of the lpd_protocolVersion structure.
*/
static char const* lpd_extractHeader(char const* s, struct lpd_protocolVersion* const ver)
{
TR_ASSERT(s != nullptr);
int major = -1;
int minor = -1;
size_t len = strlen(s);
/* something might be rotten with this chunk of data */
if (len == 0 || len > lpd_maxDatagramLength)
{
return nullptr;
}
/* now we can attempt to look up the BT-SEARCH header */
if (sscanf(s, "BT-SEARCH * HTTP/%d.%d" CRLF, &major, &minor) != 2)
{
return nullptr;
}
if (major < 0 || minor < 0)
{
return nullptr;
}
{
/* a pair of blank lines at the end of the string, no place else */
char const* const two_blank = CRLF CRLF CRLF;
char const* const end = strstr(s, two_blank);
if (end == nullptr || strlen(end) > strlen(two_blank))
{
return nullptr;
}
}
if (ver != nullptr)
{
ver->major = major;
ver->minor = minor;
}
/* separate the header, begins with CRLF */
return strstr(s, CRLF);
}
/**
* @brief Return the value of a named parameter
*
* @param[in] str Input string of "\r\nName: Value" pairs without HTTP-style method part
* @param[in] name Name of parameter to extract
* @param[in] n Maximum available storage for value to return
* @param[out] val Output parameter for the actual value
* @return Returns 1 if value could be copied successfully
*
* Extracts the associated value of a named parameter from a HTTP-style header by
* performing the following steps:
* - assemble search string "\r\nName: " and locate position
* - copy back value from end to next "\r\n"
*/
// TODO: string_view
static bool lpd_extractParam(char const* const str, char const* const name, int n, char* const val)
{
TR_ASSERT(str != nullptr);
TR_ASSERT(name != nullptr);
TR_ASSERT(val != nullptr);
auto const key = tr_strbuf<char, 30>{ CRLF, name, ": "sv };
char const* const pos = strstr(str, key);
if (pos == nullptr)
{
return false; /* search was not successful */
}
{
char const* const beg = pos + std::size(key);
char const* const new_line = strstr(beg, CRLF);
/* the value is delimited by the next CRLF */
int const len = new_line - beg;
/* if value string hits the length limit n,
* leave space for a trailing '\0' character */
n = std::min(len, n - 1);
strncpy(val, beg, n);
val[n] = 0;
}
/* we successfully returned the value string */
return true;
}
/**
* @} */
static void on_upkeep_timer(evutil_socket_t, short /*unused*/, void* /*unused*/);
/**
* @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.
*/
int tr_lpdInit(tr_session* ss, tr_address* /*tr_addr*/)
{
/* if this check fails (i.e. the definition of hashString changed), update
* string handling in tr_lpdSendAnnounce() and tr_lpdConsiderAnnounce().
* However, the code should work as long as interfaces to the rest of
* libtransmission are compatible with char* strings. */
static_assert(
std::is_same_v<std::string const&, std::remove_pointer_t<decltype(std::declval<tr_torrent>().infoHashString())>>);
struct ip_mreq mcastReq;
int const opt_on = 1;
int const opt_off = 0;
if (session != nullptr) /* already initialized */
{
return -1;
}
TR_ASSERT(lpd_announceInterval > 0);
TR_ASSERT(lpd_announceScope > 0);
lpd_port = tr_sessionGetPeerPort(ss);
if (lpd_port <= 0)
{
return -1;
}
tr_logAddDebug("Initialising Local Peer Discovery");
/* setup datagram socket (receive) */
{
lpd_socket = socket(PF_INET, SOCK_DGRAM, 0);
if (lpd_socket == TR_BAD_SOCKET)
{
goto fail;
}
if (evutil_make_socket_nonblocking(lpd_socket) == -1)
{
goto fail;
}
if (setsockopt(lpd_socket, SOL_SOCKET, SO_REUSEADDR, reinterpret_cast<char const*>(&opt_on), sizeof(opt_on)) == -1)
{
goto fail;
}
memset(&lpd_mcastAddr, 0, sizeof(lpd_mcastAddr));
lpd_mcastAddr.sin_family = AF_INET;
lpd_mcastAddr.sin_port = htons(lpd_mcastPort);
if (evutil_inet_pton(lpd_mcastAddr.sin_family, lpd_mcastGroup, &lpd_mcastAddr.sin_addr) == -1)
{
goto fail;
}
if (bind(lpd_socket, (struct sockaddr*)&lpd_mcastAddr, sizeof(lpd_mcastAddr)) == -1)
{
goto fail;
}
/* we want to join that LPD multicast group */
