[vlc-devel] [PATCH 6/7] mdns: Add basic ip address detection (Linux)

[Roland Bewick]

On Fri, 26 Jul 2019 at 12:15 AM, Rémi Denis-Courmont <remi at remlab.net>
wrote:

> Le torstaina 25. heinäkuuta 2019, 8.39.11 EEST Roland Bewick a écrit :
> > On 24/07/2019 11:18 PM, Rémi Denis-Courmont wrote:
> > > Le keskiviikkona 24. heinäkuuta 2019, 19.10.50 EEST Roland Bewick a
> écrit
> :
> > >> Hi Rémi,
> > >>
> > >> Thanks for the reply.
> > >>
> > >> On 24/07/2019 10:59 PM, Rémi Denis-Courmont wrote:
> > >>> Le keskiviikkona 24. heinäkuuta 2019, 18.43.49 EEST Roland Bewick a
> > >>> écrit
> > >>>
> > >>>> ---
> > >>>>
> > >>>>    modules/services_discovery/microdns.c | 62
> > >>>>
> > >>>> +++++++++++++++++++++++++++++++++-- 1 file changed, 59
> insertions(+), 3
> > >>>> deletions(-)
> > >>>>
> > >>>> diff --git a/modules/services_discovery/microdns.c
> > >>>> b/modules/services_discovery/microdns.c index c460126b58..fa34ffa8d8
> > >>>> 100644
> > >>>> --- a/modules/services_discovery/microdns.c
> > >>>> +++ b/modules/services_discovery/microdns.c
> > >>>> @@ -28,6 +28,12 @@
> > >>>>
> > >>>>    #include <stdatomic.h>
> > >>>>    #include <assert.h>
> > >>>>
> > >>>> +#if defined( _WIN32 )
> > >>>> +
> > >>>> +#else
> > >>>> +    #include <ifaddrs.h>
> > >>>> +#endif
> > >>>> +
> > >>>>
> > >>>>    #include <vlc_common.h>
> > >>>>    #include <vlc_plugin.h>
> > >>>>    #include <vlc_modules.h>
> > >>>>
> > >>>> @@ -868,9 +874,59 @@ CloseRD( vlc_object_t *p_this )
> > >>>>
> > >>>>        CleanDiscoveryCommon( p_sys );
> > >>>>
> > >>>>    }
> > >>>>
> > >>>> -static char * detect_ip_address()
> > >>>> +static char * detect_ip_address( vlc_object_t *p_obj )
> > >>>>
> > >>>>    {
> > >>>>
> > >>>> -    return strdup("127.0.0.1"); /* TODO: actually detect ip
> address */
> > >>>> +    char result[16] = "127.0.0.1";
> > >>>> +    bool found = false;
> > >>>> +#if defined( _WIN32 )
> > >>>> +
> > >>>> +#else
> > >>>> +    struct ifaddrs *id;
> > >>>> +    if( getifaddrs( &id ) == 0 )
> > >>>> +    {
> > >>>> +        int selected_priority = 0;
> > >>>> +
> > >>>> +        for( ; id != NULL; id = id->ifa_next )
> > >>>> +        {
> > >>>> +            if( id->ifa_addr->sa_family == AF_INET )
> > >>>> +            {
> > >>>> +                struct sockaddr_in *addr_in = (struct sockaddr_in
> > >>>> *)id->ifa_addr; +
> > >>>> +                char *ip_address = inet_ntoa( addr_in->sin_addr );
> > >>>> +
> > >>>> +                /* TODO: smarter priority function. */
> > >>>> +                int priority = 0;
> > >>>> +                if( strncmp( "eth0", id->ifa_name, strlen( "eth0"
> ) )
> > >>>> ==
> > >>>> 0
> > >>>> ) +                {
> > >>>> +                    priority = 1000;
> > >>>> +                }
> > >>>> +                else if( strncmp( "wifi0", id->ifa_name, strlen(
> > >>>> "wifi0"
> > >>>> )
> > >>>> ) == 0 ) +                {
> > >>>> +                    priority = 2000;
> > >>>> +                }
> > >>>> +
> > >>>> +                msg_Dbg( p_obj, "Assigned interface %s priority
> %d",
> > >>>> +                         id->ifa_name, priority);
> > >>>> +
> > >>>> +                if( priority > selected_priority )
> > >>>> +                {
> > >>>> +                    strcpy( result, ip_address );
> > >>>> +                    selected_priority = priority;
> > >>>> +                    found = true;
> > >>>> +                }
> > >>>> +            }
> > >>>> +        }
> > >>>> +    }
> > >>>> +    else
> > >>>> +    {
> > >>>> +        msg_Err( p_obj, "Couldn't get interface addresses");
> > >>>> +    }
> > >>>
> > >>> That could have worked in the nineties. Machines, especially laptops,
> > >>> change IP addresses and have multiple addresses routinely nowadays,
> and
> > >>> Linux desktops interfaces are not always called eth0 or wifi0.
> > >>
> > >> I will take Alexandre's suggestion.
> > >>
> > >>> Besides, you need to respond with the correct IP address for the
> > >>> interface
> > >>> on which the request came, usually with IP_PKTINFO/IP6_PKTINFO if you
> > >>> use
> > >>> UDP.
> > >>
> > >> Respond to what? I don't understand.
> > >
> > > Typically, discovery protocols wait for inbound client requests.
> >
> > Oh - so I don't even have to do the detection myself. Great!
>
> You should never do detection.
>
> For unsolicited announcements, you should either enumerate all eligible
> interfaces and spam all of them, or leave the IP stack routing table to
> pick
> the correct one at a given time.
>
> For responses to unicast, you should copy the destination IP of the
> request as
> the source IP of the response.
>
> And for responses to multicast, you should again let the IP stack routing
> table pick the correct source address for the inbound interface.
>
> In any case, that can only work with packet info ancillary data. There are
> no
> reason to write OS-specific code here.
>

In this case we are responding to multicast (works currently as you
mentioned above I think) BUT also we are providing a unicast ip address in
the A/AAAA record in the response packet.

The relationship between the unicast address and the multicast response is
that they share the same interface. Right?

To get the unicast IP address from the interface we’re responding on, I
can’t see any other way than using custom code on Windows.

Sorry, could you please correct my understanding on this?

Thanks,

Roland



> --
> Реми Дёни-Курмон
> http://www.remlab.net/
>
>
>
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