Create RAW socket?

gabrik · January 28, 2019, 12:44pm

Dear all,

I was looking at OCaml to build some pretty simple vSwitch and so I need to open a RAW socket in order to manage Ethernet frames.

But I’m unable to bind the socket to an interface.
This is how I create the socket

let create_socket iface = 
  let sock = Lwt_unix.socket Unix.PF_INET Unix.SOCK_RAW 0 in
  let sockaddr = .... ? What is supposed to be there? in
  bind sock sockaddr;
  sock

It is possible to create RAW socket in OCaml?
I’m supposed to wrap some C code like this lib ocaml-rawlink
Thanks,
Gabriele

CatseyeChandra · July 1, 2023, 12:59pm

Did you get an answer or solution to this by another mean? Got the same problem.

dangdennis · July 1, 2023, 5:41pm

Here’s an example. chatgpt’s fast becoming my new let-me-google-it-for-you tool.

Then there’s this excellent read. Modern communication: sockets

To create a socket address, you need to use one of the sockaddr variant type.

yawaramin · July 1, 2023, 5:45pm

The problem with this is that the original question was using Lwt, and ChatGPT is using Unix. To someone who is not very familiar with both of these libraries and their differences, this will lead to more confusion.

dangdennis · July 1, 2023, 5:53pm

That’s good to note. When I skimmed the original code, I saw only Unix.X .

Fortunately, lwt_unix reuses the standard Unix types.

One should be able to define socket addresses with the Unix.sockaddr types and use it with lwt’s socket apis.

Chet_Murthy · July 1, 2023, 11:44pm

Before you try to do this with lwt, why not try to do it with straight-up Unix ? That is, the unix library in OCaml? Indeed, first do it with C code, and you can then compare the strace output of your C program to that of your OCaml program written using unix. Once the latter works, you can then switch to lwt.

ETA: what I’m saying is: first get yourself a “known good” (C program version); then “baby-step” to a “known good OCaml version” (the version using unix) and only then baby-step to lwt (your current “known bad”).

haesbaert · July 3, 2023, 8:08am

Rawlink has Lwt bindings, you give the interface name and voilat:

github.com

haesbaert/rawlink/blob/master/lib/lwt_rawlink.mli#L28


      
          
              There are times when one needs to construct the full ethernet frame to
              send/receive. Most unixes support BPF (BSD Packet Filter) to achieve it, but
              linux provides the same functionality via AF_SOCKET. This API works with
              either a BPF or AF_SOCKET backend, so it should work on every usable UNIX,
              as well as linux out there. The module is designed to work with Lwt threads.
          *)
          
          type t
          
          val open_link : ?filter:string -> ?promisc:bool -> string -> t
          (** [open_link ~filter ~promisc interface]. Creates a rawlink on the
              specified [interface], a BPF program [filter] can be passed to
              filter out incoming packets. If [promisc] is true, sets [interface]
              to promiscuous mode, defaults to false. *)
          
          val close_link : t -> unit Lwt.t
          (** [close_link]. Closes a rawlink. *)
          
          val read_packet : t -> Cstruct.t Lwt.t
          (** [read_packet t]. Reads a full packet, may raise Unix.Unix_error. *)

github.com

haesbaert/rawlink/blob/master/lib/lwt_rawlink.mli#L37


      
          
          val open_link : ?filter:string -> ?promisc:bool -> string -> t
          (** [open_link ~filter ~promisc interface]. Creates a rawlink on the
              specified [interface], a BPF program [filter] can be passed to
              filter out incoming packets. If [promisc] is true, sets [interface]
              to promiscuous mode, defaults to false. *)
          
          val close_link : t -> unit Lwt.t
          (** [close_link]. Closes a rawlink. *)
          
          val read_packet : t -> Cstruct.t Lwt.t
          (** [read_packet t]. Reads a full packet, may raise Unix.Unix_error. *)
          
          val send_packet : t -> Cstruct.t -> unit Lwt.t
          (** [send_packet t]. Sends a full packet, may raise Unix.Unix_error. *)
          
          val dhcp_server_filter : unit -> string
          (** [dhcp_server_filter]. Returns a BPF program suitable to be passed in
              [open_link ~filter], it accepts UDP packets destined to
              port 67 (DHCP client). *)

follows a usage:

github.com

mirage/charrua/blob/7a874ed49d433f83442261da64bda18ac8331905/unix/charruad.ml#L98


      
            Duration.to_sec (Mtime_clock.elapsed_ns ())
          
          let maybe_gc db now gbcol =
            let open Lwt in
            if (now - gbcol) >= 60 then
              Lwt_log.debug "Garbage collecting..." >>= fun () ->
              return (Dhcp_server.Lease.garbage_collect db ~now:(Int32.of_int now), now + 60)
            else
              return (db, gbcol)
          
          let rec input config db link gbcol =
            let open Dhcp_server.Input in
            let open Lwt in
          
            Lwt_rawlink.read_packet link
            >>= fun buf ->
            let now = uptime_in_sec () in
            maybe_gc db now gbcol
            >>= fun (db, gbcol) ->
            let t = match Dhcp_wire.pkt_of_buf buf (Cstruct.length buf) with
              | Error e -> Lwt_log.error e

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Create RAW socket?

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