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Re: How to send UDP broadcast to

On 2009-06-16, Andrew Lunn <> wrote:
> On Mon, Jun 15, 2009 at 05:41:54PM +0000, Grant Edwards wrote:
>> I've been asked by one of my internal customers how to send a
>> UDP broadcast packet to IP address
> Hi Grant
> If you have a debugger handy, try putting a break point here:

I don't have one handy, but it certainly looks like the right

> src/sys/netinet/in_pcb.c
> int
> in_pcbladdr(inp, nam, plocal_sin)
>         register struct inpcb *inp;
>         struct sockaddr *nam;
>         struct sockaddr_in **plocal_sin;
> {
>         struct in_ifaddr *ia;
>         register struct sockaddr_in *sin = (struct sockaddr_in *)nam;
>         if (nam->sa_len != sizeof (*sin))
>                 return (EINVAL);
>         if (sin->sin_family != AF_INET)
>                 return (EAFNOSUPPORT);
>         if (sin->sin_port == 0)
>                 return (EADDRNOTAVAIL);
>         if (!TAILQ_EMPTY(&in_ifaddrhead)) {
>                 /*
>                  * If the destination address is INADDR_ANY,
>                  * use the primary local address.
>                  * If the supplied address is INADDR_BROADCAST,
>                  * and the primary interface supports broadcast,
>                  * choose the broadcast address for that interface.
>                  */
> #define satosin(sa)     ((struct sockaddr_in *)(sa))
> #define sintosa(sin)    ((struct sockaddr *)(sin))
> #define ifatoia(ifa)    ((struct in_ifaddr *)(ifa))
>                 if (sin->sin_addr.s_addr == INADDR_ANY)
>                     sin->sin_addr = IA_SIN(TAILQ_FIRST(&in_ifaddrhead))->sin_addr;
>                 else if (sin->sin_addr.s_addr == (u_long)INADDR_BROADCAST &&
>                   (TAILQ_FIRST(&in_ifaddrhead)->ia_ifp->if_flags & IFF_BROADCAST))
>                     sin->sin_addr = satosin(&TAILQ_FIRST(&in_ifaddrhead)->ia_broadaddr)->sin_addr;
> Unfortunately, if this is the place the address is being changed, i
> don't see an obvious way around this.

I've been doing some experiments on a FreeBSD 6.1 system, and
it won't allow applications to send a broadcast
either.  In fact, it _requires_ you to specify the interface's
broadcast address as a destination before in order to broadcast
a packet.  FreeBSD 6.1 won't "convert" into the
interface broadcast address the way that eCos does.

Still, the Linux implementation seems to be a lot more useful.
You can specify either or the interface's
broadcast address as the destination, and the packet is
broadcast to that destination address.

I found complaints dating back many years regarding FreeBSD's
lack of ability to send broadcast UDP packets to  It seems you have to use the raw socket
interface to do that.

Grant Edwards                   grante             Yow! Pardon me, but do you
                                  at               know what it means to be
                                 TRULY ONE with your BOOTH!

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