Windows uses SOCK_RAW for ICMP sockets

The MSDN reference[1] says:

> An example of the first type of socket is a socket for the ICMP protocol
> (IP protocol type = 1) or the ICMPv6 protocol (IP procotol type = 58).

[1] https://msdn.microsoft.com/en-us/library/windows/desktop/ms740548(v=vs.85).aspx

Signed-off-by: David Scott <dave@recoil.org>

src/stack-unix/icmpv4_socket.ml

@@ -9,6 +9,13 @@ type t = unit
 type error = [ `Ip of string ]
 let pp_error ppf (`Ip s) = Fmt.string ppf s
 
+let is_win32 = Sys.os_type = "Win32"
+
+let sock_icmp =
+  (* Windows uses SOCK_RAW protocol 1 for ICMP
+     Unix uses SOCK_DGRAM protocol 1 for ICMP *)
+  if is_win32 then Lwt_unix.SOCK_RAW else Lwt_unix.SOCK_DGRAM
+
 let ipproto_icmp = 1 (* according to BSD /etc/protocols *)
 let port = 0 (* port isn't meaningful in this context *)
 
@@ -29,7 +36,7 @@ let write _t ~dst buf =
   let flags = [] in
   let ipproto_icmp = 1 in (* according to BSD /etc/protocols *)
   let port = 0 in (* port isn't meaningful in this context *)
-  let fd = socket PF_INET SOCK_DGRAM ipproto_icmp in
+  let fd = socket PF_INET sock_icmp ipproto_icmp in
   let in_addr = Unix.inet_addr_of_string (Ipaddr.V4.to_string dst) in
   let sockaddr = ADDR_INET (in_addr, port) in
   Lwt.catch (fun () ->
@@ -61,7 +68,7 @@ let input t ~src ~dst:_ buf =
     | _, _ -> Lwt.return_unit
 
 let listen _t addr fn =
-  let fd = Lwt_unix.(socket PF_INET SOCK_DGRAM) ipproto_icmp in
+  let fd = Lwt_unix.socket PF_INET sock_icmp ipproto_icmp in
   let sa = Lwt_unix.ADDR_INET (Unix.inet_addr_of_string (Ipaddr.V4.to_string addr), port) in
   Lwt_unix.bind fd sa >>= fun () ->
   Log.debug (fun f -> f "Bound ICMP file descriptor to %a" pp_sockaddr sa);