diff --git a/mirage/opam_file.ml b/mirage/opam_file.ml
new file mode 100644
index 0000000..e08f252
--- /dev/null
+++ b/mirage/opam_file.ml
@@ -0,0 +1,126 @@
+let src = Logs.Src.create "opam-file.opam-mirror" ~doc:"Opam file decoding in opam-mirror"
+module Log = (val Logs.src_log src : Logs.LOG)
+
+module HM = Archive_checksum.HM
+
+let hash_to_string = Archive_checksum.Hash.to_string
+
+let hex_of_string s =
+  match Ohex.decode s with
+  | d -> Ok d
+  | exception Invalid_argument err -> Error (`Msg err)
+
+let decode_digest filename str =
+  let hex h s =
+    match hex_of_string s with
+    | Ok d -> Some (h, d)
+    | Error `Msg msg ->
+      Log.warn (fun m -> m "%s invalid hex (%s) %s" filename msg s); None
+  in
+  match String.split_on_char '=' str with
+  | [ data ] -> hex `MD5 data
+  | [ "md5" ; data ] -> hex `MD5 data
+  | [ "sha256" ; data ] -> hex `SHA256 data
+  | [ "sha512" ; data ] -> hex `SHA512 data
+  | [ hash ; _ ] -> Log.warn (fun m -> m "%s unknown hash %s" filename hash); None
+  | _ -> Log.warn (fun m -> m "%s unexpected hash format %S" filename str); None
+
+let extract_url_checksum filename items =
+  let open OpamParserTypes.FullPos in
+  let url =
+    List.find_opt
+      (function { pelem = Variable ({ pelem = "src" ; _ }, _); _ } -> true | _ -> false)
+      items
+  and archive =
+    List.find_opt
+      (function { pelem = Variable ({ pelem = "archive" ; _ }, _); _ } -> true | _ -> false)
+      items
+  and checksum =
+    List.find_opt
+      (function { pelem = Variable ({ pelem = "checksum" ; _ }, _); _ } -> true | _ -> false)
+      items
+  in
+  let url =
+    match url, archive with
+    | Some { pelem = Variable (_, { pelem = String url ; _ }) ; _ }, None -> Some url
+    | None, Some { pelem = Variable (_, { pelem = String url ; _ }); _ } -> Some url
+    | _ ->
+      Log.warn (fun m -> m "%s neither src nor archive present" filename); None
+  in
+  let csum =
+    match checksum with
+    | Some { pelem = Variable (_, { pelem = List { pelem = csums ; _ } ; _ }); _ } ->
+      let csums =
+        List.fold_left (fun acc ->
+            function
+            | { pelem = String csum ; _ } ->
+              begin match decode_digest filename csum with
+                | None -> acc
+                | Some (h, v) ->
+                  HM.update h (function
+                      | None -> Some v
+                      | Some v' when String.equal v v' -> None
+                      | Some v' ->
+                        Log.warn (fun m -> m "for %s, hash %s, multiple keys are present: %s %s"
+                                     (Option.value ~default:"NONE" url) (hash_to_string h) (Ohex.encode v) (Ohex.encode v'));
+                        None)
+                    acc
+              end
+            | _ -> acc) HM.empty csums
+      in
+      Some csums
+    | Some { pelem = Variable (_, { pelem = String csum ; _ }) ; _ } ->
+      begin match decode_digest filename csum with
+        | None -> None
+        | Some (h, v) -> Some (HM.singleton h v)
+      end
+    | _ ->
+      Log.warn (fun m -> m "couldn't decode checksum in %s" filename);
+      None
+  in
+  match url, csum with
+  | Some url, Some cs -> Some (url, cs)
+  | _ -> None
+
+let extract_checksums_and_urls filename opam =
+  let open OpamParserTypes.FullPos in
+  List.fold_left (fun acc ->
+      function
+      | { pelem = Section ({ section_kind = { pelem = "url" ; _ } ; section_items = { pelem = items ; _ } ; _ }) ; _} ->
+        begin match extract_url_checksum filename items with
+          | None -> acc
+          | Some url -> url :: acc
+        end
+      | { pelem = Section ({ section_kind = { pelem = "extra-source" ; _ } ; section_name = Some { pelem ; _ } ;  section_items = { pelem = items ; _ };  _ }) ; _} ->
+        begin
