diff --git a/chronos/apps/http/httpclient.nim b/chronos/apps/http/httpclient.nim
index 01e2bab..34089c7 100644
--- a/chronos/apps/http/httpclient.nim
+++ b/chronos/apps/http/httpclient.nim
@@ -126,6 +126,7 @@ type
     connectionsCount*: int
     socketFlags*: set[SocketFlags]
     flags*: HttpClientFlags
+    dualstack*: DualStackType
 
   HttpAddress* = object
     id*: string
@@ -263,7 +264,8 @@ proc new*(t: typedesc[HttpSessionRef],
           maxConnections = -1,
           idleTimeout = HttpConnectionIdleTimeout,
           idlePeriod = HttpConnectionCheckPeriod,
-          socketFlags: set[SocketFlags] = {}): HttpSessionRef {.
+          socketFlags: set[SocketFlags] = {},
+          dualstack = DualStackType.Auto): HttpSessionRef {.
      raises: [] .} =
   ## Create new HTTP session object.
   ##
@@ -283,7 +285,8 @@ proc new*(t: typedesc[HttpSessionRef],
     idleTimeout: idleTimeout,
     idlePeriod: idlePeriod,
     connections: initTable[string, seq[HttpClientConnectionRef]](),
-    socketFlags: socketFlags
+    socketFlags: socketFlags,
+    dualstack: dualstack
   )
   res.watcherFut =
     if HttpClientFlag.Http11Pipeline in flags:
@@ -620,7 +623,8 @@ proc connect(session: HttpSessionRef,
     let transp =
       try:
         await connect(address, bufferSize = session.connectionBufferSize,
-                      flags = session.socketFlags)
+                      flags = session.socketFlags,
+                      dualstack = session.dualstack)
       except CancelledError as exc:
         raise exc
       except CatchableError:
diff --git a/chronos/apps/http/httpserver.nim b/chronos/apps/http/httpserver.nim
index f0788e2..2ab5317 100644
--- a/chronos/apps/http/httpserver.nim
+++ b/chronos/apps/http/httpserver.nim
@@ -191,7 +191,8 @@ proc new*(htype: typedesc[HttpServerRef],
           backlogSize: int = DefaultBacklogSize,
           httpHeadersTimeout = 10.seconds,
           maxHeadersSize: int = 8192,
-          maxRequestBodySize: int = 1_048_576): HttpResult[HttpServerRef] {.
+          maxRequestBodySize: int = 1_048_576,
+          dualstack = DualStackType.Auto): HttpResult[HttpServerRef] {.
      raises: [].} =
 
   let serverUri =
@@ -206,7 +207,7 @@ proc new*(htype: typedesc[HttpServerRef],
   let serverInstance =
     try:
       createStreamServer(address, flags = socketFlags, bufferSize = bufferSize,
-                         backlog = backlogSize)
+                         backlog = backlogSize, dualstack = dualstack)
     except TransportOsError as exc:
       return err(exc.msg)
     except CatchableError as exc:
diff --git a/chronos/apps/http/shttpserver.nim b/chronos/apps/http/shttpserver.nim
index 6d321a0..0300597 100644
--- a/chronos/apps/http/shttpserver.nim
+++ b/chronos/apps/http/shttpserver.nim
@@ -92,7 +92,8 @@ proc new*(htype: typedesc[SecureHttpServerRef],
           backlogSize: int = DefaultBacklogSize,
           httpHeadersTimeout = 10.seconds,
           maxHeadersSize: int = 8192,
-          maxRequestBodySize: int = 1_048_576
+          maxRequestBodySize: int = 1_048_576,
+          dualstack = DualStackType.Auto
          ): HttpResult[SecureHttpServerRef] {.raises: [].} =
 
   doAssert(not(isNil(tlsPrivateKey)), "TLS private key must not be nil!")
@@ -110,7 +111,7 @@ proc new*(htype: typedesc[SecureHttpServerRef],
   let serverInstance =
     try:
       createStreamServer(address, flags = socketFlags, bufferSize = bufferSize,
-                         backlog = backlogSize)
+                         backlog = backlogSize, dualstack = dualstack)
     except TransportOsError as exc:
       return err(exc.msg)
     except CatchableError as exc:
diff --git a/chronos/handles.nim b/chronos/handles.nim
index 2348b33..afa57fb 100644
--- a/chronos/handles.nim
+++ b/chronos/handles.nim
@@ -21,66 +21,113 @@ const
   asyncInvalidSocket* = AsyncFD(osdefs.INVALID_SOCKET)
   asyncInvalidPipe* = asyncInvalidSocket
 
-proc setSocketBlocking*(s: SocketHandle, blocking: bool): bool =
+proc setSocketBlocking*(s: SocketHandle, blocking: bool): bool {.
+     deprecated: "Please use setDescriptorBlocking() instead".} =
   ## Sets blocking mode on socket.
-  when defined(windows) or defined(nimdoc):
-    var mode = clong(ord(not blocking))
-    if osdefs.ioctlsocket(s, osdefs.FIONBIO, addr(mode)) == -1:
-      false
-    else:
-      true
-  else:
-    let x: int = osdefs.fcntl(s, osdefs.F_GETFL, 0)
-    if x == -1:
-      false
-    else:
-      let mode =
-        if blocking: x and not osdefs.O_NONBLOCK else: x or osdefs.O_NONBLOCK
-      if osdefs.fcntl(s, osdefs.F_SETFL, mode) == -1:
-        false
-      else:
-        true
+  setDescriptorBlocking(s, blocking).isOkOr:
+    return false
+  true
 
-proc setSockOpt*(socket: AsyncFD, level, optname, optval: int): bool =
-  ## `setsockopt()` for integer options.
-  ## Returns ``true`` on success, ``false`` on error.
+proc setSockOpt2*(socket: AsyncFD,
+                  level, optname, optval: int): Result[void, OSErrorCode] =
   var value = cint(optval)
-  osdefs.setsockopt(SocketHandle(socket), cint(level), cint(optname),
-                    addr(value), SockLen(sizeof(value))) >= cint(0)
+  let res = osdefs.setsockopt(SocketHandle(socket), cint(level), cint(optname),
+                              addr(value), SockLen(sizeof(value)))
+  if res == -1:
+    return err(osLastError())
+  ok()
 
-proc setSockOpt*(socket: AsyncFD, level, optname: int, value: pointer,
-                 valuelen: int): bool =
+proc setSockOpt2*(socket: AsyncFD, level, optname: int, value: pointer,
+                  valuelen: int): Result[void, OSErrorCode] =
   ## `setsockopt()` for custom options (pointer and length).
   ## Returns ``true`` on success, ``false`` on error.
-  osdefs.setsockopt(SocketHandle(socket), cint(level), cint(optname), value,
-                    SockLen(valuelen)) >= cint(0)
+  let res = osdefs.setsockopt(SocketHandle(socket), cint(level), cint(optname),
+                              value, SockLen(valuelen))
+  if res == -1:
+    return err(osLastError())
+  ok()
 
-proc getSockOpt*(socket: AsyncFD, level, optname: int, value: var int): bool =
+proc setSockOpt*(socket: AsyncFD, level, optname, optval: int): bool {.
+     deprecated: "Please use setSockOpt2() instead".} =
+  ## `setsockopt()` for integer options.
+  ## Returns ``true`` on success, ``false`` on error.
+  setSockOpt2(socket, level, optname, optval).isOk
+
+proc setSockOpt*(socket: AsyncFD, level, optname: int, value: pointer,
+                 valuelen: int): bool {.
+     deprecated: "Please use setSockOpt2() instead".} =
+  ## `setsockopt()` for custom options (pointer and length).
+  ## Returns ``true`` on success, ``false`` on error.
+  setSockOpt2(socket, level, optname, value, valuelen).isOk
+
+proc getSockOpt2*(socket: AsyncFD,
+                  level, optname: int): Result[cint, OSErrorCode] =
+  var
+    value: cint
+    size = SockLen(sizeof(value))
+  let res = osdefs.getsockopt(SocketHandle(socket), cint(level), cint(optname),
+                              addr(value), addr(size))
+  if res == -1:
+    return err(osLastError())
+  ok(value)
+
+proc getSockOpt2*(socket: AsyncFD, level, optname: int,
+                  T: type): Result[T, OSErrorCode] =
+  var
+    value = default(T)
+    size = SockLen(sizeof(value))
+  let res = osdefs.getsockopt(SocketHandle(socket), cint(level), cint(optname),
+                              cast[ptr byte](addr(value)), addr(size))
+  if res == -1:
+    return err(osLastError())
+  ok(value)
+
+proc getSockOpt*(socket: AsyncFD, level, optname: int, value: var int): bool {.
+     deprecated: "Please use getSockOpt2() instead".} =
   ## `getsockopt()` for integer options.
   ## Returns ``true`` on success, ``false`` on error.
-  var res: cint
-  var size = SockLen(sizeof(res))
-  if osdefs.getsockopt(SocketHandle(socket), cint(level), cint(optname),
-                       addr(res), addr(size)) >= cint(0):
-    value = int(res)
-    true
-  else:
-    false
+  value = getSockOpt2(socket, level, optname).valueOr:
+    return false
+  true
 
-proc getSockOpt*(socket: AsyncFD, level, optname: int, value: pointer,
-                 valuelen: var int): bool =
+proc getSockOpt*(socket: AsyncFD, level, optname: int, value: var pointer,
+                 valuelen: var int): bool  {.
+     deprecated: "Please use getSockOpt2() instead".} =
   ## `getsockopt()` for custom options (pointer and length).
   ## Returns ``true`` on success, ``false`` on error.
   osdefs.getsockopt(SocketHandle(socket), cint(level), cint(optname),
                     value, cast[ptr SockLen](addr valuelen)) >= cint(0)
 
