get the default broadcast network address.

info.rkt

@@ -1,7 +1,7 @@
 #lang info
 
 (define pkg-authors '(hnmdijkema))
-(define version "0.1.5")
+(define version "0.1.6")
 (define license 'MIT)   
 (define collection "racket-upnp")
 (define pkg-desc "racket-upnp - UpnP and DLNA for racket")

main.rkt

@@ -2,7 +2,8 @@
 
 ;; Main public interface for racket-upnp.
 
-(require "query.rkt"
+(require "network.rkt"
+         "query.rkt"
          "service.rkt"
          "didl-lite.rkt"
          "media-renderer.rkt"
@@ -11,6 +12,7 @@
 
 (provide
  (all-from-out
+  "network.rkt"
   "query.rkt"
   "service.rkt"
   "didl-lite.rkt"

network.rkt

@@ -0,0 +1,33 @@
+#lang racket/base
+
+;; Network helpers used by UPnP and SSDP.
+
+(require racket/udp
+         simple-log)
+
+(provide upnp-default-ipv4-address)
+
+(sl-def-log upnp-network)
+
+(define ssdp-multicast-address "239.255.255.250")
+(define ssdp-multicast-port 1900)
+
+;; Return the local IPv4 address selected by the operating system for traffic
+;; to the SSDP multicast destination. No datagram is transmitted.
+(define (upnp-default-ipv4-address)
+  (let ([socket (udp-open-socket ssdp-multicast-address
+                                 ssdp-multicast-port)])
+    (dynamic-wind
+      void
+      (lambda ()
+        (udp-connect! socket ssdp-multicast-address ssdp-multicast-port)
+        (define-values (local-address local-port remote-address remote-port)
+          (udp-addresses socket #t))
+        (dbg-upnp-network
+         "Selected UPnP IPv4 address ~a for SSDP destination ~a:~a"
+         local-address
+         ssdp-multicast-address
+         ssdp-multicast-port)
+        local-address)
+      (lambda ()
+        (udp-close socket)))))

query.rkt

@@ -183,7 +183,7 @@
   (dbg-upnp-query
    "Querying UPnP devices kinds=~s interface=~a dns?=~a mx=~a timeout=~a repeat=~a"
    kinds-value
-   (or interface "default")
+   (or interface "automatic")
    dns?
    mx
    (or timeout "default")

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

ssdp.rkt

@@ -9,7 +9,8 @@
 (require racket/list
          simple-log
          racket/string
-         racket/udp)
+         racket/udp
+         "network.rkt")
 
 (provide ssdp-response?
          ssdp-response-address
@@ -179,6 +180,7 @@
 ;; Search for a UPnP search target.  The default "ssdp:all" returns all
 ;; advertised device and service targets.  #:interface may be a local IPv4
 ;; address such as "10.7.3.118" when the machine has multiple interfaces.
+;; Without #:interface, the local IPv4 address for the SSDP route is detected.
 (define (ssdp-discover [search-target "ssdp:all"]
                        #:mx [mx 3]
                        #:timeout [timeout #f]
@@ -186,24 +188,36 @@
                        #:interface [interface #f]
                        #:ttl [ttl 2]
                        #:user-agent [user-agent #f])
-  (let ([effective-timeout (or timeout (+ mx 1.0))])
+  (let* ([effective-timeout (or timeout (+ mx 1.0))]
+         [effective-interface
+          (or interface
+              (with-handlers
+                  ([exn:fail?
+                    (lambda (exception)
+                      (warn-upnp-ssdp
+                       "Could not determine the UPnP IPv4 address: ~a; using the OS-selected interface"
+                       (exn-message exception))
+                      #f)])
+                (upnp-default-ipv4-address)))])
     (check-discovery-arguments search-target
                                mx
                                effective-timeout
                                repeat
-                               interface
+                               effective-interface
                                ttl
                                user-agent)
-    (dbg-upnp-ssdp "Starting SSDP discovery target=~a mx=~a timeout=~a repeat=~a interface=~a ttl=~a" search-target mx effective-timeout repeat (or interface "default") ttl)
+    (dbg-upnp-ssdp "Starting SSDP discovery target=~a mx=~a timeout=~a repeat=~a interface=~a ttl=~a" search-target mx effective-timeout repeat (or effective-interface "OS-selected") ttl)
     (let ([socket (udp-open-socket ssdp-multicast-address ssdp-multicast-port)]
           [request (make-search-request search-target mx user-agent)])
       (dynamic-wind
         void
         (lambda ()
           (define started-at (current-inexact-milliseconds))
-          (udp-bind! socket interface 0)
-          (dbg-upnp-ssdp "SSDP socket bound to interface ~a" (or interface "default"))
-          (udp-multicast-set-interface! socket interface)
+          (udp-bind! socket effective-interface 0)
+          (dbg-upnp-ssdp
+           "SSDP socket bound to local interface ~a"
+           (or effective-interface "OS-selected"))
+          (udp-multicast-set-interface! socket effective-interface)
           (udp-multicast-set-ttl! socket ttl)
           (for ([attempt (in-range repeat)])
             (dbg-upnp-ssdp "Sending SSDP M-SEARCH attempt ~a of ~a" (add1 attempt) repeat)