i recently hit the same bug, did a ktrace(1) and found the following

 61086 rtorrent CALL  socket(AF_INET6,0x2<SOCK_DGRAM>,0)
 61086 rtorrent RET   socket 16/0x10
 61086 rtorrent CALL  setsockopt(16,41<ipv6>,27,0x71f9b0bf3e24,4)
 61086 rtorrent RET   setsockopt -1 errno 22 Invalid argument
 61086 rtorrent CALL  kbind(0x71f9b0bf3d30,24,0xa7ad56a04777c55)
 61086 rtorrent RET   kbind 0
 61086 rtorrent CALL  close(16)
 61086 rtorrent RET   close 0
 61086 rtorrent CALL  close(16)
 61086 rtorrent RET   close -1 errno 9 Bad file descriptor

so i set a breakpoint on setsockopt(2) and found in libtorrents
src/net/socket_fd.cc

bool
SocketFd::set_ipv6_v6only(bool state) {
  check_valid();

  if (!m_ipv6_socket)
    return false;

  int opt = state;
  return setsockopt(m_fd, IPPROTO_IPV6, IPV6_V6ONLY, &opt, sizeof(opt)) == 0;
}

bool
SocketFd::open_datagram() {
  m_fd = socket(rak::socket_address::pf_inet6, SOCK_DGRAM, 0);

  if (m_fd == -1) {
    m_ipv6_socket = false;
    return (m_fd = socket(rak::socket_address::pf_inet, SOCK_DGRAM, 0)) != -1;
  }

  m_ipv6_socket = true;

  if (!set_ipv6_v6only(false)) {
    close();
    return false;
  }

  return true;
}

OpenBSD does not support dualstack sockets, so setting IPV6_V6ONLY to
false fails. instead of using two sockets or at least falling back to
IPv4-only, libtorrent gives up completely and closes the socket.
however, m_fd is not set to -1, so later in TrackerUdp::close_directly()
SocketFd::is_valid() returns true and SocketFd::close() is called again,
which leads to the crash.

the patch below changes SocketFd::open_datagram() to fall back to IPv4,
like already done for SocketFd::open_stream(). proper dualstack support
would need a bigger refactoring.

diff --git net/libtorrent/Makefile net/libtorrent/Makefile
index 7bd433cd6b5..880218060e1 100644
--- net/libtorrent/Makefile
+++ net/libtorrent/Makefile
@@ -7,7 +7,7 @@ BROKEN-sh =		undefined references to __sync atomic ops
 NOT_FOR_ARCHS=		${GCC3_ARCHS}
 
 DISTNAME=		libtorrent-0.14.0
-REVISION=		1
+REVISION=		2
 EPOCH=			0
 SHARED_LIBS +=  	torrent 22.1	# .18.0
 CATEGORIES=		net devel
diff --git net/libtorrent/patches/patch-src_net_socket_fd_cc net/libtorrent/patches/patch-src_net_socket_fd_cc
index 7f0b78aa71f..ad7dc8c1350 100644
--- net/libtorrent/patches/patch-src_net_socket_fd_cc
+++ net/libtorrent/patches/patch-src_net_socket_fd_cc
@@ -1,7 +1,7 @@
 Index: src/net/socket_fd.cc
 --- src/net/socket_fd.cc.orig
 +++ src/net/socket_fd.cc
-@@ -126,21 +126,8 @@ SocketFd::get_error() const {
+@@ -126,40 +126,14 @@ SocketFd::get_error() const {
  
  bool
  SocketFd::open_stream() {
@@ -25,3 +25,24 @@ Index: src/net/socket_fd.cc
  }
  
  bool
+ SocketFd::open_datagram() {
+-  m_fd = socket(rak::socket_address::pf_inet6, SOCK_DGRAM, 0);
+-
+-  if (m_fd == -1) {
+-    m_ipv6_socket = false;
+-    return (m_fd = socket(rak::socket_address::pf_inet, SOCK_DGRAM, 0)) != -1;
+-  }
+-
+-  m_ipv6_socket = true;
+-
+-  if (!set_ipv6_v6only(false)) {
+-    close();
+-    return false;
+-  }
+-
+-  return true;
++  m_ipv6_socket = false;
++  return (m_fd = socket(rak::socket_address::pf_inet, SOCK_DGRAM, 0)) != -1;
+ }
+ 
+ bool