get the default broadcast network address.

scribblings/main.scrbl

@@ -13,6 +13,14 @@ interface.
 
 @section{Discovering devices}
 
+@defproc[(upnp-default-ipv4-address) string?]{
+Returns the local IPv4 address selected by the operating system for traffic to
+the SSDP multicast destination @tt{239.255.255.250:1900}. No datagram is sent.
+This is normally the interface through which UPnP devices on the local network
+can be reached.
+}
+
+
 @defproc[(query-upnp-devices
           [kinds (or/c 'all symbol? (listof symbol?)) 'all]
           [#:interface interface (or/c #f string?) #f]
@@ -26,7 +34,10 @@ Discovers and describes matching UPnP devices.
 @racket['all] returns all described devices. A symbol such as
 @racket['media-renderer] selects one known kind, and a list selects several
 kinds. When @racket[interface] is a local IPv4 address, SSDP multicast is sent
-through that interface. Reverse DNS lookup is only attempted when @racket[dns?] is true.
+through that interface. When it is @racket[#f],
+@racket[upnp-default-ipv4-address] is used automatically. If automatic
+detection fails, discovery falls back to the interface selected by the
+operating system. Reverse DNS lookup is only attempted when @racket[dns?] is true.
 
 @racket[mx] is the SSDP response delay advertised in the M-SEARCH request.
 The default receive window is @racket[mx] plus one second. An explicit