-proc getSocketError*(socket: AsyncFD, err: var int): bool =
+proc getSocketError*(socket: AsyncFD, err: var int): bool  {.
+     deprecated: "Please use getSocketError() instead".} =
   ## Recover error code associated with socket handle ``socket``.
-  getSockOpt(socket, cint(osdefs.SOL_SOCKET), cint(osdefs.SO_ERROR), err)
+  err = getSockOpt2(socket, cint(osdefs.SOL_SOCKET),
+                    cint(osdefs.SO_ERROR)).valueOr:
+    return false
+  true
+
+proc getSocketError2*(socket: AsyncFD): Result[cint, OSErrorCode] =
+  getSockOpt2(socket, cint(osdefs.SOL_SOCKET), cint(osdefs.SO_ERROR))
+
+proc isAvailable*(domain: Domain): bool =
+  when defined(windows):
+    let fd = wsaSocket(toInt(domain), toInt(SockType.SOCK_STREAM),
+                       toInt(Protocol.IPPROTO_TCP), nil, GROUP(0), 0'u32)
+    if fd == osdefs.INVALID_SOCKET:
+      return if osLastError() == osdefs.WSAEAFNOSUPPORT: false else: true
+    discard closeFd(fd)
+    true
+  else:
+    let fd = osdefs.socket(toInt(domain), toInt(SockType.SOCK_STREAM),
+                           toInt(Protocol.IPPROTO_TCP))
+    if fd == -1:
+      return if osLastError() == osdefs.EAFNOSUPPORT: false else: true
+    discard closeFd(fd)
+    true
 
 proc createAsyncSocket2*(domain: Domain, sockType: SockType,
-                        protocol: Protocol,
-                        inherit = true): Result[AsyncFD, OSErrorCode] =
+                         protocol: Protocol,
+                         inherit = true): Result[AsyncFD, OSErrorCode] =
   ## Creates new asynchronous socket.
   when defined(windows):
     let flags =
@@ -93,15 +140,12 @@ proc createAsyncSocket2*(domain: Domain, sockType: SockType,
     if fd == osdefs.INVALID_SOCKET:
       return err(osLastError())
 
-    let bres = setDescriptorBlocking(fd, false)
-    if bres.isErr():
+    setDescriptorBlocking(fd, false).isOkOr:
       discard closeFd(fd)
-      return err(bres.error())
-
-    let res = register2(AsyncFD(fd))
-    if res.isErr():
+      return err(error)
+    register2(AsyncFD(fd)).isOkOr:
       discard closeFd(fd)
-      return err(res.error())
+      return err(error)
 
     ok(AsyncFD(fd))
   else:
@@ -114,23 +158,20 @@ proc createAsyncSocket2*(domain: Domain, sockType: SockType,
       let fd = osdefs.socket(toInt(domain), socketType, toInt(protocol))
       if fd == -1:
         return err(osLastError())
-      let res = register2(AsyncFD(fd))
-      if res.isErr():
+      register2(AsyncFD(fd)).isOkOr:
         discard closeFd(fd)
-        return err(res.error())
+        return err(error)
       ok(AsyncFD(fd))
     else:
       let fd = osdefs.socket(toInt(domain), toInt(sockType), toInt(protocol))
       if fd == -1:
         return err(osLastError())
-      let bres = setDescriptorFlags(cint(fd), true, true)
-      if bres.isErr():
+      setDescriptorFlags(cint(fd), true, true).isOkOr:
         discard closeFd(fd)
-        return err(bres.error())
-      let res = register2(AsyncFD(fd))
-      if res.isErr():
+        return err(error)
+      register2(AsyncFD(fd)).isOkOr:
         discard closeFd(fd)
-        return err(bres.error())
+        return err(error)
       ok(AsyncFD(fd))
 
 proc wrapAsyncSocket2*(sock: cint|SocketHandle): Result[AsyncFD, OSErrorCode] =
@@ -230,3 +271,26 @@ proc createAsyncPipe*(): tuple[read: AsyncFD, write: AsyncFD] =
   else:
     let pipes = res.get()
     (read: AsyncFD(pipes.read), write: AsyncFD(pipes.write))
+
+proc getDualstack*(fd: AsyncFD): Result[bool, OSErrorCode] =
+  ## Returns `true` if `IPV6_V6ONLY` socket option set to `false`.
+  var
+    flag = cint(0)
+    size = SockLen(sizeof(flag))
+  let res = osdefs.getsockopt(SocketHandle(fd), cint(osdefs.IPPROTO_IPV6),
+                              cint(osdefs.IPV6_V6ONLY), addr(flag), addr(size))
+  if res == -1:
+    return err(osLastError())
+  ok(flag == cint(0))
+
+proc setDualstack*(fd: AsyncFD, value: bool): Result[void, OSErrorCode] =
+  ## Sets `IPV6_V6ONLY` socket option value to `false` if `value == true` and
+  ## to `true` if `value == false`.
+  var
+    flag = cint(if value: 0 else: 1)
+    size = SockLen(sizeof(flag))
+  let res = osdefs.setsockopt(SocketHandle(fd), cint(osdefs.IPPROTO_IPV6),
+                              cint(osdefs.IPV6_V6ONLY), addr(flag), size)
+  if res == -1:
+    return err(osLastError())
+  ok()
diff --git a/chronos/internal/asyncengine.nim b/chronos/internal/asyncengine.nim
index 5a46f04..ebcc278 100644
--- a/chronos/internal/asyncengine.nim
+++ b/chronos/internal/asyncengine.nim
@@ -670,6 +670,19 @@ when defined(windows):
     if not(isNil(aftercb)):
       loop.callbacks.addLast(AsyncCallback(function: aftercb, udata: param))
 
+  proc unregisterAndCloseFd*(fd: AsyncFD): Result[void, OSErrorCode] =
+    ## Unregister from system queue and close asynchronous socket.
+    ##
+    ## NOTE: Use this function to close temporary sockets/pipes only (which
+    ## are not exposed to the public and not supposed to be used/reused).
+    ## Please use closeSocket(AsyncFD) and closeHandle(AsyncFD) instead.
+    doAssert(fd != AsyncFD(osdefs.INVALID_SOCKET))
+    unregister(fd)
+    if closeFd(SocketHandle(fd)) != 0:
+      err(osLastError())
+    else:
+      ok()
+
   proc contains*(disp: PDispatcher, fd: AsyncFD): bool =
     ## Returns ``true`` if ``fd`` is registered in thread's dispatcher.
     fd in disp.handles
diff --git a/chronos/osdefs.nim b/chronos/osdefs.nim
index 78de4b7..ab07721 100644
--- a/chronos/osdefs.nim
+++ b/chronos/osdefs.nim
@@ -122,6 +122,7 @@ when defined(windows):
     SO_UPDATE_ACCEPT_CONTEXT* = 0x700B
     SO_CONNECT_TIME* = 0x700C
     SO_UPDATE_CONNECT_CONTEXT* = 0x7010
+    SO_PROTOCOL_INFOW* = 0x2005
 
     FILE_FLAG_FIRST_PIPE_INSTANCE* = 0x00080000'u32
     FILE_FLAG_OPEN_NO_RECALL* = 0x00100000'u32
@@ -258,6 +259,9 @@ when defined(windows):
     FIONBIO* = WSAIOW(102, 126)
 
     HANDLE_FLAG_INHERIT* = 1'u32
+    IPV6_V6ONLY* = 27
+    MAX_PROTOCOL_CHAIN* = 7
+    WSAPROTOCOL_LEN* = 255
 
   type
     LONG* = int32
@@ -441,6 +445,32 @@ when defined(windows):
       prefix*: SOCKADDR_INET
       prefixLength*: uint8
 
