#lang racket/base

;; Network helpers used by UPnP and SSDP.

(require racket/list
         racket/string
         racket/udp
         simple-log)

(provide upnp-default-ipv4-address)

(sl-def-log upnp-network)

;; 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)))

(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 destination port)
        (define-values (local-address local-port remote-address remote-port)
          (udp-addresses socket #t))
        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")))