Added extraction of IP_PKTINFO when reading and setting it when sending back

CMakeLists.txt

@@ -117,6 +117,9 @@ if (ENABLE_DEBUG)
 	set(ENABLE_HEAVY_LOGGING_DEFAULT ON)
 endif()
 
+# Note that the IP_PKTINFO  has a limited portability and may not work on some platforms
+# (Windows, FreeBSD, ...).
+set (ENABLE_PKTINFO_DEFAULT OFF)
 
 set(ENABLE_STDCXX_SYNC_DEFAULT OFF)
 set(ENABLE_MONOTONIC_CLOCK_DEFAULT OFF)
@@ -144,6 +147,7 @@ option(ENABLE_HEAVY_LOGGING "Should heavy debug logging be enabled" ${ENABLE_HEA
 option(ENABLE_HAICRYPT_LOGGING "Should logging in haicrypt be enabled" 0)
 option(ENABLE_SHARED "Should libsrt be built as a shared library" ON)
 option(ENABLE_STATIC "Should libsrt be built as a static library" ON)
+option(ENABLE_PKTINFO "Enable using IP_PKTINFO to allow the listener extracting the target IP address from incoming packets" ${ENABLE_PKTINFO_DEFAULT})
 option(ENABLE_RELATIVE_LIBPATH "Should application contain relative library paths, like ../lib" OFF)
 option(ENABLE_GETNAMEINFO "In-logs sockaddr-to-string should do rev-dns" OFF)
 option(ENABLE_UNITTESTS "Enable unit tests" OFF)
@@ -711,6 +715,15 @@ if (ENABLE_GETNAMEINFO)
 	list(APPEND SRT_EXTRA_CFLAGS "-DENABLE_GETNAMEINFO=1")
 endif()
 
+if (ENABLE_PKTINFO)
+	if (WIN32 OR BSD)
+		message(FATAL_ERROR "PKTINFO is not implemented on Windows or *BSD.")
+	endif()
+
+	list(APPEND SRT_EXTRA_CFLAGS "-DSRT_ENABLE_PKTINFO=1")
+endif()
+
+
 # ENABLE_EXPERIMENTAL_BONDING is deprecated. Use ENABLE_BONDING. ENABLE_EXPERIMENTAL_BONDING is be removed in v1.6.0.
 if (ENABLE_EXPERIMENTAL_BONDING)
 	message(DEPRECATION "ENABLE_EXPERIMENTAL_BONDING is deprecated. Please use ENABLE_BONDING instead.")

configure-data.tcl

@@ -43,6 +43,7 @@ set cmake_options {
     enable-logging "Should logging be enabled (default: ON)"
     enable-heavy-logging "Should heavy debug logging be enabled (default: OFF)"
     enable-haicrypt-logging "Should logging in haicrypt be enabled (default: OFF)"
+    enable-pktinfo "Should pktinfo reading and using be enabled (POSIX only) (default: OFF)"
     enable-shared "Should libsrt be built as a shared library (default: ON)"
     enable-static "Should libsrt be built as a static library (default: ON)"
     enable-relative-libpath "Should applications contain relative library paths, like ../lib (default: OFF)"