+    WSAPROTOCOLCHAIN* {.final, pure.} = object
+      chainLen*: int32
+      chainEntries*: array[MAX_PROTOCOL_CHAIN, DWORD]
+
+    WSAPROTOCOL_INFO* {.final, pure.} = object
+      dwServiceFlags1*: uint32
+      dwServiceFlags2*: uint32
+      dwServiceFlags3*: uint32
+      dwServiceFlags4*: uint32
+      dwProviderFlags*: uint32
+      providerId*: GUID
+      dwCatalogEntryId*: DWORD
+      protocolChain*: WSAPROTOCOLCHAIN
+      iVersion*: int32
+      iAddressFamily*: int32
+      iMaxSockAddr*: int32
+      iMinSockAddr*: int32
+      iSocketType*: int32
+      iProtocol*: int32
+      iProtocolMaxOffset*: int32
+      iNetworkByteOrder*: int32
+      iSecurityScheme*: int32
+      dwMessageSize*: uint32
+      dwProviderReserved*: uint32
+      szProtocol*: array[WSAPROTOCOL_LEN + 1, WCHAR]
+
     MibIpForwardRow2* {.final, pure.} = object
       interfaceLuid*: uint64
       interfaceIndex*: uint32
@@ -890,7 +920,7 @@ elif defined(macos) or defined(macosx):
                         O_NONBLOCK, SOL_SOCKET, SOCK_RAW, SOCK_DGRAM,
                         SOCK_STREAM, MSG_NOSIGNAL, MSG_PEEK,
                         AF_INET, AF_INET6, AF_UNIX, SO_ERROR, SO_REUSEADDR,
-                        SO_REUSEPORT, SO_BROADCAST, IPPROTO_IP,
+                        SO_REUSEPORT, SO_BROADCAST, IPPROTO_IP, IPPROTO_IPV6,
                         IPV6_MULTICAST_HOPS, SOCK_DGRAM, RLIMIT_NOFILE,
                         SIG_BLOCK, SIG_UNBLOCK, SHUT_RD, SHUT_WR, SHUT_RDWR,
                         SIGHUP, SIGINT, SIGQUIT, SIGILL, SIGTRAP, SIGABRT,
@@ -915,7 +945,7 @@ elif defined(macos) or defined(macosx):
          O_NONBLOCK, SOL_SOCKET, SOCK_RAW, SOCK_DGRAM,
          SOCK_STREAM, MSG_NOSIGNAL, MSG_PEEK,
          AF_INET, AF_INET6, AF_UNIX, SO_ERROR, SO_REUSEADDR,
-         SO_REUSEPORT, SO_BROADCAST, IPPROTO_IP,
+         SO_REUSEPORT, SO_BROADCAST, IPPROTO_IP, IPPROTO_IPV6,
          IPV6_MULTICAST_HOPS, SOCK_DGRAM, RLIMIT_NOFILE,
          SIG_BLOCK, SIG_UNBLOCK, SHUT_RD, SHUT_WR, SHUT_RDWR,
          SIGHUP, SIGINT, SIGQUIT, SIGILL, SIGTRAP, SIGABRT,
@@ -977,7 +1007,8 @@ elif defined(linux):
                         SOL_SOCKET, SO_ERROR, RLIMIT_NOFILE, MSG_NOSIGNAL,
                         MSG_PEEK,
                         AF_INET, AF_INET6, AF_UNIX, SO_REUSEADDR, SO_REUSEPORT,
-                        SO_BROADCAST, IPPROTO_IP, IPV6_MULTICAST_HOPS,
+                        SO_BROADCAST, IPPROTO_IP, IPPROTO_IPV6,
+                        IPV6_MULTICAST_HOPS,
                         SOCK_DGRAM, SOCK_STREAM, SHUT_RD, SHUT_WR, SHUT_RDWR,
                         POLLIN, POLLOUT, POLLERR, POLLHUP, POLLNVAL,
                         SIGHUP, SIGINT, SIGQUIT, SIGILL, SIGTRAP, SIGABRT,
@@ -1005,7 +1036,7 @@ elif defined(linux):
          SOL_SOCKET, SO_ERROR, RLIMIT_NOFILE, MSG_NOSIGNAL,
          MSG_PEEK,
          AF_INET, AF_INET6, AF_UNIX, SO_REUSEADDR, SO_REUSEPORT,
-         SO_BROADCAST, IPPROTO_IP, IPV6_MULTICAST_HOPS,
+         SO_BROADCAST, IPPROTO_IP, IPPROTO_IPV6, IPV6_MULTICAST_HOPS,
          SOCK_DGRAM, SOCK_STREAM, SHUT_RD, SHUT_WR, SHUT_RDWR,
          POLLIN, POLLOUT, POLLERR, POLLHUP, POLLNVAL,
          SIGHUP, SIGINT, SIGQUIT, SIGILL, SIGTRAP, SIGABRT,
@@ -1127,7 +1158,7 @@ elif defined(freebsd) or defined(openbsd) or defined(netbsd) or
                         O_NONBLOCK, SOL_SOCKET, SOCK_RAW, SOCK_DGRAM,
                         SOCK_STREAM, MSG_NOSIGNAL, MSG_PEEK,
                         AF_INET, AF_INET6, AF_UNIX, SO_ERROR, SO_REUSEADDR,
-                        SO_REUSEPORT, SO_BROADCAST, IPPROTO_IP,
+                        SO_REUSEPORT, SO_BROADCAST, IPPROTO_IP, IPPROTO_IPV6,
                         IPV6_MULTICAST_HOPS, SOCK_DGRAM, RLIMIT_NOFILE,
                         SIG_BLOCK, SIG_UNBLOCK, CLOCK_MONOTONIC,
                         SHUT_RD, SHUT_WR, SHUT_RDWR,
@@ -1154,7 +1185,7 @@ elif defined(freebsd) or defined(openbsd) or defined(netbsd) or
          O_NONBLOCK, SOL_SOCKET, SOCK_RAW, SOCK_DGRAM,
          SOCK_STREAM, MSG_NOSIGNAL, MSG_PEEK,
          AF_INET, AF_INET6, AF_UNIX, SO_ERROR, SO_REUSEADDR,
-         SO_REUSEPORT, SO_BROADCAST, IPPROTO_IP,
+         SO_REUSEPORT, SO_BROADCAST, IPPROTO_IP, IPPROTO_IPV6,
          IPV6_MULTICAST_HOPS, SOCK_DGRAM, RLIMIT_NOFILE,
          SIG_BLOCK, SIG_UNBLOCK, CLOCK_MONOTONIC,
          SHUT_RD, SHUT_WR, SHUT_RDWR,
@@ -1182,47 +1213,52 @@ when defined(linux):
     SOCK_CLOEXEC* = 0x80000
     TCP_NODELAY* = cint(1)
     IPPROTO_TCP* = 6
-elif defined(freebsd) or defined(netbsd) or defined(dragonfly):
+    O_CLOEXEC* = 0x80000
+    POSIX_SPAWN_USEVFORK* = 0x40
+    IPV6_V6ONLY* = 26
+elif defined(freebsd):
   const
     SOCK_NONBLOCK* = 0x20000000
     SOCK_CLOEXEC* = 0x10000000
     TCP_NODELAY* = cint(1)
     IPPROTO_TCP* = 6
+    O_CLOEXEC* = 0x00100000
+    POSIX_SPAWN_USEVFORK* = 0x00
+    IPV6_V6ONLY* = 27
+elif defined(netbsd):
+  const
+    SOCK_NONBLOCK* = 0x20000000
+    SOCK_CLOEXEC* = 0x10000000
+    TCP_NODELAY* = cint(1)
+    IPPROTO_TCP* = 6
+    O_CLOEXEC* = 0x00400000
+    POSIX_SPAWN_USEVFORK* = 0x00
+    IPV6_V6ONLY* = 27
+elif defined(dragonfly):
+  const
+    SOCK_NONBLOCK* = 0x20000000
+    SOCK_CLOEXEC* = 0x10000000
+    TCP_NODELAY* = cint(1)
+    IPPROTO_TCP* = 6
+    O_CLOEXEC* = 0x00020000
+    POSIX_SPAWN_USEVFORK* = 0x00
+    IPV6_V6ONLY* = 27
 elif defined(openbsd):
   const
     SOCK_CLOEXEC* = 0x8000
     SOCK_NONBLOCK* = 0x4000
     TCP_NODELAY* = cint(1)
     IPPROTO_TCP* = 6
+    O_CLOEXEC* = 0x10000
+    POSIX_SPAWN_USEVFORK* = 0x00
+    IPV6_V6ONLY* = 27
 elif defined(macos) or defined(macosx):
   const
     TCP_NODELAY* = cint(1)
     IP_MULTICAST_TTL* = cint(10)
     IPPROTO_TCP* = 6
-
-when defined(linux):
-  const
-    O_CLOEXEC* = 0x80000
-    POSIX_SPAWN_USEVFORK* = 0x40
-elif defined(freebsd):
-  const
-    O_CLOEXEC* = 0x00100000
-    POSIX_SPAWN_USEVFORK* = 0x00
-elif defined(openbsd):
-  const
-    O_CLOEXEC* = 0x10000
-    POSIX_SPAWN_USEVFORK* = 0x00
-elif defined(netbsd):
-  const
-    O_CLOEXEC* = 0x00400000
-    POSIX_SPAWN_USEVFORK* = 0x00
-elif defined(dragonfly):
-  const
-    O_CLOEXEC* = 0x00020000
-    POSIX_SPAWN_USEVFORK* = 0x00
-elif defined(macos) or defined(macosx):
-  const
     POSIX_SPAWN_USEVFORK* = 0x00
+    IPV6_V6ONLY* = 27
 
 when defined(linux) or defined(macos) or defined(macosx) or defined(freebsd) or
      defined(openbsd) or defined(netbsd) or defined(dragonfly):
diff --git a/chronos/oserrno.nim b/chronos/oserrno.nim
index 4f1c765..2a9f82c 100644
--- a/chronos/oserrno.nim
+++ b/chronos/oserrno.nim
@@ -1328,6 +1328,7 @@ elif defined(windows):
     ERROR_CONNECTION_REFUSED* = OSErrorCode(1225)
     ERROR_CONNECTION_ABORTED* = OSErrorCode(1236)
     WSAEMFILE* = OSErrorCode(10024)
+    WSAEAFNOSUPPORT* = OSErrorCode(10047)
     WSAENETDOWN* = OSErrorCode(10050)
     WSAENETRESET* = OSErrorCode(10052)
     WSAECONNABORTED* = OSErrorCode(10053)
diff --git a/chronos/osutils.nim b/chronos/osutils.nim
index 86505c2..f9c09f2 100644
--- a/chronos/osutils.nim
+++ b/chronos/osutils.nim
@@ -346,6 +346,10 @@ else:
         return err(osLastError())
     ok()
 
+  proc setDescriptorBlocking*(s: SocketHandle,
+                              value: bool): Result[void, OSErrorCode] =
+    setDescriptorBlocking(cint(s), value)
+
   proc setDescriptorInheritance*(s: cint,
                                  value: bool): Result[void, OSErrorCode] =
     let flags = handleEintr(osdefs.fcntl(s, osdefs.F_GETFD))
diff --git a/chronos/transports/common.nim b/chronos/transports/common.nim
index 4b4be7d..b7776e5 100644
--- a/chronos/transports/common.nim
+++ b/chronos/transports/common.nim
@@ -11,7 +11,7 @@
 
 import std/[strutils]
 import stew/[base10, byteutils]
-import ".."/[asyncloop, osdefs, oserrno]
+import ".."/[asyncloop, osdefs, oserrno, handles]
 
 from std/net import Domain, `==`, IpAddress, IpAddressFamily, parseIpAddress,
                     SockType, Protocol, Port, `$`
@@ -31,6 +31,9 @@ type
     ReuseAddr, ReusePort, TcpNoDelay, NoAutoRead, GCUserData, FirstPipe,
     NoPipeFlash, Broadcast
 
+  DualStackType* {.pure.} = enum
+    Auto, Enabled, Disabled, Default
+
   AddressFamily* {.pure.} = enum
     None, IPv4, IPv6, Unix
 
