get the default broadcast network address.

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)