+          Log.debug (fun m -> m "extracting for extra-source %s in %s" filename pelem);
+          match extract_url_checksum filename items with
+          | None -> acc
+          | Some url -> url :: acc
+        end
+      | _ -> acc)
+    [] opam.file_contents
+
+let extract_urls filename str =
+  (* in an opam file, there may be:
+     url { src: <string> checksum: [ STRING ] } <- list of hash
+     url { src: <string> checksum: STRING } <- single hash
+     url { archive: <string> checksum: STRING } <- MD5
+     extra-source NAME { src: URL checksum: [ STRING ] } (OR checksum: STRING) <- multiple occurences possible
+  *)
+  let open OpamParserTypes.FullPos in
+  let opamfile = OpamParser.FullPos.string str filename in
+  let unavailable =
+    List.exists
+      (function
+        | { pelem = Variable ({ pelem = "available" ; _ },
+                              { pelem = (Bool false | List { pelem = [{ pelem = Bool false; _ }] ; _ }); _ })
+          ; _ } -> true
+        | _ -> false)
+      opamfile.file_contents
+  in
+  if unavailable then
+    (Log.debug (fun m -> m "%s is marked unavailable, skipping" filename);
+     [])
+  else
+    extract_checksums_and_urls filename opamfile
diff --git a/mirage/unikernel.ml b/mirage/unikernel.ml
index 27ef584..51da59a 100644
--- a/mirage/unikernel.ml
+++ b/mirage/unikernel.ml
@@ -57,7 +57,7 @@ module K = struct
     let doc = "Initialize the disk with a partition table. THIS IS DESTRUCTIVE!" in
     let doc = Arg.info ~doc ["initialize-disk"] in
     Mirage_runtime.register_arg Arg.(value & flag doc)
-  
+
   let ignore_local_git =
     let doc = "Ignore restoring locally saved git repository." in
     let doc = Arg.info ~doc ["ignore-local-git"] in
@@ -90,11 +90,6 @@ module Make
 
   let hex_to_key h = Mirage_kv.Key.v (Ohex.encode h)
 
-  let hex_of_string s =
-    match Ohex.decode s with
-    | d -> Ok d
-    | exception Invalid_argument err -> Error (`Msg err)
-
   let hm_to_s hm =
     HM.fold (fun h v acc ->
         hash_to_string h ^ "=" ^ Ohex.encode v ^ "\n" ^ acc)
@@ -123,107 +118,6 @@ module Make
       in
       go store Mirage_kv.Key.empty []
 
-    let decode_digest filename str =
-      let hex h s =
-        match hex_of_string s with
-        | Ok d -> Some (h, d)
-        | Error `Msg msg ->
-          Logs.warn (fun m -> m "%s invalid hex (%s) %s" filename msg s); None
-      in
-      match String.split_on_char '=' str with
-      | [ data ] -> hex `MD5 data
-      | [ "md5" ; data ] -> hex `MD5 data
-      | [ "sha256" ; data ] -> hex `SHA256 data
-      | [ "sha512" ; data ] -> hex `SHA512 data
-      | [ hash ; _ ] -> Logs.warn (fun m -> m "%s unknown hash %s" filename hash); None
-      | _ -> Logs.warn (fun m -> m "%s unexpected hash format %S" filename str); None
-
-    let extract_urls filename str =
-      (* in an opam file, there may be:
-         url { src: <string> checksum: [ STRING ] } <- list of hash
-         url { src: <string> checksum: STRING } <- single hash
-         url { archive: <string> checksum: STRING } <- MD5
-      *)
-      let open OpamParserTypes.FullPos in
-      let opamfile = OpamParser.FullPos.string str filename in
-      let unavailable =
-        List.exists
-          (function
-            | { pelem = Variable ({ pelem = "available" ; _ },
-                                  { pelem = (Bool false | List { pelem = [{ pelem = Bool false; _ }] ; _ }); _ })
-              ; _ } -> true
-            | _ -> false)
-          opamfile.file_contents
-      in
-      if unavailable then
-        (Logs.debug (fun m -> m "%s is marked unavailable, skipping" filename);
-         None)
-      else
-      let url_section =
-        List.find_opt (function
-            | { pelem = Section ({ section_kind = { pelem = "url" ; _ } ; _ }) ; _} -> true | _ -> false)
-          opamfile.file_contents
-      in
-      match url_section with