@@ -76,6 +79,7 @@ when defined(windows) or defined(nimdoc):
       asock*: AsyncFD               # Current AcceptEx() socket
       errorCode*: OSErrorCode       # Current error code
       abuffer*: array[128, byte]    # Windows AcceptEx() buffer
+      dualstack*: DualStackType     # IPv4/IPv6 dualstack parameters
       when defined(windows):
         aovl*: CustomOverlapped     # AcceptEx OVERLAPPED structure
 else:
@@ -90,6 +94,7 @@ else:
       bufferSize*: int              # Size of internal transports' buffer
       loopFuture*: Future[void]     # Server's main Future
       errorCode*: OSErrorCode       # Current error code
+      dualstack*: DualStackType     # IPv4/IPv6 dualstack parameters
 
 type
   TransportError* = object of AsyncError
@@ -720,3 +725,75 @@ proc raiseTransportError*(ecode: OSErrorCode) {.
       raise getTransportTooManyError(ecode)
     else:
       raise getTransportOsError(ecode)
+
+proc isAvailable*(family: AddressFamily): bool =
+  case family
+  of AddressFamily.None:
+    raiseAssert "Invalid address family"
+  of AddressFamily.IPv4:
+    isAvailable(Domain.AF_INET)
+  of AddressFamily.IPv6:
+    isAvailable(Domain.AF_INET6)
+  of AddressFamily.Unix:
+    isAvailable(Domain.AF_UNIX)
+
+proc getDomain*(socket: AsyncFD): Result[AddressFamily, OSErrorCode] =
+  ## Returns address family which is used to create socket ``socket``.
+  ##
+  ## Note: `chronos` supports only `AF_INET`, `AF_INET6` and `AF_UNIX` sockets.
+  ## For all other types of sockets this procedure returns
+  ## `EAFNOSUPPORT/WSAEAFNOSUPPORT` error.
+  when defined(windows):
+    let protocolInfo = ? getSockOpt2(socket, cint(osdefs.SOL_SOCKET),
+                                     cint(osdefs.SO_PROTOCOL_INFOW),
+                                     WSAPROTOCOL_INFO)
+    if protocolInfo.iAddressFamily == toInt(Domain.AF_INET):
+      ok(AddressFamily.IPv4)
+    elif protocolInfo.iAddressFamily == toInt(Domain.AF_INET6):
+      ok(AddressFamily.IPv6)
+    else:
+      err(WSAEAFNOSUPPORT)
+  else:
+    var
+      saddr = Sockaddr_storage()
+      slen = SockLen(sizeof(saddr))
+    if getsockname(SocketHandle(socket), cast[ptr SockAddr](addr saddr),
+                   addr slen) != 0:
+      return err(osLastError())
+    if int(saddr.ss_family) == toInt(Domain.AF_INET):
+      ok(AddressFamily.IPv4)
+    elif int(saddr.ss_family) == toInt(Domain.AF_INET6):
+      ok(AddressFamily.IPv6)
+    elif int(saddr.ss_family) == toInt(Domain.AF_UNIX):
+      ok(AddressFamily.Unix)
+    else:
+      err(EAFNOSUPPORT)
+
+proc setDualstack*(socket: AsyncFD, family: AddressFamily,
+                   flag: DualStackType): Result[void, OSErrorCode] =
+  if family == AddressFamily.IPv6:
+    case flag
+    of DualStackType.Auto:
+      # In case of `Auto` we going to ignore all the errors.
+      discard setDualstack(socket, true)
+      ok()
+    of DualStackType.Enabled:
+      ? setDualstack(socket, true)
+      ok()
+    of DualStackType.Disabled:
+      ? setDualstack(socket, false)
+      ok()
+    of DualStackType.Default:
+      ok()
+  else:
+    ok()
+
+proc setDualstack*(socket: AsyncFD,
+                   flag: DualStackType): Result[void, OSErrorCode] =
+  let family =
+    case flag
+    of DualStackType.Auto:
+      getDomain(socket).get(AddressFamily.IPv6)
+    else:
+      ? getDomain(socket)
+  setDualstack(socket, family, flag)
diff --git a/chronos/transports/datagram.nim b/chronos/transports/datagram.nim
index af29c2a..aec18ae 100644
--- a/chronos/transports/datagram.nim
+++ b/chronos/transports/datagram.nim
@@ -11,7 +11,7 @@
 
 import std/deques
 when not(defined(windows)): import ".."/selectors2
-import ".."/[asyncloop, osdefs, oserrno, handles]
+import ".."/[asyncloop, osdefs, oserrno, osutils, handles]
 import "."/common
 
 type
@@ -247,57 +247,65 @@ when defined(windows):
                                   udata: pointer,
                                   child: DatagramTransport,
                                   bufferSize: int,
-                                  ttl: int): DatagramTransport {.
+                                  ttl: int,
+                                  dualstack = DualStackType.Auto
+                                 ): DatagramTransport {.
       raises: [TransportOsError].} =
-    var localSock: AsyncFD
-    doAssert(remote.family == local.family)
     doAssert(not isNil(cbproc))
-    doAssert(remote.family in {AddressFamily.IPv4, AddressFamily.IPv6})
-
     var res = if isNil(child): DatagramTransport() else: child
 
-    if sock == asyncInvalidSocket:
-      localSock = createAsyncSocket(local.getDomain(), SockType.SOCK_DGRAM,
-                                    Protocol.IPPROTO_UDP)
-
-      if localSock == asyncInvalidSocket:
-        raiseTransportOsError(osLastError())
-    else:
-      if not setSocketBlocking(SocketHandle(sock), false):
-        raiseTransportOsError(osLastError())
-      localSock = sock
-      let bres = register2(localSock)
-      if bres.isErr():
-        raiseTransportOsError(bres.error())
+    let localSock =
+      if sock == asyncInvalidSocket:
+        let proto =
+          if local.family == AddressFamily.Unix:
+            Protocol.IPPROTO_IP
+          else:
+            Protocol.IPPROTO_UDP
+        let res = createAsyncSocket2(local.getDomain(), SockType.SOCK_DGRAM,
+                                     proto)
+        if res.isErr():
+          raiseTransportOsError(res.error)
+        res.get()
+      else:
+        setDescriptorBlocking(SocketHandle(sock), false).isOkOr:
+          raiseTransportOsError(error)
+        register2(sock).isOkOr:
+          raiseTransportOsError(error)
+        sock
 
     ## Apply ServerFlags here
     if ServerFlags.ReuseAddr in flags:
-      if not setSockOpt(localSock, osdefs.SOL_SOCKET, osdefs.SO_REUSEADDR, 1):
-        let err = osLastError()
+      setSockOpt2(localSock, SOL_SOCKET, SO_REUSEADDR, 1).isOkOr:
         if sock == asyncInvalidSocket:
           closeSocket(localSock)
-        raiseTransportOsError(err)
+        raiseTransportOsError(error)
 
     if ServerFlags.ReusePort in flags:
-      if not setSockOpt(localSock, osdefs.SOL_SOCKET, osdefs.SO_REUSEPORT, 1):
-        let err = osLastError()
+      setSockOpt2(localSock, SOL_SOCKET, SO_REUSEPORT, 1).isOkOr:
         if sock == asyncInvalidSocket:
           closeSocket(localSock)
-        raiseTransportOsError(err)
+        raiseTransportOsError(error)
 
     if ServerFlags.Broadcast in flags:
-      if not setSockOpt(localSock, osdefs.SOL_SOCKET, osdefs.SO_BROADCAST, 1):
-        let err = osLastError()
+      setSockOpt2(localSock, SOL_SOCKET, SO_BROADCAST, 1).isOkOr:
         if sock == asyncInvalidSocket:
           closeSocket(localSock)
-        raiseTransportOsError(err)
+        raiseTransportOsError(error)
 
       if ttl > 0:
-        if not setSockOpt(localSock, osdefs.IPPROTO_IP, osdefs.IP_TTL, ttl):
-          let err = osLastError()
+        setSockOpt2(localSock, osdefs.IPPROTO_IP, osdefs.IP_TTL, ttl).isOkOr:
           if sock == asyncInvalidSocket:
             closeSocket(localSock)
-          raiseTransportOsError(err)
+          raiseTransportOsError(error)
+
+    ## IPV6_V6ONLY
+    if sock == asyncInvalidSocket:
+      setDualstack(localSock, local.family, dualstack).isOkOr:
+        closeSocket(localSock)
+        raiseTransportOsError(error)
+    else:
+      setDualstack(localSock, dualstack).isOkOr:
+        raiseTransportOsError(error)
 
     ## Fix for Q263823.
     var bytesRet: DWORD
@@ -457,70 +465,75 @@ else:
                                   udata: pointer,
                                   child: DatagramTransport,
                                   bufferSize: int,
-                                  ttl: int): DatagramTransport {.
+                                  ttl: int,
+                                  dualstack = DualStackType.Auto
+                                 ): DatagramTransport {.
       raises: [TransportOsError].} =
-    var localSock: AsyncFD
-    doAssert(remote.family == local.family)
     doAssert(not isNil(cbproc))
-
     var res = if isNil(child): DatagramTransport() else: child
 
-    if sock == asyncInvalidSocket:
-      let proto =
-        if local.family == AddressFamily.Unix:
-          Protocol.IPPROTO_IP
-        else:
-          Protocol.IPPROTO_UDP
-      localSock = createAsyncSocket(local.getDomain(), SockType.SOCK_DGRAM,
-                                    proto)
-      if localSock == asyncInvalidSocket:
-        raiseTransportOsError(osLastError())
-    else:
-      if not setSocketBlocking(SocketHandle(sock), false):
-        raiseTransportOsError(osLastError())
-      localSock = sock
-      let bres = register2(localSock)
-      if bres.isErr():
-        raiseTransportOsError(bres.error())
+    let localSock =
+      if sock == asyncInvalidSocket:
+        let proto =
+          if local.family == AddressFamily.Unix:
+            Protocol.IPPROTO_IP
+          else:
+            Protocol.IPPROTO_UDP
+        let res = createAsyncSocket2(local.getDomain(), SockType.SOCK_DGRAM,
+                                     proto)
+        if res.isErr():
+          raiseTransportOsError(res.error)
+        res.get()
+      else:
+        setDescriptorBlocking(SocketHandle(sock), false).isOkOr:
+          raiseTransportOsError(error)
+        register2(sock).isOkOr:
+          raiseTransportOsError(error)
+        sock
 