memset(&mcastReq, 0, sizeof(mcastReq));
mcastReq.imr_multiaddr = lpd_mcastAddr.sin_addr;
mcastReq.imr_interface.s_addr = htonl(INADDR_ANY);
if (setsockopt(lpd_socket, IPPROTO_IP, IP_ADD_MEMBERSHIP, reinterpret_cast<char const*>(&mcastReq), sizeof(mcastReq)) ==
-1)
{
goto fail;
}
if (setsockopt(lpd_socket, IPPROTO_IP, IP_MULTICAST_LOOP, reinterpret_cast<char const*>(&opt_off), sizeof(opt_off)) ==
-1)
{
goto fail;
}
}
/* setup datagram socket (send) */
{
unsigned char const scope = lpd_announceScope;
lpd_socket2 = socket(PF_INET, SOCK_DGRAM, 0);
if (lpd_socket2 == TR_BAD_SOCKET)
{
goto fail;
}
if (evutil_make_socket_nonblocking(lpd_socket2) == -1)
{
goto fail;
}
/* configure outbound multicast TTL */
if (setsockopt(lpd_socket2, IPPROTO_IP, IP_MULTICAST_TTL, reinterpret_cast<char const*>(&scope), sizeof(scope)) == -1)
{
goto fail;
}
if (setsockopt(lpd_socket2, IPPROTO_IP, IP_MULTICAST_LOOP, reinterpret_cast<char const*>(&opt_off), sizeof(opt_off)) ==
-1)
{
goto fail;
}
}
session = ss;
/* Note: lpd_unsolicitedMsgCounter remains 0 until the first timeout event, thus
* any announcement received during the initial interval will be discarded. */
lpd_event = event_new(ss->event_base, lpd_socket, EV_READ | EV_PERSIST, event_callback, nullptr);
event_add(lpd_event, nullptr);
upkeep_timer = evtimer_new(ss->event_base, on_upkeep_timer, ss);
tr_timerAdd(*upkeep_timer, UpkeepIntervalSecs, 0);
tr_logAddDebug("Local Peer Discovery initialised");
return 1;
fail:
{
int const save = errno;
evutil_closesocket(lpd_socket);
evutil_closesocket(lpd_socket2);
lpd_socket = lpd_socket2 = TR_BAD_SOCKET;
session = nullptr;
tr_logAddWarn(fmt::format(
_("Couldn't initialize LPD: {error} ({error_code})"),
fmt::arg("error", tr_strerror(save)),
fmt::arg("error_code", save)));
errno = save;
}
return -1;
}
void tr_lpdUninit(tr_session* ss)
{
if (session != ss)
{
return;
}
tr_logAddTrace("Uninitialising Local Peer Discovery");
event_free(lpd_event);
lpd_event = nullptr;
evtimer_del(upkeep_timer);
upkeep_timer = nullptr;
/* just shut down, we won't remember any former nodes */
evutil_closesocket(lpd_socket);
evutil_closesocket(lpd_socket2);
tr_logAddTrace("Done uninitialising Local Peer Discovery");
session = nullptr;
}
/**
* @endcond */
/**
* @defgroup LdsProto LPD announcement processing
* @{
*/
/**
* @brief Announce the given torrent on the local network
*
* @param[in] t Torrent to announce
* @return Returns true on success
*
* 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 tr_lpdSendAnnounce(tr_torrent const* t)
{
if (t == nullptr)
{
return false;
}
auto const query = fmt::format(
FMT_STRING("BT-SEARCH * HTTP/{:d}.{:d}" CRLF "Host: {:s}:{:d}" CRLF "Port: {:d}" CRLF "Infohash: {:s}" CRLF CRLF CRLF),
1,
1,
lpd_mcastGroup,
lpd_mcastPort,
lpd_port,
tr_strupper(t->infoHashString()));
// send the query out using [lpd_socket2]
// destination address info has already been set up in tr_lpdInit(),
// so we refrain from preparing another sockaddr_in here
if (auto const res = sendto(
lpd_socket2,
std::data(query),
std::size(query),
0,
(struct sockaddr const*)&lpd_mcastAddr,
sizeof(lpd_mcastAddr));
res != static_cast<int>(std::size(query)))
{
return false;
}
tr_logAddTraceTor(t, "LPD announce message away");
return true;
}
/**
* @brief Process incoming unsolicited messages and add the peer to the announced
* torrent if all checks are passed.
*
* @param[in,out] peer Address information of the peer to add
* @param[in] msg The announcement message to consider
* @return Returns 0 if any input parameter or the announce was invalid, 1 if the peer
* was successfully added, -1 if not; a non-null return value indicates a side-effect to
* the peer in/out parameter.
*
* @note The port information gets added to the peer structure if tr_lpdConsiderAnnounce
* is able to extract the necessary information from the announce message. That is, if
* return != 0, the caller may retrieve the value from the passed structure.