docs/build/build-options.md

@@ -39,13 +39,14 @@ Option details are given further below.
 | [`ENABLE_HEAVY_LOGGING`](#enable_heavy_logging)              | 1.3.0 | `BOOL`    | OFF        | Enables heavy logging instructions in the code that occur often and cover many detailed aspects of library behavior. Default: OFF in release mode.   |
 | [`ENABLE_INET_PTON`](#enable_inet_pton)                      | 1.3.2 | `BOOL`    | ON         | Enables usage of the `inet_pton` function used to resolve the network endpoint name into an IP address.                                              |
 | [`ENABLE_LOGGING`](#enable_logging)                          | 1.2.0 | `BOOL`    | ON         | Enables normal logging, including errors.                                                                                                            |
-| [`ENABLE_MONOTONIC_CLOCK`](#enable_monotonic_clock)          | 1.4.0 | `BOOL`    | ON*        | Enforces the use of `clock_gettime` with a monotonic clock that is independent of the currently set time in the system.                              |
+| [`ENABLE_MONOTONIC_CLOCK`](#enable_monotonic_clock)          | 1.4.0 | `BOOL`    | ON\*        | Enforces the use of `clock_gettime` with a monotonic clock that is independent of the currently set time in the system.                              |
 | [`ENABLE_PROFILE`](#enable_profile)                          | 1.2.0 | `BOOL`    | OFF        | Enables code instrumentation for profiling (only for GNU-compatible compilers).                                                                      |
 | [`ENABLE_RELATIVE_LIBPATH`](#enable_relative_libpath)        | 1.3.2 | `BOOL`    | OFF        | Enables adding a relative path to a library for linking against a shared SRT library by reaching out to a sibling directory.                         |
 | [`ENABLE_SHARED`](#enable_shared--enable_static)             | 1.2.0 | `BOOL`    | ON         | Enables building SRT as a shared library.                                                                                                            |
 | [`ENABLE_SHOW_PROJECT_CONFIG`](#enable_show_project_config)  | 1.5.0 | `BOOL`    | OFF        | When ON, the project configuration is displayed at the end of the CMake Configuration Step.                                                          |
 | [`ENABLE_STATIC`](#enable_shared--enable_static)             | 1.3.0 | `BOOL`    | ON         | Enables building SRT as a static library.                                                                                                            |
-| [`ENABLE_STDCXX_SYNC`](#enable_stdcxx_sync)                  | 1.4.2 | `BOOL`    | ON*        | Enables the standard C++11 `thread` and `chrono` libraries to be used by SRT instead of the `pthreads`.                                              |
+| [`ENABLE_STDCXX_SYNC`](#enable_stdcxx_sync)                  | 1.4.2 | `BOOL`    | ON\*       | Enables the standard C++11 `thread` and `chrono` libraries to be used by SRT instead of the `pthreads`.                                              |
+| [`ENABLE_PKTINFO`](#enable_pktinfo)                          | 1.5.2 | `BOOL`    | OFF\*      | Enables using `IP_PKTINFO` to allow the listener extracting the target IP address from incoming packets                                              |
 | [`ENABLE_TESTING`](#enable_testing)                          | 1.3.0 | `BOOL`    | OFF        | Enables compiling of developer testing applications (`srt-test-live`, etc.).                                                                         |
 | [`ENABLE_THREAD_CHECK`](#enable_thread_check)                | 1.3.0 | `BOOL`    | OFF        | Enables `#include <threadcheck.h>`, which implements `THREAD_*` macros" to  support better thread debugging.                                         |
 | [`ENABLE_UNITTESTS`](#enable_unittests)                      | 1.3.2 | `BOOL`    | OFF        | Enables building unit tests.                                                                                                                         |
@@ -421,6 +422,55 @@ to use either of those (`ENABLE_STDCXX_SYNC` excludes `ENABLE_MONOTONIC_CLOCK`).
 When ON, this option enables the standard C++ `thread` and `chrono` libraries 
 (available since C++11) to be used by SRT instead of the `pthreads` libraries.
 
+#### ENABLE_PKTINFO
+**`--enable-pktinfo`** (default: OFF)
+
+This allows the use of `IP_PKTINFO` control message which can be used to
+extract the true target IP address from the incoming UDP packets to a listener
+bound to a so-called "any" address (`0.0.0.0` for IPv4, AKA `INADDR_ANY`, or
+`::` for IPv6, AKA `in6addr_any`, or simply when the application clears the
+`sockaddr*` structure and only sets the port number), and then set this address
+as source IP address when sending packets to the peer. This solves the problem,
+when the routing rules in the agent's host make that sending a packet to a
+given IP address would use a different source IP address than the one that was
+set as a target IP address in the UDP packet from the peer. The peer will
+reject such a packet as a suspected MITM attempt, and this will lead to a
+connection failure.
+
+This problem has been observed in the case, when the agent's host has at least
+two IP addresses that share the same broadcast prefix and it is being contacted
+by the peer using other than the first of these addresses. For example:
+
+The host has set the following local IP addresses:
+
+* 192.168.10.5 - routing to 192.168.10.1
+* 10.10.5.10 - routing to 10.10.5.1
+* 10.10.5.20 - routing to 10.10.5.1
+
+In all of them the netmask is 255.255.255.0, which means that the second
+and third IP address share the same broadcast prefix, which is 10.10.5.0.
+The problematic case is when an agent running on this host will be contacted
+by a peer using the address 10.10.5.20.
+
+In such a case the source address set in the UDP packet sent back to the peer
+will always be the first of these addresses (10.10.5.10), which will be
+different than the one the peer used to contact (10.10.5.20). The peer will
+then rejeect such a packet by having the source address different than the one
+it tries to contact.
+
+This problem occurs only with listener sockets bound to "any" address - when it
+is bound to a specific IP address, this will be always set as source address
+for outgoing packets.
+
+By enabling this feature SRT mitigates this problem by first reading the real
+target IP address from the incoming handshake packet, and then setting this
+very address forcefully to the source address field of every UDP packet sent
+from this socket. This behavior is also consistent with TCP.
+
+Note that this feature is available only on certain platforms. Notably the BSD
+systems are known to not provide this feature and the current implementation
+doesn't support this feature on Windows systems.
+
 