     ## Apply ServerFlags here
     if ServerFlags.ReuseAddr in flags:
-      if not setSockOpt(localSock, osdefs.SOL_SOCKET, osdefs.SO_REUSEADDR, 1):
-        let err = osLastError()
+      setSockOpt2(localSock, SOL_SOCKET, SO_REUSEADDR, 1).isOkOr:
         if sock == asyncInvalidSocket:
           closeSocket(localSock)
-        raiseTransportOsError(err)
+        raiseTransportOsError(error)
 
     if ServerFlags.ReusePort in flags:
-      if not setSockOpt(localSock, osdefs.SOL_SOCKET, osdefs.SO_REUSEPORT, 1):
-        let err = osLastError()
+      setSockOpt2(localSock, SOL_SOCKET, SO_REUSEPORT, 1).isOkOr:
         if sock == asyncInvalidSocket:
           closeSocket(localSock)
-        raiseTransportOsError(err)
+        raiseTransportOsError(error)
 
     if ServerFlags.Broadcast in flags:
-      if not setSockOpt(localSock, osdefs.SOL_SOCKET, osdefs.SO_BROADCAST, 1):
-        let err = osLastError()
+      setSockOpt2(localSock, SOL_SOCKET, SO_BROADCAST, 1).isOkOr:
         if sock == asyncInvalidSocket:
           closeSocket(localSock)
-        raiseTransportOsError(err)
+        raiseTransportOsError(error)
 
       if ttl > 0:
-        let tres =
-          if local.family == AddressFamily.IPv4:
-            setSockOpt(localSock, osdefs.IPPROTO_IP, osdefs.IP_MULTICAST_TTL,
-                       cint(ttl))
-          elif local.family == AddressFamily.IPv6:
-            setSockOpt(localSock, osdefs.IPPROTO_IP, osdefs.IPV6_MULTICAST_HOPS,
-                       cint(ttl))
-          else:
-            raiseAssert "Unsupported address bound to local socket"
+        if local.family == AddressFamily.IPv4:
+          setSockOpt2(localSock, osdefs.IPPROTO_IP, osdefs.IP_MULTICAST_TTL,
+                      cint(ttl)).isOkOr:
+            if sock == asyncInvalidSocket:
+              closeSocket(localSock)
+            raiseTransportOsError(error)
+        elif local.family == AddressFamily.IPv6:
+          setSockOpt2(localSock, osdefs.IPPROTO_IP, osdefs.IPV6_MULTICAST_HOPS,
+                      cint(ttl)).isOkOr:
+            if sock == asyncInvalidSocket:
+              closeSocket(localSock)
+            raiseTransportOsError(error)
+        else:
+          raiseAssert "Unsupported address bound to local socket"
 
-        if not tres:
-          let err = osLastError()
-          if sock == asyncInvalidSocket:
-            closeSocket(localSock)
-          raiseTransportOsError(err)
+    ## IPV6_V6ONLY
+    if sock == asyncInvalidSocket:
+      setDualstack(localSock, local.family, dualstack).isOkOr:
+        closeSocket(localSock)
+        raiseTransportOsError(error)
+    else:
+      setDualstack(localSock, dualstack).isOkOr:
+        raiseTransportOsError(error)
 
     if local.family != AddressFamily.None:
       var saddr: Sockaddr_storage
@@ -594,8 +607,9 @@ proc newDatagramTransport*(cbproc: DatagramCallback,
                            udata: pointer = nil,
                            child: DatagramTransport = nil,
                            bufSize: int = DefaultDatagramBufferSize,
-                           ttl: int = 0
-                           ): DatagramTransport {.
+                           ttl: int = 0,
+                           dualstack = DualStackType.Auto
+                          ): DatagramTransport {.
     raises: [TransportOsError].} =
   ## Create new UDP datagram transport (IPv4).
   ##
@@ -610,7 +624,7 @@ proc newDatagramTransport*(cbproc: DatagramCallback,
   ## ``ttl`` - TTL for UDP datagram packet (only usable when flags has
   ## ``Broadcast`` option).
   newDatagramTransportCommon(cbproc, remote, local, sock, flags, udata, child,
-                             bufSize, ttl)
+                             bufSize, ttl, dualstack)
 
 proc newDatagramTransport*[T](cbproc: DatagramCallback,
                               udata: ref T,
@@ -620,13 +634,15 @@ proc newDatagramTransport*[T](cbproc: DatagramCallback,
                               flags: set[ServerFlags] = {},
                               child: DatagramTransport = nil,
                               bufSize: int = DefaultDatagramBufferSize,
-                              ttl: int = 0
-                              ): DatagramTransport {.
+                              ttl: int = 0,
+                              dualstack = DualStackType.Auto
+                             ): DatagramTransport {.
     raises: [TransportOsError].} =
   var fflags = flags + {GCUserData}
   GC_ref(udata)
   newDatagramTransportCommon(cbproc, remote, local, sock, fflags,
-                             cast[pointer](udata), child, bufSize, ttl)
+                             cast[pointer](udata), child, bufSize, ttl,
+                             dualstack)
 
 proc newDatagramTransport6*(cbproc: DatagramCallback,
                             remote: TransportAddress = AnyAddress6,
@@ -636,8 +652,9 @@ proc newDatagramTransport6*(cbproc: DatagramCallback,
                             udata: pointer = nil,
                             child: DatagramTransport = nil,
                             bufSize: int = DefaultDatagramBufferSize,
-                            ttl: int = 0
-                            ): DatagramTransport {.
+                            ttl: int = 0,
+                            dualstack = DualStackType.Auto
+                           ): DatagramTransport {.
     raises: [TransportOsError].} =
   ## Create new UDP datagram transport (IPv6).
   ##
@@ -652,7 +669,7 @@ proc newDatagramTransport6*(cbproc: DatagramCallback,
   ## ``ttl`` - TTL for UDP datagram packet (only usable when flags has
   ## ``Broadcast`` option).
   newDatagramTransportCommon(cbproc, remote, local, sock, flags, udata, child,
-                             bufSize, ttl)
+                             bufSize, ttl, dualstack)
 
 proc newDatagramTransport6*[T](cbproc: DatagramCallback,
                                udata: ref T,
@@ -662,13 +679,15 @@ proc newDatagramTransport6*[T](cbproc: DatagramCallback,
                                flags: set[ServerFlags] = {},
                                child: DatagramTransport = nil,
                                bufSize: int = DefaultDatagramBufferSize,
-                               ttl: int = 0
-                               ): DatagramTransport {.
+                               ttl: int = 0,
+                               dualstack = DualStackType.Auto
+                              ): DatagramTransport {.
     raises: [TransportOsError].} =
   var fflags = flags + {GCUserData}
   GC_ref(udata)
   newDatagramTransportCommon(cbproc, remote, local, sock, fflags,
-                             cast[pointer](udata), child, bufSize, ttl)
+                             cast[pointer](udata), child, bufSize, ttl,
+                             dualstack)
 
 proc join*(transp: DatagramTransport): Future[void] =
   ## Wait until the transport ``transp`` will be closed.
diff --git a/chronos/transports/stream.nim b/chronos/transports/stream.nim
index f96650c..8982b99 100644
--- a/chronos/transports/stream.nim
+++ b/chronos/transports/stream.nim
@@ -639,7 +639,8 @@ when defined(windows):
                 child: StreamTransport = nil,
                 localAddress = TransportAddress(),
                 flags: set[SocketFlags] = {},
-                ): Future[StreamTransport] =
+                dualstack = DualStackType.Auto
+               ): Future[StreamTransport] =
     ## Open new connection to remote peer with address ``address`` and create
     ## new transport object ``StreamTransport`` for established connection.
     ## ``bufferSize`` is size of internal buffer for transport.
@@ -658,24 +659,33 @@ when defined(windows):
 
       toSAddr(raddress, saddr, slen)
       proto = Protocol.IPPROTO_TCP
-      sock = createAsyncSocket(raddress.getDomain(), SockType.SOCK_STREAM,
-                               proto)
-      if sock == asyncInvalidSocket:
-        retFuture.fail(getTransportOsError(osLastError()))
+      sock = createAsyncSocket2(raddress.getDomain(), SockType.SOCK_STREAM,
+                                proto).valueOr:
+        retFuture.fail(getTransportOsError(error))
         return retFuture
 
+      if address.family in {AddressFamily.IPv4, AddressFamily.IPv6}:
+        if SocketFlags.TcpNoDelay in flags:
+          setSockOpt2(sock, osdefs.IPPROTO_TCP, osdefs.TCP_NODELAY, 1).isOkOr:
+            sock.closeSocket()
+            retFuture.fail(getTransportOsError(error))
+            return retFuture
+
       if SocketFlags.ReuseAddr in flags:
-        if not(setSockOpt(sock, SOL_SOCKET, SO_REUSEADDR, 1)):
-          let err = osLastError()
+        setSockOpt2(sock, SOL_SOCKET, SO_REUSEADDR, 1).isOkOr:
           sock.closeSocket()
-          retFuture.fail(getTransportOsError(err))
+          retFuture.fail(getTransportOsError(error))
           return retFuture
       if SocketFlags.ReusePort in flags:
-        if not(setSockOpt(sock, SOL_SOCKET, SO_REUSEPORT, 1)):
-          let err = osLastError()
+        setSockOpt2(sock, SOL_SOCKET, SO_REUSEPORT, 1).isOkOr:
           sock.closeSocket()
-          retFuture.fail(getTransportOsError(err))
+          retFuture.fail(getTransportOsError(error))
           return retFuture
+      # IPV6_V6ONLY.
+      setDualstack(sock, address.family, dualstack).isOkOr:
+        sock.closeSocket()
+        retFuture.fail(getTransportOsError(error))
+        return retFuture
 