*/
static int tr_lpdConsiderAnnounce(tr_pex* peer, char const* const msg)
{
auto constexpr MaxValueLen = int{ 25 };
auto constexpr MaxHashLen = int{ SIZEOF_HASH_STRING };
auto ver = lpd_protocolVersion{ -1, -1 };
char value[MaxValueLen] = { 0 };
char hashString[MaxHashLen] = { 0 };
int res = 0;
int peerPort = 0;
if (peer != nullptr && msg != nullptr)
{
tr_torrent* tor = nullptr;
char const* params = lpd_extractHeader(msg, &ver);
if (params == nullptr || ver.major != 1) /* allow messages of protocol v1 */
{
return 0;
}
/* save the effort to check Host, which seems to be optional anyway */
if (!lpd_extractParam(params, "Port", MaxValueLen, value))
{
return 0;
}
/* determine announced peer port, refuse if value too large */
if (sscanf(value, "%d", &peerPort) != 1 || peerPort > (in_port_t)-1)
{
return 0;
}
peer->port = htons(peerPort);
res = -1; /* signal caller side-effect to peer->port via return != 0 */
if (!lpd_extractParam(params, "Infohash", MaxHashLen, hashString))
{
return res;
}
tor = session->torrents().get(hashString);
if (tr_isTorrent(tor) && tor->allowsLpd())
{
/* we found a suitable peer, add it to the torrent */
tr_peerMgrAddPex(tor, TR_PEER_FROM_LPD, peer, 1);
tr_logAddDebugTor(tor, fmt::format("Found a local peer from LPD ({})", peer->addr.to_string(peerPort)));
/* periodic reconnectPulse() deals with the rest... */
return 1;
}
tr_logAddDebug(fmt::format("Cannot serve torrent #{}", hashString));
}
return res;
}
/**
* @} */
/**
* @note Since it possible for tr_lpdAnnounceMore to get called from outside the LPD module,
* the function needs to be informed of the externally employed housekeeping interval.
* Further, by setting interval to zero (or negative) the caller may actually disable LPD
* announces on a per-interval basis.
*
* TODO: since this function's been made private and is called by a periodic timer,
* most of the previous paragraph isn't true anymore... we weren't using that functionality
* before. are there cases where we should? if not, should we remove the bells & whistles?
*/
static int tr_lpdAnnounceMore(time_t const now, int const interval)
{
int announcesSent = 0;
if (!tr_isSession(session))
{
return -1;
}
if (tr_sessionAllowsLPD(session))
{
for (auto* const tor : session->torrents())
{
int announcePrio = 0;
if (!tor->allowsLpd())
{
continue;
}
/* issue #3208: prioritize downloads before seeds */
switch (tr_torrentGetActivity(tor))
{
case TR_STATUS_DOWNLOAD:
announcePrio = 1;
break;
case TR_STATUS_SEED:
announcePrio = 2;
break;
default:
break;
}
if (announcePrio > 0 && tor->lpdAnnounceAt <= now)
{
if (tr_lpdSendAnnounce(tor))
{
announcesSent++;
}
tor->lpdAnnounceAt = now + lpd_announceInterval * announcePrio;
break; /* that's enough; for this interval */
}
}
}
/* perform housekeeping for the flood protection mechanism */
{
int const maxAnnounceCap = interval * lpd_announceCapFactor;
if (lpd_unsolicitedMsgCounter < 0)
{
tr_logAddTrace(fmt::format(
"Dropped {} announces in the last interval (max. {} allowed)",
-lpd_unsolicitedMsgCounter,
maxAnnounceCap));
}
lpd_unsolicitedMsgCounter = maxAnnounceCap;
}
return announcesSent;
}
static void on_upkeep_timer(evutil_socket_t /*s*/, short /*type*/, void* /*user_data*/)
{
time_t const now = tr_time();
tr_lpdAnnounceMore(now, UpkeepIntervalSecs);
tr_timerAdd(*upkeep_timer, UpkeepIntervalSecs, 0);
}
/**
* @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* /*user_data*/)
{
TR_ASSERT(tr_isSession(session));
/* do not allow announces to be processed if LPD is disabled */
if (!tr_sessionAllowsLPD(session))
{
return;
}
if ((type & EV_READ) != 0)
{
struct sockaddr_in foreignAddr;
int addrLen = sizeof(foreignAddr);
char foreignMsg[lpd_maxDatagramLength + 1];
/* process local announcement from foreign peer */
int res = recvfrom(
lpd_socket,
foreignMsg,
lpd_maxDatagramLength,
0,
(struct sockaddr*)&foreignAddr,
(socklen_t*)&addrLen);
/* besides, do we get flooded? then bail out! */
if (--lpd_unsolicitedMsgCounter < 0)
{
return;
}
if (res > 0 && res <= lpd_maxDatagramLength)
{
auto foreignPeer = tr_pex{};
/* be paranoid enough about zero terminating the foreign string */
foreignMsg[res] = '\0';
foreignPeer.addr.addr.addr4 = foreignAddr.sin_addr;
if (tr_lpdConsiderAnnounce(&foreignPeer, foreignMsg) != 0)
{
return; /* OK so far, no log message */
}
}
tr_logAddTrace("Discarded invalid multicast message");
}
}
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