diff --git a/config.ml b/config.ml
new file mode 100644
index 0000000..5707160
--- /dev/null
+++ b/config.ml
@@ -0,0 +1,90 @@
+open Ipaddr
+
+type zone =
+  | ZoneIpv4 of V4.Prefix.t
+  | ZoneIpv6 of V6.Prefix.t
+  | Zone of string
+  | ZoneList of zone list
+  | ZoneExclude of zone
+
+type addrs =
+  | ZoneName of string
+  | Ipv4 of V4.Prefix.t
+  | Ipv6 of V6.Prefix.t
+
+type l4_rule =
+  | TcpRule of { sport: int list; dport: int list }
+  | UdpRule of { sport: int list; dport: int list }
+  | IcmpRule
+
+type rule = { src: addrs list; dest: addrs list; l4: l4_rule }
+
+type config = { zones: (string * zone) list; rules: rule list }
+
+
+(* TODO *)
+let zone_of_json _ =
+  Zone "xxx"
+
+let addrs_of_json json =
+  let open Yojson.Basic.Util in
+  let value = json |> to_string in
+  let addrs = ZoneName value in
+  let addrs = match V6.Prefix.of_string value with
+    | Ok p -> Ipv6 p
+    | Error _ -> addrs
+  in
+  match V4.Prefix.of_string value with
+  | Ok p -> Ipv4 p
+  | Error _ -> addrs
+
+let addrs_list_of_json json =
+  let open Yojson.Basic.Util in
+  let elems = json |> to_list in
+  List.map addrs_of_json elems
+
+let to_list_force = function
+  | `List l -> l
+  | _ -> []
+
+let to_int_list json =
+  let open Yojson.Basic.Util in
+  json |> to_list_force |> List.map to_int
+
+let tcp_rule_of_json json =
+  let open Yojson.Basic.Util in
+  let sport = json |> member "sport" |> to_int_list in
+  let dport = json |> member "dport" |> to_int_list in
+  TcpRule { sport; dport }
+
+let udp_rule_of_json json =
+  let open Yojson.Basic.Util in
+  let sport = json |> member "sport" |> to_int_list in
+  let dport = json |> member "dport" |> to_int_list in
+  UdpRule { sport; dport }
+
+let l4_rule_of_json json =
+  let open Yojson.Basic.Util in
+  let proto = json |> member "proto" |> to_string in
+  match proto with
+  | "tcp" -> tcp_rule_of_json json
+  | "udp" -> udp_rule_of_json json
+  | "icmp" -> IcmpRule
+  | _ -> assert false
+
+let rule_of_json json =
+  let open Yojson.Basic.Util in
+  let src = addrs_list_of_json (json |> member "src") in
+  let dest = addrs_list_of_json (json |> member "dest") in
+  let l4 = l4_rule_of_json json in
+  { src; dest; l4 }
+
+let zones_of_json json =
+  let open Yojson.Basic.Util in
+  json |> to_assoc |> List.map (fun (n, z) -> (n, zone_of_json z))
+
+let config_of_json json =
+  let open Yojson.Basic.Util in
+  let zones = json |> member "zones" |> zones_of_json in
+  let rules = json |> member "rules" |> to_list |> List.map rule_of_json in
+  { zones; rules }