       if localAddress != TransportAddress():
         if localAddress.family != address.family:
@@ -966,14 +976,9 @@ when defined(windows):
         if server.status notin {ServerStatus.Stopped, ServerStatus.Closed}:
           server.apending = true
           # TODO No way to report back errors!
-          server.asock =
-            block:
-              let sock = createAsyncSocket(server.domain, SockType.SOCK_STREAM,
-                                           Protocol.IPPROTO_TCP)
-              if sock == asyncInvalidSocket:
-                raiseOsDefect(osLastError(),
-                              "acceptLoop(): Unablet to create new socket")
-              sock
+          server.asock = createAsyncSocket2(server.domain, SockType.SOCK_STREAM,
+                                            Protocol.IPPROTO_TCP).valueOr:
+            raiseOsDefect(error, "acceptLoop(): Unablet to create new socket")
 
           var dwBytesReceived = DWORD(0)
           let dwReceiveDataLength = DWORD(0)
@@ -1167,15 +1172,13 @@ when defined(windows):
     if server.local.family in {AddressFamily.IPv4, AddressFamily.IPv6}:
       # TCP Sockets part
       var loop = getThreadDispatcher()
-      server.asock = createAsyncSocket(server.domain, SockType.SOCK_STREAM,
-                                       Protocol.IPPROTO_TCP)
-      if server.asock == asyncInvalidSocket:
-        let err = osLastError()
-        case err
+      server.asock = createAsyncSocket2(server.domain, SockType.SOCK_STREAM,
+                                        Protocol.IPPROTO_TCP).valueOr:
+        case error
         of ERROR_TOO_MANY_OPEN_FILES, WSAENOBUFS, WSAEMFILE:
-          retFuture.fail(getTransportTooManyError(err))
+          retFuture.fail(getTransportTooManyError(error))
         else:
-          retFuture.fail(getTransportOsError(err))
+          retFuture.fail(getTransportOsError(error))
         return retFuture
 
       var dwBytesReceived = DWORD(0)
@@ -1468,7 +1471,8 @@ else:
                 child: StreamTransport = nil,
                 localAddress = TransportAddress(),
                 flags: set[SocketFlags] = {},
-                ): Future[StreamTransport] =
+                dualstack = DualStackType.Auto,
+               ): Future[StreamTransport] =
     ## Open new connection to remote peer with address ``address`` and create
     ## new transport object ``StreamTransport`` for established connection.
     ## ``bufferSize`` - size of internal buffer for transport.
@@ -1483,36 +1487,37 @@ else:
       else:
         Protocol.IPPROTO_TCP
 
-    let sock = createAsyncSocket(address.getDomain(), SockType.SOCK_STREAM,
-                                 proto)
-    if sock == asyncInvalidSocket:
-      let err = osLastError()
-      case err
+    let sock = createAsyncSocket2(address.getDomain(), SockType.SOCK_STREAM,
+                                  proto).valueOr:
+      case error
       of oserrno.EMFILE:
         retFuture.fail(getTransportTooManyError())
       else:
-        retFuture.fail(getTransportOsError(err))
+        retFuture.fail(getTransportOsError(error))
       return retFuture
 
     if address.family in {AddressFamily.IPv4, AddressFamily.IPv6}:
       if SocketFlags.TcpNoDelay in flags:
-        if not(setSockOpt(sock, osdefs.IPPROTO_TCP, osdefs.TCP_NODELAY, 1)):
-          let err = osLastError()
+        setSockOpt2(sock, osdefs.IPPROTO_TCP, osdefs.TCP_NODELAY, 1).isOkOr:
           sock.closeSocket()
-          retFuture.fail(getTransportOsError(err))
+          retFuture.fail(getTransportOsError(error))
           return retFuture
+
     if SocketFlags.ReuseAddr in flags:
-      if not(setSockOpt(sock, SOL_SOCKET, SO_REUSEADDR, 1)):
-        let err = osLastError()
+      setSockOpt2(sock, SOL_SOCKET, SO_REUSEADDR, 1).isOkOr:
         sock.closeSocket()
-        retFuture.fail(getTransportOsError(err))
+        retFuture.fail(getTransportOsError(error))
         return retFuture
     if SocketFlags.ReusePort in flags:
-      if not(setSockOpt(sock, SOL_SOCKET, SO_REUSEPORT, 1)):
-        let err = osLastError()
+      setSockOpt2(sock, SOL_SOCKET, SO_REUSEPORT, 1).isOkOr:
         sock.closeSocket()
-        retFuture.fail(getTransportOsError(err))
+        retFuture.fail(getTransportOsError(error))
         return retFuture
+    # IPV6_V6ONLY.
+    setDualstack(sock, address.family, dualstack).isOkOr:
+      sock.closeSocket()
+      retFuture.fail(getTransportOsError(error))
+      return retFuture
 
     if localAddress != TransportAddress():
       if localAddress.family != address.family:
@@ -1532,17 +1537,14 @@ else:
 
     proc continuation(udata: pointer) =
       if not(retFuture.finished()):
-        var err = 0
-
-        let res = removeWriter2(sock)
-        if res.isErr():
+        removeWriter2(sock).isOkOr:
           discard unregisterAndCloseFd(sock)
-          retFuture.fail(getTransportOsError(res.error()))
+          retFuture.fail(getTransportOsError(error))
           return
 
-        if not(sock.getSocketError(err)):
+        let err = sock.getSocketError2().valueOr:
           discard unregisterAndCloseFd(sock)
-          retFuture.fail(getTransportOsError(res.error()))
+          retFuture.fail(getTransportOsError(error))
           return
 
         if err != 0:
@@ -1578,10 +1580,9 @@ else:
         # http://www.madore.org/~david/computers/connect-intr.html
         case errorCode
         of oserrno.EINPROGRESS, oserrno.EINTR:
-          let res = addWriter2(sock, continuation)
-          if res.isErr():
+          addWriter2(sock, continuation).isOkOr:
             discard unregisterAndCloseFd(sock)
-            retFuture.fail(getTransportOsError(res.error()))
+            retFuture.fail(getTransportOsError(error))
             return retFuture
           retFuture.cancelCallback = cancel
           break
@@ -1782,11 +1783,13 @@ proc connect*(address: TransportAddress,
               bufferSize = DefaultStreamBufferSize,
               child: StreamTransport = nil,
               flags: set[TransportFlags],
-              localAddress = TransportAddress()): Future[StreamTransport] =
+              localAddress = TransportAddress(),
+              dualstack = DualStackType.Auto
+             ): Future[StreamTransport] =
   # Retro compatibility with TransportFlags
   var mappedFlags: set[SocketFlags]
   if TcpNoDelay in flags: mappedFlags.incl(SocketFlags.TcpNoDelay)
-  address.connect(bufferSize, child, localAddress, mappedFlags)
+  connect(address, bufferSize, child, localAddress, mappedFlags, dualstack)
 
 proc close*(server: StreamServer) =
   ## Release ``server`` resources.
@@ -1848,7 +1851,8 @@ proc createStreamServer*(host: TransportAddress,
                          bufferSize: int = DefaultStreamBufferSize,
                          child: StreamServer = nil,
                          init: TransportInitCallback = nil,
-                         udata: pointer = nil): StreamServer {.
+                         udata: pointer = nil,
+                         dualstack = DualStackType.Auto): StreamServer {.
     raises: [TransportOsError].} =
   ## Create new TCP stream server.
   ##
@@ -1874,42 +1878,48 @@ proc createStreamServer*(host: TransportAddress,
   elif defined(windows):
     # Windows
     if host.family in {AddressFamily.IPv4, AddressFamily.IPv6}:
-      if sock == asyncInvalidSocket:
-        serverSocket = createAsyncSocket(host.getDomain(),
-                                         SockType.SOCK_STREAM,
-                                         Protocol.IPPROTO_TCP)
-
-        if serverSocket == asyncInvalidSocket:
-          raiseTransportOsError(osLastError())
-      else:
-        let bres = setDescriptorBlocking(SocketHandle(sock), false)
-        if bres.isErr():
-          raiseTransportOsError(bres.error())
-        let wres = register2(sock)
-        if wres.isErr():
-          raiseTransportOsError(wres.error())
-        serverSocket = sock
-      # SO_REUSEADDR is not useful for Unix domain sockets.
+      serverSocket =
+        if sock == asyncInvalidSocket:
+          # TODO (cheatfate): `valueOr` generates weird compile error.
+          let res = createAsyncSocket2(host.getDomain(), SockType.SOCK_STREAM,
+                                       Protocol.IPPROTO_TCP)
+          if res.isErr():
+            raiseTransportOsError(res.error())
+          res.get()
+        else:
+          setDescriptorBlocking(SocketHandle(sock), false).isOkOr:
+            raiseTransportOsError(error)
+          register2(sock).isOkOr:
+            raiseTransportOsError(error)
+          sock
+      # SO_REUSEADDR
       if ServerFlags.ReuseAddr in flags:
-        if not(setSockOpt(serverSocket, SOL_SOCKET, SO_REUSEADDR, 1)):
-          let err = osLastError()
+        setSockOpt2(serverSocket, SOL_SOCKET, SO_REUSEADDR, 1).isOkOr:
           if sock == asyncInvalidSocket:
             discard closeFd(SocketHandle(serverSocket))
-          raiseTransportOsError(err)
+          raiseTransportOsError(error)
+      # SO_REUSEPORT
       if ServerFlags.ReusePort in flags:
-        if not(setSockOpt(serverSocket, SOL_SOCKET, SO_REUSEPORT, 1)):
-          let err = osLastError()
+        setSockOpt2(serverSocket, SOL_SOCKET, SO_REUSEPORT, 1).isOkOr:
           if sock == asyncInvalidSocket:
             discard closeFd(SocketHandle(serverSocket))
-          raiseTransportOsError(err)
-      # TCP flags are not useful for Unix domain sockets.
+          raiseTransportOsError(error)
+      # TCP_NODELAY
       if ServerFlags.TcpNoDelay in flags:
-        if not(setSockOpt(serverSocket, osdefs.IPPROTO_TCP,
-                          osdefs.TCP_NODELAY, 1)):
-          let err = osLastError()
+        setSockOpt2(serverSocket, osdefs.IPPROTO_TCP,
+                    osdefs.TCP_NODELAY, 1).isOkOr:
           if sock == asyncInvalidSocket:
             discard closeFd(SocketHandle(serverSocket))
-          raiseTransportOsError(err)
+          raiseTransportOsError(error)
+      # IPV6_V6ONLY.
+      if sock == asyncInvalidSocket:
+        setDualstack(serverSocket, host.family, dualstack).isOkOr:
+          discard closeFd(SocketHandle(serverSocket))
+          raiseTransportOsError(error)
+      else:
+        setDualstack(serverSocket, dualstack).isOkOr:
+          raiseTransportOsError(error)
+
       host.toSAddr(saddr, slen)
       if bindSocket(SocketHandle(serverSocket),
                     cast[ptr SockAddr](addr saddr), slen) != 0:
@@ -1936,47 +1946,54 @@ proc createStreamServer*(host: TransportAddress,
       serverSocket = AsyncFD(0)
   else:
     # Posix
-    if sock == asyncInvalidSocket:
-      let proto = if host.family == AddressFamily.Unix:
-        Protocol.IPPROTO_IP
+    serverSocket =
+      if sock == asyncInvalidSocket:
+        let proto = if host.family == AddressFamily.Unix:
+          Protocol.IPPROTO_IP
+        else:
+          Protocol.IPPROTO_TCP
+        # TODO (cheatfate): `valueOr` generates weird compile error.
+        let res = createAsyncSocket2(host.getDomain(), SockType.SOCK_STREAM,
+                                     proto)
+        if res.isErr():
+          raiseTransportOsError(res.error())
+        res.get()
       else:
-        Protocol.IPPROTO_TCP
-      serverSocket = createAsyncSocket(host.getDomain(),
-                                       SockType.SOCK_STREAM,
-                                       proto)
-      if serverSocket == asyncInvalidSocket:
-        raiseTransportOsError(osLastError())
-    else:
-      let bres = setDescriptorFlags(cint(sock), true, true)
-      if bres.isErr():
-        raiseTransportOsError(osLastError())
-      let rres = register2(sock)
-      if rres.isErr():
-        raiseTransportOsError(osLastError())
-      serverSocket = sock
+        setDescriptorFlags(cint(sock), true, true).isOkOr:
+          raiseTransportOsError(error)
+        register2(sock).isOkOr:
+          raiseTransportOsError(error)
+        sock
 
