network detection better.

network.rkt

@@ -2,32 +2,73 @@
 
 ;; Network helpers used by UPnP and SSDP.
 
-(require racket/udp
+(require racket/list
+         racket/string
+         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)
+;; These destinations are only used to make the operating system select its
+;; normal outbound IPv4 route. UDP connect does not transmit a datagram.
+(define route-probes
+  '(("1.1.1.1" 53)
+    ("8.8.8.8" 53)))
 
-;; 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)])
+(define (loopback-ipv4-address? address)
+  (string-prefix? address "127."))
+
+(define (usable-ipv4-address? address)
+  (and (string? address)
+       (not (string=? address "0.0.0.0"))
+       (not (loopback-ipv4-address? address))))
+
+(define (local-address-for destination port)
+  (let ([socket (udp-open-socket destination port)])
     (dynamic-wind
       void
       (lambda ()
-        (udp-connect! socket ssdp-multicast-address ssdp-multicast-port)
+        (udp-connect! socket destination 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)))))
+
+;; Return the local non-loopback IPv4 address selected by the operating system
+;; for normal outbound traffic. This is also the interface normally suitable
+;; for SSDP multicast. No datagram is transmitted.
+(define (upnp-default-ipv4-address)
+  (or
+   (for/or ([probe (in-list route-probes)])
+     (define destination (first probe))
+     (define port (second probe))
+     (with-handlers
+         ([exn:fail?
+           (lambda (exception)
+             (dbg-upnp-network
+              "Could not determine local IPv4 address using route to ~a:~a: ~a"
+              destination
+              port
+              (exn-message exception))
+             #f)])
+       (define address (local-address-for destination port))
+       (cond
+         [(usable-ipv4-address? address)
+          (dbg-upnp-network
+           "Selected UPnP IPv4 address ~a using route to ~a:~a"
+           address
+           destination
+           port)
+          address]
+         [else
+          (dbg-upnp-network
+           "Rejected unusable local IPv4 address ~a selected for route to ~a:~a"
+           address
+           destination
+           port)
+          #f])))
+   (error 'upnp-default-ipv4-address
+          "could not determine a non-loopback IPv4 address")))