From 91c6755bb16747f88f23ac3fcb79d033deef5c93 Mon Sep 17 00:00:00 2001
From: Hans Dijkema <hans@dijkewijk.nl>
Date: Mon, 3 Aug 2026 16:48:29 +0200
Subject: [PATCH] network detection better.

---
 info.rkt               |  2 +-
 network.rkt            | 69 +++++++++++++++++++++++++++++++++---------
 scribblings/main.scrbl |  8 ++---
 3 files changed, 60 insertions(+), 19 deletions(-)

diff --git a/info.rkt b/info.rkt
index abeed9d..2c40b67 100644
--- a/info.rkt
+++ b/info.rkt
@@ -1,7 +1,7 @@
 #lang info
 
 (define pkg-authors '(hnmdijkema))
-(define version "0.1.6")
+(define version "0.1.7")
 (define license 'MIT)   
 (define collection "racket-upnp")
 (define pkg-desc "racket-upnp - UpnP and DLNA for racket")
diff --git a/network.rkt b/network.rkt
index a6f20fe..cb57be8 100644
--- a/network.rkt
+++ b/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")))
diff --git a/scribblings/main.scrbl b/scribblings/main.scrbl
index 837f717..243aeb4 100644
--- a/scribblings/main.scrbl
+++ b/scribblings/main.scrbl
@@ -14,10 +14,10 @@ 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.
+Returns a non-loopback local IPv4 address selected by the operating system for
+normal outbound IPv4 traffic. No datagram is sent. The selected address is
+normally also the interface through which SSDP multicast should be sent.
+Loopback and wildcard addresses are rejected.
 }