     if host.family in {AddressFamily.IPv4, AddressFamily.IPv6}:
-      # SO_REUSEADDR and SO_REUSEPORT are not useful for Unix domain sockets.
+      # SO_REUSEADDR
       if ServerFlags.ReuseAddr in flags:
-        if not(setSockOpt(serverSocket, SOL_SOCKET, SO_REUSEADDR, 1)):
-          let err = osLastError()
+        setSockOpt2(serverSocket, SOL_SOCKET, SO_REUSEADDR, 1).isOkOr:
           if sock == asyncInvalidSocket:
             discard unregisterAndCloseFd(serverSocket)
-          raiseTransportOsError(err)
+          raiseTransportOsError(error)
+      # SO_REUSEPORT
       if ServerFlags.ReusePort in flags:
-        if not(setSockOpt(serverSocket, SOL_SOCKET, SO_REUSEPORT, 1)):
-          let err = osLastError()
+        setSockOpt2(serverSocket, SOL_SOCKET, SO_REUSEPORT, 1).isOkOr:
           if sock == asyncInvalidSocket:
             discard unregisterAndCloseFd(serverSocket)
-          raiseTransportOsError(err)
-      # TCP flags are not useful for Unix domain sockets.
+          raiseTransportOsError(error)
+      # TCP_NODELAY
       if ServerFlags.TcpNoDelay in flags:
-        if not(setSockOpt(serverSocket, osdefs.IPPROTO_TCP,
-                          osdefs.TCP_NODELAY, 1)):
-          let err = osLastError()
+        setSockOpt2(serverSocket, osdefs.IPPROTO_TCP,
+                    osdefs.TCP_NODELAY, 1).isOkOr:
           if sock == asyncInvalidSocket:
             discard unregisterAndCloseFd(serverSocket)
-          raiseTransportOsError(err)
+          raiseTransportOsError(error)
+      # IPV6_V6ONLY
+      if sock == asyncInvalidSocket:
+        setDualstack(serverSocket, host.family, dualstack).isOkOr:
+          discard closeFd(SocketHandle(serverSocket))
+          raiseTransportOsError(error)
+      else:
+        setDualstack(serverSocket, dualstack).isOkOr:
+          raiseTransportOsError(error)
+
     elif host.family in {AddressFamily.Unix}:
       # We do not care about result here, because if file cannot be removed,
       # `bindSocket` will return EADDRINUSE.
@@ -2016,6 +2033,7 @@ proc createStreamServer*(host: TransportAddress,
   sres.status = Starting
   sres.loopFuture = newFuture[void]("stream.transport.server")
   sres.udata = udata
+  sres.dualstack = dualstack
   if localAddress.family == AddressFamily.None:
     sres.local = host
   else:
@@ -2029,8 +2047,7 @@ proc createStreamServer*(host: TransportAddress,
       cb = acceptPipeLoop
 
     if not(isNil(cbproc)):
-      sres.aovl.data = CompletionData(cb: cb,
-                                        udata: cast[pointer](sres))
+      sres.aovl.data = CompletionData(cb: cb, udata: cast[pointer](sres))
     else:
       if host.family == AddressFamily.Unix:
         sres.sock =
@@ -2055,10 +2072,11 @@ proc createStreamServer*(host: TransportAddress,
                          bufferSize: int = DefaultStreamBufferSize,
                          child: StreamServer = nil,
                          init: TransportInitCallback = nil,
-                         udata: pointer = nil): StreamServer {.
+                         udata: pointer = nil,
+                         dualstack = DualStackType.Auto): StreamServer {.
     raises: [CatchableError].} =
   createStreamServer(host, nil, flags, sock, backlog, bufferSize,
-                     child, init, cast[pointer](udata))
+                     child, init, cast[pointer](udata), dualstack)
 
 proc createStreamServer*[T](host: TransportAddress,
                             cbproc: StreamCallback,
@@ -2068,12 +2086,13 @@ proc createStreamServer*[T](host: TransportAddress,
                             backlog: int = DefaultBacklogSize,
                             bufferSize: int = DefaultStreamBufferSize,
                             child: StreamServer = nil,
-                            init: TransportInitCallback = nil): StreamServer {.
+                            init: TransportInitCallback = nil,
+                            dualstack = DualStackType.Auto): StreamServer {.
     raises: [CatchableError].} =
   var fflags = flags + {GCUserData}
   GC_ref(udata)
   createStreamServer(host, cbproc, fflags, sock, backlog, bufferSize,
-                     child, init, cast[pointer](udata))
+                     child, init, cast[pointer](udata), dualstack)
 
 proc createStreamServer*[T](host: TransportAddress,
                             flags: set[ServerFlags] = {},
@@ -2082,12 +2101,13 @@ proc createStreamServer*[T](host: TransportAddress,
                             backlog: int = DefaultBacklogSize,
                             bufferSize: int = DefaultStreamBufferSize,
                             child: StreamServer = nil,
-                            init: TransportInitCallback = nil): StreamServer {.
+                            init: TransportInitCallback = nil,
+                            dualstack = DualStackType.Auto): StreamServer {.
     raises: [CatchableError].} =
   var fflags = flags + {GCUserData}
   GC_ref(udata)
   createStreamServer(host, nil, fflags, sock, backlog, bufferSize,
-                     child, init, cast[pointer](udata))
+                     child, init, cast[pointer](udata), dualstack)
 
 proc getUserData*[T](server: StreamServer): T {.inline.} =
   ## Obtain user data stored in ``server`` object.
diff --git a/tests/testdatagram.nim b/tests/testdatagram.nim
index 7db04f9..ae7ab23 100644
--- a/tests/testdatagram.nim
+++ b/tests/testdatagram.nim
@@ -533,6 +533,54 @@ suite "Datagram Transport test suite":
 