 #### ENABLE_TESTING
 **`--enable-testing`** (default: OFF)

srtcore/channel.cpp

@@ -139,7 +139,24 @@ static int set_cloexec(int fd, int set)
 
 srt::CChannel::CChannel()
     : m_iSocket(INVALID_SOCKET)
+#ifdef SRT_ENABLE_PKTINFO
+    , m_bBindMasked(true)
+#endif
 {
+#ifdef SRT_ENABLE_PKTINFO
+   // Do the check for ancillary data buffer size, kinda assertion
+   static const size_t CMSG_MAX_SPACE = sizeof (CMSGNodeAlike);
+
+   if (CMSG_MAX_SPACE < CMSG_SPACE(sizeof(in_pktinfo))
+           || CMSG_MAX_SPACE < CMSG_SPACE(sizeof(in6_pktinfo)))
+   {
+       LOGC(kmlog.Fatal, log << "Size of CMSG_MAX_SPACE="
+               << CMSG_MAX_SPACE << " too short for cmsg "
+               << CMSG_SPACE(sizeof(in_pktinfo)) << ", "
+               << CMSG_SPACE(sizeof(in6_pktinfo)) << " - PLEASE FIX");
+       throw CUDTException(MJ_SETUP, MN_NONE, 0);
+   }
+#endif
 }
 
 srt::CChannel::~CChannel() {}
@@ -207,6 +224,9 @@ void srt::CChannel::open(const sockaddr_any& addr)
         throw CUDTException(MJ_SETUP, MN_NORES, NET_ERROR);
 
     m_BindAddr = addr;
+#ifdef SRT_ENABLE_PKTINFO
+    m_bBindMasked = m_BindAddr.isany();
+#endif
     LOGC(kmlog.Debug, log << "CHANNEL: Bound to local address: " << m_BindAddr.str());
 
     setUDPSockOpt();
@@ -247,6 +267,12 @@ void srt::CChannel::open(int family)
     }
     m_BindAddr = sockaddr_any(res->ai_addr, (sockaddr_any::len_t)res->ai_addrlen);
 
+#ifdef SRT_ENABLE_PKTINFO
+    // We know that this is intentionally bound now to "any",
+    // so the requester-destination address must be remembered and passed.
+    m_bBindMasked = true;
+#endif
+
     ::freeaddrinfo(res);
 
     HLOGC(kmlog.Debug, log << "CHANNEL: Bound to local address: " << m_BindAddr.str());
@@ -472,6 +498,19 @@ void srt::CChannel::setUDPSockOpt()
     if (0 != ::setsockopt(m_iSocket, SOL_SOCKET, SO_RCVTIMEO, (char*)&tv, sizeof(timeval)))
         throw CUDTException(MJ_SETUP, MN_NORES, NET_ERROR);
 #endif
+
+#ifdef SRT_ENABLE_PKTINFO
+    if (m_bBindMasked)
+    {
+        HLOGP(kmlog.Debug, "Socket bound to ANY - setting PKTINFO for address retrieval");
+        const int on = 1, off SRT_ATR_UNUSED = 0;
+        ::setsockopt(m_iSocket, IPPROTO_IP, IP_PKTINFO, (char*)&on, sizeof(on));
+        ::setsockopt(m_iSocket, IPPROTO_IPV6, IPV6_RECVPKTINFO, &on, sizeof(on));
+
+        // XXX Unknown why this has to be off. RETEST.
+        //::setsockopt(m_iSocket, IPPROTO_IPV6, IPV6_V6ONLY, &off, sizeof(off));
+    }
+#endif
 }
 
 void srt::CChannel::close() const
@@ -610,11 +649,16 @@ void srt::CChannel::getPeerAddr(sockaddr_any& w_addr) const
     w_addr.len = namelen;
 }
 
