diff --git a/proto/scanner.py b/proto/scanner.py
index 8211dd8..c4ba011 100755
--- a/proto/scanner.py
+++ b/proto/scanner.py
@@ -71,6 +71,12 @@ class Argument:
     signature: str = attr.ib(converter=proto_to_type)
     summary: str = attr.ib()
     enum: Optional["Enum"] = attr.ib()
+    interface: Optional["Interface"] = attr.ib()
+
+    @interface.validator  # type: ignore
+    def _validate_interface(self, attribute, value):
+        if value is not None and self.signature not in ["n", "o"]:
+            raise ValueError("Interface may only be set for object types")
 
     @property
     def as_arg(self) -> str:
@@ -107,9 +113,14 @@ class Argument:
 
     @classmethod
     def create(
-        cls, name: str, signature: str, summary: str = "", enum: Optional["Enum"] = None
+        cls,
+        name: str,
+        signature: str,
+        summary: str = "",
+        enum: Optional["Enum"] = None,
+        interface: Optional["Interface"] = None,
     ) -> "Argument":
-        return cls(name, signature, summary, enum)
+        return cls(name, signature, summary, enum, interface=interface)
 
 
 @attr.s
@@ -291,6 +302,8 @@ class Protocol(xml.sax.handler.ContentHandler):
     current_interface: Optional[Interface] = attr.ib(init=False, default=None)
     current_message: Optional[Union[Message, Enum]] = attr.ib(init=False, default=None)
 
+    run: int = attr.ib(init=False, default=0)
+
     def xmlerror(self, msg) -> None:
         line = self._locator.getLineNumber()  # type: ignore
         col = self._locator.getColumnNumber()  # type: ignore
@@ -303,6 +316,15 @@ class Protocol(xml.sax.handler.ContentHandler):
         col = self._locator.getColumnNumber()  # type: ignore
         return line, col
 
+    def interface_by_name(self, name) -> Interface:
+        try:
+            return [iface for iface in self.interfaces if iface.name == name].pop()
+        except IndexError:
+            raise XmlError(*self.location, f"Unable to find interface {name}")
+
+    def startDocument(self):
+        self.run += 1
+
     def startElement(self, element: str, attrs: dict):
         if element == "interface":
             if self.current_interface is not None:
@@ -321,10 +343,21 @@ class Protocol(xml.sax.handler.ContentHandler):
 
             if name.startswith("ei"):
                 name = f"{self.component}{name[2:]}"
-            intf = Interface.create(name=name, version=version, mode=self.component)
+            # We only create the interface on the first run, in subsequent runs we
+            # re-use them so we can cross reference correctly
+            if self.run > 1:
+                intf = self.interface_by_name(name)
+            else:
+                intf = Interface.create(name=name, version=version, mode=self.component)
+                self.interfaces.append(intf)
+
             self.current_interface = intf
-            self.interfaces.append(intf)
-        elif element == "request":
+
+        # first run only parses interfaces
+        if self.run <= 1:
+            return
+
+        if element == "request":
             if self.current_interface is None:
                 raise XmlError(
                     *self.location,
@@ -419,6 +452,13 @@ class Protocol(xml.sax.handler.ContentHandler):
             name = attrs["name"]
             sig = attrs["type"]
             summary = attrs.get("summary", "")
+            interface_name = attrs.get("interface", None)
+            if interface_name is not None:
+                if interface_name.startswith("ei"):
+                    interface_name = f"{self.component}{interface_name[2:]}"
+                interface = self.interface_by_name(interface_name)
+            else:
+                interface = None
             enum = attrs.get("enum", None)
             if enum is not None and enum not in [
                 e.name for e in self.current_interface.enums
@@ -427,7 +467,13 @@ class Protocol(xml.sax.handler.ContentHandler):
                     *self.location,
                     f"Failed to find enum '{self.current_interface.name}.{enum}'",
                 )
-            arg = Argument.create(name=name, signature=sig, summary=summary, enum=enum)
+            arg = Argument.create(
+                name=name,
+                signature=sig,
+                summary=summary,
+                enum=enum,
+                interface=interface,
+            )
             self.current_message.add_argument(arg)
         elif element == "entry":
             if self.current_interface is None:
@@ -452,7 +498,12 @@ class Protocol(xml.sax.handler.ContentHandler):
         if name == "interface":
             assert self.current_interface is not None
             self.current_interface = None
-        elif name == "request":
+
+        # first run only parses interfaces
+        if self.run <= 1:
+            return
+
+        if name == "request":
             assert isinstance(self.current_message, Request)
             self.current_message = None
         elif name == "event":
@@ -474,6 +525,8 @@ class Protocol(xml.sax.handler.ContentHandler):
 def parse(protofile: Path, component: str) -> Protocol:
     proto = Protocol.create(component=component)
     xml.sax.parse(os.fspath(protofile), proto)
+    # We parse two times, once to fetch all the interfaces, then to parse the details
+    xml.sax.parse(os.fspath(protofile), proto)
     return proto