     result = res
 
+  proc performDualstackTest(
+         sstack: DualStackType, saddr: TransportAddress,
+         cstack: DualStackType, caddr: TransportAddress
+       ): Future[bool] {.async.} =
+    var
+      expectStr = "ANYADDRESS MESSAGE"
+      event = newAsyncEvent()
+      res = 0
+
+    proc process1(transp: DatagramTransport,
+                  raddr: TransportAddress): Future[void] {.async.} =
+      var bmsg = transp.getMessage()
+      var smsg = cast[string](bmsg)
+      if smsg == expectStr:
+        inc(res)
+      event.fire()
+
+    proc process2(transp: DatagramTransport,
+                  raddr: TransportAddress): Future[void] {.async.} =
+      discard
+
+    let
+      sdgram = newDatagramTransport(process1, local = saddr,
+                                    dualstack = sstack)
+      localcaddr =
+        if caddr.family == AddressFamily.IPv4:
+          AnyAddress
+        else:
+          AnyAddress6
+
+      cdgram = newDatagramTransport(process2, local = localcaddr,
+                                    dualstack = cstack)
+
+    var address = caddr
+    address.port = sdgram.localAddress().port
+
+    try:
+      await cdgram.sendTo(address, addr expectStr[0], len(expectStr))
+    except CatchableError:
+      discard
+    try:
+      await event.wait().wait(500.milliseconds)
+    except CatchableError:
+      discard
+
+    await allFutures(sdgram.closeWait(), cdgram.closeWait())
+    res == 1
+
   test "close(transport) test":
     check waitFor(testTransportClose()) == true
   test m1:
@@ -557,5 +605,83 @@ suite "Datagram Transport test suite":
     check waitFor(testBroadcast()) == 1
   test "0.0.0.0/::0 (INADDR_ANY) test":
     check waitFor(testAnyAddress()) == 6
+  asyncTest "[IP] getDomain(socket) [SOCK_DGRAM] test":
+    if isAvailable(AddressFamily.IPv4) and isAvailable(AddressFamily.IPv6):
+      block:
+        let res = createAsyncSocket2(Domain.AF_INET, SockType.SOCK_DGRAM,
+                                     Protocol.IPPROTO_UDP)
+        check res.isOk()
+        let fres = getDomain(res.get())
+        check fres.isOk()
+        discard unregisterAndCloseFd(res.get())
+        check fres.get() == AddressFamily.IPv4
+
+      block:
+        let res = createAsyncSocket2(Domain.AF_INET6, SockType.SOCK_DGRAM,
+                                     Protocol.IPPROTO_UDP)
+        check res.isOk()
+        let fres = getDomain(res.get())
+        check fres.isOk()
+        discard unregisterAndCloseFd(res.get())
+        check fres.get() == AddressFamily.IPv6
+
+      when not(defined(windows)):
+        block:
+          let res = createAsyncSocket2(Domain.AF_UNIX, SockType.SOCK_DGRAM,
+                                       Protocol.IPPROTO_IP)
+          check res.isOk()
+          let fres = getDomain(res.get())
+          check fres.isOk()
+          discard unregisterAndCloseFd(res.get())
+          check fres.get() == AddressFamily.Unix
+    else:
+      skip()
+  asyncTest "[IP] DualStack [UDP] server [DualStackType.Auto] test":
+    if isAvailable(AddressFamily.IPv4) and isAvailable(AddressFamily.IPv6):
+      let serverAddress = initTAddress("[::]:0")
+      check:
+        (await performDualstackTest(
+           DualStackType.Auto, serverAddress,
+           DualStackType.Auto, initTAddress("127.0.0.1:0"))) == true
+      check:
+        (await performDualstackTest(
+           DualStackType.Auto, serverAddress,
+           DualStackType.Auto, initTAddress("127.0.0.1:0").toIPv6())) == true
+      check:
+        (await performDualstackTest(
+           DualStackType.Auto, serverAddress,
+           DualStackType.Auto, initTAddress("[::1]:0"))) == true
+    else:
+      skip()
+  asyncTest "[IP] DualStack [UDP] server [DualStackType.Enabled] test":
+    if isAvailable(AddressFamily.IPv4) and isAvailable(AddressFamily.IPv6):
+      let serverAddress = initTAddress("[::]:0")
+      check:
+        (await performDualstackTest(
+           DualStackType.Enabled, serverAddress,
+           DualStackType.Auto, initTAddress("127.0.0.1:0"))) == true
+        (await performDualstackTest(
+           DualStackType.Enabled, serverAddress,
+           DualStackType.Auto, initTAddress("127.0.0.1:0").toIPv6())) == true
+        (await performDualstackTest(
+           DualStackType.Enabled, serverAddress,
+           DualStackType.Auto, initTAddress("[::1]:0"))) == true
+    else:
+      skip()
+  asyncTest "[IP] DualStack [UDP] server [DualStackType.Disabled] test":
+    if isAvailable(AddressFamily.IPv4) and isAvailable(AddressFamily.IPv6):
+      let serverAddress = initTAddress("[::]:0")
+      check:
+        (await performDualstackTest(
+           DualStackType.Disabled, serverAddress,
+           DualStackType.Auto, initTAddress("127.0.0.1:0"))) == false
+        (await performDualstackTest(
+           DualStackType.Disabled, serverAddress,
+           DualStackType.Auto, initTAddress("127.0.0.1:0").toIPv6())) == false
+        (await performDualstackTest(
+           DualStackType.Disabled, serverAddress,
+           DualStackType.Auto, initTAddress("[::1]:0"))) == true
+    else:
+      skip()
   test "Transports leak test":
     checkLeaks()
diff --git a/tests/teststream.nim b/tests/teststream.nim
index 762e996..b042792 100644
--- a/tests/teststream.nim
+++ b/tests/teststream.nim
@@ -1372,6 +1372,42 @@ suite "Stream Transport test suite":
     if not(sleepFut.finished()):
       await cancelAndWait(sleepFut)
 
+  proc performDualstackTest(
+         sstack: DualStackType, saddr: TransportAddress,
+         cstack: DualStackType, caddr: TransportAddress
+       ): Future[bool] {.async.} =
+    let server = createStreamServer(saddr, dualstack = sstack)
+    var address = caddr
+    address.port = server.localAddress().port
+    var acceptFut = server.accept()
+    let
+      clientTransp =
+        try:
+          let res = await connect(address,
+                                  dualstack = cstack).wait(500.milliseconds)
+          Opt.some(res)
+        except CatchableError:
+          Opt.none(StreamTransport)
+      serverTransp =
+        if clientTransp.isSome():
+          let res = await acceptFut
+          Opt.some(res)
+        else:
+          Opt.none(StreamTransport)
+
+    let testResult = clientTransp.isSome() and serverTransp.isSome()
+    var pending: seq[FutureBase]
+    if clientTransp.isSome():
+      pending.add(closeWait(clientTransp.get()))
+    if serverTransp.isSome():
+      pending.add(closeWait(serverTransp.get()))
+    else:
+      pending.add(cancelAndWait(acceptFut))
+    await allFutures(pending)
+    server.stop()
+    await server.closeWait()
+    testResult
+
   markFD = getCurrentFD()
 
   for i in 0..<len(addresses):
@@ -1469,6 +1505,97 @@ suite "Stream Transport test suite":
     waitFor(testConnectCancelLeaksTest())
   test "[IP] accept() cancellation leaks test":
     waitFor(testAcceptCancelLeaksTest())
+  asyncTest "[IP] getDomain(socket) [SOCK_STREAM] test":
+    if isAvailable(AddressFamily.IPv4) and isAvailable(AddressFamily.IPv6):
+      block:
+        let res = createAsyncSocket2(Domain.AF_INET, SockType.SOCK_STREAM,
+                                     Protocol.IPPROTO_TCP)
+        check res.isOk()
+        let fres = getDomain(res.get())
+        check fres.isOk()
+        discard unregisterAndCloseFd(res.get())
+        check fres.get() == AddressFamily.IPv4
+
+      block:
+        let res = createAsyncSocket2(Domain.AF_INET6, SockType.SOCK_STREAM,
+                                     Protocol.IPPROTO_TCP)
+        check res.isOk()
+        let fres = getDomain(res.get())
+        check fres.isOk()
+        discard unregisterAndCloseFd(res.get())
+        check fres.get() == AddressFamily.IPv6
+
+      when not(defined(windows)):
+        block:
+          let res = createAsyncSocket2(Domain.AF_UNIX, SockType.SOCK_STREAM,
+                                       Protocol.IPPROTO_IP)
+          check res.isOk()
+          let fres = getDomain(res.get())
+          check fres.isOk()
+          discard unregisterAndCloseFd(res.get())
+          check fres.get() == AddressFamily.Unix
+    else:
+      skip()
+  asyncTest "[IP] DualStack [TCP] server [DualStackType.Auto] test":
+    if isAvailable(AddressFamily.IPv4) and isAvailable(AddressFamily.IPv6):
+      let serverAddress = initTAddress("[::]:0")
+      check:
+        (await performDualstackTest(
+           DualStackType.Auto, serverAddress,
+           DualStackType.Auto, initTAddress("127.0.0.1:0"))) == true
+        (await performDualstackTest(
+           DualStackType.Auto, serverAddress,
+           DualStackType.Auto, initTAddress("127.0.0.1:0").toIPv6())) == true
+        (await performDualstackTest(
+           DualStackType.Auto, serverAddress,
+           DualStackType.Auto, initTAddress("[::1]:0"))) == true
+    else:
+      skip()
+  asyncTest "[IP] DualStack [TCP] server [DualStackType.Enabled] test":
+    if isAvailable(AddressFamily.IPv4) and isAvailable(AddressFamily.IPv6):
+      let serverAddress = initTAddress("[::]:0")
+      check:
+        (await performDualstackTest(
+           DualStackType.Enabled, serverAddress,
+           DualStackType.Auto, initTAddress("127.0.0.1:0"))) == true
+        (await performDualstackTest(
+           DualStackType.Enabled, serverAddress,
+           DualStackType.Auto, initTAddress("127.0.0.1:0").toIPv6())) == true
+        (await performDualstackTest(
+           DualStackType.Enabled, serverAddress,
+           DualStackType.Auto, initTAddress("[::1]:0"))) == true
+    else:
+      skip()
+  asyncTest "[IP] DualStack [TCP] server [DualStackType.Disabled] test":
+    if isAvailable(AddressFamily.IPv4) and isAvailable(AddressFamily.IPv6):
+      let serverAddress = initTAddress("[::]:0")
+      check:
+        (await performDualstackTest(
+           DualStackType.Disabled, serverAddress,
+           DualStackType.Auto, initTAddress("127.0.0.1:0"))) == false
+        (await performDualstackTest(
+           DualStackType.Disabled, serverAddress,
+           DualStackType.Auto, initTAddress("127.0.0.1:0").toIPv6())) == false
+        (await performDualstackTest(
+           DualStackType.Disabled, serverAddress,
+           DualStackType.Auto, initTAddress("[::1]:0"))) == true
+    else:
+      skip()
+  asyncTest "[IP] DualStack [TCP] connect [IPv4 mapped address] test":
+    if isAvailable(AddressFamily.IPv4) and isAvailable(AddressFamily.IPv6):
+      let serverAddress = initTAddress("[::]:0")
+      check:
+        (await performDualstackTest(
+           DualStackType.Auto, serverAddress,
+           DualStackType.Disabled, initTAddress("127.0.0.1:0"))) == true
+        (await performDualstackTest(
+           DualStackType.Auto, serverAddress,
+           DualStackType.Disabled, initTAddress("127.0.0.1:0").toIPv6())) == false
+        (await performDualstackTest(
+           DualStackType.Auto, serverAddress,
+           DualStackType.Disabled, initTAddress("[::1]:0"))) == true
+    else:
+      skip()
   test "Leaks test":
     checkLeaks()
   test "File descriptors leak test":