Add primitive iptables wrapper which can set up port forwards

Signed-off-by: David Scott <dave.scott@docker.com>

alpine/packages/iptables/.gitignore

@@ -0,0 +1 @@
+iptables

alpine/packages/iptables/Dockerfile

@@ -1,5 +1,7 @@
 FROM ocaml/opam:alpine
+RUN sudo apk add m4
+RUN opam install ocamlfind astring -y
 WORKDIR /app
 ADD . /app
 RUN sudo chown -R opam /app
-RUN ocamlopt -o iptables main.ml
+RUN opam config exec -- ocamlfind ocamlopt -package unix,astring -linkpkg -o iptables main.ml

alpine/packages/iptables/main.ml

@@ -1,2 +1,82 @@
-let () =
-  print_string "Hello world!\n";;
+(* ocamlfind ocamlopt -package unix,astring -linkpkg -o iptables iptables.ml *)
+
+(*
+--wait -t nat -I DOCKER-INGRESS -p tcp --dport 80 -j DNAT --to-destination 172.18.0.2:80
+--wait -t nat -D DOCKER-INGRESS -p tcp --dport 80 -j DNAT --to-destination 172.18.0.2:80
+*)
+
+let _iptables = "/sbin/iptables"
+let _proxy = "/usr/bin/docker-proxy"
+let _pid_dir = "/var/run/service-port-opener"
+
+type port = {
+  proto: string;
+  dport: string; (* host port *)
+  ip:    string; (* container ip *)
+  port:  string; (* container port *)
+}
+
+let log_fd = Unix.openfile "/var/run/log/service-port-opener.log" [ Unix.O_WRONLY; Unix.O_APPEND; Unix.O_CREAT ] 0o0644
+
+let logf fmt =
+  Printf.ksprintf (fun s ->
+    let s = s ^ "\n" in
+    let rec loop ofs remaining =
+      if remaining > 0 then begin
+        let n = Unix.write log_fd s ofs remaining in
+        loop (ofs + n) (remaining - n)
+      end in
+    loop 0 (String.length s)
+  ) fmt
+
+let pid_filename { proto; dport; ip; port } =
+  Printf.sprintf "%s/%s.%s.%s.%s.pid" _pid_dir proto dport ip port
+
+let insert ({ proto; dport; ip; port } as p) =
+  let filename = pid_filename p in
+  logf "insert: creating a proxy for %s" filename;
+  let args = [ _proxy; "-proto"; proto; "-container-ip"; ip; "-container-port"; port; "-host-ip"; "0.0.0.0"; "-host-port"; dport; "-i" ] in
+  let pid = Unix.fork () in
+  if pid != 0 then begin
+    (* write pid to a file (not atomically) *)
+    let oc = open_out filename in
+    output_string oc (string_of_int pid);
+    close_out oc;
+    logf "binary = %s args = %s" _proxy (String.concat "; " args);
+    (try Unix.execv _proxy (Array.of_list args) with e -> logf "Failed with %s" (Printexc.to_string e));
+    exit 1
+  end
+
+let delete ({ proto; dport; ip; port } as p) =
+  let filename = pid_filename p in
+  logf "delete: removing a proxy for %s" filename;
+  (* read the pid from a file *)
+  try
+    let ic = open_in filename in
+    let pid = int_of_string (input_line ic) in
+    logf "Sending SIGTERM to %d" pid;
+    Unix.kill Sys.sigterm pid
+  with e ->
+    logf "delete: failed to remove proxy for %s: %s" filename (Printexc.to_string e);
+    raise e
+
+let parse_ip_port ip_port = match Astring.String.cut ~sep:":" ip_port with
+  | None ->
+    failwith ("Failed to parse <ip:port>:" ^ ip_port)
+  | Some (ip, port) ->
+    ip, port
+
+let _ =
+  ( try Unix.mkdir _pid_dir 0o0755 with Unix.Unix_error(Unix.EEXIST, _, _) -> () );
+  logf "intercepted arguments [%s]" (String.concat "; " (Array.to_list Sys.argv));
+  ( match Array.to_list Sys.argv with
+    | [ _; "--wait"; "-t"; "nat"; "-I"; "DOCKER-INGRESS"; "-p"; proto; "--dport"; dport; "-j"; "DNAT"; "--to-destination"; ip_port ] ->
+      let ip, port = parse_ip_port ip_port in
+      insert { proto; dport; ip; port }
+    | [ _; "--wait"; "-t"; "nat"; "-D"; "DOCKER-INGRESS"; "-p"; proto; "--dport"; dport; "-j"; "DNAT"; "--to-destination"; ip_port ] ->
+      let ip, port = parse_ip_port ip_port in
+      delete { proto; dport; ip; port }
+    | _ ->
+      ()
+  );
+  Unix.execv _iptables Sys.argv