-int srt::CChannel::sendto(const sockaddr_any& addr, CPacket& packet) const
+int srt::CChannel::sendto(const sockaddr_any& addr, CPacket& packet, const sockaddr_any& source_addr SRT_ATR_UNUSED) const
 {
     HLOGC(kslog.Debug,
           log << "CChannel::sendto: SENDING NOW DST=" << addr.str() << " target=@" << packet.m_iID
-              << " size=" << packet.getLength() << " pkt.ts=" << packet.m_iTimeStamp << " " << packet.Info());
+              << " size=" << packet.getLength() << " pkt.ts=" << packet.m_iTimeStamp
+#ifdef SRT_ENABLE_PKTINFO
+        << " sourceIP="
+        << (m_bBindMasked && !source_addr.isany() ? source_addr.str() : "default")
+#endif
+        << " " << packet.Info());
 
 #ifdef SRT_TEST_FAKE_LOSS
 
@@ -683,8 +727,28 @@ int srt::CChannel::sendto(const sockaddr_any& addr, CPacket& packet) const
     mh.msg_namelen    = addr.size();
     mh.msg_iov        = (iovec*)packet.m_PacketVector;
     mh.msg_iovlen     = 2;
-    mh.msg_control    = NULL;
-    mh.msg_controllen = 0;
+    bool have_set_src = false;
+
+#ifdef SRT_ENABLE_PKTINFO
+    if (m_bBindMasked && !source_addr.isany())
+    {
+        if (!setSourceAddress(mh, source_addr))
+        {
+            LOGC(kslog.Error, log << "CChannel::setSourceAddress: source address invalid family #" << source_addr.family() << ", NOT setting.");
+        }
+        else
+        {
+            HLOGC(kslog.Debug, log << "CChannel::setSourceAddress: setting as " << source_addr.str());
+            have_set_src = true;
+        }
+    }
+#endif
+
+    if (!have_set_src)
+    {
+        mh.msg_control    = NULL;
+        mh.msg_controllen = 0;
+    }
     mh.msg_flags      = 0;
 
     const int res = ::sendmsg(m_iSocket, &mh, 0);
@@ -725,15 +789,33 @@ srt::EReadStatus srt::CChannel::recvfrom(sockaddr_any& w_addr, CPacket& w_packet
     }
 
 #ifndef _WIN32
+    msghdr mh; // will not be used on failure
+
     if (select_ret > 0)
     {
-        msghdr mh;
         mh.msg_name       = (w_addr.get());
         mh.msg_namelen    = w_addr.size();
         mh.msg_iov        = (w_packet.m_PacketVector);
         mh.msg_iovlen     = 2;
+
+        // Default
         mh.msg_control    = NULL;
         mh.msg_controllen = 0;
+
+#ifdef SRT_ENABLE_PKTINFO
+        // Without m_bBindMasked, we don't need ancillary data - the source
+        // address will always be the bound address.
+        if (m_bBindMasked)
+        {
+            // Extract the destination IP address from the ancillary
+            // data. This might be interesting for the connection to
+            // know to which address the packet should be sent back during
+            // the handshake and then addressed when sending during connection.
+            mh.msg_control = (m_acCmsgRecvBuffer);
+            mh.msg_controllen = sizeof m_acCmsgRecvBuffer;
+        }
+#endif
+
         mh.msg_flags      = 0;
 
         recv_size = ::recvmsg(m_iSocket, (&mh), 0);
@@ -779,6 +861,17 @@ srt::EReadStatus srt::CChannel::recvfrom(sockaddr_any& w_addr, CPacket& w_packet
         goto Return_error;
     }
 
+#ifdef SRT_ENABLE_PKTINFO
+    if (m_bBindMasked)
+    {
+        // Extract the address. Set it explicitly; if this returns address that isany(),
+        // it will simply set this on the packet so that it behaves as if nothing was
+        // extracted (it will "fail the old way").
+        w_packet.m_DestAddr = getTargetAddress(mh);
+        HLOGC(krlog.Debug, log << CONID() << "(sys)recvmsg: ANY BOUND, retrieved DEST ADDR: " << w_packet.m_DestAddr.str());
+    }
+#endif
+
 #else
     // XXX REFACTORING NEEDED!
     // This procedure uses the WSARecvFrom function that just reads