#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)))))