-      | Some { pelem = Section ({ section_items = { pelem = items ; _ }; _}) ; _ } ->
-        begin
-          let url =
-            List.find_opt
-              (function { pelem = Variable ({ pelem = "src" ; _ }, _); _ } -> true | _ -> false)
-              items
-          and archive =
-            List.find_opt
-              (function { pelem = Variable ({ pelem = "archive" ; _ }, _); _ } -> true | _ -> false)
-              items
-          and checksum =
-            List.find_opt
-              (function { pelem = Variable ({ pelem = "checksum" ; _ }, _); _ } -> true | _ -> false)
-              items
-          in
-          let url =
-            match url, archive with
-            | Some { pelem = Variable (_, { pelem = String url ; _ }) ; _ }, None -> Some url
-            | None, Some { pelem = Variable (_, { pelem = String url ; _ }); _ } -> Some url
-            | _ ->
-              Logs.warn (fun m -> m "%s neither src nor archive present" filename); None
-          in
-          let csum =
-            match checksum with
-            | Some { pelem = Variable (_, { pelem = List { pelem = csums ; _ } ; _ }); _ } ->
-              let csums =
-                List.fold_left (fun acc ->
-                    function
-                    | { pelem = String csum ; _ } ->
-                      begin match decode_digest filename csum with
-                        | None -> acc
-                        | Some (h, v) ->
-                          HM.update h (function
-                              | None -> Some v
-                              | Some v' when String.equal v v' -> None
-                              | Some v' ->
-                                Logs.warn (fun m -> m "for %s, hash %s, multiple keys are present: %s %s"
-                                              (Option.value ~default:"NONE" url) (hash_to_string h) (Ohex.encode v) (Ohex.encode v'));
-                                None)
-                            acc
-                      end
-                    | _ -> acc) HM.empty csums
-              in
-              Some csums
-            | Some { pelem = Variable (_, { pelem = String csum ; _ }) ; _ } ->
-              begin match decode_digest filename csum with
-                | None -> None
-                | Some (h, v) -> Some (HM.singleton h v)
-              end
-            | _ ->
-              Logs.warn (fun m -> m "couldn't decode checksum in %s" filename);
-              None
-          in
-          match url, csum with
-          | Some url, Some cs -> Some (url, cs)
-          | _ -> None
-        end
-      | _ -> Logs.debug (fun m -> m "no url section for %s" filename); None
-
     let find_urls store =
       find_contents store >>= fun paths ->
       let opam_paths =
@@ -234,8 +128,8 @@ module Make
           | Ok data ->
             (* TODO report parser errors *)
             (try
-               let url_csums = extract_urls (Mirage_kv.Key.to_string path) data in
-               Option.fold ~none:acc ~some:(fun (url, csums) ->
+               let url_csums = Opam_file.extract_urls (Mirage_kv.Key.to_string path) data in
+               List.fold_left (fun acc (url, csums) ->
                    if HM.cardinal csums = 0 then
                      (Logs.warn (fun m -> m "no checksums for %s, ignoring" url); acc)
                    else
@@ -252,7 +146,7 @@ module Make
                              Logs.warn (fun m -> m "mismatching hashes for %s: %s vs %s"
                                            url (hm_to_s csums') (hm_to_s csums));
                              None
-                           end) acc) url_csums
+                           end) acc) acc url_csums
              with _ ->
                Logs.warn (fun m -> m "some error in %a, ignoring" Mirage_kv.Key.pp path);
                acc)